A New Solution to Climate Science’s Biggest Mystery

Read: This land is the only land there is

This week, a team of 25 researchers—drawn from across the earth sciences and descended from the Bavaria effort—published the first new answer in 41 years. Their estimate of this value, called “climate sensitivity,” significantly reduces the amount of uncertainty involved in forecasting climate change. “It helps us answer this fundamental question, which is: How warm is it gonna get?” Kate Marvel, a climate scientist at NASA and an author of the paper, told me.

Since Arrhenius first tried to calculate climate sensitivity, scientists have talked about it by estimating how much temperatures would rise if CO₂ doubled. The new paper finds that doubling carbon dioxide will likely increase Earth’s average temperature by 2.6 to 3.9 degrees Celsius (about 5 to 7 degrees Fahrenheit). That’s much narrower than the old estimate, which said that a doubling of CO₂ would raise temperatures by 1.5 to 4.5 degrees Celsius (about 3 to 8 degrees Fahrenheit).

The biggest change in the new study is on the lower end: It zeroes out any chance that Earth isn’t sensitive to carbon emissions. There’s no way to double the planet’s carbon-dioxide content and avoid fewer than 2.3 degrees Celsius (4 degrees Fahrenheit) of warming, the authors found. But they have also reduced their estimate of the chance of a major overshoot, with a doubling of CO₂ leading to 6 degrees Celsius (about 10 degrees Fahrenheit) or more of global warming. “We’ve ruled out ‘We’ll be fine,’ and we don’t think ‘doom’ is very likely,” Marvel said.

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The paper is considered a landmark by other researchers. “It’s a really important study,” Bjorn Stevens, an atmospheric scientist at the Max Planck Institute for Meteorology, told me. In a 2015 paper, he laid out a method for how climate-sensitivity might be refined, and he helped organize the gathering at the castle, but he is not an author of the new study. “It tells us we understand the Earth system better than we thought we did.”

“This study probably shouldn’t do much to affect our baseline view of climate change,” Joseph Majkut, a climate scientist and the director of climate policy at the Niskanen Center, a nonpartisan think tank, told me by email. “Despite the controversy about the possible bounds of warming, the central estimates for climate sensitivity have been pretty consistent (around 3 [degrees Celsius] for doubling CO2). This study doesn’t find different, but shows that basically any way you cut the evidence, expected warming is pretty similar.”

One group that the refined result does not bode well for is “self-styled lukewarmers,” Majkut said, “who claim that global warming is real but not too much of a problem.” The paper undercuts their claims, he said, by showing that “when you carefully calibrate your estimate and confront it with other pieces of evidence in a rigorous way, very low climate sensitivity appears really unlikely.”