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Climate Change Part 2
What is Causing the Warming?
Recent Climate Change• When analyzing the recent warming, it is
important to separate natural from anthropogenic (human) forcings that result in climate change.
• Natural Climate Change– Long Term: Orbital Cycles– Short Term: Sunspot Activity, Volcanic
Eruptions
• Anthropogenic– Changes in Greenhouse Gas Concentrations– Changes in Land Use
Long Term Orbital Cycles
• The graph on the next page shows the insolation at the north pole for the past 15,000 years and prediction of the future insolation based on the known cycles of the orbital changes
• Note the decrease in insolation starting 11,000 years ago.
• We should back to ice age conditions with glacial ice accumulating at the north pole.
• However, global temperature is warming!
North-Pole Summer Insolation
ICE AGE
Short-Term Climate Change
• Three possible short term forcing mechanisms that may have caused the recent warming and temporally prevented an ice age.
1. Sun Spot Activity
2. Volcanic Eruptions
3. Anthropogenic (Human) Activities
• Sunspot activity has played a major role in long-term climate change.
• The Maunder Minimum most likely caused the Little Ice Age
• The Recent Modern Maximum peaked in 1960.
Sunspots and Temperature
• Only 0.1°C of the 0.8°C of warming since the late 1800s is due to solar irradiance.
• Since direct satellite measurements (1980 –present) solar contribution to the observed rapid warming is negligible.
• In fact, the sun has been WEAKER while the climate WARMS since 1960.
• There is no evidence that variations in the strength of the sun are the cause of the modern day climate change.
Sunspots and Temperature
Solar Activity and Atmospheric Heating
• While the troposphere (the lower region of the atmosphere) has warmed, the stratosphere, just above it, has cooled.
• If solar changes provided the dominant forcing, warming would be expected in both atmospheric layers.
Volcanic Emissions over the last 150 years
Natural short term factors cannot explain recent warming. The climate should be cooling!
Recent warming requires anthropogenic (human) activities!
Anthropogenic Forcing
• The next slide shows computer models comparing the natural forcing and human (anthropogenic) forcing with real world observations.
• Models using only natural forcings (blue) do not reflect the actual increases in temperature.
• When both natural and human-induced forcings (orange) are included, however, the models reproduce the real-world rise in temperature, both on a global scale and on a continental scale.
• Changes are shown relative to the average for 1901–1950.
Climate Change Skeptics
• “It’s not human greenhouse gas emissions that have caused the recent warming. It’s natural cycles of sunspots, the earth’s orbit, clouds, humidity, unpatriotic tax-loving liberal scientists...”
• Shifting blame to something else would require two lines of evidence:
1) Another mechanism to account for the recent warming; AND
2) an explanation of why CO2 would not be trapping the heat as we expect it would be doing.
Think of it like a murder mystery
• The butler (CO2) was caught with a smoking gun in his hand in the room with the dead guy. – The bullets came from the gun. – The gun was purchased by the butler. – Everything checks out.
• But now your partner Bob argues that it was really the chauffer did it.
• Actually, you find out that the chauffer was at his sister's wedding on the other side of town for the whole time and lots of people saw him.
Think of it like a murder mystery
• But Bob says, maybe there is some way he did it but you're just not smart enough to figure it out.
• But if Bob is going to convict the chauffer, he has to think of a way to “unconvict” the butler. He would have to come up with an innocent explanation for the butler's smoking gun, and the bullets.
• The bottom line is that there are no competing theories or climate models that can account for the recent warming without incorporating human activities.
Humans and Climate
• Anthropogenic factors are acts by humans that change the environment and influence climate.
• There are two main anthropogenic factors in the climate system
1. Greenhouse Gas Emissions (Carbon Dioxide, Methane, Nitrous Oxides, CFCs)
2. Land Use (Agriculture, Deforestation)
Table 9-1, p. 290
Greenhouse Gas Trends
These graphs show long term trends in the three primary anthropogenic (human) greenhouse gases.
Greenhouse Gas Trends
• In all but the most recent part of the record the data are obtained from analyzing air samples trapped in ice cores.
• Direct measurements have been made since the mid 1950s and fit nicely with the ice core record.
• For each gas, concentration was very constant from 1000-1800. From 1800-1900 a linear increase in concentration is noted for each gas.
• After 1900 the concentrations all increase exponentially.
Carbon Dioxide
• Prior to industrialization the Earth’s atmosphere contained about 280 parts per million (280 CO2 molecules for every 1,000,000 molecules in the atmosphere).
• This carbon dioxide was maintained in the atmosphere via volcanic and biological activity.
• Before 1960, scientists thought that CO2 levels in atmosphere remain unchanged because CO2 was dissolving in ocean sink.
Carbon Dioxide
• Charles Keeling monitored CO2 levels on Mauna Loa, Hawaii
• Expected the level to be constant but he found CO2 levels dropped during late spring and summer, and rose in the fall and winter.
• Keeling's sensitive measurements had picked up the Earth's respiration. Every spring, plants bloomed, sucking carbon dioxide out of the air. Every fall, CO2 was released back into the air as the plants decayed
Carbon Dioxide
• But Keeling noticed that year by year, the total concentration of CO2 increased. Superimposed on the annual cycle was a rising curve. – Each year, more CO2 was inhaled and exhaled by the
biosphere. – Over 40 years, the concentrations rose from 330
parts per million to almost 370 parts per million.
• The Keeling curve appeared to prove what many had suspected, that humans were changing the composition of the atmosphere.
Past Carbon Dioxide Levels
• What about CO2 levels before 1957?
• Scientists can determine the amount of carbon dioxide that was in the air during different time periods by analyzing gas bubbles trapped in ice cores.
• Up to 600,000 years of CO2 data trapped in the ice
Past Carbon Dioxide Levels
• Throughout most of the past 600,000 years the concentration of CO2 has ranged between 180 and 280 parts per million (ppm).
• When the concentration was at its highest the world was warmer.
• There is a close correspondence between average global temperature and CO2 concentration.
Past Carbon Dioxide Levels
• The 100,000 year CO2 cycles are believed to be caused by the orbital changes associated with the ice ages.
• A major reason for the drop in atmospheric CO2 as temperatures decrease is that colder oceans are able to dissolve more CO2.
• When the orbital pattern changes, the oceans warm and the CO2 is released
Recent Carbon Dioxide Levels
• The last 100-150 years of the 20th Century show a significant rise in CO2.
• The current concentration of carbon dioxide, 380 ppm, is 35% higher than at any time during the past 400,000 year period -- and it is steadily increasing
Recent Carbon Dioxide Levels
• Analysis of isotopes, which can distinguish among sources of emissions, demonstrates that the majority of the increase in carbon dioxide comes from combustion of fossil fuels (coal, oil and natural gas).
• Geographic differences in concentrations reveal that sources occur predominantly over land in the more heavily populated Northern Hemisphere.
Recent Carbon Dioxide Levels• Note large increase since 1850 when
human fossil fuel use began.
Methane• Methane has increased 150% since 1750
to its highest levels in at least 420,000 years
• About 75% of the increase in methane concentrations results from human agriculture (livestock, particularly cows, rice paddies).
• Fossil fuel burning contributes another 25%.
• Smaller contributions come from landfills and domestic sewage systems.
Methane• Methane increase significant because it absorbs
infrared radiation 21 times more than CO2
Nitrous Oxide
• Nitrous Oxide also occurs naturally in the ocean and as a result of microbial activity in the soil.
• Has increased 16% since 1750 to its highest levels in at least 1,000 years.
• Nitrous oxide from agriculture and industry has increased by 15%
• The anthropogenic increases are primarily the result of enhanced loss from agricultural soils due to fertilizer applications.
• Industrial sources, feedlots and biomass burning also contribute to the increase.
Nitrous Oxide
• Nitrous Oxide increase is significant because it absorbs infrared radiation 310 times more than CO2
Deforestation: The hidden cause of global warming
• After carbon emissions caused by humans, deforestation is the second principle cause of atmospheric CO2.
• Deforestation is responsible for 20-25% of all carbon emissions entering the atmosphere, by the burning and cutting of about 34 million acres of trees each year.
• In the next 24 hours, deforestation will release as much CO2 into the atmosphere as 8 million people flying from London to New York.
Why Forests are so Important
• Trees are sinks for atmospheric carbon dioxide. Fewer trees mean less CO2 is removed from the atmosphere.
• If the trees are subsequently burned the CO2 is added back to the atmosphere.
• Dark-colored forests also absorb more solar energy and keep the region warmer and more moist than do the light-colored areas left when the trees are gone.
• Large rainforests (and their humidity) contribute to the formation of low (high albedo) clouds which reflect solar radiation
2007 IPCC Report
“Most of the observed increase in globally averaged temperatures
since the mid-20th century is very likely (90% certainty) due to the
observed increase in anthropogenic greenhouse gas concentrations.”
Are Humans Causing the Recent Warming?
VERYLikely
Based on peer reviewed scientific evidence……..
• The Ruddiman hypothesis suggests that humans began contributing significant quantities of greenhouse gases to the atmosphere thousands of years earlier by clearing forests and irrigating fields to grow crops such as rice and wheat.
• As a result, humans have kept the planet notably warmer than it would have been otherwise—and possibly even averted the start of a new ice age about 5,000 years ago.
When Did Humans Begin to Change Climate?
Humans and CO2
• 8,000 years ago: Late Stone Age Europeans begin clearing forests to grow wheat, barley, peas and other nonindigenous crop plants
• 8,000 years ago: CO2 trend, which has been falling for 2,500 years, bottoms out and suddenly reverses direction
Humans and Methane
• 5,000 years ago: Farmers in the south of China begin flooding lowlands near rivers to grow rice
• 5,000 years ago: Methane trend, which has been falling for 6,000 years, suddenly reverses direction
The Ruddiman Hypothesis
The Ruddiman Hypothesis• Early human agricultural activities produced
enough greenhouse gases to offset most of the natural cooling trend during preindustrial times (yellow), warming the planet by an average of almost 0.8 degree Celsius.
The Ruddiman Hypothesis
• That early warming effect (yellow) rivals the 0.6 degree Celsius warming measured in the past century of rapid industrialization (orange).
The Ruddiman Hypothesis• Once most fossil fuels are depleted and the
temperature rise caused by human greenhouse gases peaks, the earth will cool toward the next glaciation—now thousands of years overdue.
