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'Storyline' Models Reveal Global Warming's Influence on Severe Weather Events

Introduction: Storm Boris and its Impact on Central and Eastern Europe

Recently, storm 'Boris' brought intense rainfall, triggering severe flooding and disruption in Central and Eastern Europe. Analysis Wegener Institute indicates that in a cooler climate, Boris would have delivered around 9% less rain.

New Modeling Approach: The 'Storyline' Technique

What is the 'Storyline' Approach?

A new modeling approach called 'storylines' has enabled these findings, showcasing its near-real-time potential in Communications Earth & Environment. Additionally, the AWI team introduced a free online tool that allows users to detect climate change impact in extreme weather events and design their own comparative graphics.

Real-World Application: Storm Boris Case Study

In mid-September, storm 'Boris' brought intense rainfall and severe flooding to Poland, the Czech Republic, Austria, and Romania. In several areas, the rainfall marked one of the highest five-day totals on record. Tragically, at least 27 lives were lost, and countless families were displaced.

Was the Event Driven by Global Climate Change?

The Debate: Climate Change and Extreme Weather

Meanwhile, the situation has improved, and cleanup efforts continue around the clock. However, new weather extremes in Spain are already raising concerns. Once again, a central question is being debated in public, political, and media circles: Was this catastrophe driven by global climate change?

The Science of Attribution

"In recent years, science has provided solid answers to this entirely valid question," states Dr. Marylou Athanase, lead author and physicist at the Climate Dynamics Section of the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI).

"Within just one or two weeks of an event, probabilistic attribution studies can provide initial insights into how much more likely the event was due to climate change."

The Challenge: Making Climate Impact Tangible

The Issue with Abstract Probabilities

The main issue is that probabilities tend to be abstract, especially when they intersect with tangible, exceptional real—world events. In external communications—whether to the public or policymakers-the scientific community has yet to possess a tool that can effectively demonstrate the impact of global climate change on local weather in a way that is both compelling and easy to understand.

Introducing the "What if?" Principle

"This is why, at the AWI, we've dedicated ourselves to developing an entirely new method—the 'storyline' approach," says Dr. Antonio Sánchez-Benítez, a physicist in the Climate Dynamics Section and co-lead author of the study.

"In essence, we use the 'what if?' principle—what would a specific catastrophe look like in a world without climate change? And how would it differ in a warmer climate? By contrasting these hypothetical scenarios with actual outcomes, we can clearly pinpoint the distinct impacts of climate change—not only for extreme weather but also for everyday weather patterns."

How the 'Storyline' Approach Works: The Storm Boris Example

9% Less Rainfall in a Cooler Climate

Using Storm Boris as a case study, AWI experts have now showcased the capabilities of the new approach. Their scenario comparison reveals that, in the absence of global warming, Boris would have brought approximately 9% less rainfall.

In reality, as the storm traveled from the eastern Mediterranean and the Black Sea toward Central Europe, it gained strength due to the water temperature being approximately two degrees Celsius higher than preindustrial levels. This increase led to a higher concentration of water vapor in the air over the region.

Why Even Small Changes Matter

While a 9% change may appear negligible, the true effects of intense rainfall are determined by how much water accumulates on the ground and how it is channeled. Can existing infrastructure—such as rivers, dams, or sewage systems—manage the volume, or does it overflow, resulting in extensive damage?

Bridging the Gap: Nudging for Accurate Weather Simulations

How 'Nudging' Improves Weather Models

The question arises: How did experts bridge the gap between climate models, which focus on long-term trends, and actual local weather? According to Dr. Helge Gößling, a climate physicist and team leader at AWI, the solution lies in a technique known as 'nudging.'

"Climate models typically generate a quasi-random sequence of weather patterns that align with the physical laws embedded in their programming. To detect climate variations, one must examine whether the average values and distributions shift over an extended period and across a broad range of weather conditions."

Similarly, in weather models, the simulated conditions become increasingly detached from reality after just a few weeks, limiting the accuracy of weather predictions.

Simulating a World Without Climate Change

Through 'nudging,' we incorporate real-time wind data, including jet stream patterns, to guide the model toward actual observations, enabling us to accurately replicate real-world weather within the current climate.

"Next, we modify the model's background climate, such as by simulating a world unaffected by climate change, lowering greenhouse gas concentrations, and adjusting other relevant factors, before repeating the experiment."

The Technical Backbone: AWI's Climate Model

CMIP6 and ERA5 Data Integration

The AWI climate model used is the CMIP6 version, which served as part of the foundational data for the IPCC's Sixth Assessment Report. The wind data integrated into the model is sourced from the ERA5 reanalysis by the European Centre for Medium-Range Weather Forecasts (ECMWF).

Automated Weather Analyses on Supercomputers

As Marylou Athanase explains, "We have now automated the system to the extent that daily weather analyses are conducted on the supercomputer at the German Climate Computing Center (DKRZ)."

Free Public Access: The Climate Storylines Tools

Exploring the 'Climate Changes Signal of the Day'

The data is subsequently transferred to an online tool hosted on AWI's servers, which is freely accessible to the public at climate-storylines. Analyses are performed with a three-day delay and made available online thereafter.

"Consequently, users can log in at any time to explore the 'Climate Changes Signal of the Day,' which displays global extreme and regular weather patterns. The information is presented interactively via maps and timelines, with data from January 1, 2024, onward focusing on temperature and precipitation."

Enhancing Public Understanding of Climate Change

"Our objective is to enhance understanding of the links between climate change and extreme weathers events, providing clear and timely responses that can be utilized in media coverage."

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