It's known that water vapor and clouds are by far the major contributors to the 'greenhouse effect' on Earth but since those have had a predictable range the planet's temperature ultimately hinges on atmospheric levels of carbon dioxide, according to a new model by the Goddard Institute for Space Studies (GISS) in New York, which sought to analyze the nature of Earth's greenhouse effect and clarify 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 because without non-condensing greenhouse gases, water vapor and clouds would be unable to provide the feedback mechanisms that amplify the 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 their computer simulation was run forward in time to see what would happen to the greenhouse effect. 

In their numerical model, 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 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," said Andrew Lacis of GISS. 

The model uses the period in which the geologic record shows which carbon dioxide levels 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) change in global temperature over the past century, they estimate that the global mean temperature difference between the extremes of the ice age and interglacial periods was 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, also of GISS. "Today we are in uncharted territory as carbon dioxide approaches 390 parts per million in what has been referred to as the 'superinterglacial.'" 

A different study led by Gavin Schmidt of GISS says 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 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."

Citation: Andrew A. Lacis, Gavin A. Schmidt, David Rind, Reto A. Ruedy, 'Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature', Science 15 October 2010 330: 356-359 DOI: 10.1126/science.1190653