Increasing probability of extreme records in heavy precipitation

It comes as no surprise that the future holds record-breaking weather and climate events. As global warming continues, temperature records will continue to be broken. Also heavy precipitation records are likely to be broken due to the increased water holding capacity of the atmosphere, in combination with changing atmospheric stability and circulation patterns. Improved estimates on the range of possible record-breaking precipitation events – now and in the future – are a first step to inform adequate adaptation policies for heavy precipitation. Of particular interest are events that break records by large margins – record-shattering events –, since these are likely to incur most damage and losses. 

In order to improve estimates of record shattering precipitation events in the present and future climate we use initial condition large ensemble simulation data (CESM2, SSP370) and statistical models. We evaluate record-shattering events in Rx1d (day with most precipitation per chosen time period (year or season)). In a stationary climate, the probability of Rx1d record-breaking is known to decrease with the number of data points since the start of measurements (inversely proportional). We find, however, that in our nonstationary climate, the decay in Rx1d record breaking and shattering probability is slowed down and even reversed in most world regions. Regional changes in record shattering probability are attributable to a changing underlying probability distribution of Rx1d, which also is region specific. We elucidate the contributions of changes in mean (distribution shift), and in variability (distribution widening/narrowing) to increasing record shattering probability by using a statistical model to create counterfactual realities representative of the regions of interest.

We focus on regions of a size relevant for national and cross-border policy that show differently driven changes in record shattering precipitation probabilities. For example, the annual probability of a record shattering precipitation event somewhere in the Benelux-Germany region which was hit by severe floods in summer 2021 increases from ~2% now to ~4.5% at the end of the century in CESM2. This increase results from a non-linear interaction between mean and variability increases, and is primarily driven by increasing variability. At lower latitudes, for example in Central America, the effect of variability is even stronger, where we find increasing record shattering probability despite a negative long-term trend in Rx1d levels.

Very unlikely events are, paradoxically, arguably the most important to know about, since their unimaginability often means that critical infrastructure is not sized to withstand these events. Our results may thus prove invaluable for regional policy.