Almost 200 years after the greenhouse effect was discovered, and 150 years after its experimental proof, NASA scientists have finally demonstrated that CO2 is the most important greenhouse gas. That’s despite the fact that it only accounts for around one-fifth of the Earth’s greenhouse effect, whereas water vapour accounts for about half, and clouds – water in its solid or liquid forms – contribute a quarter.
“It often is stated that water vapour is the chief greenhouse gas (GHG) in the atmosphere,” write NASA’s Andrew Lacis, Gavin Schmidt, David Rind and Reto Ruedy in top journal Science on Thursday. “This would imply that changes in atmospheric CO2 are not important influences on the natural greenhouse capacity of Earth, and that the continuing increase in CO2 due to human activity is therefore not relevant to climate change. This misunderstanding is resolved through simple examination of the terrestrial greenhouse.”
To establish CO2’s key role, the team from NASA’s Goddard Institute for Space Studies (GISS) in New York separated the components of the greenhouse effect into two groups, “feedbacks” and “forcings”. The feedbacks include water vapour and clouds, which are “highly active components of the climate system that respond rapidly to changes in temperature and air pressure”, the GISS scientists say. Their ability to change between gas, liquid and solid, condensing to form rain, for example, allows them to move quickly around the climate. By contrast, forcings such as CO2 don’t condense, instead staying well-mixed in the atmosphere, providing a stable temperature structure that controls the amount of water vapour and cloud.
As its name suggests, the greenhouse effect involves both feedback and noncondensing forcing groups trapping heat that the Earth has already absorbed from the Sun, but which would otherwise escape into space afterwards. Lacis and colleagues used computer climate models to look at what would happen to this process if forcing components like CO2 are removed. They found that without their contribution, more energy escaped into space, reducing temperatures and causing the water vapour “feedback” to quickly precipitate from the atmosphere. The lower temperatures also reduce how much water vapour returns to the atmosphere through evaporation.
In the model, atmospheric water vapour levels then fall, causing the greenhouse effect to collapse, and global mean surface temperature to fall by 4.6°C during the first year alone. After 50 years, the model Earth had plunged into an icebound state, with global temperature reaching –21°C, a decrease of 34.8°C, clearly showing that water vapour alone can’t keep the Earth’s greenhouse effect going.
“Because CO2, ozone, nitrous oxide, methane, and chlorofluorocarbons (CFCs) do not condense and precipitate, noncondensing GHGs constitute the key 25% [contribution] that supports and sustains the entire terrestrial greenhouse effect, the remaining 75% coming as fast feedback contributions from water vapour and clouds,” the team writes. These feedbacks amplify any effect that the noncondensing gases might have alone, which is likely to be especially important given the high levels of CO2 actually measured in the atmosphere today.
“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,” explained Rind. The scientists note that measurements of ice taken from the poles suggest that atmospheric CO2 concentrations have varied naturally between 180 and 300 parts per million over the past 650,000 years, causing temperature variations of around 5ºC. However, current concentrations are both above these level and the 350 parts per million target beyond which scientists consider human interference in the climate dangerous. “Today we are in uncharted territory as carbon dioxide [concentration in the atmosphere] approaches 390 parts per million in what has been referred to as the ‘superinterglacial,’” Rind said.
“The bottom line is that atmospheric carbon dioxide acts as a thermostat in regulating the temperature of Earth,” added Lacis. “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.”