Robust ecological drought projections for drylands in the 21st century

(Bradford) Dryland ecosystems may be especially vulnerable to expected 21st century increases in temperatures and aridity because they are tightly controlled by patterns of moisture availability. However, climate impact assessments in drylands are difficult because ecological dynamics are dictated by drought conditions that are difficult to define and complex to estimate from climate conditions alone. In addition, precipitation projections vary substantially among climate models, enhancing variation in overall trajectories for aridity. Here, we constrain this uncertainty by utilizing an ecosystem water balance model to quantify drought conditions with recognized ecological importance, and by identifying changes in ecological drought conditions that are robust among climate models. Despite limited evidence for robust changes in precipitation, changes in ecological drought are robust over large portions of N. American drylands. Our results suggest strong regional differences in long-term drought trajectories, epitomized by chronic drought increases in southern areas and decreases in the north. However, we also found that exposure to hot-dry stress is both increasing faster than mean annual temperature and, surprisingly, most pronounced in northern areas. Robust shifts in seasonal patterns of soil moisture availability are identified in most regions, although the directions of change and implications for ecosystems vary geographically. These results provide useful insights about the likely impact of climate change on dryland ecosystems in N. America. More broadly, this approach of identifying robust changes in ecological drought may be useful for other assessment of climate change impacts in drylands and may provide a more rigorous foundation for making long-term strategic resource management decisions.