Everyday weather in a warmer world

How would the weather of the past unfold today, in a warmer, wetter world? Would every day be warmer and wetter everywhere, or would changes vary across the distribution and in space? We present a novel reanalysis-based method (ReBASE) to explore this question, for the test case of 1903. We first reconstruct the weather of 1903 using the global 20th Century Reanalysis system (20CRv3), which assimilates only surface pressure observations and is forced by observed sea surface temperatures. We then re-run the model, assimilating the same pressure observations, but with an added +2K perturbation to the sea surface temperature boundary conditions. This gives us a warmer counterfactual version of the weather of 1903. Storyline approaches have previously been used to study the changing impacts of individual extreme weather events. However, the ReBASE method also offers a unique chance to study much longer counterfactual storylines, including changes in the ‘everyday weather’ on non-extreme days.

 

We find that land warms more than 2K globally, with cold days warming the most. Daily precipitation becomes more variable, globally and in four regions with high observation density in 1903. We also find changes in the frequency of precipitation occurrence, with increases in dry and extreme days at the expense of moderate precipitation days. The reanalysis experiments thus provide an independent line of evidence supporting several well-known features of the climate response to warming, in the unique setting of simulations where the large-scale circulation is constrained through the assimilation of station pressure observations. Future experiments are planned for different historical periods, with the data to be made openly available.