Global Warming Warren Wiscombe NASA Goddard. Apr 2005Meto 4012 Movie GWE-Intro.mov

Global WarmingGlobal Warming

Warren Wiscombe

NASA Goddard

Apr 2005 Meto 401 2

Movie GWE-Intro.movMovie GWE-Intro.mov

Apr 2005 Meto 401 3

It’s a hot-It’s a hot-button button

political issue!political issue!

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Why the big concern?Why the big concern?

Because Earth is near its carrying capacity for humans. Slight changes in climate cause massive disruptions in agriculture, commerce, etc.

A unique situation. Before, mankind adapted by• migration• war• population control

Apr 2005 Meto 401 5

Aspects of Global WarmingAspects of Global Warming

• Is it really happening? the “skeptics”…

• Carbon cycle (predict future levels of CO2)

• Climatic impact (temperature, precip, sea ice, ...)

• Range of plausible scenarios (energy use, ...)

• Society’s response, if any

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Is it really happening? - 1Is it really happening? - 1

• Atmosphere– global average temperature — up– polar temperature — up even faster– stratospheric temperature — down fast– H2O vapor — up

• Ocean– sea surface temperature — up– sea level (and day length) — up

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Is it really happening? - 2Is it really happening? - 2

• Cryosphere– Arctic sea ice — receding and thinning– Mountain glaciers — disappearing– snow line — rising

• Growing season — lengthening

• The problem is...even though we see these effects, are they:– due to other causes than humans?– due to natural internal variability?

Apr 2005 Meto 401 8

The following slides are adapted The following slides are adapted from a presentation given to Pres. from a presentation given to Pres. Bush and his cabinet by a NOAA Bush and his cabinet by a NOAA

scientist (hence the use of scientist (hence the use of Fahrenheit rather than Celsius).Fahrenheit rather than Celsius).

It was so well done that some of the It was so well done that some of the slides were reproduced in the slides were reproduced in the

newspapers.newspapers.

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Greenhouse Basics – 1Greenhouse Basics – 1

• If an object is bathed in visible light...– it warms up, and...– emits infrared radiation

• Water vapor (2%) and CO2 (0.03%) have been part of our atmopshere for millions of years.

• Their presence gives us an average surface temperature of ~60° F

• Without them, the average would be ~5° F.

• This is the natural greenhouse effect.

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Greenhouse effect under a treeGreenhouse effect under a tree

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Movie: Greenhouse_Effect.movMovie: Greenhouse_Effect.mov

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• Greenhouse gases are increasing in the atmosphere, and increasingly trapping more heat

• The rise in CO2 is impeccably documented back at least 1000 years (from bubbles in ice cores); it was constant until 1850, since then has risen ~30%

• Methane (CH4) has also increased and has 1/3 the effect of the additional CO2

• All of the CO2 increase and most of the CH4 increase are human-caused

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• Most indicators point to a warming world

• Temperature rise is 0.7-1.4° F over past 100 years

• Glacial retreat ubiquitous

• Snow-cover decrease

• Freeze-free periods lengthened

• Sea level rise of 4-8 inches (thermal expansion)

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• Most warming over past 30 yr is likely due to greenhouse gas increases

• Reasons: comparisons of simulated vs observed temperatures

– simulations including both natural and human factors give best match to observations

– the correspondences increase with time

– probability is loow that a “natural-only” Earth would have such correspondences

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• A continued growth in greenhouse gases is projected to lead to very significant increases in global temperatures and sea level

• CO2 abundance will likely double before 2100– cuts in emissions needed just to stabilize

there

• For a range of future emission scenarios:– global temperature will rise by 2.5–10° F

• (if so, this would exceed the natural changes over the past 10,000 years)

– corresponding sea level rise: 4–35 inches

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• A greenhouse warming could be reversed only very slowly– Reason: the oceans are sluggish

• It is a complex planet of which we have imperfect knowledge, so prediction of further details suffers– regional changes cannot yet be predicted

reliably– but some projections are likely robust:

• land areas warm more than oceans (N. North America 40% above average), and night more than day

• increased mid-continental soil drying• more vigorous hydrological cycle

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• More extreme events?– likely heavier rains, with more rain in

extreme events, and larger variance between regions

– hurricanes? don’t know…

• Abrupt shifts?– has happened before (but mainly in ice

ages)– climate is a non-linear system; such

systems are subject to chaos and regime changes

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• The vast-majority scientific viewpoint:– the issue is a real one– the first signs of human-caused climate

change have likely occurred already– some degree of further change appears

inevitable– exactly where, when (rate of change), how

much is hard to predict– human-caused climate change would be

slow to reverse — 100s of years at the least

Apr 2005 Meto 401 19

The Three Sisters: Differing The Three Sisters: Differing GreenhousesGreenhouses

Decrease in temperature with increasing distance from Sun (units of 10^6 km) for non-reflecting planets with no atmospheres. Open circles take account of the actual planetary reflection of sunlight. Solid circles : actual surface temperatures. The length of each dashed line is a measure of the greenhouse effect.

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History of the COHistory of the CO22 Theory Theoryof Climate Changeof Climate Change

Warren Wiscombe

NASA Goddard

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“A great deal has been written about the influence of the absorption of the atmosphere

upon the climate.”

Was this written by a present-day climate scientist, weary of trying to keep up with the thousands of papers on this subject?

No, it is the first sentence of the classic 1896 paper of Nobel-Prize winning chemist Svante Arrhenius.

Has the CO2-climate problem, then, riveted the attention of scientists for so long?

Far from it. Whole decades passed without the slightest attention being paid to it. It is characteristic of the CO2 theory of climate change that it has repeatedly been incinerated by its critics, only to rise again from its own ashes.

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Became interested in dissipation of heat while serving Napoleon in Egypt.

Proposed that the Earth’s atmosphere behaves like a “hothouse”.

Misunderstood how a hothouse works — assumed that the glass let in the sun's infrared rays and reflected heat back into the structure

Fourier (1768-1830), French mathematician

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John Tyndall (1820-1893)

Investigated radiant (infrared) heat in 1859. Was first to measure absorptive powers of gases such as H2O, CO2, O3 and hydrocarbons (all greenhouse gases).

Demonstrated that H2O, CO2, O3 are some of the best absorbers of infrared radiation.

Apr 2005 Meto 401 25

Wrote that without water vapor, Earth’s Wrote that without water vapor, Earth’s atmosphere would be “held fast in the iron atmosphere would be “held fast in the iron

grip of frost”grip of frost”

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Tyndall Speculates on ClimateTyndall Speculates on Climate

“Now if, as the above experiments indicate, the chief influence be exercised by the aqueous vapor, every variation of this constituent must produce a change of climate.

Similar remarks would apply to the carbonic acid (CO2) diffused through the air, while an almost inappreciable admixture of any of the hydrocarbon vapors would produce great effects on the terrestrial [infrared] rays and corresponding changes of climate...

A slight change in these variable constituents may have produced all the mutations of climate which the researches of geologists reveal.”

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Svante Svante Arrhenius Arrhenius

(1859-1927),(1859-1927),Swedish Swedish physical physical chemistchemist

Said rising atmospheric CO2 due to human industry.

Predicted doubling CO2 would increase average global temperature by ~5°C.

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Movie: GlobalWarming(1.15).movMovie: GlobalWarming(1.15).mov

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Arrhenius’ Arrhenius’ radiation balance equations — the first climate model

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Arrhenius anticipatedArrhenius anticipated:

Role of carbon cycle, including coal-burning (rediscovered 1950s)

H2O feedback with fixed relative humidity (rediscovered 1960s)

Ice-albedo feedback (rediscovered 1960s)

but he lacked:

– Heat transport terms in his equations

Apr 2005 Meto 401 31

Disbelief Period #1: 1900-1930Disbelief Period #1: 1900-1930

CO2 infrared absorption bands are “saturated”

H2O absorbs in same IR spectral region (“overlap

problem”)

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Overlapping gas absorption bands are Overlapping gas absorption bands are made up of lines which don’t really made up of lines which don’t really

interfere with one anotherinterfere with one another

Gasabsorption

(normalized)

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Callendar (1938):Callendar (1938):Temperature Change vs. CO2 concentrationTemperature Change vs. CO2 concentration

Apr 2005 Meto 401 34

Callendar answers his critics at the Callendar answers his critics at the Royal Meteorological SocietyRoyal Meteorological Society

Mr. Dines:

“Was Mr. Callendar satisfied the change in the temperature of the air which he had found [in the historical record] was significant, and not merely a casual variation?”

Callendar replies:

“If any substance is added to the atmosphere which delays the transfer of low temperature radiation, without interfering with the arrival or distribution of the heat supply, some rise of temperature appears to be inevitable in those parts which are furthest from outer space.”

Apr 2005 Meto 401 35

Disbelief Period #2: 1940-1970Disbelief Period #2: 1940-1970

Rising temperature trend from 1880 reversed around 1940 and a cooling commenced. Nature itself had rejected the CO2 theory.

Apr 2005 Meto 401 36

Roger Revelle (1909-1991): CO2 in the Roger Revelle (1909-1991): CO2 in the OceansOceans

In 1936 Revelle began studying the chemical interaction of CO2 with seawater. He left the subject, returning two decades later (here in 1958) to discover that the absorption of CO2 was taking place at a much lower rate than he or anyone else had thought. His first published statements to this effect were taped on to a manuscript just before publication (revealed from his archives at Scripps).

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1957: Revelle and Hans Suess, a founder of radiocarbon dating, demonstrated in a famous article published in Tellus that CO2 had increased in the air as a result of the use of fossil fuels.

Revelle chaired National Academy of Sciences Energy and Climate Panel in 1977, which found that ~ 40% of the anthropogenic CO2 has remained in the air, 2/3 of that from fossil fuel, and 1/3 from the clearing of forests.

Revelle wrote the now legendary "human beings are now carrying out a large scale geophysical

experiment"

Apr 2005 Meto 401 38

Manabe & Wetherald invented Manabe & Wetherald invented “radiative-convective modeling” “radiative-convective modeling”

(1960’s)(1960’s)

(considers whole atmosphere, not just surface)

Two positive feedbacks:(1) more H2O vapor in air (~ constant relative

humidity)

(2) troposphere warms

which both radiate more IR to surface.

Net result: 4x amplification of bare CO2 effect

Apr 2005 Meto 401 39

1970’s: CO2 a lesser concern; eclipsed 1970’s: CO2 a lesser concern; eclipsed by...by...

– Aerosols: the “Human Volcano”

– Ozone layer in danger:* spray cans (CFCs)* supersonic transports

– Nitrogen fertilizers

– Contrail cirrus

– Stratospheric H2O

– Surface albedo changes (desertification)

– Glaciers are coming! (cooling trend continues...)

Apr 2005 Meto 401 40

Highlights of the modern period – 1Highlights of the modern period – 1

1975 Ramanathan: CFC greenhouse effectGlobal cooling stops; warming begins

1978 Clouds begin to eclipse aerosols; Oxford meeting begins 20+ year focus on clouds

1986-9 Hansen says greenhouse warming already here (testimony to U.S. Congress)

1990 IPCC Reports begin

IPCC = Intergovernmental Panelon Climate Change

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Ramanathan’s famous 1976 graph of Ramanathan’s famous 1976 graph of global surface temperature vs. CFC global surface temperature vs. CFC

concentrationconcentration

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Fraction of global warming Fraction of global warming contributed by each trace gascontributed by each trace gas

Apr 2005 Meto 401 43

Properties of trace greenhouse gases Properties of trace greenhouse gases – 1– 1

Gas CO2 CH4 N2O

Human sources Fossil fuels; deforestation

rice culture; cattle; burning

fertilizer; land use conversion

Natural sources (balanced in nature)

wetlands soils; tropical forests

Atmos’c lifetime 50–200 yr 10 yr 150 yr

Concen. (ppb) 370,000 1800 320

Preindustrial concen.

280,000 790 288

Annual rate of increase

0.5% 0.9% 0.3%

Relative contrib. to greenh effect

60% 15% 5%

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Properties of trace greenhouse gases Properties of trace greenhouse gases – 2– 2

Gas CFCs O3, troposphere

H2O

Human sources Refrigerants; aerosols; industry

Hydrocarbons (with NOx); biomass burning

Land conversion; irrigation

Natural sources (none) Hydrocarbons Evapo-transpiration

Atmos’c lifetime 60–100 yr weeks to months

days

Concen. (ppb) CFC-11: 0.28CFC-12: 0.48

20–40 3000-6000 in stratosphere

Preindustrial concen.

0 10 unknown

Annual rate of increase

4% 0.5–2% unknown

Relative contrib. to greenh effect

12% 8% unknown

Apr 2005 Meto 401 45

Highlights of the modern period – 2Highlights of the modern period – 2

1992 Rio Climate Treaty; U.S. signs

1995 IPCC report says anthropogenic greenhouse warming has been detected; attacked

1997 Kyoto agreement; legally binding emission targets for greenhouse gases (developed countries only); U.S. won’t sign

2001 1990’s warmest decade in recorded history

SHEBA, submarines find Arctic sea ice disappearing at alarming rate

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SO2 has no greenhouse effect of its SO2 has no greenhouse effect of its own...own...

but it converts to sulfate particles in the atmosphere in a few days …

which, being relatively transparent, can cause a cooling by reflecting sunlight back to space– Pinatubo SO2 emissions did this

Charlson et al. estimated sulfate aerosols counteracted 20–30% of greenhouse gas warming

Human emissions of SO2 are twice natural ones (volcanic and biological “exhalations”)

Apr 2005 Meto 401 47

Aerosol studies died after a burst of study Aerosol studies died after a burst of study in 1960’s, but revived in the late 1980sin 1960’s, but revived in the late 1980s

Big discovery — most aerosol important for climate is anthropogenic; not realized pre-1985

Climate models can only get agreement with measured temperatures by including sulfate aerosols– supposedly solves 40s cooling

Aerosol “indirect effect” (aerosol changes cloud drop size and liquid water content and therefore cloud reflection) also revived

Apr 2005 Meto 401 48

1980’s Earth Radiation Budget 1980’s Earth Radiation Budget Experiment Experiment

(3 satellites) settled an issue dating (3 satellites) settled an issue dating back to Arrhenius: do clouds warm or back to Arrhenius: do clouds warm or

cool the Earth?cool the Earth?

• Answer was...a large net cooling, 15–20 W/m2. That is, cloud solar reflection beats their infrared heat-trapping effect.

• Around 1989, a comparison of 19 GCM’s revealed that most of the range of temperature response (2-5 C) to 2xCO2 was due to differing model treatment of clouds.

Apr 2005 Meto 401 49

Evidence for Global WarmingEvidence for Global Warming

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The warming trend suffered a reversal in The warming trend suffered a reversal in 19921992

Pinatubo!

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The warming trend recovered from Pinatubo

The ending years of the century were the warmest on record.

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The newly developed (1980s) datasets for ocean surface temperatures did not

fundamentally change the result for land temperatures alone

Sea Surface Temp.Night Marine Air Temp.Land Surface Air Temp.

each point on a curve is roughly a 10-yr average

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The red and orange colors show that temperatures warmed in most regions of the

world

change from 1950-80 mean

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Remarks on warming mapsRemarks on warming maps

Warming has been widespread, but it is not present everywhere. The largest warming is in Northern Canada, Alaska and Siberia.

The lower 48 United States have become warmer recently, but only enough to make the temperatures comparable to what they were in the 1930s. This has caused much misunderstanding among greenhouse skeptics.

Apr 2005 Meto 401 55

The Arctic warming record is very The Arctic warming record is very noisy due to the paucity of stationsnoisy due to the paucity of stations

newly-released Russian data out on sea ice has helped a lot

satellites are badly hampered in Arctic by clouds and blinded by high surface albedo

Apr 2005 Meto 401 56

NASA researchers used maps of urban areas derived from city lights data to account for the “heat island” effect of

cities

from DMSP satellite, OLS instrument

Apr 2005 Meto 401 57

Earth lights - biomass burning in red, Earth lights - biomass burning in red, gas flares in yellowgas flares in yellow

What are the lights off Japan?

Apr 2005 Meto 401 58

More More Night Night LightsLights

gas flares also

Apr 2005 Meto 401 59

Other Temperatures (than Surface)Other Temperatures (than Surface)

• Borehole (clear warming signal)

• Stratosphere (clear cooling signal)

• Troposphere (ambiguous warming signal)

• Ocean mixed layer (50–100 m thick)

Apr 2005 Meto 401 60

Borehole Temperature Record – 1Borehole Temperature Record – 1

• Uses ~600 boreholes from 200 to 1000 m deep– mostly in N. America and Europe

• Relate profiles of temperature with depth to history of temp. change at ground surface– integrates out fluctuations at surface

• Can go back 100–500 yr

• Sensitivity to model assumptions grows w. depth– (snow can esp. mess things up, since it has a

different thermal conductivity than bare soil)

Apr 2005 Meto 401 61

Borehole Temperature Record – 2Borehole Temperature Record – 2

jagged curve is global surface air temperature; gray area is ± 2 standard deviations

Apr 2005 Meto 401 62

Big controversy over tropospheric Big controversy over tropospheric temperature changetemperature change

• NASA people, using satellite Microwave Sounding Unit data since 1980, said troposphere NOT warming

• Radiosonde data seemed to agree, somewhat

• Models show troposphere fairly tightly coupled to surface, so it should have warmed

Apr 2005 Meto 401 63

Ocean Ocean Heat Heat

Content, Content, Top 300 mTop 300 m

change in global ocean heat content from 1950s to 1990s is equivalent to a net downwards surface heat flux of 0.3 W/m^2 over whole period

Apr 2005 Meto 401 64

IPCC “Indicators of Warming”IPCC “Indicators of Warming”

Apr 2005 Meto 401 65

IPCC Indicators of Hydrological Cycle and Storminess IPCC Indicators of Hydrological Cycle and Storminess ChangeChange

much more uncertainty than for temperature effects

Documents

Global Warming Warren Wiscombe NASA Goddard. Apr 2005Meto 4012 Movie GWE-Intro.mov