“We have much clearer evidence on changes in climate extremes as a result of human-induced climate change,” said Sonia Seneviratne, a panel coordinating lead author and climate scientist at the Swiss Federal Institute of Technology in Zurich.
The report also highlights a probable increase in “compound” extreme events, when one type of weather disaster feeds into and intensifies another. As a sobering example of this in late June, the British Columbia village of Lytton set Canada’s national temperature record of 121 degrees and then burned to the ground in a wildfire the next day.
Previously, scientists shied away from attributing individual weather events to climate change. Now, the report explains, they’ve gained confidence in connecting the dots and quantifying the impact of the rising temperatures. After the historic heat wave in the Pacific Northwest and Canada in late June, a group of scientists found human-caused climate change made it at least 150 times as likely.
The changes in extremes seen so far have occurred as the Earth has warmed by about 2 degrees (1.1 Celsius), just a small fraction of what could be in the pipeline. The panel projects up to another 6 degrees of warming by the end of this century if emissions of heat-trapping gases are not slashed.
“With every additional increment of global warming, changes in extremes continue to become larger,” the report cautions.
Of the impacts of climate change, the increases in extreme events loom especially large because of the suffering they cause and the cost to society. Heat is the leading cause of weather-related fatalities in the United States and was blamed for hundreds of excess deaths in the Pacific Northwest during June. Last year brought a record 22 weather-climate disasters to the United States, totaling $98.9 billion in damage. This year, through June, eight such events have occurred in the United States.
The toll of the extremes we’ve seen so far is a warning sign, said Jane Lubchenco, deputy director for climate and environment at the White House Office of Science and Technology Policy.
“All we need to do is look at the extreme events that are happening now and see how devastating those are, and then to comprehend that it’s going to get worse if we don’t get our act together,” she said.
Below, we describe the panel’s findings for the four major types of weather extremes that the report finds are profoundly affected by climate change.
Hotter heat waves
In June 2020, the temperature in the town of Verkhoyansk in Russia soared to 100 degrees Fahrenheit (38 degrees Celsius) — the hottest temperature recorded in the Arctic. In 2018, several record-breaking heat waves plagued North America, Europe and Asia at the same time during the spring and summer. The panel finds that these specific heat events, as well as others, would have been “extremely unlikely to occur without human influence on the climate system.”
“Temperatures [in Siberia] were very extreme, and studies have shown that basically it had near-zero probability without human-induced climate change,” Seneviratne said. “The probability of having all those heat waves [across the Northern Hemisphere in 2018] at the same time was also near zero.”
Projections show hot weather will increase in intensity through the 21st century, even if global warming is held to 2.7 degrees Fahrenheit (1.5 degrees Celsius) above preindustrial levels. The chance of a 1-in-50-year heat event increases 8.6 times for this amount of warming compared with a planet without such a temperature increase. If the Earth warms 7.2 degrees Fahrenheit (4 degrees Celsius), the chance of such a heat wave increases 39.2 times.
“We find that changes in heat waves very clearly also become more intense and more frequent with increasing global warming,” Seneviratne said.
When temperatures rise, they increase evaporation and make more water available for storms to draw from. The panel states heavy precipitation events have increased since the 1950s over most land areas.
“Many studies have been able to show that the severity of some recent unprecedented extremes is only possible due to human caused warming of climate,” Richard Allan, a panel lead author and professor at the University of Reading, wrote in an email.
With more warming, the panel projects, heavy precipitation events will intensify further, increasing by 7 percent for every 1.8 degrees (1 Celsius) of warming.
While downpours strengthen, how much they increase flooding is a more involved question. “Such precipitation increases may lead to greater river flood risk but that increased risk varies in different regions as the complex factors leading to floods are very locally dependent,” Michael Wehner, a report lead author and climate scientist at Lawrence Berkeley National Laboratory, wrote in an email.
Although increasing evaporation from rising temperatures makes more water available for storms, it also speeds up the drying of the land surface, which can increase drought in the absence of storms.
Observations showed drought is increasing across substantial land areas, but not everywhere.
Seneviratne said drought events in western North America and the Mediterranean have been particularly strong in recent decades.
The Western United States is enduring one of its worst droughts on record. As of last week, around 98 percent of the West was experiencing some level of drought. Water levels at Lake Powell, the nation’s second-largest reservoir, on the Arizona-Utah border, and Lake Oroville, California’s second-biggest reservoir, have both dropped to historic lows this summer.
Projections show that drying regions will get even drier as temperatures rise, the number of regions experiencing drought increasing with incremental changes in global warming.
“What we find is that there is a certain number of regions projected to get drier,” Seneviratne said. “And [in] those regions, we clearly see an increase in intensity and frequency of the droughts.”
The panel also projects an increase in the number of compound extreme events involving drought, heat waves and fires at the same time. Drought events paired with extreme heat waves increase the fire risk by drying out the land surface and making it more combustible. Fire weather is expected to become more probable in Southern Europe, Eurasia, parts of the United States and Australia, all places that have endured sieges of devastating fires in recent years.
Windier and rainier hurricanes
Multiple major hurricanes, rated Category 3 or higher on the Saffir-Simpson wind scale, have struck the United States, the Caribbean and Central America in recent years, demolishing coastal property and upending the lives of residents.
Hurricanes feed on warm water and, as ocean temperatures have increased, the panel finds increasing evidence that these major storms have turned more intense.
“Our confidence is higher now that tropical cyclones have become stronger, particularly the major (Category 3-5) tropical cyclones that create the most risk, and they have more rain falling out of them as the planet warms,” Jim Kossin, a contributing lead author and a senior scientist at the Climate Service, wrote in an email.
The tropical Atlantic has seen a recent flurry of Category 5 hurricanes. At least one of these top-tier storms developed in four straight years from 2016 to 2019, the longest such streak on record. The powerhouses Matthew, Irma, Maria, Michael and Dorian were among them.
The storms, Kossin added, also seem to be slowing down when moving over land.
“[This] increases the likelihood of having storms that ‘stall’ and do massive damage, such as Hurricanes Harvey, Dorian, Sally, Florence, Eta, Iota, and other recent storms,” he wrote. “The combination of slower movement and more rain falling out of them increase coastal and inland flooding risk tremendously.”
The panel states that with increasing global warming, a greater percentage of hurricanes will reach categories 4 and 5, which are by far the most dangerous. And, with warming-induced rising seas, the ocean surge from landfalling storms will inundate even more coastal territory.
Brady Dennis contributed to this report.