Climate change worsens extreme weather. A revolution in attribution science proved it.

There’s a cliché that has popped up for years in discussions of climate and weather disasters: You can’t blame any individual event on climate change. Climate is all about trends and statistics, the reasoning goes, so you can’t necessarily draw meaningful conclusions from a single data point, be it a heat wave, a hurricane, or a drought.

But in recent years, climate scientists have been pushing back against this notion: Bolstered with better data and even clearer trends, they’re no longer reluctant to point the finger back at humanity for worsening these calamities. In the latest report from the Intergovernmental Panel on Climate Change (IPCC), a team of leading researchers convened by the United Nations presented some of the most robust research that connects the dots. It shows how some greater weather extremes can be traced back to rising average temperatures, which in turn stem from emissions of greenhouse gases, mainly from countries and corporations burning fossil fuels.

“On a case-by-case basis, scientists can now quantify the contribution of human influences to the magnitude and probability of many extreme events,” according to the report.

Scientists are not just confident that humans are warming the climate — that much is “unequivocal.” Now they’ve concluded it’s “an established fact” that signals of this warming shine bright in events like the severe heat and torrential rainfall that we’re seeing now, increasing their frequency, severity, or both. That’s why scientists only needed a few days to implicate climate change in recent Pacific Northwest heat waves, for example.

The field of extreme weather attribution has made enormous strides since the last major IPCC assessment in 2013, and it’s one of the most significant components of the new report. “What’s new in this report is that we can now attribute many more changes at the global and regional level to human influence — and better project future changes we will see from different amounts of emissions,” said Ko Barrett, vice chair of the IPCC and senior adviser for climate at the National Oceanic and Atmospheric Administration, during a press conference.

At the same time, the stunning, record-breaking disasters of recent memory have again forced a public discussion about the role of climate change. This year alone, the world has seen extreme fires in California, Siberia, Greece, and Turkey; devastating flooding in China and Germany; and heat waves and record drought across the US West, to name just a few.

The question is not whether these individual events were “caused” by climate change — such disasters arise from a confluence of factors and could still happen even if humans weren’t spewing heat-trapping gases into the sky. The question is: How has human-caused climate change altered their likelihood and their severity?

“Over the last 10 years, we’ve seen a shift in the ways scientists answer that question,” said Kevin Reed, an associate professor at Stony Brook University who studies climate attribution but was not involved in the IPCC report. Rather than invoking general trends, scientists are now keen to quantify climate change’s role in specific events, and increasingly coming up with estimates while a given disaster is still fresh in people’s minds.

The revolution in attribution research highlights that climate change is not just a problem for the future; it’s being felt right now. As humanity’s role becomes increasingly clear, it could have far-reaching consequences for how we prepare for the future and for the countries and corporations that will shoulder the blame for climate disasters.

The science linking weather extremes to climate change is stronger than ever

The planet has warmed by at least 1.1 degrees Celsius on average, the first installment of the new IPCC report concludes, and the effects are visible now. The report dedicates a whole chapter just to attributing elements of extreme weather to humanity’s actions.

Here are some of the findings in how human influence is playing out now:

• It is “virtually certain” that the frequency and intensity of heat waves has shot up globally since the 1950s due to greenhouse gas emissions from burning fossil fuels.
• The report says there is “high confidence” that tropical cyclones like hurricanes are dishing out more rainfall due to climate change.
• There is “medium confidence” that warming is leading to less water availability over most land areas as it increases evaporation.
• Fire weather conditions — hot, dry, windy — have become more probable in some regions, scientists can say with “medium confidence.”

“Evidence of observed changes and attribution to human influence has strengthened for several types of extremes … in particular for extreme precipitation, droughts, tropical cyclones and compound extremes,” including fires, the IPCC report says. Much of this is due to emissions of carbon dioxide from burning coal, oil, and gas, but changes in land use and aerosol pollutants play a role as well. And as average global temperatures continue to rise, the magnitude of these impacts will grow.

There are also aspects of extreme weather that don’t exhibit human influence, or haven’t been studied enough to yield conclusions. For example, there does not appear to be a distinct global or regional trend in the number of tropical cyclones, also known as hurricanes and typhoons. However, scientists do expect a trend toward higher wind speeds in these storms over the coming century.

Drought is another complicated phenomenon with different mechanisms at play, so it’s hard to make general conclusions about how climate change is influencing them. But scientists have found droughts that overlap with heat waves have become more frequent, and looking ahead, they have “high confidence” that drought will get more frequent and more severe in many parts of the world.

Some scientists have taken this research further by calculating the degree of human influence in specific events, often soon after they occur. The World Weather Attribution initiative, an international research consortium founded in 2014, examined the June 2021 heat wave in the Pacific Northwest of North America. Within a week, they concluded that the searing heat in the region “was virtually impossible without human-caused climate change.”

There are other examples of how scientists have traced the influence of climate change in extreme weather in the past few years:

• The WWA team looked at Australia’s massive bushfires in 2020 and found the likelihood of the severe heat, dryness, and winds that fueled the blazes has increased by at least 30 percent.
• The June 2020 heat wave in Siberia, in which temperatures north of the Arctic Circle topped 100 degrees Fahrenheit (38°C), was “almost impossible” without climate change.
• Since 1900, a storm like 2019’s Tropical Storm Imelda, which struck Texas with heavy floods, is now 1.6 to 2.6 times more likely and between 9 and 17 percent more intense.
• Another research team examined the 2017 record-breaking fire season in British Columbia. They determined that climate change increased the likelihood of fire conditions two- to fourfold and expanded the burned area between seven- and 11-fold.
• Warming since 1980 increased the record rainfall of 2017’s Hurricane Harvey by about 20 percent.

With more disasters looming on the horizon, scientists are preparing to conduct even more assessments and aim to come up with more precise attributions at a faster pace.

How climate change attribution science works, and how it’s improved

When a storm, heat wave, or flood strikes, scientists ask counterfactual questions: What would this event have looked like if humans didn’t change the climate? Or how often would you see an event like this in a world that hadn’t warmed?

There are several reasons scientists are better able to answer these questions now. For one thing, scientists have been able to observe climate change in real time. Atmospheric concentrations of carbon dioxide have risen from 400 parts per million in 2013 to almost 420 ppm this year. Computer models of the global climate system have been refined, and the computers themselves have gotten faster.

“The level of simulation and analysis we can do has improved,” said Reed. “Related to that, we have, in some cases, 10 years more of observations, which means we can build better models.”

The tricky thing is that climate change doesn’t just alter extreme weather — it changes everything. Every sunny day and rainstorm that’s occurring now is happening in a world that’s been warmed up. “Climate change is changing the characteristics of the weather we experience on a day-to-day basis,” Reed said. “Every storm is different because of climate change, because the underlying states are different.”

So to figure out how climate change has influenced a given disaster, scientists have to compare it to a baseline without climate change. One way to do this is to look at historical climate records prior to the Industrial Revolution in the 1800s, before humans started spewing gargantuan amounts of carbon dioxide into the air. Another way is to construct computer models that show what would have happened if people didn’t emit greenhouse gases.

The difference in the likelihood or severity of the disaster between what’s observed and what’s calculated without human influence tells scientists just how much climate change influenced the event.

Attribution of extreme weather has real-world consequences

The more researchers know about the impacts of climate change, the more prepared people can be — not only to survive heat waves and hurricanes, but also to implement new climate policies and hold the worst contributors accountable for their actions.

For example, when a hurricane sends a storm surge inland, researchers can compare the flooding to the expected flood levels in a world where warming had not occurred. The First Street Foundation, a nonprofit that studies climate risks, did such an analysis of the flooding following Hurricane Florence’s landfall in the Carolinas in 2018. They found that sea level rise since the 1980s flooded over 11,000 additional homes than would have been inundated if water levels held steady. Findings like this could help planners identify new climate risks and prepare for them.

Extreme weather attribution could also play a role in lawsuits. “As confidence levels grow in the findings that scientists are making, it’s quite possible that it will have legal implications in the courtroom and elsewhere,” said Michael Burger, executive director of the Sabin Center for Climate Change Law at Columbia Law School. “That’s the sort of scientific conclusion that can have direct impacts on the outcome of litigation.”

There’s already a wave of climate change litigation in the US and in several other countries. Some cities have filed suit against oil companies for selling products that contribute to climate change while misleading the public about their harms. Several groups of youth activists are making the case that governments are not doing an adequate job of protecting against worsening climate change, thereby depriving them of a safe climate.

Will the improvements in extreme weather attribution help plaintiffs win these lawsuits? It’s not clear yet. “We haven’t really seen a battle of the experts in the courtroom as to whether an individual event can be attributed to climate change, and if so, what the legal repercussions of that would be,” Burger said.

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On the other hand, the limiting factor in these lawsuits may not be the science but the law itself. For the most part, courts in the US have largely accepted the fundamentals of climate change and the conclusions of the IPCC in particular. The main hang-ups in many cases are whether the plaintiffs have standing to sue, and whether courts are actually the proper venue for directing climate policy. Some courts that have agreed climate change is an urgent threat say entirely new laws are needed to properly limit emissions.

But it doesn’t take a judge to rule on what the science now clearly shows. When it comes to climate change and the ensuing turmoil, humanity is guilty as hell.