Articles - Models underestimating climate signal from winter rainfall

 

Understanding how and why winter rainfall is changing is challenging because natural variability masks the signal. But new research through the Willis Research Network shows that a clear climate change signal is now detectable, with important implications for risk and resilience, and on the way we think about the future of winter rainfall across Europe.

 

We take you through four ways U.K. winter storms are changing, what’s driving these shifts, and why it matters for how we model and manage future risk.

 

When we think about what contributes to wet winters, there are two main drivers at play:

Dynamical drivers– these are about where storms go. They are shaped by the jet stream, the fast-moving river of air high above the Atlantic that steers storm systems toward or away from Europe.

Thermodynamical drivers– these are about how much water storms can carry. A warmer atmosphere holds more moisture, which means that when storms arrive, they can deliver heavier rainfall.

 

One way to picture this is to imagine storms as delivery trucks. The dynamical drivers decide which roads the trucks take and which towns they visit. The thermodynamical drivers decide how full those trucks are when they arrive. 

Both dynamical and thermodynamical drivers are expected to change with global warming, although thermodynamical changes are more robust. Physics dictates that the amount of moisture the atmosphere can hold increases at 7% per degree of warming. Changes to dynamical drivers are more complex and uncertain, although models indicate a likely strengthening of the Atlantic jet stream in the winter. Understanding observed and projected changes in winter rainfall requires understanding both these drivers separately, since they are influenced by different processes.

 

In recently published research from the University of Newcastle and Willis Research Network, the contribution from dynamical and thermodynamical drivers in U.K. winter rainfall is separated via a methodology called dynamical adjustment. By separating these two drivers, a clear pattern is found:

 

The thermodynamical signal– the increase in atmospheric moisture linked to rising temperatures – is already detectable.This strengthening is directly tied to human-driven climate change, which is warming both the oceans and the atmosphere.

 

While the results of this study are for the U.K., the processes which drive this intensification will likely also influence Europe more generally and other mid-latitudes such as the USA and Japan. Further research is ongoing to assess changes in these regions.

 

Climate models also predict this intensification, but more importantly the observed increase is happening faster than the models suggest.

 

In other words, the “trucks” of winter storms are not just carrying more rain, they are filling up more quickly than global climate models had anticipated. For the insurance and risk management industry, this matters because many climate risk assessments often rely on those models. If the models are underestimating how quickly thermodynamical drivers are intensifying, then there is a risk of underestimating potential losses from heavy winter rainfall in the near term.

 

To address this gap, the industry and scientific community are increasingly looking at alternative approaches:

 

Convection-permitting models: These are high-resolution climate simulations that explicitly represent storm processes like heavy rainfall, rather than smoothing them out. They tend to give a more realistic picture of extremes, though they are computationally intensive. Research from Newcastle University, a Willis Research Network partner, has shed new light onto rainfall,wind, and hail  extremes using these models.

Storyline approaches: Instead of relying only on model averages, storylines explore “what if” scenarios of past or plausible future events under today’s warmer conditions. For example, what if the 2023/24 winter happened again with the extra moisture the atmosphere can now hold? This can provide decision-makers with concrete narratives of risk.

 

By combining these approaches with traditional modelling, reinsurers can gain a more robust view of evolving rainfall risks. The goal is not to replace models but to broaden the toolkit, bringing science closer to the realities of risk management.

 

 

For reinsurers and their clients, the key message is that winter rainfall is intensifying, and the pace of change is faster than global climate models suggest.