Plants put the brakes on global warming but clouds – sometimes suggested as a climate stabiliser – amplify it, scientists have shown this week. In the first case, NASA researchers have developed more detailed computer models of how plants respond to the increased amount of CO2 that humans are putting into the atmosphere. To do this, Lahouari Bounoua and his colleagues incorporated a previously-overlooked way in which plants grow more efficiently in these circumstances into their simulations.
When more CO2 is available, plants are able to use less water yet maintain previous levels of the photosynthesis process that drives their growth, in a process called “down-regulation”. Meanwhile, higher temperatures would increase leaf growth, and in turn raise the movement of water into the atmosphere from plants and land known as evapotranspiration on a global scale. Water cools surfaces it evaporates from, which is why we sweat, and so this creates an additional cooling effect. Consequently plants provide more help in keeping the Earth’s temperature down through “cooling feedbacks” in simulations including this process than existing models predicted.
“What we did is improve plants’ physiological response in the model by including down-regulation,” Bounoua said. “The end result is a stronger feedback than previously thought.” By including the additional cooling effect the extra plant growth contributed, the NASA team’s model predicted that a doubling of CO2 in the atmosphere would cause a 1.94ºC temperature increase. This is fractionally below the climate sensitivity figure of 2-4.5ºC normally put forward by scientists, the team noted in their Geophysical Research Letters paper published Tuesday. “This feedback slows but does not alleviate the projected warming,” Bounoua said.
Meanwhile, Andrew Dessler at Texas A&M University said that the climate feedback from clouds has a warming rather than a cooling impact. This is what models like Bounoua’s typically simulate, but until recently it was impossible to test the models using observations. “Climate models disagree on the magnitude of the cloud feedback, simulating a range of feedbacks in response to long-term global warming,” Dessler wrote. “This spread is the single most important reason for the large spread in the climate sensitivities among climate models.”
Clouds both reflect incoming solar radiation back to space, which tends to cool the climate, and trap outgoing infrared radiation, which tends to warm the climate. Currently, the net effect of clouds is cooling, but the cloud feedback reflects how this changes as the planet warms. A negative feedback would see the cooling effect increase, offsetting warming, while a positive effect would see it decrease, amplifying warming.
Dessler’s calculations of the cloud feedback published in top journal Science on Friday, used measurements from the “Clouds and the Earth’s Radiant Energy System” (CERES) instrument onboard NASA’s Terra satellite. These provided the amount of energy trapped by clouds as the climate varied over the last decade. He supplemented these with information from meteorological analyses provided by NASA and the European Center for Medium-Range Weather Forecasts. Altogether, Dessler showed that over the last decade short-term cloud feedback decreased clouds’ cooling effect by adding 0.54 watts of energy flow per square metre of the Earth’s surface for each 1°C temperature rise. However, he found a large margin of error – plus or minus 0.74 watts per square metre per 1°C.
Despite the uncertainties, Dessler said that these figures show that clouds will not have enough of a cooling feedback to stabilise recent warming, as some critics of current climate models suggest. “A small negative feedback is possible, but one large enough to cancel the climate’s positive feedbacks is not supported by these observations,” he wrote. “The chances that clouds will save us from dramatic climate change is pretty low,” he added.
While the feedbacks from clouds and plants pull the climate in opposite directions, the predictive models used by scientists bring them together and balance them against each other. The NASA modellers note that each new finding, like their revised plant growth cooling feedback, adds to the gradual improvement of their predictions. “As we learn more about how these systems react, we can learn more about how the climate will change,” said Bounoua’s co-author Forrest Hall. “Each year we get better and better.”