Exploring the relationship between climate change and extreme weather

This summer, communities from Lake Como in Italy to China, Germany and the United States have witnessed deadly flooding and mudslides that caused widespread loss of lives, homes and other property.

The torrential rains whose impacts have upended life in so many places serve as a stark reminder that climate change is making weather more extreme across the globe, with wildfires also burning bigger and stronger across Europe and North America.

Canada and the western U.S. states, meanwhile, faced record-smashing high temperatures in late June and July that killed hundreds, while even Siberia in Russia's far north experienced a heatwave and forest fires.

Climate scientists say global warming is making weather extremes more likely and frequent - with a flagship report from the Intergovernmental Panel on Climate Change warning this month the situation is likely to worsen even if bigger efforts are made to rein in emissions of planet-heating gases.

In a study, an international team of scientists found climate change has made extreme rainfall events similar to those that led to July's floods in Germany, Belgium, the Netherlands and Luxembourg - which killed at least 220 people - between 1.2 and 9 times more likely to happen.

They also found that such downpours in Western Europe are now 3-19% heavier because of human-caused warming.

“These floods have shown us that even developed countries are not safe from severe impacts of extreme weather that we have seen and (that are) known to get worse with climate change," said Friederike Otto, associate director of the Environmental Change Institute at the University of Oxford.

"This is an urgent global challenge and we need to step up to it. The science is clear and has been for years,” she added in a statement on the study.

But how much can we say in general about the role of climate change in a specific weather event? Here's what scientists told us:

Climate change is never the sole cause of an extreme weather event, but it can sometimes be a significant contributing factor, according to specialists.

"An event is the result of many drivers. There are many factors that influence and may favour an extreme event," said Nikos Christidis, a senior scientist at Britain's Met Office.

But global warming is making the world's weather hotter and more volatile in general, making it more likely that extreme events take place, said Maarten van Aalst, director of the Red Cross Red Crescent Climate Centre.

Extreme events happen when climate change and natural weather variability come together - and may be influenced by other geographical factors, such as whether nearby rivers and lakes are empty enough to absorb water from heavy rains.

"The only way you can say if something is changing due to climate change is to say whether the odds of that really bad day are changing," said van Aalst.

"There's always an element of bad luck, but the chances of bad luck are increasing – quite rapidly in some cases."

Researchers at the World Weather Attribution (WWA) project say they can confidently assess how much more likely climate change has made individual major weather events such as heatwaves, intense rains and storms.

They gather key data, such as temperature readings or rainwater measurements.

They compare them to historical trends for similar weather in the area, and run computer simulations that model expected weather norms both with and without climate change.

The process takes between a week and two months depending on the complexity, said Geert Jan van Oldenborgh, one of the leaders of the WWA initiative.

The project's findings can be stark. For instance, the extreme heat that killed hundreds of people in the Pacific Northwest earlier this month would have been "virtually impossible" without global warming, they found.

While it is not possible to pin the full blame for any single event on a heating planet, the wider trend is clear, with experts pointing to increasingly volatile weather and a sharp rise in climate-linked disasters over recent decades.

Global warming can also bring a few benefits, with harmful extreme cold events reducing even as record heatwaves become more common, said van Oldenborgh.

On average, though, "the net balance is very damaging", he said.

Understanding how a warming world is driving extremes and how those trends could develop in the future is vital to ensure we can adapt to coming conditions and try to reduce the risks, climate scientists said.

"These events are the wake-up calls that the world apparently needs," said van Aalst.

With more people living in cities - many of them very vulnerable to floods, heatwaves, storms and other risks - the combination of strengthening climate change and growing populations in harm's way needs more attention, he said.

"We're setting ourselves up potentially for a double-whammy of rising climate risk, plus rising exposure and vulnerability," he said. "But there's also an opportunity there if we can increase awareness."

Clear evidence of increasing risk can help unlock funding to curb threats and ensure officials create plans to protect their residents, said van Oldenborgh.

Evidence of climate risks also can help tackle scepticism among some people about the need to act on climate change, and drive wider action to bring emissions down.

"Showing that climate change is not some abstract thing in the future but is really affecting us right now helps to build support for mitigation - for reducing CO2 emissions," he said.

"Of course, the big impacts will be in the future but showing there are already sizeable impacts now ... makes it more concrete to people."

This article was updated on Tuesday, August 24 2021, to include new research about the influence of climate change on Europe's recent floods.