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Warming of the planet by 2 degrees Celsius is often seen as a “tipping point” that people should try to avoid by limiting greenhouse gas emissions. But the Earth is very likely to exceed that by the end of the century.
“Our analysis shows that the goal of 2 degrees is very much a best-case scenario.”
A new study shows only a 5 percent chance that Earth will warm 2 degrees or less in that time—and shows a mere 1 percent chance that warming could be at or below 1.5 degrees, the target set by the 2016 Paris Agreement.
“Our analysis shows that the goal of 2 degrees is very much a best-case scenario,” says lead author Adrian Raftery, professor of statistics and sociology at the University of Washington. “It is achievable, but only with major, sustained effort on all fronts over the next 80 years.”
The new, statistically-based projections, that appear in Nature Climate Change, show a 90 percent chance that temperatures will increase this century by 2.0 to 4.9 C.
“Our analysis is compatible with previous estimates, but it finds that the most optimistic projections are unlikely to happen,” Raftery says. “We’re closer to the margin than we think.”
The most recent report from the Intergovernmental Panel on Climate Change included future warming rates based on four scenarios for future carbon emissions. The scenarios ranged from “business-as-usual” emissions from growing economies, to serious worldwide efforts to transition away from fossil fuels.
“The IPCC was clear that these scenarios were not forecasts,” Raftery says. “The big problem with scenarios is that you don’t know how likely they are, and whether they span the full range of possibilities or are just a few examples. Scientifically, this type of storytelling approach was not fully satisfying.”
The new paper focuses instead on three quantities that underpin the scenarios for future emissions: total world population, gross domestic product per person and the amount of carbon emitted for each dollar of economic activity, known as carbon intensity.
“…an abrupt change of course is needed to achieve these goals.”
Using statistical projections for each of these three quantities based on 50 years of past data in countries around the world, the study finds a median value of 3.2 C (5.8 F) warming by 2100, and a 90 percent chance that warming this century will fall between 2.0 to 4.9 C (3.6 to 8.8 F).
“Countries argued for the 1.5 C target because of the severe impacts on their livelihoods that would result from exceeding that threshold. Indeed, damages from heat extremes, drought, extreme weather and sea level rise will be much more severe if 2 C or higher temperature rise is allowed,” says coauthor Dargan Frierson, associate professor of atmospheric sciences. “Our results show that an abrupt change of course is needed to achieve these goals.”