Earth's temperature depends on the level of carbon dioxide in atmosphere

A new atmosphere-ocean climate modelling study has shown that the planet's temperature ultimately depends on the atmospheric level of carbon dioxide and not only on water vapour and clouds.

The study conducted by Andrew Lacis and colleagues at NASA's Goddard Institute for Space Studies (GISS) in New York examined the nature of the 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 vapour and clouds would be unable to provide the feedback mechanisms that amplify the greenhouse effect.

A companion study led by GISS co-author Gavin Schmidt that has been accepted for publication in the Journal of Geophysical Research, showed that carbon dioxide accounts for about 20% of the greenhouse effect, while water vapour and clouds together account for 75%, and minor gases and aerosols make up the remaining 5%.

But it is the 25% non-condensing greenhouse gas component, including carbon dioxide, which is the key factor in sustaining the earth's greenhouse effect.

From this it was clear that carbon dioxide is responsible for 80% 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 support of the non-condensing greenhouse gases, the earth's greenhouse effect collapsed as water vapour quickly precipitated from the atmosphere, plunging the model Earth into an icebound state — a clear demonstration that water vapour, although contributing 50% of the total greenhouse warming, acts as a feedback process, and as such, cannot by itself uphold 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 study's results have been published in Science.