Scientists uncover arid region rainfall trends that challenge commonly held climate change theories

An analysis of 50 years of rainfall data in arid regions by researchers at Cardiff University and the University of Bristol in the UK has shown a decline in rainfall intensity, despite an increase in total rainfall. The findings run counter to research that suggests that global warming causes heavier rainfall, because a hotter atmosphere can hold more moisture, and warmer oceans evaporate faster, thus feeding the atmosphere with more moisture. Lead author of the study, Dr Michael Singer from the School of Earth and Ocean Sciences at Cardiff University, said, “In drylands, convective rainfall controls water supply, flood risk and soil moisture, but we have had little information on how atmospheric warming will affect the characteristics of such rainstorms, given the limited moisture in these areas.” Until this study, the link between climate warming and heavy rainfall has only been examined in regions where moisture availability is relatively high. No research has been undertaken to examine the relationship between climate change and rainfall in dryland regions where short, sharp rainstorms are the dominant source of precipitation and where moisture availability on land is extremely limited. To explore the links, scientists analysed more than 50 years of detailed rainfall data, measured every minute, from a semi-arid drainage basin in south-east Arizona exhibiting an upward trend in temperatures during that period. The analysis demonstrated a decline in rainfall intensity, despite an increase in total rainfall over the years. The study shows that there is a long-term decline in heavy rainfall events, greater than 25mm/h, and an associated increase in the number of smaller storms each delivering less rainfall. Co-author Dr Katerina Michaelides, from the School of Geographical Sciences and Cabot Institute at the University of Bristol, said, “Our findings are consistent with previous research in the Colorado Basin, which revealed a decline in runoff in the upper part of the basin. Our work demonstrates that there is a more regional decline in water resources in this dryland region, which may be found in other dryland regions of the world.” Since trends in convective rainfall are not easily detected in daily rainfall records, or well-simulated by global or regional climate models, the researchers created a new tool to assess the effects of climate change on rainfall patterns and trends in dryland areas. Their model, STORM, simulates individual rainstorms and their expression over a river basin, and it can represent different classes of climate change over many decades. Dr Singer said, “We see this model as a useful tool to simulate climate change in regions and cases where traditional models and methods don’t capture the trends.”