Clouds aren’t expected to dampen global warming— one reason why the planet IN DEPTH is likely to respond sharply to carbon emissions. ATMOSPHERIC SCIENCE Downloaded from Earth’s climate destiny finally seen more clearly Landmark study narrows bounds for “climate sensitivity,” ruling out benign warming http://science.sciencemag.org/ By Paul Voosen agree with the result. Whoever shepherded 2021 or 2022. The estimate will also inform this deserves our gratitude.” projections for sea-level rise, economic dam- t seems like such a simple question: Humanity has already emitted enough age, and much else. A clearer picture of those How hot is Earth going to get? Yet for CO2 to be halfway to the doubling point consequences could do much to spur local 40 years, climate scientists have re- of 560 parts per million, and many emis- governments to cut emissions and adapt peated the same unsatisfying answer: sions scenarios have the planet reaching to warming, says Diana Reckien, a climate If humans double atmospheric carbon that threshold by 2060. The report under- planning expert at the University of Twente. dioxide (CO2) from preindustrial levels, scores the risks of that course: It rules out “The decreasing uncertainty could poten- Ithe planet will eventually warm between the milder levels of warming sometimes tially motivate more jurisdictions to act.” 1.5°C and 4.5°C—a temperature range that invoked by those who would avoid emis- The study dispels uncertainty introduced on July 26, 2020 encompasses everything from a merely sions cuts. “For folks hoping for something by the latest climate models. Models have troubling rise to a catastrophic one. better, those hopes are less grounded in re- historically been used to estimate sensitiv- Now, in a landmark effort, a team of 25 sci- ality,” says David Victor, a climate policy re- ity, beginning in 1979, with the world’s first entists has significantly narrowed the bounds searcher at the University of California, San comprehensive assessment of CO2-driven on this critical factor, known as climate sen- Diego, who was not part of the study. climate change. That summer, at a meet- sitivity. The assessment, conducted under the The WCRP sensitivity estimate is designed ing in Woods Hole, Massachusetts, led by World Climate Research Programme (WCRP) to be used by the United Nations’s Intergov- Jule Charney, scientists produced a paper, and published this week in Reviews of Geo- ernmental Panel on Climate Change (IPCC) known ever since as the Charney report, physics, relies on three strands of evidence: when it publishes its next major report in that predicted between 1.5°C and 4.5°C ; trends indicated by contemporary warm- warming for a CO2 doubling. Those ing, the latest understanding of the feed- numbers—based in part on a model SCIENCE back effects that can slow or accelerate A hot number Hansen had developed—stuck around climate change, and lessons from ancient Bounds on Earth’s climate sensitivity—how far temperatures far longer than anyone imagined: The climates. They support a likely warming will eventually rise for a doubling of atmospheric carbon latest IPCC report, from 2013, gave the range of between 2.6°C and 3.9°C, says dioxide—did not narrow for 40 years. Using new lines of same range. Steven Sherwood, one of the study’s lead evidence, a major study now says substantial warming is likely. Recent models suggest the range authors and a climate scientist at the might even go higher. They run hot, University of New South Wales. “This is 5 some predicting warming of more than ) C the number that really controls how bad ° 4 5°C for a CO2 doubling (Science, 19 April ( y global warming is going to be.” t 2019, p. 222), apparently because of the i 6, EABA1981 (2020) (2020) EABA1981 6, v i 3 The new study is the payoff of de- it way they render clouds, especially over s n cades of advances in climate science, e the Southern Ocean. Yet these high-end s 2 e says James Hansen, the famed retired t models struggle to accurately re-create a ET AL., SCI. ADV., AL., SCI. ET NASA climate scientist who helped craft lim 1 the climate of the 20th century, under- the first sensitivity range in 1979. “It is C mining their credibility. Such models 0 an impressive, comprehensive study, Charney IPCC IPCC IPCC IPCC IPCC WCRP play only a supporting role in the new and I am not just saying that because I (1979) (1990) (1995) (2001) (2007) (2013) (2020) assessment, says Robert Kopp, a climate BICKEL/ C. (GRAPHIC) EXPEDITION 7 CREW/EOL/NASA; ISS (PHOTO) CREDITS: MEEHL (DATA) 354 24 JULY 2020 • VOL 369 ISSUE 6502 sciencemag.org SCIENCE Published by AAAS NEWS scientist at Rutgers University, New Bruns- clouds. Satellites have since observed these ARCHAEOLOGY wick, who was not involved in the effort. dynamics in warmer-than-average parts of “We now have enough independent lines of the atmosphere. “There’s a growing consen- evidence that we don’t need to use the cli- sus that the [cloud] feedback is positive, but Tools suggest mate models as their own line.” not super-large,” says Thorsten Mauritsen, a The WCRP study arose out of a 2015 climate scientist at Stockholm University. workshop at Schloss Ringberg, a castle in Finally, the team looked at records from people reached the Bavarian Alps. Many participants were two past climates—20,000 years ago, at the dissatisfied with the IPCC process and peak of the last ice age, and a warm period wanted to look at how physical mechanisms 3 million years ago, the last time atmo- Americas early might set the boundaries of the sensitivity spheric CO2 levels were similar to today’s. range. “Work on the ends, rather than on Recent work suggests climate sensitivity Skeptics question Mexican the middle,” says Bjorn Stevens, a cloud is not a fixed property of the planet, but finds that suggest settlement scientist at the Max Planck Institute for changes over time. During warm periods, Meteorology, who edited the WCRP report for instance, the absence of ice sheets prob- during the last ice age with Sandrine Bony of the Pierre Simon La- ably raised sensitivity. Records of ancient place Institute. Sherwood and Mark Webb, a temperatures and CO2 levels enabled the climate scientist at the United team to pin down sensitivi- By Andrew Curry Kingdom’s Met Office, agreed ties of 2.5°C and 3.2°C for to lead the effort. “We’re light-years the cold and warm periods, t first glance, Chiquihuite Cave in The first line of evidence respectively. “It’s really com- Mexico’s Zacatecas state is an un- Downloaded from they considered was modern- ahead of where prehensive,” says Jessica likely place to find signs of early hu- day warming. Since record Tierney, a paleoclimatologist mans, let alone evidence that might keeping began in the 1800s, we were in 1979.” at the University of Arizona, change the story of the peopling of average surface temperatures Reto Knutti, ETH Zurich who was not part of the re- the Americas. It sits a daunting 1000 have risen by 1.1°C. Continu- port. Even for the coldest Ameters above a valley, overlooking a des- ing that trend into the future would lead climate state, she says, the possibility of a ert landscape in the mountains north of http://science.sciencemag.org/ to warming on the lower end of the range. sensitivity below 2°C seems negligible. Zacatecas. Getting there requires a 4- or But recent observations have shown the Assembling the three lines of evidence 5-hour uphill scramble over a moonscape of planet is not warming uniformly; in par- was a huge task. But wiring them together jagged boulders. ticular, warming has barely touched parts for a unified prediction was even tougher, But in the soil below the cave’s floor, a of the eastern Pacific Ocean and Southern Marvel says. The team used Bayesian sta- team led by archaeologist Ciprian Ardelean Ocean, where cold, deep waters well up and tistics to churn through its assembled data, of the Autonomous University of Zacatecas, absorb heat. Eventually, models and paleo- which allowed the researchers to test how University City Siglo XXI, dug up almost climate records suggest, these waters will their assumptions influence the results. 2000 stone objects that researchers think warm—not only eliminating a heat sink, “The real advantage” of Bayesian statistics, are tools. By combining state-of-the-art dat- but also spurring the formation of clouds Tierney says, is how it allows uncertainties ing methods, the team argues that humans on July 26, 2020 above them that will trap more heat. Ad- at each stage to feed into a final result. Co- were at the site at least 26,000 years ago— justing the temperature projections for this authors often butted heads, Marvel says. “It more than 10,000 years before any other fact rules out low-sensitivity estimates, says was such a long and painful process.” The known human occupation in the region. Kate Marvel, a climate scientist at NASA’s final range represents a 66% confidence in- “Chiquihuite is a solitary dot” of human oc- Goddard Institute for Space Studies. terval, matching IPCC’s traditional “likely” cupation, Ardelean says. Second, the team probed individual cli- range. The WCRP team also calculated a The dates place humans there during the mate feedbacks. Some of these, like the 90% confidence interval, which ranges from height of the last ice age, when ice covered warming effect of water vapor, are well 2.3°C to 4.7°C, leaving a slight chance of a much of what is now Canada and sea levels known.