Researchers have confirmed that Earth is likely to warm by more than 2 degrees by the end of the century, an increase often cited as a "tipping point" by climate scientists—and one that people should try to avoid by limiting greenhouse gas emissions.
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In findings published Monday in Nature Climate Change, University of Washington researchers show a 90 percent chance that temperatures will have increased by 3.6 to 8.8 degrees Fahrenheit (2.0 to 4.9 degrees Celsius) by the end of the 21st century. Using statistical projections based on 50 years' worth of past data in countries around the world, they found just a 5 percent chance that Earth will warm by 2 degrees or less in the next eight decades. As far as staying within the target set by the 2016 Paris agreement—an increase of 1.5 degrees or less—the researchers put the chances of that becoming a reality at a mere 1 percent.
"Our analysis shows that the goal of 2 degrees is very much a best-case scenario," lead author Adrian Raftery, a UW professor of statistics and sociology, said in a statement. "It is achievable, but only with major, sustained effort on all fronts over the next 80 years."
The most recent report from the Intergovernmental Panel on Climate Change (IPCC) included warming rates based on four scenarios for future carbon emissions, which included "business-as-usual" emissions from growing economies as well as serious worldwide efforts to transition away from fossil fuels.
Raftery points out that the IPCC scenarios are not forecasts, meaning they don't necessarily advance climate science. "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," he said. "Scientifically, this type of storytelling approach was not fully satisfying."
The new paper focuses instead on three quantities that will shape 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. A study Raftery worked on in 2014 showed that the world population is unlikely to stabilize in this century. The planet likely will reach 11 billion people by 2100.
The researchers say damages from heat extremes, drought, extreme weather and sea-level rise will be much more severe if a 2 degrees Celsius or higher temperature rise is allowed.
"Our analysis is compatible with previous estimates, but it finds that the most optimistic projections are unlikely to happen," said Raftery, who worked on United Nations projections for future world population. "We're closer to the margin than we think."
The U.N. has warned that the global threat of climate change will increase if leaders don't rein in the release of greenhouse gases into the atmosphere. Two years ago, representatives from 195 countries around the world reached an historic international agreement to combat the effects of global warming; then-President Barack Obama was among the world leaders who agreed. The pact was set to take effect in 2020, with the U.S. committed to reducing its greenhouse gas emissions by 2025.
In June, though, President Donald Trump abandoned the U.S. commitment to the Paris climate agreement, a decision that has stirred international controversy. At the G-20 summit earlier this month, French President Emmanuel Macron said he hoped to persuade Trump to rejoin the landmark accord.
"Overall, the goals expressed in the Paris agreement are ambitious but realistic," Raftery said. "The bad news is, they are unlikely to be enough to achieve the target of keeping warming at or below 1.5 degrees."
