Every time the U.N. Intergovernmental Panel on Climate Change (IPCC) issues a new report surveying the science of global warming, the alarm sounds louder. Now, 8 years after its last report, the message of IPCC’s latest assessment, released this week, is urgent and unequivocal: The window for mitigating the worst projected impacts of global climate change is closing. Average global temperatures are now 1.1°C above preindustrial records, and under every scenario for future greenhouse gas emissions that the panel examined, average warming of 1.5°C—a target set by the 2015 Paris climate accord—will very likely be reached within the next 20 years.
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“This is a critical decade for keeping the 1.5°C target within reach,” says Jane Lubchenco, deputy director for climate and the environment at the White House Office of Science and Technology Policy. The projections mean countries should come to the U.N. Climate Change Conference, scheduled for November, with the most “aggressive, ambitious” targets for reducing emissions possible, she says.
It is “unequivocal” and “established fact” that human activities are causing Earth to warm, says the report, which was assembled by hundreds of scientists from around the world, and climate change’s impact on the planet is “unprecedented.” That blunt language reflects, in part, the substantial scientific advances that have occurred since IPCC issued its last major assessment in 2013. Climate models are more detailed and powerful, and observational records cover more ground—including the rapidly warming Arctic—as well as eight additional years of warming. They have enabled researchers to better “see the climate change signal developing,” says Nerilie Abram, a climate scientist at Australian National University.
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Growing evidence from ancient climates has also helped researchers constrain estimates of what is called climate sensitivity—the amount of warming expected if concentrations of atmospheric carbon dioxide (CO2) rise twice as high as in preindustrial times, to 560 parts per million (ppm). (Current levels are about 415 ppm and climbing fast.) The panel now estimates that a CO2 doubling would boost temperatures by 2.5°C to 4°C, a narrower range than the previously estimated 1.5°C to 4.5°C.
In the best case scenario, with the world cutting net emissions to zero by 2050, CO2 will fall short of doubling and warming is projected to peak midcentury at 1.6°C above preindustrial levels. Even in this scenario, Arctic sea ice is likely to vanish completely in at least one summer by 2050. In the worst case scenario, warming will very likely reach 2.4°C by midcentury and rise to 4.4°C—and potentially 5.7°C—by 2100. At higher emissions levels, “low-likelihood, high-impact” consequences—such as mass ice sheet loss in the Antarctic or the stalling of ocean currents—become more likely. The probability of these abrupt, irreversible changes is not well-understood, the report says, but they cannot be ruled out. Current emissions are on IPCC’s mid- to higher trajectories, Imperial College London climate scientist Joeri Rogelj said at a briefing, and countries’ pledges still fall short of achieving the lowest emissions scenario. “Let’s be clear,” he said, “about the work that still needs to be done.”
For the first time, the report elaborates on how each increment of warming could play out in different regions, stoking extreme events such as flooding, heat waves, droughts, and fire. Past reports focused on averages, Abram notes, but “people don’t live in the global average.” One forecast is that climate change will give extra potency to existing natural variability in temperatures and precipitation. With 1.5°C of warming, for example, high daytime temperatures that would be rare without climate change could occur four times a decade; at 4°C of warming, such heat extremes could come nearly every year. Such projections illustrate that “every little bit of warming counts,” says Claudia Tebaldi, a climate scientist at Pacific Northwest National Laboratory and an author of the report. But she and others emphasize that targets like 1.5°C should not be seen as precipices beyond which there is no redemption or hope.
It is now “established fact” that warming is fueling extreme events, the report says. And since IPCC’s last report, scientific advances have made it possible to link climate change to specific events, such as the recent heat wave in northwestern North America, says Francisco Doblas Reyes, a climate scientist at the Barcelona Supercomputing Center and a report author. It’s clear that “climate change is here now,” he says. “No region is spared,” adds Sonia Seneviratne, a climate scientist at ETH Zürich and a report author. New extremes in heat, precipitation, or drought have been observed in nearly every global region. “We are starting to see events which would have had near zero probability of happening without human-induced climate change,” Seneviratne says.
Some global changes are already locked in, the report notes, regardless of how much humanity reduces emissions in coming decades. Melting of glaciers and ice sheets and thawing of permafrost is now “irreversible” for decades or centuries to come, it says. The warming, acidification, and deoxygenation that are already damaging the world’s oceans will continue for centuries to millennia. Continued sea level rise, now estimated at 3.7 millimeters each year between 2006 and 2018, is also inevitable: Over the next 2000 years, oceans will likely rise by 2 to 3 meters if the planet warms by 1.5°C, and up to 22 meters with 5°C of warming.
Because of the ongoing COVID-19 pandemic, the hundreds of scientists who wrote the assessment had to meet online to wrestle with how to convey the extent of the climate crisis and the urgent need to act. It was uncanny to see “the echoes of one crisis in another,” Tebaldi says. And for many researchers, the work isn’t done. The science assessment—the sixth produced by IPCC since 1990—is just the first of three major reports that IPCC’s 195 member states will release over the coming year. The next reports will examine how climate policies can reduce emissions, and what actions will be needed to adapt to extreme events such as flooding, heat waves, and drought.
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