Climate scientists say previous estimates of how much the planet will warm if the amount of carbon dioxide in the atmosphere doubles—known as climate sensitivity—may be flawed.
“Over the past few years, a debate has raged in the scientific community over climate sensitivity,” says Andrew Dessler, professor of atmospheric sciences at Texas A&M University. “Many researchers found that the 20th-century warming suggested a lower climate sensitivity of about 4 degrees F, while climate models and the paleoclimate record suggested a higher climate sensitivity of 6 degrees F,” Dessler says.
“This difference may not sound like much,” says graduate student John Kummer, lead author of the study published in Geophysical Research Letters. “But it could have a profound effect on the impacts of climate change, such as heat waves, drought, and sea level rise.”
Ozone and aerosols
The researchers calculated climate sensitivity based on the 20th-century warming, but with one key difference.
“Previous estimates have assumed that all human emissions are equally effective at altering the climate,” Kummer says. “However, greenhouse gases like carbon dioxide are well mixed in the atmosphere—this means that, no matter where you go, the amount of carbon dioxide is about the same.
“Ozone and aerosols, on the other hand, have short atmospheric lifetimes, so they are mainly found near where they are emitted. This means that the heating from these constituents are highly localized. Recent studies using climate models have suggested that this highly localized forcing is more effective at changing the climate.”
“What we did was to take the 20th-century observational record and calculate the climate sensitivity under the assumption that ozone and aerosols were about one-third more effective at warming the climate than well mixed greenhouse gases like carbon dioxide,” Dessler says.
“We found that this yielded a climate sensitivity of 6 degrees F, in good agreement with other estimates of climate sensitivity.”
Dessler is confident that the new method of calculation is an advance over previous calculations.
“We can now resolve the dispute over these different estimates of climate sensitivity,” he says. “As the evidence for a large climate sensitivity accumulates and the evidence that it is low evaporates, our society needs to accept the science and begin thinking about what to do about it.”
NASA funded the study.
Source: Texas A&M University