A Scout is Brave Campaign Day 3 = 2013-12-04

8/13/2019 A Scout is Brave Campaign Day 3 = 2013-12-04

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A Scout Is Brave Campaign - Day 3

Yesterday morning as I was deciding to go to Irving a day earlier thanoriginally planned, an email from Jim Hansen popped into my inbox. I didnot take time to read it then, but last night I went back to it and readit quickly. Its about a paper he has been preparing for months in support

of lawsuits led by Kids vs Global Warming and Our Childrens Trust .The paper is authored by Dr. James E. Hansen and 17 colleagues, and itstitle is Assessing Dangerous Climate Change: Required Reduction ofCarbon Emissions to Protect Young People, Future Generations and Nature .(Click here for a short synopsis.)

The paper is a dire wa rning. It underscores the urgency of starting thereduction of CO2 emissions immediately -- by all means necessary, whichincludes government policies and regulations, industry leaders making ashift from conventional paradigms that have led to the current crisis to

proactive changes from burning carbon-based fossil fuels to replacingold dirty technology with life promoting clean carbon-free energytechnologies.

Rex Tillerson is one of the leaders who must start the shift -- freezingfunds for fossil fuel infrastructure, spending money to expand renewableenergy and dismantling and cleaning up the toxic remains of the carboninfrastructure.

Rex Tillersons staff yesterday told me that he is too busy to meet withme; that he only has time for scheduled business-related meetings.

In view of the urgency that RexTillerson address the fate of theplanet, I will continue to press him fora meeting to discuss the contents ofmy letters, which I handed to hissecurity guard yesterday in the formof A Scout Is B rave.Following is a copy of my em ail with

Jim Ha nsens paper and synopsis. I see

no alte rnative but to use the personalemail address prov ided by his staff toinject the most crit ical information into the Ofce of the Chairman.

Attachments: My email to Rex Tillerson dated today Jim Hansens email received yesterday Assessing "Dangerous Climate Change": Required Reduction of Carbon

Emissions to Protect Young People, Future Generations and Nature (6-page synopsis and the complete 26-page paper)

December 4, 2013
http://bit.ly/TellRexhttp://bit.ly/TellRexhttp://bit.ly/TellRexhttp://www.columbia.edu/~jeh1/mailings/2013/20131202_PopularSciencePlosOneE.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.columbia.edu/~jeh1/mailings/2013/20131202_PopularSciencePlosOneE.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.imatteryouth.org/http://www.imatteryouth.org/http://ourchildrenstrust.org/http://bit.ly/TellRexhttp://bit.ly/TellRexhttp://www.columbia.edu/~jeh1/mailings/2013/20131202_PopularSciencePlosOneE.pdfhttp://www.columbia.edu/~jeh1/mailings/2013/20131202_PopularSciencePlosOneE.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://www.plos.org/wp-content/uploads/2013/05/pone-8-12-hansen.pdfhttp://ourchildrenstrust.org/http://ourchildrenstrust.org/http://www.imatteryouth.org/http://www.imatteryouth.org/


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Dear Mr. Tillerson,

Yesterday, I delivered a copy of my book " A Scout Is Brave " to the security guard at your entrance gate.

Within minutes, coincidentally, I received a phone call from Sean in your office who explained that you are very busy, don't accept drop byrequests for meetings, only accept business-related requests, and that you will not be able to meet with me.

Yesterday, I also received an email from former NASA climatologist, Dr. James E. Hansen, in which he announced the completion of a paperthat he and 17 colleagues have been preparing in support of Kids vs Global Warming and Our Children's Trust lawsuits against several stategovernments.

Dr. Hansen et al. are issuing a dire warning that you would be well advised to heed. You have the position and the power to actually changethe course of history, if you were to take bold and courageous action.

I hope you will take time to familiarize yourself with the attachments, and give your staff the responsibility to quickly make an assessmentof the implications for ExxonMobil Corporation as well as your personal portfolio and wealth in the scenario that the science presented by Dr.

Hansen et al. is correct.

If there is even a remote likelihood that they are right and your own assessments are incorrect, would you risk humanity by continuing abusiness plan of unfettered expansion of the infrastructure that is now slowly but surely making our planet unsuitable for human life?

This is the email from Jim Hansen:

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Assessing "Dangerous Climate Change"

The paper 'Assessing "Dangerous Climate Change": RequiredReduction of Carbon Emissions to Protect Young People, FutureGenerations and Nature' is being published today in the leading open-access journal PLOS ONE. A 2-page paper summary + 4-page opinion(Hansen & Kharecha) re policy i mplications is available here or frommy web site .

The paper was written to provide the scientific basis for legal actionsagainst federal and state governments, in the United States and othernations, for not doing their job of protecting the rights of youngpeople. The legal actions being filed by Our Children's Trust ask thecourts to require the government to provide a plan for how they willreduce fossil fuel emissions consistent with stabilizing climate.

We dispute the common assumption that the world necessarily isgoing to develop all fossil fuels that can be found, thus making largeglobal warming inevitable. Humanity does not need to be a bunch of lemmings headed over a cliff. Indeed, appropriate policies that phaseout fossil fuel emissions over decades would be economically andenvironmentally beneficial. The editors of PLOS ONE, noting ourstatement "...there is still an opportunity for humanity to exercise free will", are establishing a "Responding to Climate Change" Collection inthe journal PLOS ONE. They invite paper submissions in all areas of research and a broad range of disciplines aimed at returning Earth toa state of energy balance.

The paper draws attention to the moral and ethical issues caused by the inertia of the climate system, which causes most of the impacts of climate change to be felt by young people and future generations, as aconsequence of action or inaction of the current generation. Besidesthis moral issue, we point out that effective government policies,collecting a rising carbon fee from the fossil fuel industry that madefossil fuels pay their costs to society, would be a path to economicprosperity, while business-as-usual only assures economic decline.

~Jim3 December 2013

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Dr. James E. Hansenhttp://www.columbia.edu/~jeh1

James Hansen [email protected]

James Hansen Assessing "Dangerous Climate Change"

Douglas Grandt Rex Tillerson

Re: Meeting request to present a copy of my book to you

4 Attachments, 3.8 MB


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. | |

. .: . .

This is his synopsis of the paper Assessing Dangerous Climate Change: Required Reduction of Carbon Emissions to Protect Young People,Future Generations and Nature

Hansen - 20df (231 KB )

This is the complete paper Assessing Dangerous Climate Change: Required Reduction of Carbon Emissions to Protect Young People, FutureGenerations and Nature

Hansen - As pdf (2.8 MB)

Please read " A Scout Is Brave " which I left for you -- I believe you will decide to invite me back for coffee to discuss its contents.

In case you do not take the time to open and read my book, I would like to repeat here the inscription that I wrote to you on the title page:

I wrote this book to you -- it started as a le tter requesting a meeting so I could explain my vision for you as the one person who can changethe course of history. I still believe in that vision. May we meet Eagle Scout to Eagle Scout? I am anxious to talk with you -- see my firstletter, March 25, 2012

I continue to believe that you will understand the mutual value we have in meeting some time soon.

Very best regards,

Doug Grandt510-432-1452

Begin forwarded message:

From: Douglas GrandtSubject: Meeting request to present a copy of my book to youDate: December 2, 2013 10:10:53 AM CSTTo: Rex Tillerson

Dear Mr. Tillerson,

This email is a follow-up to my phone request last week to meet with you sometime in December.

This morning, I spoke with a receptionist in your Office of the Chairman, and explained my follow-up, suggesting the likelihood that myrequest of last Monday might have been overlooked during Thanksgiving festivities. He gave me your email address and suggested that Icontact you directly.

I would like to meet with you -- perhaps over coffee -- and give you a copy of my book A Scout Is Brave which I published November 12,2013.

The two-volume set is a compilation of over 350 letters that I have sent to you over the past twenty months. It is likely that you are notaware of my letters and my message. I believe it is possible that your staff have filed them and not bothered you with my persistence toconvey to you what is an urgent need to change course.

The only reason I published the tome was to create a simple way to give my letters to you in a format convenient for your perusal.

I am in the DFW area for the month of December and have two complete sets of the two-volume tome to present to you. I am available atyour convenience to explain in brief terms what it is that I envision, as a fellow Eagle, former Exxon (Humble Oil) employee, andgrandparent.
http://bit.ly/TellRex


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Please find time to have coffee with me.

You may email me at [email protected] or call me at 510-432-1452.

Sincerely yours,

Doug Grandt
mailto:[email protected]


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Assessing "Dangerous Climate Change"

The paper 'Assessing "Dangerous Climate Change": RequiredReduction of Carbon Emissions to Protect Young People, FutureGenerations and Nature' is being published today in the leading open-access journal PLOS ONE. A 2-page paper summary + 4-page opinion(Hansen & Kharecha) re policy implications is available here or frommy web site .

The paper was written to provide the scientific basis for legal actionsagainst federal and state governments, in the United States and othernations, for not doing their job of protecting the rights of youngpeople. The legal actions being filed by Our Children's Trust ask thecourts to require the government to provide a plan for how they willreduce fossil fuel emissions consistent with stabilizing climate.

We dispute the common assumption that the world necessarily isgoing to develop all fossil fuels that can be found, thus making largeglobal warming inevitable. Humanity does not need to be a bunch of lemmings headed over a cliff. Indeed, appropriate policies that phaseout fossil fuel emissions over decades would be economically andenvironmentally beneficial. The editors of PLOS ONE, noting ourstatement "...there is still an opportunity for humanity to exercise free will", are establishing a "Responding to Climate Change" Collection inthe journal PLOS ONE. They invite paper submissions in all areas of research and a broad range of disciplines aimed at returning Earth toa state of energy balance.

The paper draws attention to the moral and ethical issues caused by the inertia of the climate system, which causes most of the impacts of climate change to be felt by young people and future generations, as aconsequence of action or inaction of the current generation. Besidesthis moral issue, we point out that effective government policies,collecting a rising carbon fee from the fossil fuel industry that madefossil fuels pay their costs to society, would be a path to economicprosperity, while business-as-usual only assures economic decline.

~Jim3 December 2013

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Dr. James E. Hansenhttp://www.columbia.edu/~jeh1

James Hansen [email protected]

James Hansen Assessing "Dangerous Climate Change"


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Assessing "Dangerous Climate Change": Required Reduction of Carbon Emissionsto Protect Young People, Future Generations and NatureJames Hansen, Pushker Kharecha, Makiko Sato, Valerie Masson-Delmotte, Frank Ackerman, David J. Beerling,Paul J. Hearty, Ove Hoegh-Guldberg, Shi-Ling Hsu, Camille Parmesan, Johan Rockstrom, Eelco J. Rohling, JeffreySachs, Pete Smith, Konrad Steffen, Lise Van Susteren, Karina von Schuckmann, James C. Zachos

This paper, by an international team of scientists, is being published today (3/12/2013) in the open access journal PLOS ONE, where it is freely available. The paper points out the clear and present danger thattoday's children may be handed a deteriorating climate with consequences out of their control. However,despite the fact that governments today seem to allow and encourage extraction of almost every fossil fuelthat can be found, we suggest that "there is still opportunity for humanity to exercise free will." 1

Here we (Hansen and Kharecha) first summarize some of the main conclusions reached in the paper.Then in a following discussion we provide our opinion concerning more detailed policy implications

Summary. We conclude that the widely accepted target of limiting human-made global climate warming to 2 degreesCelsius (3.6 degrees Fahrenheit) above the preindustrial level is too high and would subject young people,future generations and nature to irreparable harm. Carbon dioxide (CO 2) emissions from fossil fuel usemust be reduced rapidly to avoid irreversible consequences such as sea level rise large enough to inundatemost coastal cities and extermination of many of today's species. Unabated global warming would alsoworsen climate extremes. In association with summer high pressure systems, warming causes strongersummer heat waves, more intense droughts, and wildfires that burn hotter. Yet because warming causesthe atmosphere to hold more water vapor, which is the fuel that drives thunderstorms, tornadoes andtropical storms, it also leads to the possibility of stronger storms as well as heavier rainfall and floods.Observational data reveal that some climate extremes are already increasing in response to warming ofseveral tenths of a degree in recent decades; these extremes would likely be much enhanced with warmingof 2C or more.

We use evidence from Earth's climate history and measurements of Earth's present energy imbalance asour principal tools for inferring climate sensitivity and the safe level of global warming. The inferredwarming limit leads to a limit on cumulative fossil fuel emissions.

It is assessed that humanity must aim to keep global temperature close to the range occurring in the past10,000 years, the Holocene epoch, a time of relatively stable climate and stable sea level during whichcivilization developed. The world cooled slowly over the last half of the Holocene, but warming of 0.8C(1.4F) in the past 100 years has brought global temperature back near the Holocene maximum.

We note that policies should emphasize fossil fuel carbon, not mixing in carbon from forest changes as ifit were equivalent. Most of the carbon from fossil fuel burning will stay in the climate system for of order100,000 years. Of course carbon dioxide from deforestation also causes warming and policies mustaddress that carbon source, but good land use policies could restore most of that carbon to the biosphereon a time scale of decades to centuries. However, maximum biospheric restoration is likely to be onlycomparable to the past deforestation source, so fossil fuel sources must be strictly limited.

We conclude that human-made warming could be held to about 1C (1.8F) if cumulative industrial-erafossil fuel emissions are limited to 500 GtC (gigatons of carbon, where a gigaton is one billion metrictons) and if policies are pursued to restore 100 GtC into the biosphere, including the soil. This scenarioleads to reduction of atmospheric CO 2 to 350 ppm by 2100, as needed to restore Earth's energy balanceand approximately stabilize climate.

1 With this theme, PLOS ONE is initiating a Collection of papers "Responding to Climate Change". Papers are sought in areas ofresearch aimed at returning the Earth to a state of energy balance, including: Atmospheric Chemistry, Geoengineering,Alternative Energy, Science Policy, Economics, Behavioral Psychology, Conservation Biology. Articles will be publishedimmediately after passing peer review and acceptance. Information available at PLOS ONE booth #301 at the AGU meeting.


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In contrast, we conclude that the target to limit global warming to 2C, confirmed by the 2009Copenhagen Accord of the 15th Conference of the Parties of the United Nations Framework Conventionon Climate Change, would lead to disastrous consequences. For example, Earth's history shows that 2Cglobal warming is likely to result in eventual sea level rise of the order of six meters (20 feet). Moreover,we note that such a warming level would induce "slow amplifying feedbacks".

These amplifying feedbacks include a reduction of ice sheet area, vegetation changes including growth of

forests in high latitudes of Asia and North America that are now sparsely vegetated, and an increase ofatmospheric gases such as nitrous oxide and methane. These slow feedbacks are small if climate stayswithin the Holocene range, but substantial if warming reaches 2C or more.

Cumulative fossil fuel emissions through 2012 are 370 GtC and increasing almost 10 GtC per year. Thecurrent emission rate would need to decrease 6% per year to limit emissions to 500 GtC. If reductionshad begun in 1995, the required reduction rate would have been 2.1% per year, or 3.5% per year ifreductions had begun in 2005. If emissions continue to grow until 2020, reductions must be 15% per yearto stay within the 500 GtC limit, which emphasizes the urgency of initiating emission reductions.

The huge fossil fuel energy infrastructure now in place makes it practically certain that the 500 GtC limitwill be exceeded. However, the need to come as close as possible to that target is made clear by thespecter of likely climate impacts from 2C warming. Although it is difficult to predict the timing of

consequences such as large sea level rise, its eventual occurrence likely will be locked in if we allowwarming to reach a level as high as 2C. The situation would then be out of humanity's control, because,even if the atmospheric CO 2 amount declines, it would take many centuries for the ocean to cool down.

We draw attention to the difficulty, and possible impracticality, of extracting much CO 2 from the air, onceit becomes clear that an acceptable level of CO 2 has been overshot. Specifically, we note that theAmerican Physical Society estimates a cost for air capture of 1 GtC with current technology as about $2trillion, thus about $200 trillion to remove 100 GtC. Improved technologies might reduce this cost, butfundamental energy considerations imply that extraction will be very costly and very unlikely to bedeployed at a sufficient scale in the required time frame. At most CO 2 extraction might help alleviate amodest overshoot of the safe CO 2 level at a high cost to future generations.

The research team, which includes three economists, covers a broad range of fields and does not shy away

from "connecting the dots" all the way to policy implications. It concludes that the essential underlying policy, albeit not sufficient, is a rising price on carbon emissions that allows the costs of pollution andclimate change to be internalized within the economics of energy use. We note that a rising carbon feecollected from fossil fuel companies would improve economic efficiency, as it allows energy efficiencyand alternative low-carbon and no-carbon energies to compete on equal footing. The resulting energytransformations would generate many jobs, especially benefitting nations still in economic recession.

An advantage of a carbon fee or tax is the relative ease with which it can be made global. An agreementamong even a few of the largest economies (United States, China, European Union, Japan) could spurnear-global agreement. Countries agreeing to have a rising carbon fee would likely place border duties on

products from countries without a carbon fee, thus providing strong incentive for other countries to join.

Governments should also support technology research, development and demonstration of carbon-free

energy including advanced generation nuclear power as well as renewable energy, especially in view ofthe urgency with which emissions from coal and unconventional fossil fuels must be eliminated.(Unconventional fossil fuels include tar sands, shale-derived oil and gas, and methane hydrates.)


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Fig. 1. World energy consumption for indicated fuels, excludes wood [2].

Opinion: Further Policy Implications.

In the following pages we discuss policy implications of the science in more detail. We have noindication that the conclusions drawn would be especially controversial with the co-authors on our paper, but the discussion goes well beyond that in the paper, so we do not want to imply that we have discussedthese topics and reached a consensus on these specific conclusions.

The huge task of phasing out fossil fuel emissions is illustrated in Fig. 1, which shows that fossil fuels provide more than 85% of global energy today. Non-hydro renewable energy, despite laws in manynations that require utilities to employ increasing amounts of renewable energy, still provides only about2% of global energy. Thus cumulative efforts over several decades to expand renewable energy useoffset only about one year's current growth in global energy use and CO 2 emissions (Fig. 2).

The urgency of halting the rapid growth of emissions (Fig. 2), reversing that trend, and phasing down CO 2 emissions implies a need for an extraordinary change of energy systems and international cooperation. Itis crucial that the major international powers today realize that we are all in the same boat together andwe will all sink together or sail together. At such a time, when true leadership is needed, and on the 50thanniversary of John F. Kennedy's death, it is worth recalling words from his 1963 Peace speech: "No

problem of human destiny is beyond human beings. Man's reason and spirit have often solved theseemingly unsolvable--and we believe they can do it again."

The solution of a challenging problem in science is often revealed by clear objective problem definition.Indeed, an outline of the political approach required to stabilize climate is implied by the basic sciencedescribed in the present paper. The principal task is to limit the amount of CO 2 injected into the climatesystem by burning fossil fuels. Two subsidiary but important tasks are: (1) to achieve a net storage in the

biosphere and soil of at least ~100 GtC via reforestation and improved agricultural and forestry practices,and (2) to at least stabilize human-made non-CO 2 climate forcings (no net increase of present forcing).

Fig. 2 helps define what is needed to phase down CO 2 emissions. The main cause of growing emissionsis coal use, especially in China and developing countries. The biggest use of coal is electricity generation.Electricity is a clean energy carrier, the fastest growing form of energy use, and a key to solution of boththe climate and pollution problems. With abundant affordable carbon-free pollution-free sources ofelectricity, it becomes feasible not only to eliminate most of the CO 2 emissions from coal (nowapproaching half of the CO 2 emissions), but also to phase down oil and gas use, because electricity can beused for heating and for vehicles or to produce liquid fuels for vehicles.

Coal is used for electricity generation because it seems to be cheap, but that is only because it does not pay its costs to society. Those costs include not only climate impacts but air and water pollution, which


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Fig. 2. CO 2 annual emissions from fossil fuel use and cement manufacture, based on data of BritishPetroleum [2] concatenated with data of Boden et al. [3]. (A) is log scale and (B) is linear.

kills enough people in China, via increased incidence of cardiorespiratory mortality, to reduce averagelifespan by more than 5 years [5]. China is initiating large scale production of syngas, synthetic natural

gas extracted from coal. Syngas power plants produce less air pollution than pulverized-coal-fired power plants, but life-cycle greenhouse gas emissions are 36-82% higher than direct burning of coal [6].

A preferable approach, for the sake of both global climate and local pollution reduction, would be acombination of renewable energy and advanced (3rd and 4th) generation nuclear power plants 2.Abundant affordable clean energy is essential to provide the energy needed to raise billions of people outof poverty, which empirical evidence indicates is a requirement for reducing fertility rates, thus loweringhuman population, and giving hope that we can provide the opportunity of a good life to all humanitywhile allowing other life on the planet to flourish.

When the world's leading nations recognize the urgency of phasing out fossil fuel emissions, and realizethat we are all in the same boat, it should be possible to agree on cooperative technology development anddeployment. History, including World War II and the Apollo program, reveal how rapidly technology

can be developed and deployed. Phase-out of most coal emissions and a substantial reduction of oil andgas use could be achieved rapidly. This would require agreement among leading nations not only to havecommon internal rising carbon fees, but also an agreement to cooperate in rapid technology development.

Surely rapid phase-down of coal emissions requires a major role for advanced-generation safer nuclear power. Nuclear technology has advanced significantly over the past few decades such that there is nowthe potential to produce modular 3rd generation light-water reactors that are passively safe, i.e., reactorsthat would shut down automatically in case of an anomaly such as an earthquake and hav e the ability tokeep the nuclear fuel cool without an external power source. The same concept, modular 3 simplifiedreactor design with factory production and shipping to the utility site, is appropriate for 4th generationreactors, and these should also be pursued to deal with nuclear waste, utilizing the waste as fuel.

2 The existing fleet of nuclear power plants, largely 40-year-old 2nd generation technology, itself saved a large number of livesand reduced carbon emissions by replacing fossil fuel plants [7]. However, 3rd generation reactors incorporate additional safetyfeatures including automatic shutdown in case of anomalies and the ability to cool the nuclear fuel without external power. 4thgeneration power plants add an ability to utilize more than 99% of the nuclear fuel (compared with about 1% in earliergenerations) including the ability to "burn" nuclear waste, depleted uranium, and excess weapons material as fuel. Stockpiles ofthese latter materials contain enough fuel to power 4th generation reactors for centuries. Japan and the United States havedemonstrated that nuclear fuel can be sieved from the ocean, where there is enough nuclear material to last billions of years.Thus it will be possible to eliminate mining of uranium on land, should there be strong incentives to do that.3 An example of 3rd generation modular reactor is Gen III++ mPower (http://en.wikipedia.org/wiki/B&W_mPower). Anexample of 4th generation is the Integral Fast Reactor, Till and Chang [8]


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Fig. 3. (A) Fossil fuel CO 2 2012 emissions by source region and (B) cumulative 1751-2012 emissions(update of Fig. 10 of [4] using data from [3]). Emissions include gas flaring and cement manufacture.

Fortunately, the place where deployment of advanced nuclear technology is most urgently needed, China,

is also the place that has the potential to rapidly build and grow the manufacturing capability. What isneeded is cooperation with nations that have developed relevant technical abilities, especially the UnitedStates. Such cooperation has potential for enormous mutual and global benefits via development ofscalable affordable carbon-free energy. Contrary to assertions of dedicated anti-nuke activists, such

technology can be made more resistant than existing technology to exploitation by terrorists who mayseek weapons material. Dangers from rogue states or terrorists will always exist, and the best way tominimize such danger is to cooperate in developing the safest technology, not to pretend that anti-nuclearactivism will cause nuclear technology to disappear from the planet.

The principal policy allowing renewable energies to grow to almost 2% of global energy use has beenlaws imposing specified "renewable energy portfolio standards" (RPS) on utilities or other mandates forrenewable energy use. These policies have aided growth of renewables, and by spreading costs among allutility customers of feed-in tariffs, added transmission lines, and the backup power needed for intermittentrenewables (usually fossil fuel based), the electricity cost has been bearable as long as the portion ofrenewables is small. Now for the sake of moving rapidly to carbon-free power while minimizingelectricity costs, the need is for "clean energy portfolio standards" (CPS), thus allowing nuclear energy tocompete with renewable energies.

Every energy source has environmental and economic costs, and these need to weighed and considered bylocal populations, but given the enormous challenge of stabilizing climate it is inappropriate to eliminateany candidates a priori on a global basis. Some nations and states, e.g., Germany and California, seem tohave a preference to eliminate nuclear power a priori and they have populations that are willing to payhigh electricity costs. However, such nations and states need to understand that they must phase out fossilfuels entirely, and environmentalists need to recognize that attempts to force all-renewable policies on allof the world will only assure that fossil fuels continue to reign for baseload electric power, making it

unlikely that abundant affordable power will exist and implausible that fossil fuels will be phased out.As mentioned above, stabilizing climate requires, besides phasing down fossil fuel use: (1) storing at least~ 100 GtC in the biosphere and soil, and (2) stabilizing or even reducing human-made non-CO 2 climateforcings. Much of the solution of these matters necessarily will occur in developing countries and it isappropriate that the countries who caused the climate problem provide financial and other assistance toachieve these goals.

The burden for climate change causation is accurately specified by cumulative fossil fuel emissions [4](Fig. 3B). It is important for this burden to be recognized now and for all nations to realize that their


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proportion of the burden will continue to change as emissions continue. Thus, for example, if Chinachooses to build extensive syngas plants, China will incur a heavy future obligation, which should befactored into their decision-making process. Obligations implied by human-made climate change tend to

be ignored by today's political leaders, but the leaders need to become aware that this matter and theglobal cacophony for reparations will grow as climate change impacts accelerate.

We do not attempt to define specific agreements that must be reached to achieve the goals of increasing

biospheric/soil carbon storage and decreasing non-CO 2 climate forcings. However, certain generalcharacteristics are apparent. Financial assistance deserved by any given nation will be dependent uponclimate impacts that are being suffered by that nation. However, the support delivered should bedependent on another factor: the cooperation of that nation in achieving the goals of biospheric/soilstorage of carbon and reduction of non-CO 2 (and CO 2) climate forcings.

In other words, developing countries have a legitimate claim for assistance to deal with climate change.However, for their own good as well as for the good of all other nations, they should be encouraged tocontribute to the actions that minimize climate change. Thus the support provided to participating nationsshould be continually evaluated and the level of support to any nation adjusted in proportion to theirsuccess in contributing to the carbon and non-CO 2 objectives in the preceding reporting period. Althoughgoals and assessments for biospheric/soil carbon and non-CO 2 forcings are not as easy to quantify asfossil fuel carbon emissions, they can be set and assessed based on best available and developing science.

Our paper [1] was initiated to provide the scientific basis for legal actions against national and stategovernments for not doing their job of protecting the rights of young people and future gener ations. Alower court ruling in the case against the U.S. federal government, suggesting that the "trust" 4 doctrine [9]does not give the court a constitutional basis for ordering actions on the executive branch, is now beingappealed to a higher court. The appeal places greater emphasis on "equal protection of the laws" and "due

process", which the U.S. Constitution guarantees to all people. Amici briefs have been filed with thecourt concerning both the scientific [10] and legal [11] aspects.

James Hansen and Pushker Kharecha03 December 2013

References

[1] Hansen, J, P Kharecha, M Sato, V Masson-Delmotte, F. Ackerman, DJ Beerling, PJ Hearty, O Hoegh-Guldberg, S-L Hsu, C Parmesan, LVan Susteren, K von Schuckmann, JC Zachos (2013) Climate change and intergenerational justice: rapid reduction of carbon emissionsrequired to protect young people, future generations and nature. Plos One, 3 December 2013 (in press).

[2] BP Statistical Review of World Energy 2012.http://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdf

[3] Boden TA, G Marland, RJ Andres (2012) Global, Regional, and National Fossil-Fuel CO 2 http://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdf

[4] Hansen, J, M Sato, R Ruedy, P Kharecha, A Lacis, et al. (2007) Dangerous human-made interference with climate: a GISS modelE study.Atmos Chem Phys 7, 1189-1202.

[5] Chen Y, A Ebenstein, M Greenstone, H Li (2013) Evidence on the impact of sustained exposure to air pollution on life expectancy fromChina's Huai River policy. Proc Natl Acad Sci USA www.pnas.org/cgi/doi/10.1073/pnas.1300018110

[6] Yang, C, R Jackson (2013) China's synthetic natural gas revolution. Nature Clim Chan 3, 852-854.[7] Kharecha PA, JE Hansen (2013) Prevented mortality and greenhouse gas emissions from historical and projected nuclear power . Environ

Sci Technol 47, 4889-4895, doi:10.1021/es3051197.[8] Till, CE, YI Chang, Plentiful energy: the story of the integral fast reactor. ISBN:978:-1466384606, 116 pp.[9] Wood MC, Nature's Trust: Environmental Law for a New Ecological Age, Cambridge University Press, 2013.[10] Hansen, J, D Beerling, PJ Hearty, O Hoegh-Guldberg, P Kharecha, V Masson-Delmotte, C. Parmesan, EJ Rohling, M Sato, P Smith, L

Van Susteren (2013) U.S. Court of Appeals for District of Columbia Circuit, Alec L et al. versus Gina McCarthy et al. Brief of Scientistsas Amici Curiae in support of Plaintiffs-Appellants seeking reversal.

[11] Rodgers, WH, MC Wood, E Chemerinsky, M Blumm, J Davidson, et al. (2013) U.S. Court of Appeals for District of Columbia Circuit,Alec L et al. versus Gina McCarthy et al. Brief of Law Professors as Amici Curiae in support of Plaintiffs-Appellants seeking reversal.

4 The concept that the current generation has a usufruct (use and enjoyment) obligation to deliver an undamaged environment tothe next generation was recognized by U.S. founding fathers, e.g., Thomas Jefferson argued that the soils must be left in equally

productive condition. Other cultures have similar concepts, e.g., Native Americans speak of obligation to the 'seventh generation.'
http://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdfhttp://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdfhttp://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdfhttp://pubs.giss.nasa.gov/abs/kh05000e.htmlhttp://pubs.giss.nasa.gov/abs/kh05000e.htmlhttp://pubs.giss.nasa.gov/abs/kh05000e.htmlhttp://pubs.giss.nasa.gov/abs/kh05000e.htmlhttp://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdfhttp://www.bp.com/assets/bp_internet/globalbp/globalbp_uk_english/reports_and_publications/statistical_energy_review_2011/STAGING/local_assets/pdf/statistical_review_of_world_energy_full_report_2012.pdf


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European Union in 1996 proposed to limit global warming to 2u

Crelative to pre-industrial times [10], based partly on evidence thatmany ecosystems are at risk with larger climate change. The 2 u Ctarget was reaffirmed in the 2009 Copenhagen Accordemerging from the 15th Conference of the Parties of theFramework Convention [11], with specific language We agreethat deep cuts in global emissions are required according toscience, as documented in the IPCC Fourth Assessment Reportwith a view to reduce global emissions so as to hold the increase inglobal temperature below 2 degrees Celsius.

A global warming target is converted to a fossil fuel emissionstarget with the help of global climate-carbon-cycle models, whichreveal that eventual warming depends on cumulative carbonemissions, not on the temporal history of emissions [12]. Theemission limit depends on climate sensitivity, but central estimates[1213], including those in the upcoming Fifth Assessment of theIntergovernmental Panel on Climate Change [14], are that a 2 u Cglobal warming limit implies a cumulative carbon emissions limitof the order of 1000 GtC. In comparing carbon emissions, notethat some authors emphasize the sum of fossil fuel anddeforestation carbon. We bookkeep fossil fuel and deforestationcarbon separately, because the larger fossil fuel term is knownmore accurately and this carbon stays in the climate system forhundreds of thousands of years. Thus fossil fuel carbon is thecrucial human input that must be limited. Deforestation carbon ismore uncertain and potentially can be offset on the century timescale by storage in the biosphere, including the soil, viareforestation and improved agricultural and forestry practices.

There are sufficient fossil fuel resources to readily supply 1000

GtC, as fossil fuel emissions to date (370 GtC) are only a smallfraction of potential emissions from known reserves and potentiallyrecoverable resources (Fig. 2). Although there are uncertainties inreserves and resources, ongoing fossil fuel subsidies and continuing technological advances ensure that more and more of these fuelswill be economically recoverable. As we will show, Earthspaleoclimate record makes it clear that the CO 2 produced byburning all or most of these fossil fuels would lead to a verydifferent planet than the one that humanity knows.

Our evaluation of a fossil fuel emissions limit is not based onclimate models but rather on observational evidence of globalclimate change as a function of global temperature and on the fact

that climate stabilization requires long-term planetary energybalance. We use measured global temperature and Earthsmeasured energy imbalance to determine the atmospheric CO 2level required to stabilize climate at todays global temperature,which is near the upper end of the global temperature range in thecurrent interglacial period (the Holocene). We then examineclimate impacts during the past few decades of global warming and in paleoclimate records including the Eemian period,concluding that there are already clear indications of undesirableimpacts at the current level of warming and that 2 u C warming would have major deleterious consequences. We use simplerepresentations of the carbon cycle and global temperature,consistent with observations, to simulate transient global temper-ature and assess carbon emission scenarios that could keep globalclimate near the Holocene range. Finally, we discuss likely over-shooting of target emissions, the potential for carbon extractionfrom the atmosphere, and implications for energy and economicpolicies, as well as intergenerational justice.

Global Temperature and Earths Energy Balance

Global temperature and Earths energy imbalance provide ourmost useful measuring sticks for quantifying global climate changeand the changes of global climate forcings that would be requiredto stabilize global climate. Thus we must first quantify knowledgeof these quantities.

TemperatureTemperature change in the past century (Fig. 3; update of figures

in [16]) includes unforced variability and forced climate change.The long-term global warming trend is predominantly a forcedclimate change caused by increased human-made atmosphericgases, mainly CO 2 [1]. Increase of greenhouse gases such as CO 2has little effect on incoming sunlight but makes the atmospheremore opaque at infrared wavelengths, causing infrared (heat)radiation to space to emerge from higher, colder levels, which thusreduces infrared radiation to space. The resulting planetary energyimbalance, absorbed solar energy exceeding heat emitted to space,causes Earth to warm. Observations, discussed below, confirm thatEarth is now substantially out of energy balance, so the long-termwarming will continue.

Figure 1. CO 2 annual emissions from fossil fuel use and cement manufacture, based on data of British Petroleum [4 ] concatenatedwith data of Boden et al. [5 ]. (A) is log scale and (B) is linear.doi:10.1371/journal.pone.0081648.g001

Assessing Dangerous Climate Change

PLOS ONE | www.plosone.org 2 December 2013 | Volume 8 | Issue 12 | e81648


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Global temperature appears to have leveled off since 1998 (Fig.3a). That plateau is partly an illusion due to the 1998 globaltemperature spike caused by the El Nino of the century that year.The 11-year (132-month) running mean temperature (Fig. 3b)shows only a moderate decline of the warming rate. The 11-yearaveraging period minimizes the effect of variability due to the 10 12 year periodicity of solar irradiance as well as irregular El Nino/La Nina warming/cooling in the tropical Pacific Ocean. Thecurrent solar cycle has weaker irradiance than the several prior

solar cycles, but the decreased irradiance can only partiallyaccount for the decreased warming rate [17]. Variability of the ElNino/La Nina cycle, described as a Pacific Decadal Oscillation,largely accounts for the temporary decrease of warming [18], aswe discuss further below in conjunction with global temperaturesimulations.

Assessments of dangerous climate change have focused onestimating a permissible level of global warming. The Intergov-ernmental Panel on Climate Change [1,19] summarized broad-based assessments with a burning embers diagram, whichindicated that major problems begin with global warming of 2 3u C. A probabilistic analysis [20], still partly subjective, found amedian dangerous threshold of 2.8 u C, with 95% confidencethat the dangerous threshold was 1.5 u C or higher. Theseassessments were relative to global temperature in year 1990, so

add 0.6u

C to these values to obtain the warming relative to 1880 1920, which is the base period we use in this paper forpreindustrial time. The conclusion that humanity could tolerateglobal warming up to a few degrees Celsius meshed with commonsense. After all, people readily tolerate much larger regional andseasonal climate variations.

Figure 2. Fossil fuel CO 2 emissions and carbon content (1 ppm atmospheric CO 2 2.12 GtC). Estimates of reserves (profitable to extractat current prices) and resources (potentially recoverable with advanced technology and/or at higher prices) are the mean of estimates of EnergyInformation Administration (EIA) [7], German Advisory Council (GAC) [8], and Global Energy Assessment (GEA) [9]. GEA [9] suggests the possibility of .

15,000 GtC unconventional gas. Error estimates (vertical lines) are from GEA and probably underestimate the total uncertainty. We convert energycontent to carbon content using emission factors of Table 4.2 of [15] for coal, gas and conventional oil, and, also following [15], emission factor of unconventional oil is approximated as being the same as for coal. Total emissions through 2012, including gas flaring and cement manufacture, are384 GtC; fossil fuel emissions alone are , 370 GtC.doi:10.1371/journal.pone.0081648.g002

Figure 3. Global surface temperature relative to 18801920 mean. B shows the 5 and 11 year means. Figures are updates of [16] using datathrough August 2013.doi:10.1371/journal.pone.0081648.g003




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The fallacy of this logic emerged recently as numerous impactsof ongoing global warming emerged and as paleoclimateimplications for climate sensitivity became apparent. Arctic seaice end-of-summer minimum area, although variable from year to year, has plummeted by more than a third in the past few decades,at a faster rate than in most models [21], with the sea ice thicknessdeclining a factor of four faster than simulated in IPCC climatemodels [22]. The Greenland and Antarctic ice sheets began to

shed ice at a rate, now several hundred cubic kilometers per year,which is continuing to accelerate [2325]. Mountain glaciers arereceding rapidly all around the world [2629] with effects onseasonal freshwater availability of major rivers [3032]. The hotdry subtropical climate belts have expanded as the troposphere haswarmed and the stratosphere cooled [3336], contributing toincreases in the area and intensity of drought [37] and wildfires[38]. The abundance of reef-building corals is decreasing at a rateof 0.52%/year, at least in part due to ocean warming andpossibly ocean acidification caused by rising dissolved CO 2 [39 41]. More than half of all wild species have shown significantchanges in where they live and in the timing of major life events[4244]. Mega-heatwaves, such as those in Europe in 2003, theMoscow area in 2010, Texas and Oklahoma in 2011, Greenlandin 2012, and Australia in 2013 have become more widespread

with the increase demonstrably linked to global warming [4547].These growing climate impacts, many more rapid than

anticipated and occurring while global warming is less than 1 u C,imply that society should reassess what constitutes a dangerouslevel of global warming. Earths paleoclimate history provides a valuable tool for that purpose.

Paleoclimate TemperatureMajor progress in quantitative understanding of climate change

has occurred recently by use of the combination of data from highresolution ice cores covering time scales of order several hundredthousand years [4849] and ocean cores for time scales of orderone hundred million years [50]. Quantitative insights on globaltemperature sensitivity to external forcings [5152] and sea level

sensitivity to global temperature [5253] are crucial to ouranalyses. Paleoclimate data also provide quantitative informationabout how nominally slow feedback processes amplify climatesensitivity [5152,5456], which also is important to our analyses.

Earths surface temperature prior to instrumental measurementsis estimated via proxy data. We will refer to the surfacetemperature record in Fig. 4 of a recent paper [52]. Global meantemperature during the Eemian interglacial period (120,000 yearsago) is constrained to be 2 u C warmer than our pre-industrial(18801920) level based on several studies of Eemian climate [52].The concatenation of modern and instrumental records [52] isbased on an estimate that global temperature in the first decade of the 21st century ( + 0.8 u C relative to 18801920) exceeded theHolocene mean by 0.25 6 0.25 u C. That estimate was based in parton the fact that sea level is now rising 3.2 mm/yr (3.2 m/millennium) [57], an order of magnitude faster than the rateduring the prior several thousand years, with rapid change of icesheet mass balance over the past few decades [23] and Greenlandand Antarctica now losing mass at accelerating rates [2324]. Thisconcatenation, which has global temperature 13.9 u C in the baseperiod 19511980, has the first decade of the 21st century slightly( , 0.1 u C) warmer than the early Holocene maximum. A recentreconstruction from proxy temperature data [55] concluded thatglobal temperature declined about 0.7 u C between the Holocenemaximum and a pre-industrial minimum before recent warming brought temperature back near the Holocene maximum, which isconsistent with our analysis.

Climate oscillations evident in Fig. 4 of Hansen et al. [52] wereinstigated by perturbations of Earths orbit and spin axis tiltrelative to the orbital plane, which alter the geographical andseasonal distribution of sunlight on Earth [58]. These forcingschange slowly, with periods between 20,000 and 400,000 years,and thus climate is able to stay in quasi-equilibrium with theseforcings. Slow insolation changes initiated the climate oscillations,but the mechanisms that caused the climate changes to be so large

were two powerful amplifying feedbacks: the planets surfacealbedo (its reflectivity, literally its whiteness) and atmospheric CO 2amount. As the planet warms, ice and snow melt, causing thesurface to be darker, absorb more sunlight and warm further. Asthe ocean and soil become warmer they release CO 2 and othergreenhouse gases, causing further warming. Together with fastfeedbacks processes, via changes of water vapor, clouds, and the vertical temperature profile, these slow amplifying feedbacks wereresponsible for almost the entire glacial-to-interglacial temperaturechange [5962].

The albedo and CO 2 feedbacks amplified weak orbital forcings,the feedbacks necessarily changing slowly over millennia, at thepace of orbital changes. Today, however, CO 2 is under the controlof humans as fossil fuel emissions overwhelm natural changes. Atmospheric CO 2 has increased rapidly to a level not seen for at

least 3 million years [56,63]. Global warming induced byincreasing CO 2 will cause ice to melt and hence sea level to riseas the global volume of ice moves toward the quasi-equilibriumamount that exists for a given global temperature [53]. As icemelts and ice area decreases, the albedo feedback will amplifyglobal warming.

Earth, because of the climate systems inertia, has not yet fullyresponded to human-made changes of atmospheric composition.The oceans thermal inertia, which delays some global warming for decades and even centuries, is accounted for in global climatemodels and its effect is confirmed via measurements of Earthsenergy balance (see next section). In addition there are slowclimate feedbacks, such as changes of ice sheet size, that occurmainly over centuries and millennia. Slow feedbacks have littleeffect on the immediate planetary energy balance, instead coming into play in response to temperature change. The slow feedbacksare difficult to model, but paleoclimate data and observations of ongoing changes help provide quantification.

Earths Energy Imbalance At a time of climate stability, Earth radiates as much energy to

space as it absorbs from sunlight. Today Earth is out of balancebecause increasing atmospheric gases such as CO 2 reduce Earthsheat radiation to space, thus causing an energy imbalance, as thereis less energy going out than coming in. This imbalance causesEarth to warm and move back toward energy balance. Thewarming and restoration of energy balance take time, however,because of Earths thermal inertia, which is due mainly to theglobal ocean.

Earth warmed about 0.8 u C in the past century. That warming increased Earths radiation to space, thus reducing Earths energyimbalance. The remaining energy imbalance helps us assess howmuch additional warming is still in the pipeline. Of courseincreasing CO 2 is only one of the factors affecting Earths energybalance, even though it is the largest climate forcing. Otherforcings include changes of aerosols, solar irradiance, and Earthssurface albedo.

Determination of the state of Earths climate therefore requiresmeasuring the energy imbalance. This is a challenge, because theimbalance is expected to be only about 1 W/m 2 or less, soaccuracy approaching 0.1 W/m 2 is needed. The most promising




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approach is to measure the rate of changing heat content of theocean, atmosphere, land, and ice [64]. Measurement of ocean heat

content is the most critical observation, as nearly 90 percent of theenergy surplus is stored in the ocean [6465].

Observed Energy ImbalanceNations of the world have launched a cooperative program to

measure changing ocean heat content, distributing more than3000 Argo floats around the world ocean, with each floatrepeatedly diving to a depth of 2 km and back [66]. Oceancoverage by floats reached 90% by 2005 [66], with the gapsmainly in sea ice regions, yielding the potential for an accurateenergy balance assessment, provided that several systematicmeasurement biases exposed in the past decade are minimized[6769].

Argo data reveal that in 20052010 the oceans upper 2000 m

gained heat at a rate equal to 0.41 W/m2

averaged over Earthssurface [70]. Smaller contributions to planetary energy imbalanceare from heat gain by the deeper ocean ( + 0.10 W/m 2 ), energyused in net melting of ice ( + 0.05 W/m 2 ), and energy taken up bywarming continents ( + 0.02 W/m 2 ). Data sources for theseestimates and uncertainties are provided elsewhere [64]. Theresulting net planetary energy imbalance for the six years 2005 2010 is + 0.58 6 0.15 W/m 2 .

The positive energy imbalance in 20052010 confirms that theeffect of solar variability on climate is much less than the effect of human-made greenhouse gases. If the sun were the dominantforcing, the planet would have a negative energy balance in 2005 2010, when solar irradiance was at its lowest level in the period of accurate data, i.e., since the 1970s [64,71]. Even though much of the greenhouse gas forcing has been expended in causing observed0.8

u

C global warming, the residual positive forcing overwhelmsthe negative solar forcing. The full amplitude of solar cycle forcing is about 0.25 W/m 2 [64,71], but the reduction of solar forcing dueto the present weak solar cycle is about half that magnitude as weillustrate below, so the energy imbalance measured during solarminimum (0.58 W/m 2 ) suggests an average imbalance over thesolar cycle of about 0.7 W/m 2 .

Earths measured energy imbalance has been used to infer theclimate forcing by aerosols, with two independent analyses yielding a forcing in the past decade of about 2 1.5 W/m 2 [64,72],including the direct aerosol forcing and indirect effects via inducedcloud changes. Given this large (negative) aerosol forcing, precise

monitoring of changing aerosols is needed [73]. Public reaction toincreasingly bad air quality in developing regions [74] may lead to

future aerosol reductions, at least on a regional basis. Increase of Earths energy imbalance from reduction of particulate airpollution, which is needed for the sake of human health, can beminimized via an emphasis on reducing absorbing black soot [75],but the potential to constrain the net increase of climate forcing byfocusing on black soot is limited [76].

Energy Imbalance Implications for CO2 TargetEarths energy imbalance is the most vital number character-

izing the state of Earths climate. It informs us about the globaltemperature change in the pipeline without further change of climate forcings and it defines how much greenhouse gases mustbe reduced to restore Earths energy balance, which, at least to agood approximation, must be the requirement for stabilizing

global climate. The measured energy imbalance accounts for allnatural and human-made climate forcings, including changes of atmospheric aerosols and Earths surface albedo.

If Earths mean energy imbalance today is + 0.5 W/m 2 , CO 2must be reduced from the current level of 395 ppm (global-meanannual-mean in mid-2013) to about 360 ppm to increase Earthsheat radiation to space by 0.5 W/m 2 and restore energy balance.If Earths energy imbalance is 0.75 W/m 2 , CO 2 must be reducedto about 345 ppm to restore energy balance [64,75].

The measured energy imbalance indicates that an initial CO 2target , 350 ppm would be appropriate, if the aim is to stabilizeclimate without further global warming. That target is consistentwith an earlier analysis [54]. Additional support for that target isprovided by our analyses of ongoing climate change andpaleoclimate, in later parts of our paper. Specification now of aCO 2 target more precise than , 350 ppm is difficult andunnecessary, because of uncertain future changes of forcingsincluding other gases, aerosols and surface albedo. More preciseassessments will become available during the time that it takes toturn around CO 2 growth and approach the initial 350 ppm target.

Below we find the decreasing emissions scenario that wouldachieve the 350 ppm target within the present century. Specifically,we want to know the annual percentage rate at which emissionsmust be reduced to reach this target, and the dependence of this rateupon the date at which reductions are initiated. This approach iscomplementary to the approach of estimating cumulative emissionsallowed to achieve a given limit on global warming [12].

Figure 4. Decay of atmospheric CO 2 perturbations. (A) Instantaneous injection or extraction of CO 2 with initial conditions at equilibrium. (B)Fossil fuel emissions terminate at the end of 2015, 2030, or 2050 and land use emissions terminate after 2015 in all three cases, i.e., thereafter there isno net deforestation.doi:10.1371/journal.pone.0081648.g004




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If the only human-made climate forcing were changes of atmospheric CO 2 , the appropriate CO 2 target might be close tothe pre-industrial CO 2 amount [53]. However, there are otherhuman forcings, including aerosols, the effect of aerosols onclouds, non-CO 2 greenhouse gases, and changes of surface albedothat will not disappear even if fossil fuel burning is phased out. Aerosol forcings are substantially a result of fossil fuel burning [1,76], but the net aerosol forcing is a sensitive function of various

aerosol sources [76]. The indirect aerosol effect on clouds is non-linear [1,76] such that it has been suggested that even the modestaerosol amounts added by pre-industrial humans to an otherwisepristine atmosphere may have caused a significant climate forcing [59]. Thus continued precise monitoring of Earths radiationimbalance is probably the best way to assess and adjust theappropriate CO 2 target.

Ironically, future reductions of particulate air pollution mayexacerbate global warming by reducing the cooling effect of reflective aerosols. However, a concerted effort to reduce non-CO 2forcings by methane, tropospheric ozone, other trace gases, andblack soot might counteract the warming from a decline inreflective aerosols [54,75]. Our calculations below of future globaltemperature assume such compensation, as a first approximation.To the extent that goal is not achieved, adjustments must be madein the CO 2 target or future warming may exceed calculated values.

Climate Impacts

Determination of the dangerous level of global warming inherently is partly subjective, but we must be as quantitative aspossible. Early estimates for dangerous global warming based onthe burning embers approach [1,1920] have been recognizedas probably being too conservative [77]. A target of limiting warming to 2 u C has been widely adopted, as discussed above. Wesuspect, however, that this may be a case of inching toward abetter answer. If our suspicion is correct, then that gradualapproach is itself very dangerous, because of the climate systemsinertia. It will become exceedingly difficult to keep warming belowa target smaller than 2 u C, if high emissions continue much longer.

We consider several important climate impacts and useevidence from current observations to assess the effect of 0.8 u Cwarming and paleoclimate data for the effect of larger warming,especially the Eemian period, which had global mean temperatureabout + 2u C relative to pre-industrial time. Impacts of specialinterest are sea level rise and species extermination, because theyare practically irreversible, and others important to humankind.

Sea LevelThe prior interglacial period, the Eemian, was at most , 2u C

warmer than 18801920 (Fig. 3). Sea level reached heights severalmeters above todays level [7880], probably with instances of sealevel change of the order of 1 m/century [8183]. Geologicshoreline evidence has been interpreted as indicating a rapid sealevel rise of a few meters late in the Eemian to a peak about 9meters above present, suggesting the possibility that a criticalstability threshold was crossed that caused polar ice sheet collapse[8485], although there remains debate within the researchcommunity about this specific history and interpretation. Thelarge Eemian sea level excursions imply that substantial ice sheetmelting occurred when the world was little warmer than today.

During the early Pliocene, which was only , 3u C warmer thanthe Holocene, sea level attained heights as much as 1525 metershigher than today [53,8689]. Such sea level rise suggests thatparts of East Antarctica must be vulnerable to eventual melting with global temperature increase of a few degrees Celsius. Indeed,

satellite gravity data and radar altimetry reveal that the TottenGlacier of East Antarctica, which fronts a large ice mass groundedbelow sea level, is now losing mass [90].

Greenland ice core data suggest that the Greenland ice sheetresponse to Eemian warmth was limited [91], but the fifth IPCCassessment [14] concludes that Greenland very likely contributedbetween 1.4 and 4.3 m to the higher sea level of the Eemian. TheWest Antarctic ice sheet is probably more susceptible to rapid

change, because much of it rests on bedrock well below sea level[9293]. Thus the entire 34 meters of global sea level containedin that ice sheet may be vulnerable to rapid disintegration,although arguments for stability of even this marine ice sheet havebeen made [94]. However, Earths history reveals sea levelchanges of as much as a few meters per century, even though thenatural climate forcings changed much more slowly than thepresent human-made forcing.

Expected human-caused sea level rise is controversial in partbecause predictions focus on sea level at a specific time, 2100. Sealevel on a given date is inherently difficult to predict, as it dependson how rapidly non-linear ice sheet disintegration begins. Focus ona single date also encourages people to take the estimated result asan indication of what humanity faces, thus failing to emphasizethat the likely rate of sea level rise immediately after 2100 will bemuch larger than within the 21

st

century, especially if CO 2emissions continue to increase.

Recent estimates of sea level rise by 2100 have been of the orderof 1 m [9596], which is higher than earlier assessments [26], butthese estimates still in part assume linear relations betweenwarming and sea level rise. It has been argued [9798] thatcontinued business-as-usual CO 2 emissions are likely to spur anonlinear response with multi-meter sea level rise this century.Greenland and Antarctica have been losing mass at rapidlyincreasing rates during the period of accurate satellite data [23];the data are suggestive of exponential increase, but the records aretoo short to be conclusive. The area on Greenland with summermelt has increased markedly, with 97% of Greenland experiencing melt in 2012 [99].

The important point is that the uncertainty is not about whethercontinued rapid CO 2 emissions would cause large sea level rise,submerging global coastlines it is about how soon the largechanges would begin. The carbon from fossil fuel burning willremain in and affect the climate system for many millennia,ensuring that over time sea level rise of many meters will occur tens of meters if most of the fossil fuels are burned [53]. That orderof sea level rise would result in the loss of hundreds of historicalcoastal cities worldwide with incalculable economic consequences,create hundreds of millions of global warming refugees fromhighly-populated low-lying areas, and thus likely cause majorinternational conflicts.

Shifting Climate ZonesTheory and climate models indicate that the tropical overturn-

ing (Hadley) atmospheric circulation expands poleward withglobal warming [33]. There is evidence in satellite and radiosondedata and in observational data for poleward expansion of thetropical circulation by as much as a few degrees of latitude sincethe 1970s [3435], but natural variability may have contributed tothat expansion [36]. Change in the overturning circulation likelycontributes to expansion of subtropical conditions and increasedaridity in the southern United States [30,100], the Mediterraneanregion, South America, southern Africa, Madagascar, andsouthern Australia. Increased aridity and temperature contributeto increased forest fires that burn hotter and are more destructive[38].




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Despite large year-to-year variability of temperature, decadalaverages reveal isotherms (lines of a given average temperature)moving poleward at a typical rate of the order of 100 km/decadein the past three decades [101], although the range shifts forspecific species follow more complex patterns [102]. This rapidshifting of climate zones far exceeds natural rates of change.Movement has been in the same direction (poleward, and upwardin elevation) since about 1975. Wild species have responded to

climate change, with three-quarters of marine species shifting theirranges poleward as much as 1000 km [44,103] and more than half of terrestrial species shifting ranges poleward as much as 600 kmand upward as much as 400 m [104].

Humans may adapt to shifting climate zones better than manyspecies. However, political borders can interfere with humanmigration, and indigenous ways of life already have been adverselyaffected [26]. Impacts are apparent in the Arctic, with melting tundra, reduced sea ice, and increased shoreline erosion. Effects of shifting climate zones also may be important for indigenous Americans who possess specific designated land areas, as well asother cultures with long-standing traditions in South America, Africa, Asia and Australia.

Human Extermination of SpeciesBiodiversity is affected by many agents including overharvest-

ing, introduction of exotic species, land use changes, nitrogenfertilization, and direct effects of increased atmospheric CO 2 onplant ecophysiology [43]. However, an overriding role of climatechange is exposed by diverse effects of rapid warming on animals,plants, and insects in the past three decades.

A sudden widespread decline of frogs, with extinction of entiremountain-restricted species attributed to global warming [105 106], provided a dramatic awakening. There are multiple causesof the detailed processes involved in global amphibian declines andextinctions [107108], but global warming is a key contributorand portends a planetary-scale mass extinction in the making unless action is taken to stabilize climate while also fighting biodiversitys other threats [109].

Mountain-restricted and polar-restricted species are particularly vulnerable. As isotherms move up the mountainside and poleward,so does the climate zone in which a given species can survive. If global warming continues unabated, many of these species will beeffectively pushed off the planet. There are already reductions inthe population and health of Arctic species in the southern parts of the Arctic, Antarctic species in the northern parts of the Antarctic,and alpine species worldwide [43].

A critical factor for survival of some Arctic species is retention of all-year sea ice. Continued growth of fossil fuel emissions will causeloss of all Arctic summer sea ice within several decades. Incontrast, the scenario in Fig. 5A, with global warming peaking justover 1 u C and then declining slowly, should allow summer sea iceto survive and then gradually increase to levels representative of recent decades.

The threat to species survival is not limited to mountain andpolar species. Plant and animal distributions reflect the regionalclimates to which they are adapted. Although species attempt tomigrate in response to climate change, their paths may be blockedby human-constructed obstacles or natural barriers such as coastlines and mountain ranges. As the shift of climate zones [110]becomes comparable to the range of some species, less mobilespecies can be driven to extinction. Because of extensive speciesinterdependencies, this can lead to mass extinctions.

Rising sea level poses a threat to a large number of uniquelyevolved endemic fauna living on islands in marine-dominatedecosystems, with those living on low lying islands being especially

vulnerable. Evolutionary history on Bermuda offers numerousexamples of the direct and indirect impact of changing sea level onevolutionary processes [111112], with a number of taxa being extirpated due to habitat changes, greater competition, and islandinundation [113]. Similarly, on Aldahabra Island in the IndianOcean, land tortoises were exterminated during sea level highstands [114]. Vulnerabilities would be magnified by the speed of human-made climate change and the potentially large sea level

rise [115].IPCC [26] reviewed studies relevant to estimating eventualextinctions. They estimate that if global warming exceeds 1.6 u Cabove preindustrial, 931 percent of species will be committed toextinction. With global warming of 2.9 u C, an estimated 2152percent of species will be committed to extinction. A compre-hensive study of biodiversity indicators over the past decade [116]reveals that, despite some local success in increasing extent of protected areas, overall indicators of pressures on biodiversityincluding that due to climate change are continuing to increaseand indicators of the state of biodiversity are continuing todecline.

Mass extinctions occurred several times in Earths history [117 118], often in conjunction with rapid climate change. New speciesevolved over millions of years, but those time scales are almostbeyond human comprehension. If we drive many species toextinction we will leave a more desolate, monotonous planet forour children, grandchildren, and more generations than we canimagine. We will also undermine ecosystem functions (e.g.,pollination which is critical for food production) and ecosystemresilience (when losing keystone species in food chains), as well asreduce functional diversity (critical for the ability of ecosystems torespond to shocks and stress) and genetic diversity that plays animportant role for development of new medicines, materials, andsources of energy.

Coral Reef EcosystemsCoral reefs are the most biologically diverse marine ecosystem,

often described as the rainforests of the ocean. Over a millionspecies, most not yet described [119], are estimated to populatecoral reef ecosystems generating crucial ecosystem services for atleast 500 million people in tropical coastal areas. These ecosystemsare highly vulnerable to the combined effects of ocean acidificationand warming.

Acidification arises as the ocean absorbs CO 2 , producing carbonic acid [120], thus making the ocean more corrosive to thecalcium carbonate shells (exoskeletons) of many marine organ-isms. Geochemical records show that ocean pH is already outsideits range of the past several million years [121122]. Warming causes coral bleaching, as overheated coral expel symbiotic algaeand become vulnerable to disease and mortality [123]. Coralbleaching and slowing of coral calcification already are causing mass mortalities, increased coral disease, and reduced reef

carbonate accretion, thus disrupting coral reef ecosystem health[40,124].

Local human-made stresses add to the global warming andacidification effects, all of these driving a contraction of 12% per year in the abundance of reef-building corals [39]. Loss of thethree-dimensional coral reef frameworks has consequences for allthe species that depend on them. Loss of these frameworks also hasconsequences for the important roles that coral reefs play insupporting fisheries and protecting coastlines from wave stress.Consequences of lost coral reefs can be economically devastating for many nations, especially in combination with other impactssuch as sea level rise and intensification of storms.




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Climate ExtremesChanges in the frequency and magnitude of climate extremes,

of both moisture and temperature, are affected by climate trendsas well as changing variability. Extremes of the hydrologic cycleare expected to intensify in a warmer world. A warmeratmosphere holds more moisture, so precipitation can be heavierand cause more extreme flooding. Higher temperatures, on theother hand, increase evaporation and can intensify droughts whenthey occur, as can expansion of the subtropics, as discussed above.Global models for the 21st century find an increased variability of precipitation minus evaporation [P-E] in most of the world,especially near the equator and at high latitudes [125]. Somemodels also show an intensification of droughts in the Sahel,driven by increasing greenhouse gases [126].

Observations of ocean salinity patterns for the past 50 yearsreveal an intensification of [P-E] patterns as predicted by models,

but at an even faster rate. Precipitation observations over landshow the expected general increase of precipitation poleward of the subtropics and decrease at lower latitudes [1,26]. An increaseof intense precipitation events has been found on much of theworlds land area [127129]. Evidence for widespread droughtintensification is less clear and inherently difficult to confirm withavailable data because of the increase of time-integrated precip-itation at most locations other than the subtropics. Data analyseshave found an increase of drought intensity at many locations[130131] The magnitude of change depends on the droughtindex employed [132], but soil moisture provides a good means toseparate the effect of shifting seasonal precipitation and confirmsan overall drought intensification [37].

Global warming of , 0.6 u C since the 1970s (Fig. 3) has alreadycaused a notable increase in the occurrence of extreme summer heat[46]. The likelihood of occurrence or the fractional area covered by3-standard-deviation hot anomalies, relative to a base period (1951 1980) that was still within the range of Holocene climate, hasincreased by more than a factor of ten. Large areas around Moscow,the Mediterranean region, the United States and Australia haveexperienced such extreme anomalies in the past three years. Heatwaves lasting for weeks have a devastating impact on human health:the European heat wave of summer 2003 caused over 70,000 excessdeaths [133]. This heat record for Europe was surpassed already in2010 [134]. The number of extreme heat waves has increasedseveral-fold due to global warming [4546,135] and will increasefurther if temperatures continue to rise.

Human HealthImpacts of climate change cause widespread harm to human

health, with children often suffering the most. Food shortages,polluted air, contaminated or scarce supplies of water, anexpanding area of vectors causing infectious diseases, and moreintensely allergenic plants are among the harmful impacts [26].More extreme weather events cause physical and psychologicalharm. World health experts have concluded with very highconfidence that climate change already contributes to the globalburden of disease and premature death [26].

IPCC [26] projects the following trends, if global warming continue to increase, where only trends assigned very highconfidence or high confidence are included: (i) increasedmalnutrition and consequent disorders, including those relatedto child growth and development, (ii) increased death, disease andinjuries from heat waves, floods, storms, fires and droughts, (iii)

increased cardio-respiratory morbidity and mortality associatedwith ground-level ozone. While IPCC also projects fewer deathsfrom cold, this positive effect is far outweighed by the negativeones.

Growing awareness of the consequences of human-causedclimate change triggers anxiety and feelings of helplessness [136 137]. Children, already susceptible to age-related insecurities, faceadditional destabilizing insecurities from questions about how theywill cope with future climate change [138139]. Exposure tomedia ensures that children cannot escape hearing that theirfuture and that of other species is at stake, and that the window of opportunity to avoid dramatic climate impacts is closing. Thepsychological health of our children is a priority, but denial of thetruth exposes our children to even greater risk.

Health impacts of climate change are in addition to directeffects of air and water pollution. A clear illustration of directeffects of fossil fuels on human health was provided by aninadvertent experiment in China during the 19501980 period of central planning, when free coal for winter heating was providedto North China but not to the rest of the country. Analysis of theimpact was made [140] using the most comprehensive data fileever compiled on mortality and air pollution in any developing country. A principal conclusion was that the 500 million residentsof North China experienced during the 1990s a loss of more than2.5 billion life years owing to the added air pollution, and anaverage reduction in life expectancy of 5.5 years. The degree of airpollution in China exceeded that in most of the world, yet

Figure 5. Atmospheric CO 2 if fossil fuel emissions reduced. (A) 6% or 2% annual cut begins in 2013 and 100 GtC reforestation drawdownoccurs in 20312080, (B) effect of delaying onset of emission reduction.doi:10.1371/journal.pone.0081648.g005




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assessments of total health effects must also include other fossil fuelcaused air and water pollutants, as discussed in the following section on ecology and the environment.

The Text S1 has further discussion of health impacts of climatechange.

Ecology and the EnvironmentThe ecological impact of fossil fuel mining increases as the

largest, easiest to access, resources are depleted [141]. A constantfossil fuel production rate requires increasing energy input, butalso use of more land, water, and diluents, with the production of more waste [142]. The increasing ecological and environmentalimpact of a given amount of useful fossil fuel energy is a relevantconsideration in assessing alternative energy strategies.

Coal mining has progressively changed from predominantlyunderground mining to surface mining [143], including moun-taintop removal with valley fill, which is now widespread in the Appalachian ecoregion in the United States. Forest cover andtopsoil are removed, explosives are used to break up rocks toaccess coal, and the excess rock is pushed into adjacent valleys,where it buries existing streams. Burial of headwater streamscauses loss of ecosystems that are important for nutrient cycling and production of organic matter for downstream food webs[144]. The surface alterations lead to greater storm runoff [145]with likely impact on downstream flooding. Water emerging from valley fills contain toxic solutes that have been linked to declines inwatershed biodiversity [146]. Even with mine-site reclamationintended to restore pre-mined surface conditions, mine-derivedchemical constituents are found in domestic well water [147].Reclaimed areas, compared with unmined areas, are found tohave increased soil density with decreased organic and nutrientcontent, and with reduced water infiltration rates [148].Reclaimed areas have been found to produce little if any regrowthof woody vegetation even after 15 years [149], and, although thisdeficiency might be addressed via more effective reclamationmethods, there remains a likely significant loss of carbon storage[149].

Oil mining has an increasing ecological footprint per unitdelivered energy because of the decreasing size of new fields andtheir increased geographical dispersion; transit distances aregreater and wells are deeper, thus requiring more energy input[145]. Useful quantitative measures of the increasing ecologicalimpacts are provided by the history of oil development in Alberta,Canada for production of both conventional oil and tar sandsdevelopment. The area of land required per barrel of produced oilincreased by a factor of 12 between 1955 and 2006 [150] leading to ecosystem fragmentation by roads and pipelines needed tosupport the wells [151]. Additional escalation of the mining impactoccurs as conventional oil mining is supplanted by tar sandsdevelopment, with mining and land disturbance from the latterproducing land use-related greenhouse gas emissions as much as23 times greater than conventional oil production per unit area[152], but with substantial variability and uncertainty [152153].Much of the tar sands bitumen is extracted through surface mining that removes the overburden (i.e., boreal forest ecosystems) andtar sand from large areas to a depth up to 100 m, with ecologicalimpacts downstream and in the mined area [154]. Althoughmined areas are supposed to be reclaimed, as in the case of mountaintop removal, there is no expectation that the ecological value of reclaimed areas will be equivalent to predevelopmentcondition [141,155]. Landscape changes due to tar sands mining and reclamation cause a large loss of peatland and stored carbon,while also significantly reducing carbon sequestration potential[156]. Lake sediment cores document increased chemical

pollution of ecosystems during the past several decades traceableto tar sands development [157] and snow and water samplesindicate that recent levels of numerous pollutants exceeded localand national criteria for protection of aquatic organisms [158].

Gas mining by unconventional means has rapidly expanded inrecent years, without commensurate understanding of theecological, environmental and human health consequences[159]. The predominant approach is hydraulic fracturing (frack-

ing) of deep shale formations via injection of millions of gallons of water, sand and toxic chemicals under pressure, thus liberating methane [155,160]. A large fraction of the injected water returnsto the surface as wastewater containing high concentrations of heavy metals, oils, greases and soluble organic compounds [161].Management of this wastewater is a major technical challenge,especially because the polluted waters can continue to backflowfrom the wells for many years [161]. Numerous instances of groundwater and river contamination have been cited [162]. Highlevels of methane leakage from fracking have been found [163], aswell as nitrogen oxides and volatile organic compounds [159].Methane leaks increase the climate impact of shale gas, butwhether the leaks are sufficient to significantly alter the climateforcing by total natural gas development is uncertain [164].Overall, environmental and ecologic threats posed by unconven-tional gas extraction are uncertain because of limited research,however evidence for groundwater pollution on both local andriver basin scales is a major concern [165].

Today, with cumulative carbon emissions , 370 GtC from allfossil fuels, we are at a point of severely escalating ecological andenvironmental impacts from fossil fuel use and fossil fuel mining,as is apparent from the mountaintop removal for coal, tar sandsextraction of oil, and fracking for gas. The ecological andenvironmental implications of scenarios with carbon emissions of 1000 GtC or greater, as discussed below, would be profound andshould influence considerations of appropriate energy strategies.

Summary: Climate ImpactsClimate impacts accompanying global warming of 2 u C or more

would be highly deleterious. Already there are numerousindications of substantial effects in response to warming of thepast few decades. That warming has brought global temperatureclose to if not slightly above the prior range of the Holocene. Weconclude that an appropriate target would be to keep globaltemperature at a level within or close to the Holocene range.Global warming of 2 u C would be well outside the Holocene rangeand far into the dangerous range.

Transient Climate Change

We must quantitatively relate fossil fuel emissions to globaltemperature in order to assess how rapidly fossil fuel emissionsmust be phased down to stay under a given temperature limit.Thus we must deal with both a transient carbon cycle andtransient global climate change.

Global climate fluctuates stochastically and also responds tonatural and human-made climate forcings [1,1

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