Elk wade in the East Fork of Bitterroot River in Montana during a wildfire in August 2000.
Elk wade in the East Fork of Bitterroot River in Montana during a wildfire in August 2000. Greenhouse gases from fossil fuels have increased the frequency of the hot, dry, and windy weather that fuels wildfires. Credit: John McColgan/Bureau of Land Management/Alaska Fire Service
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For the first time, the Intergovernmental Panel on Climate Change’s (IPCC) report includes a chapter on extreme weather and climate.

Chapter 11 of the 4,000-page Working Group I Sixth Assessment Report explains the connection between rising global temperatures and recent extreme weather events such as heat waves, heavy precipitation, floods, droughts, and storms. Extreme events are increasingly likely and intense because of fossil fuel emissions warming the planet.

The report does not contain studies published after 31 January of this year, so it does not assess recent events, including the floods in central Europe and China or wildfires in the Mediterranean and western United States. But it points out events of several years ago, like the 2018 May–August heat waves and droughts that swept across the Northern Hemisphere and melted roads, wilted crops, and killed thousands of people. The events would not have happened without human-induced climate change.

“Using sports terms, one could say that the atmosphere has been exposed to doping,” said Petteri Taalas, secretary-general of the World Meteorological Organization (WMO), which sponsors the IPCC report with the United Nations.

The previous IPCC assessment, in 2013, concluded that human influences had been detected in changes of some climate extremes. Now, that’s an “established fact,” according to the report released 9 August.

Scientific advancements in the past decade made the difference in being able to assess the relationship between climate change and extreme weather. Researchers have a better physical understanding of Earth’s processes than ever before. Scientists have better access to different types of climate models and better data sets. Even a relatively benign sounding improvement—a finer horizontal resolution in computer models that simulate extreme events—bolstered scientists’ grasp on extreme weather.

The explosive growth in the field of attribution science gave the report’s authors many more studies to pull from, too. Attribution science started in earnest as recently as 2004, with a breakthrough study in Nature on a heat wave in Europe. Since then, the field has taken off. Just a week after the end of a record-breaking heat wave in the western United States, a World Weather Attribution study found that climate change made the heat wave 150 times more likely and increased peak temperature by 2°C.

Heat extremes are increasing across more than 80% of the world’s land regions.

Unlike individual studies, the IPCC report combines the results of 14,000 published references to paint a picture of global change. It found profound and extensive changes to the planet’s extreme weather patterns.

Heat extremes are increasing across more than 80% of the world’s land regions. The area of land affected by droughts has been growing.

The atmosphere is now holding more moisture, and increased precipitation has caused more flash floods and surface water floods. Weather systems like atmospheric rivers and tropical cyclones now carry more water. Peak wind speeds in tropical cyclones have increased.

Weather conditions contributing to wildfires are now more probable in some regions, and compound flooding—when a storm surge couples with extreme rainfall and river flow­—has grown more common.

“It is indisputable that human activities have caused and are causing climate change. But what’s new in this report is that we now have a much more advanced understanding of the connections between the emissions that were released under the rise in global surface temperature and the change to weather and the climate that we are seeing around the world,” said Panmao Zhai, cochair of the latest IPCC report.

What the Future Will Bring

The planet has warmed 1.1°C since the 1850s. The IPCC report estimates that global temperatures will surpass 1.5°C by the 2030s and that we are currently heading toward a 2°C or 3°C warming overall.

At 4°C, 50% of all land will be affected by droughts.

In a 2°C warmer world, precipitation and drought events that previously happened once every 10 years will be approximately twice as likely to occur. For extreme heat, once-in-a-decade events will be 5 times more likely.

The effects would be even more severe for 4°C warming, which is possible if fossil fuel emissions increase over the next century instead of tapering off. At 4°C warmer, 50% of all land will be affected by droughts. The frequency of 10-year precipitation events would double, and the occurrence of 50-year events would triple.

Jonathan London, a researcher at the University of California, Davis, studies the effects of drought on health and equity in California. He said the drought has far-reaching consequences: Low-income workers in agricultural or outdoor jobs lose employment when less water is allocated to farms. Home wells go dry. The state’s Central Valley already has poor air quality, and droughts make it worse as dust blows off dry fields.

“Every fraction of a degree [of warming] matters for human health,” said Mona Sarfaty, executive director of the Medical Society Consortium on Climate and Health, a coalition of medical societies representing more than half of all U.S. doctors. “America’s doctors are already seeing the health toll of climate change.”

Although scientific understanding has made significant progress, researchers aren’t sure how to determine the likelihood of extreme events that have tremendous societal consequences but are very unusual—like Hurricane Harvey in the U.S. Gulf Coast in 2017, Tropical Cyclone Idai in Mozambique in 2019, and bushfires in Australia in 2019–2020. The data for low-probability, high-impact events are so sparse that the report gave the scientific understanding of these events a “low confidence” rating. Still, the authors warned that these events would become more likely as the climate enters a new normal, especially in a 4°C warmer world.

It is “extremely important” that these factors be considered for risk management, said IPCC Working Group I cochair Valérie Masson-Delmotte. Only half of WMO’s 193 member countries have early-warning weather systems, an important tool for saving lives and property during extreme events.

“We still have a chance to stop the negative climate trend during the middle of the century,” said WMO’s Taalas, “especially by limiting the use of fossil fuels and by stopping deforestation.”

—Jenessa Duncombe (@jrdscience), Staff Writer

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Duncombe, J. (2021), Climate change and extreme weather linked in U.N. climate report, Eos, 102, https://doi.org/10.1029/2021EO162326. Published on 25 August 2021.

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