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How carbon dioxide controls earth’s temperature

NASA’s Lacis: “There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.”

A study by GISS climate scientists recently published in the journal Science shows that atmospheric CO2 operates as a thermostat to control the temperature of Earth….

CO2 is the key atmospheric gas that exerts principal control (80% of the non-condensing GHG forcing) over the strength of the terrestrial greenhouse effect. Water vapor and clouds are fast-acting feedback effects, and as such, they are controlled by the radiative forcing supplied by the non-condensing GHGs….

There is no viable alternative to counteract global warming except through direct human effort to reduce the atmospheric CO2 level.

NASA’s Goddard Institute for Space Studies has posted three articles on their website explaining two important new studies, “Atmospheric CO2: Principal control knob governing Earth’s temperature” (subs. req’d) in Science by Andrew Lacis et al. and “The attribution of the present-day total greenhouse effect” (subs. req’d) in JGR by Gavin Schmidt et al. Together they make a terrific tutorial on the critical role human-caused CO2 plays in climate change.

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Schmidt is best known as a key contributor to the must-read blog, Real Climate. Lacis may be best known as the NASA climatologist whose 2005 critique of the IPCC Fourth Assessment draft — “There is no scientific merit to be found in the Executive Summary” — was embraced by the anti-science disinformers until it was revealed he thought the IPCC consensus was in fact some watered down, least-common denominator piece of wishy-washiness that understates our scientific understanding, which it is (see “Disputing the ‘consensus’ on global warming”).

It may be obvious to CP readers and all those who follow the science, but the core conclusion of the Science article bears repeating again and again by all of us who communicate on global warming:

Ample physical evidence shows that carbon dioxide (CO2) is the single most important climate-relevant greenhouse gas in Earth’s atmosphere. This is because CO2, like ozone, N2O, CH4, and chlorofluorocarbons, does not condense and precipitate from the atmosphere at current climate temperatures, whereas water vapor can and does. Noncondensing greenhouse gases, which account for 25% of the total terrestrial greenhouse effect, thus serve to provide the stable temperature structure that sustains the current levels of atmospheric water vapor and clouds via feedback processes that account for the remaining 75% of the greenhouse effect. Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.

Indeed, absent greenhouse gases, the planet would be about 60°F colder.

Here is NASA’s Research News piece, “How Carbon Dioxide Controls Earth’s Temperature” summarizing the two studies:

Water vapor and clouds are the major contributors to Earth’s greenhouse effect, but a new atmosphere-ocean climate modeling study shows that the planet’s temperature ultimately depends on the atmospheric level of carbon dioxide.

The study, conducted by Andrew Lacis and colleagues at NASA’s Goddard Institute for Space Studies (GISS) in New York, examined the nature of Earth’s greenhouse effect and clarified the role that greenhouse gases and clouds play in absorbing outgoing infrared radiation. Notably, the team identified non-condensing greenhouse gases “” such as carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons “” as providing the core support for the terrestrial greenhouse effect.

Without non-condensing greenhouse gases, water vapor and clouds would be unable to provide the feedback mechanisms that amplify the greenhouse effect. The study’s results will be published Friday, Oct. 15, in Science.

A companion study led by GISS co-author Gavin Schmidt that has been accepted for publication in the Journal of Geophysical Research shows that carbon dioxide accounts for about 20 percent of the greenhouse effect, water vapor and clouds together account for 75 percent, and minor gases and aerosols make up the remaining five percent. However, it is the 25 percent non-condensing greenhouse gas component, which includes carbon dioxide, that is the key factor in sustaining Earth’s greenhouse effect. By this accounting, carbon dioxide is responsible for 80 percent of the radiative forcing that sustains the Earth’s greenhouse effect.

The climate forcing experiment described in Science was simple in design and concept “” all of the non-condensing greenhouse gases and aerosols were zeroed out, and the global climate model was run forward in time to see what would happen to the greenhouse effect.

Without the sustaining support by the non-condensing greenhouse gases, Earth’s greenhouse effect collapsed as water vapor quickly precipitated from the atmosphere, plunging the model Earth into an icebound state “” a clear demonstration that water vapor, although contributing 50 percent of the total greenhouse warming, acts as a feedback process, and as such, cannot by itself uphold the Earth’s greenhouse effect.

“Our climate modeling simulation should be viewed as an experiment in atmospheric physics, illustrating a cause and effect problem which allowed us to gain a better understanding of the working mechanics of Earth’s greenhouse effect, and enabled us to demonstrate the direct relationship that exists between rising atmospheric carbon dioxide and rising global temperature,” Lacis said.

The study ties in to the geologic record in which carbon dioxide levels have oscillated between approximately 180 parts per million during ice ages, and about 280 parts per million during warmer interglacial periods. To provide perspective to the nearly 1°C (1.8°F) increase in global temperature over the past century, it is estimated that the global mean temperature difference between the extremes of the ice age and interglacial periods is only about 5°C (9°F).

“When carbon dioxide increases, more water vapor returns to the atmosphere. This is what helped to melt the glaciers that once covered New York City,” said co-author David Rind, of NASA’s Goddard Institute for Space Studies. “Today we are in uncharted territory as carbon dioxide approaches 390 parts per million in what has been referred to as the ‘superinterglacial.’”

“The bottom line is that atmospheric carbon dioxide acts as a thermostat in regulating the temperature of Earth,” Lacis said. “The Intergovernmental Panel on Climate Change has fully documented the fact that industrial activity is responsible for the rapidly increasing levels of atmospheric carbon dioxide and other greenhouse gases. It is not surprising then that global warming can be linked directly to the observed increase in atmospheric carbon dioxide and to human industrial activity in general.”

Here is Lacis’s explanatory Science Brief, “The Thermostat that Controls Earth’s Temperature”:

For an excellent talk on the role of CO2, see In must-see AGU video, Richard Alley explains “The Biggest Control Knob: Carbon Dioxide in Earth’s Climate History.”

Here is Schmidt’s explanatory Science Brief, “Taking the Measure of the Greenhouse Effect”:

For more on attribution, see “10 indicators of a human fingerprint on climate change.”

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