How much does climate change influence extreme weather events?

The devastating rainfall from Storm Boris, which caused severe flooding across Central and Eastern Europe, could have been nearly nine percent less intense without the current levels of global warming. This is the conclusion of a recent analysis from the Alfred Wegener Institute (AWI).

This finding was made possible by a novel modeling technique called “storylines,” which provides a new way to trace the fingerprints of climate change on extreme weather events in real time. 

This approach is described in the journal Communications Earth & Environment and is accessible via a new online tool developed by the Alfred Wegener Institute.

The impact of a warmer climate

In mid-September, Storm Boris unleashed torrential rain across Poland, the Czech Republic, Austria, and Romania, resulting in catastrophic flooding and displacing countless families. 

Boris produced one of the highest rainfall totals ever recorded in several areas over a five-day period, and the floods claimed at least 27 lives. While cleanup is underway, public and media inquiries are focusing on whether global climate change played a role in this tragic event.

“For the past few years, science has been able to provide robust answers to this absolutely legitimate question,” said lead author Marylou Athanase, a climate physicist at AWI. 

“As early as one or two weeks after a given event, so-called probabilistic attribution studies can be used to draw initial conclusions on how much more probable the event was, due to climate change.”

Connecting climate trends and weather events

Yet explaining these probabilities to the public has often proven challenging. Probabilistic studies, while valuable, may lack clarity when connecting global climate trends with specific weather events. 

“That’s why we at the AWI have worked so hard to advance a totally new way – the ‘storyline’ approach,” explained co-lead author Antonio Sánchez-Benítez. 

“Essentially, we apply the ‘what if?’ principle. What would a given catastrophe look like in a world without climate change? And what about in a climate that was even warmer?”

Climate impact on specific storms

Using the storm as a case study, AWI’s team illustrated the power of this approach. Their analysis showed that, if not for climate change, Storm Boris would have dropped about nine percent less rain. 

On its journey from the eastern Mediterranean toward Central Europe, Boris intensified as it encountered water roughly two degrees Celsius warmer than preindustrial levels, which increased the water vapor in the atmosphere. 

While nine percent may sound minimal, it can make a significant difference in areas where the additional water exceeds the capacity of rivers, dams, or drainage systems, leading to potentially devastating consequences.

Climate models and real-world weather

One of the key challenges was to accurately connect long-term climate modeling with the actual, observed conditions of specific weather events. 

To address this, the team used a process called “nudging,” where actual wind data, including jet stream information, is fed into the model to align it more closely with real-world conditions. 

“Climate models normally simulate a specific, quasi-random sequence of weather conditions,” explained Helge Gößling, a climate physicist and head of storyline research at AWI. 

“With nudging, we provide the model with actually observed wind data, and we nudge the model a bit in the direction of the actually observed wind. In this way, we can accurately reproduce real weather in the real climate.”

Quantifying the influence of global warming 

In their models, the researchers could simulate a pre-climate change world by reducing greenhouse gas levels and adjusting other variables. 

By rerunning the scenario, they produced a side-by-side comparison of current and hypothetical climates. This approach allows scientists to quantify the influence of global warming on specific events, from extreme storms to more typical weather patterns.

Advanced climate data and technology

The researchers used the CMIP6 version of the AWI climate model, which contributed to the IPCC’s Sixth Assessment Report, incorporating real-time wind data from the European Center for Medium-Range Weather Forecasts (ECMWF). 

“We have since automated the system to the point where daily analyses on the current weather are run on the supercomputer at the German Climate Computing Center (DKRZ),” Athanase said

The results are then made available to the public on AWI’s online platform. This tool, updated with a three-day lag from real-time, provides users with access to maps and interactive timelines showing the “Climate Change Signal of the Day” for extreme and daily weather events worldwide.

Climate science and public understanding 

By offering a visual and interactive way to examine climate change’s influence, AWI hopes to bridge the gap between complex climate science and public understanding. 

For now, the tool tracks temperature and precipitation data starting from January 2024, allowing users to explore climate impacts on recent weather events. 

“Our goal is to promote a better understanding of the connections between climate change and extreme weather events, and to supply concrete and timely answers that can also be used in media coverage of these events,” Athanase said.

The AWI’s storyline model and online tool represent an innovative step forward in climate science communication, helping researchers, decision-makers, and the public see, in real terms, how global warming affects weather patterns and extreme events.

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