The researchers analyzed the trajectory and impact on global warming of three factors: total world population, GDP per person and carbon intensity, or carbon emitted per dollar of economic activity.
Global carbon emission projections through the year 2100, per year (left) and cumulative (right).
"Our analysis shows that the goal of 2 degrees is very much a best-case scenario", said lead author Adrian Raftery, a UW professor of statistics and sociology.
"The consequences of not [acting] are even higher with these results than they were before, when we could think about 1.5 degrees as being in the realm of possibility - which I think, realistically, it's not", he said, urging more investments in research, a tax on carbon and other established paths to emissions reductions. Dotted lines show the four "scenarios" from the latest IPCC climate report.
Raftery said that while their new analysis sits well with previous estimates, the most optimistic projections are not likely to occur, with the world being "closer to the margin" than believed. "Scientifically, this type of storytelling approach was not fully satisfying".
A new study has revealed a 90 percent likelihood that global temperatures will rise anywhere from 2 degrees to 4.9 degrees Celsius by 2100, negating most optimism around climate change targets set by the world's nations.
In the Paris agreement, the worldwide community committed to limit global temperature rise "well below 2°C" and to "continue efforts to limit the increase to 1.5°C", relative to the level before the industrial Revolution in order to avoid the devastating consequences of climate change (droughts, rising oceans, storms, etc.).
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"Our results show that an abrupt change of course is needed to achieve these goals", said Frierson.
Scientists have warned for years that an increase in global temperatures of 2 degrees Celsius could cause unsafe weather patterns, crunch the global food supply and force entire populations to migrate. A study that Raftery worked on in 2014 showed that world population is unlikely to stabilize this century.
It has been revealed that scientists had been using wrong baseline temperature; they started measuring global warming in 1900 - after nearly a hundred years since industrial revolution began when fossil fuels were burnt in large amounts. The planet likely will reach 11 billion people by 2100. Instead, he was surprised to learn that population has a fairly small impact. As technologies improve and governments curb emissions, carbon intensity has been dropping in many advanced economies over the past few decades.
The research conducted by Pincus and atmospheric scientist Thorsten Mauritsen of Germany's Max Planck Institute for Meteorology did not rely on computer model simulations, the basis of earlier climate change research.
President Trump's decision to yank the United States from the Paris climate agreement spurred a rallying cry from environmentalists committed to meeting the accord's goals anyway.
Meanwhile, Andrew Dessler, a climate scientist at Texas A&M University who had no associations with the research, said, "I agree that staying below 2-degrees Celsius and 1.5-degrees Celsius are unlikely and very, very unlikely, respectively".