Increasing seasonal disparity in water availability across the Northern Hemisphere

The water cycle, including the seasonal variations in land water availability (precipitation minus actual evapotranspiration (P–ET)), is intensifying with climate warming. These changes in P–ET, particularly the fluctuations between wet and dry seasons, have profound implications for ecosystem functioning and services. Understanding the dynamics and drivers of these changes is crucial for effective water resource management and climate adaptation strategies. Here, we examined seasonal P–ET changes based on multiple datasets across the extratropical Northern Hemisphere over the past two decades. Our results demonstrate a substantial increasing tendency in annual range between wet and dry season P–ET. Specifically, we observed that this trend is driven by both an increase in wet season P–ET and a decrease in dry season P–ET. Further analysis indicated that the changes in wet season P–ET were primarily due to changes in P, whereas the reductions in dry season P–ET were largely attributed to changes in ET. The results reveal a pronounced intra-hemispheric heterogeneity delineating P-dominated versus ET-dominated regimes. Evaluation of 32 state-of-the-art Earth System Models shows that most models capture these seasonal trends during recent decades, although some fail to do so. Our results highlight an enhanced seasonality in hemispheric water availability, driven by distinct changes in wet and dry season dynamics.