How much will greenhouse gases heat the planet?
For more than 40 years, scientists have expressed the answer as a range of possible temperature increases, between 2.7 and 8.1 degrees Fahrenheit, that will result from carbon dioxide levels doubling from preindustrial times. Now, a team of researchers has sharply narrowed the range of temperatures, tightening it to between 4.9 and 7.4 degrees.
Steven Sherwood, a climate scientist at the University of New South Wales in Sydney and an author of the new report, said that the group’s research suggested that these temperature shifts, which are referred to as “climate sensitivity” because they reflect how sensitive the planet is to rising carbon dioxide levels, are now unlikely below the low end of the range. The research also suggests that the “alarmingly high sensitivities” of 9 degrees Fahrenheit or higher are less likely, although they are “not impossible,” Sherwood said.
What remains, however, is still an array of effects that mean worldwide disaster if emissions are not sharply reduced in coming years.
Masahiro Watanabe, a professor in the atmosphere and ocean research institute at the University of Tokyo and an author of the report, said that determining an accurate range of temperatures was important for international efforts to address global warming.
“Narrowing the uncertainty is relevant not only for climate science but also for society that is responsible for solid decisionmaking,” he said.
The new paper, published Wednesday in the journal Reviews of Geophysics, narrowed the range of temperatures considerably and shifted it toward warmer outcomes. The researchers determined that there was less than a 5% chance of a temperature shift below 3.6 degrees, but a 6% to 18% chance of a higher temperature change than 8.1 degrees.
If the effects of carbon dioxide are at the low end of the range or even below it, then climate change will be affected less by emissions and the planet will warm more slowly.
The scientists noted that the Earth’s temperature is already about 2.2 degrees above preindustrial levels, and that, if current emissions trends continue, the doubling of atmospheric carbon dioxide could happen well before the end of this century.
Andrew Dessler, a climate scientist at Texas A&M University, who was not an author of the report but who was one of its earlier outside reviewers, called the paper “a real tour de force,” adding that “this is probably the most important paper I’ve read in years.”
The paper, produced under an international science organization, the World Climate Research Program, brought together three broad fields of climate evidence: temperature records since the industrial revolution, records of prehistoric temperatures preserved in things like sediment samples and tree rings, as well as satellite observations and computer models of the climate system.
“This paper is really the first to try and include all of those disparate sources of observational evidence in a coherent package that actually makes sense,” said Gavin A. Schmidt, director of the NASA Goddard Institute for Space Studies and an author of the paper.