Carbon dioxide controls Earth’s temperature

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

The study published in Science, 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 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 published in the Journal of Geophysical Research shows that carbon dioxide accounts for about 20 per cent of the greenhouse effect, water vapour and clouds together account for 75 per cent, and minor gases and aerosols make up the remaining five percent. However, it is the 25 per cent 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 per cent 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 of the non-condensing greenhouse gases, Earth’s greenhouse effect collapsed as water vapour quickly precipitated from the atmosphere, plunging the model Earth into an ice-bound state – a clear demonstration that water vapour, although contributing 50 per cent of the total greenhouse warming, acts as a feedback process, and as such, cannot by itself uphold the Earth’s greenhouse effect.

“Our climate modelling 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 with the geological 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 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.

“When carbon dioxide increases, more water vapour 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.”

Based on text by NASA and Science Daily