How does science test whether humans are responsible for climate extremes?

Assuming there is no human influence on the climate, will there also be weather-related disasters? This is what climate science studies.

An emerging field of climate science that analyzes extreme weather events is behind the claim that the unprecedented heat waves sweeping the globe are the result of human-caused climate change. Extreme event attribution studies the human footprint on weather-related disasters by comparing our current world—with its increasing number of weather anomalies—with an ideal world in which there is no human influence on climate.

To do this, researchers run computer programs called climate models that simulate weather patterns over time, similar to models used for seven-day local forecasts. However, the weather in these models is modeled over decades or centuries and not just hours or days.

What would the world look like without global warming?

“The really great thing about climate models is that you have a computer scientist that you can run experiments on,” says Andrew Pershing, vice president of science at the nonprofit research organization Climate Central. “And you can literally do an experiment on what this world would look like if there was no global warming.

The relationship between climate change and human activities goes back to the work of Nobel laureates in physics Seyokuro Manabe and Klaus Hasselmann, who have pioneered the development of climate models since 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.

Using climate models, scientists recreate the Earth’s past few hundred years while removing all human-caused greenhouse gas emissions. By comparing this imaginary world to our own, they can determine whether extreme events such as floods, droughts or cold spells look different and what impact these emissions have on our weather.

Without climate change, heatwaves would be “almost impossible”.

A July study found that heatwaves in North America and Europe were “nearly 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, scientists say.

They also warn that heat waves will accelerate every two to five years starting in the mid-2030s if humans continue to produce as much emissions as they do today.

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.

Linking individual events to climate change

Until recently, scientists have largely avoided linking individual events to climate change, believing that weather is inherently unpredictable and has no single cause. But in 2004, the first study of extreme events found that climate change had at least doubled the risk of the heatwave Europe had experienced the previous year, in which more than 70,000 people had died.

Almost any weather event could happen by chance, but the authors say climate models can be used to figure out the role of humans in the likelihood of such extreme heat. They simulated the climate with and without human emissions thousands of times, and counted the number of times an extreme heat wave like the one in 2003 occurred. Although the event was rare in both cases, it has only happened twice in the world due to human emissions.

Since that pioneering initial study, scientists have studied more than 500 weather-related disasters around the world. 71% of them are becoming more likely or more dangerous due to human-caused climate change. With the help of faster computers and more accurate climate models, researchers can now perform these analyzes in days rather than months.

The World Weather Attribution (WWA) Initiative, founded by an international team of climate scientists in 2015, has conducted more than 50 attribution studies, most of them post-event or during an event. WWA was responsible for analyzing the extreme temperatures seen in July around the world, an analysis that was completed in just five days.

Global warming as the main cause of droughts, heat waves and floods

Many researchers see attribution of extreme events as a communication tool that can link climate change to people’s everyday experiences. “Real-time mapping is very useful for people to stay informed while focusing on an extreme event,” said Robert Vautar, director of the Pierre-Simon Laplace Institute and a member of the WWA team.

WWA has identified global warming as the primary cause of drought in East Africa since 2020, a heatwave in South America in 2022, and floods in Pakistan in 2022.

However, in the case of the 2019 Madagascar drought, the World Water Association found that reduced rainfall was mainly due to natural climate fluctuations, despite the United Nations claiming otherwise.

Climate Central Methods developed by WWA to create the Climate Shift Index (CSI), a measure of how daily weather patterns have changed due to climate change. Pershing and his colleagues averaged the results of 22 climate models and calculated the probability of local and daily temperatures with and without historical greenhouse gas emissions.

Their latest analysis that climate change has made July hotter for more than 6.5 billion people, or 81 percent of the world’s population, studied data from 4,700 cities and 200 countries. “Almost no place on Earth escaped the impact of climate change in July,” Pershing said.

What is the impact of humans on extreme weather events?

The Reed Lab specializes in event attribution studies that specifically address the effects of hurricanes. In each study, he ran 40 simulations over the past 150 years, each with a slightly different sea surface, atmospheric temperature and humidity to add an element of chance in influencing weather conditions. He is also conducting a “pre-industrial observation tour” that reflects the climate of 1850, a time before human emissions rose.

After zooming in to a specific time and area to capture a specific tornado, Reed issues a seven-day weather forecast. To learn about the human footprint, they compared the characteristics of this week’s hurricanes, such as rainfall rates, rainfall amounts, intensity, and size.

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

We are currently testing machine translations. This article was automatically translated from English to German.

This article was first published in English on September 11, 2023 on “washingtonpost.com” was published as part of the collaboration, and is now also available in translation for readers of the IPPEN.MEDIA portals.