How scientists are tracking humanity’s role in extreme climate events

An emerging field of climate science that analyzes extreme weather events is behind the assertion that the unprecedented heat waves sweeping the globe are the result of human-induced climate change.

Extreme event attribution studies the human footprint on weather-related disasters by comparing our current world—and its increasing amount of weather anomalies—with an ideal world, where human influence on climate never occurred.

To do this, researchers run computer programs known as climate models that simulate weather patterns over time, not unlike those used for local seven-day forecasts. But they recreate weather over decades or centuries, not hours or days.

“The really cool thing about climate models is that you have a scientist in a computer, and you can run experiments on it,” said Andrew Pershing, vice president of science at the nonprofit think tank Climate Central. “And you can literally do an experiment on what this world would look like if global warming never happened.”

Linking a changing climate to human activity goes back to the work of two Nobel laureates in physics, Seyokuro Manabe and Klaus Haselmann, who pioneered the development of climate models starting in the 1960s. Climate models help us understand how the climate has changed in the past and how it may change in the future. They solve mathematical equations that describe how energy and matter interact in different parts of the ocean, atmosphere, and land.

Attribution scientists use climate models to reconstruct the past few hundred years on Earth, removing all greenhouse gas emissions from humans. By comparing this imaginary world to our own, they can see if extreme events such as floods, droughts or cold snaps look different and what impact those emissions have on our weather.

For example, a study in July found that heat waves in North America and Europe were “almost impossible” in a world without climate change.

If humans had not warmed the planet by burning fossil fuels, such heat waves would still be rare. But in reality, we can expect them to occur every 15 years in North America and every 10 years in Europe, according to attribution scientists.

They also warn that if humans continue to produce emissions at today’s rate, these emissions will accelerate every two to five years starting in the mid-2030s.

Kevin A. said: “This is really important information for water resource managers, urban planners and policy makers in terms of climate adaptation and resilience,” said Dr. Reid, professor of marine and atmospheric sciences at Stony Brook University.

Until recently, scientists have largely avoided linking any single event to climate change, with the idea that weather is inherently unpredictable and has no single cause. But in 2004, what was considered the first study of extreme events found that climate change had “at least doubled the risk” of Europe’s heatwave the previous year, which killed more than 70,000 people.

Almost any weather phenomenon can happen by chance, but the authors argued that climate models can be used to extrapolate the role humanity has played in increasing the likelihood of such extreme heat. They simulated the climate with or without human emissions thousands of times, calculating the number of times an extreme heat wave like the one in 2003 has occurred. Although rare in both cases, this event has occurred twice in the world due to human emissions.

Since that first pivotal study, scientists have investigated more than 500 weather-related disasters worldwide, with 71% found to be made more likely or more dangerous based on human-caused climate change. With the help of faster computers and more accurate climate models, researchers can now perform these analyzes in a matter of days rather than months.

The World Weather Attribution (WWA) Initiative, established by an international team of climate scientists in 2015, has conducted more than 50 attribution studies, most of them in the aftermath of events or during them. WWA was responsible for analyzing July extreme temperatures around the world, which took just five days to compile.

Many researchers see attribution of extreme events as a communication tool that has the potential to link climate change to people’s everyday experiences. “Doing real-time attribution is what will be most useful for people to get information while there is a focus on an extreme event,” said WWA team member Robert Vautar, director of the Pierre-Simon Laplace Institute.

The World Water Association cited global warming as the main driver of the drought in East Africa since 2020, the 2022 heatwave in South America, and the 2022 floods in Pakistan.

But in the case of the 2019 Madagascar drought, the WWA found that the decrease in precipitation was mostly due to natural climate change, despite the United Nations claiming otherwise.

Climate Central adapted methods developed by WWA to create the Climate Shift Index (CSI), a measure that reveals how daily weather conditions have changed due to climate change. Pershing and his colleagues average results from 22 climate models and calculate the probability of daily local temperatures with and without historical greenhouse gas emissions.

Last analysis Climate change made July hotter for more than 6.5 billion people, or 81 percent of the Earth’s population, and they looked at data for 4,700 cities and 200 countries. “Almost no place on Earth has escaped the impact of climate change” in July, Pershing said.

The Reed Lab specializes in event attribution studies that specifically look at the impact on hurricanes. In each study, he ran 40 simulations over the past 150 years, with sea surface, atmospheric temperature and atmospheric humidity varying slightly to add an element of chance in influencing weather conditions. It also includes a “pre-industrial control pathway” that depicts the climate of 1850, or the time before human emissions rose.

After zooming in to a specific time and area to capture a target tornado, Reid runs a seven-day weather forecast. To get a sense of the human footprint, they compared hurricane characteristics during that week, such as rainfall rates, accumulation amounts, intensity, and size.

His study of the 2020 North Atlantic hurricane season, one of the most active seasons on record, found that climate change increased precipitation rates by 11 percent and rainfall amounts by 8 percent.

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