Extreme weather: Is climate change always to blame?

Weather attribution studies are connecting human-caused climate change to a wave of extreme heat, storms and wildfires. But it's complicated. From coast-to-coast Canadian wildfires to biblical flooding in Libya and scorching northern hemisphere heatwaves, catastrophic extreme weather events have dominated global headlines this summer.

As the planet heats due to still-rising greenhouse gas emissions, climate change has largely taken the blame for an apparent rise in extreme weather disasters.

"The dog days of summer are not just barking, they are biting. Climate breakdown has begun," said Antonio Guterres, UN Secretary-General, after it was revealed that June to August 2023 were the hottest months ever recorded in the northern hemisphere.

But how much of a heatwave or huge storm is down to global heating, and how much is just natural weather variability?

This question is being answered by the relatively new science of weather attribution. It sets out to assess the extent to which human-caused climate change — driven primarily by burning fossil fuels — increases the likelihood and intensity of an extreme weather event.

"No hurricane is 100% caused by climate change, but it's impacted by climate change in many different ways," Delta Merner, lead scientist at the Science Hub for Climate Litigation at the Union of Concerned Scientists, told DW. "Attribution science can help us to really tease out the role of climate change in these different events."

Canadian wildfires twice as likely due to climate change

When wildfires spread from the Canadian east to west coast in the summer of 2023, they burned near twice as much area than the previous record.

In response, UK-based researchers World Weather Attribution (WWA) undertook a typically rapid study to determine the extent to which human-driven climate change increased the likelihood of the unprecedented inferno.

Focused on the province of Quebec, the report concluded that climate change helped create dry, "fire-prone" weather about 20-50% more intense than average. It more than doubled the likelihood of extreme fire weather conditions in eastern Canada.

The hotter and drier weather caused snow to melt more rapidly, for example, bringing forward the start of the fire season and increasing its duration.

WWA says that advances in climate modeling and better access to weather data has improved the confidence and precision of studies that gauge the probability of extreme weather events with or without climate change.

Italian floods: Climate not responsible

The climate crisis can't always be directly blamed for extreme weather events.

In May 2023 in the northern Italian region of Emilia-Romagna, three rainstorms sparked widespread landslides and flooding that were said to be the worst in a century.

But while the high waters align with rising incidences of climate-driven extreme weather globally, researchers concluded this was an isolated event.

After analyzing rainfall records dating back to 1960 in Emilia-Romagna, scientists including Friederike Otto, a climatologist at Imperial College London and co-founder of WWA, found that spring rainfall is neither becoming more nor less intense with climate change in the region.

This particular 21-day period of rainfall — a one in a 200-year event with only a 0.5% chance of happening annually — could have ocurred with or without climate change, the researchers found.

The flooding was caused by highly unique and unusual weather conditions "driven by an unprecedented sequence of three low-pressure systems in the central Mediterranean," said Davide Faranda, an Italian researcher at the Institute Pierre-Simon Laplace and an author of the study.

Libya and Greece floods: Climate impact can be ambiguous

After Storm Daniel triggered flooding that caused two dams to burst and killed thousands in Libya in early September, a WWA study found that human-induced planetary heating made the torrential rainfall up to 50 times more likely. Massive flooding in central Greece spawned from the same storm was up to 10 times more likely.

Following a summer of record heatwaves and wildfires with a "very clear climate change fingerprint, quantifying the contribution of global warming to these floods proved more challenging," said lead author Friederike Otto.

To figure out if temperature rise had spurred heavier rain in the region, scientists compared weather data from the pre-1880s climate with the current climate that has warmed 1.2 degrees Celsius (2.16 degrees Fahrenheit) since then.

The report acknowledged that "large mathematical uncertainties" were built into the analysis as the weather patterns covered relatively small areas, and "most climate models do not represent rainfall on these small scales well."

However, it added that "studies project heavier rain in the region as temperatures rise," and that local weather station data in Greece, for example, showed a trend toward heavier rain.

A warmer atmosphere can hold more moisture, so from 1.2 Celsius of warming alone "we would expect a 10% increase" in rainfall intensity, said Otto.

Record summer heatwaves in 2023 have climate stamp

As opposed to rainfall, the link between temperature extremes and global heating is much clearer.

WWA published a study showing that extreme heat in the US/Mexico region and Southern Europe in July "would have been virtually impossible to occur … if humans had not warmed the planet by burning fossil fuels."

More than 6.5 billion people were exposed during July 2023 to one or more days of heat made at least three times more likely by climate change, according to attribution analysis by US-based climate think tank, Climate Central. That's around 80% of the world's population.

The analysis assessed 4,700 cities and 200 countries, finding that residents in 15 major cities with populations above 6 million were exposed to high average monthly temperatures made more likely by global heating. These included Mexico City, Cairo, Kolkata, Lagos, Hong Kong, Miami and Khartoum.

"Human-caused climate change influenced July temperatures for the vast majority of humanity," said Andrew Pershing, vice president for science at Climate Central. "Across the entire planet, the average person was exposed to 11 days in which carbon pollution made the local temperature at least three times more likely. Virtually no place on Earth escaped the influence of climate change."