Can we explain why arid conditions are spreading worldwide? Or why the western United States is getting increasingly arid since the 1980s while the African Sahel has recovered from its prolonged drought? Are these two regional events independent or are they connected by large-scale mechanisms? At the opposite end of the extreme event spectrum, what are the underlying causes of the recent observed changes in extreme precipitation?

Based on CMIP5 climate simulations and a novel fingerprinting analysis, we found that since 1950, human-produced greenhouse gases and particulate atmospheric pollution have influenced, together, global changes in temperature, precipitation and regional aridity in two distinct ways. Like two songs playing simultaneously out of a noisy background, these two human “fingerprints” – signatures of large-scale mechanisms conducive to regional drying – are statistically identifiable in observations.

One song is louder and clearer: The dominant fingerprint is characterized by global warming, intensified wet-dry patterns of precipitation, and progressive large-scale continental aridity – all largely driven by a slowly evolving increase in greenhouse gas emissions. The second and more subtle fingerprint captures a temperature contrast between the Northern and Southern Hemispheres, controlled by the cooling influence of particulate pollution emitted from Europe and North America up until the 1980s. This temperature contrast moved the tropical rain belt southwards, away from the cooler Northern Hemisphere, causing more rainfall over the Western U.S. and less over the Sahel and India. After pollution regulations were put place in following the Clean Air Act, the tropical rain belt shifted back northward, bringing less rainfall to the Western U.S. and more to the Sahel.

In this presentation, I will update these results by employing the climate simulations from the most recent phase of the Coupled Model Intercomparison Project (CMIP6). We will also investigate the nature and causes of recent changes in extreme precipitations. We will explore the impact of aerosol forcing uncertainties, and of observational uncertainties.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.