On How Human Activities, ENSO, and Plant Physiology Influence Historical and Future Terrestrial Aridity

Most climate projections show a common signal of twenty-first-century increases in atmospheric moisture demand and aridity in response to CO₂-induced warming. These projections, however, often rely on aridity metrics based on potential evaporation (PE) or on potential evapotranspiration (PET) without considering important feedbacks from soil water and vegetation. In this study, we use different sets of atmospheric model simulations to identify regions where projected changes in aridity are large enough to overwhelm the interannual variability in drying or moistening associated with ENSO variability. To provide a comprehensive assessment of relative contributions to future drought, we consider a variety of hydroclimatic variables, including changes in P, PET, and ET (evapotranspiration), and several aridity indices. The latter are based on soil moisture, runoff, or on combinations of P and PET or P and ET. For each variable, the combined effects arising from mean-state changes and ENSO variability are examined for three components of greenhouse warming: 1) the so-called slow ocean warming, 2) the ocean warming and the response of plant physiology to rising atmospheric CO₂levels, and 3) ocean warming and plant physiology effects, together with the radiative forcing from enhanced CO₂. Finally, we explore whether we can identify a human influence in the historical changes in aridity using formal or derived detection and attribution techniques. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work is released as LLNL-ABS-697898