In the north-west UK the period of heaviest rain is also likely to shift to autumn by century's end, this time from a winter peak.

"The detected shifts in the peak times of the annual cycle of heavy precipitation will likely influence the probability and severity of floods," Anne Schindler of the University of Giessen told environmentalresearchweb. "The risk of flooding in autumn is likely to increase for the described regions, if the simulated scenario becomes true. This will not only have an impact on infrastructure and agriculture but also insurance companies, tourism and water management will be affected."

According to Schindler, the shifts will coincide with the time of year when river catchments are at their maximum water capacity. "This is the time of the year when, on average, the most floods occur," she said. "Therefore, you can conclude that risk increases when the timing of the near-field capacity and the probability for most extreme rainfall coincides."

Schindler and colleagues from the University of Giessen and GEOMAR Helmholtz Centre for Ocean Research Kiel analysed 12 regional climate models between 2021–2060 and 2061–2100 using the IPCC's A1B emissions scenario. They used simulations from the ENSEMBLES project because "it represents a unique collection of model runs at 25 km resolution".

The team decided to take advantage of Extreme Value Theory, which has been specifically developed for the characterisation of extremes, said Schindler. "Within Extreme Value Theory, the Poisson-process approach offered a flexible and elegant way to describe precipitation extremes in the UK," she added. "Furthermore, the implemented procedure can be easily modified and adapted to other case studies and/or regions."

The researchers found that the transition will emerge slowly, with the signal weak and disturbed by internal climate fluctuations over the next few decades. Heavy rainfall in other regions of the UK did not show a clear trend. The team believes that knowledge about the characteristics of changes in the hydrological cycle is necessary to develop adequate risk-reduction strategies.

But what will cause the shifts in rainfall timing? According to Schindler, different mechanisms affect the two regions. "Extreme precipitation in the north-west is strongly influenced by westerly airflow and in the south-east the highest precipitation events are influenced by easterly flows from the North Sea," she said. "The shifts we have projected could be caused, among other factors, by changes in these large-scale circulation systems; however, this needs further investigation. For instance, we know there are deficits in the representation of rainfall in climate models and we do not know how the peak times vary from year to year without any man-made climate change."

Now the team is trying to understand if their findings are model dependent or if there is a real process behind them. "Then we are going to analyse projections for other northern-European regions to investigate the presence of similar changes," said Schindler. "Finally, in future we will work on the theoretical development of the statistical model."

Schindler and colleagues reported the study in Environmental Research Letters.