Driven in part by large-scale atmospheric climate modes, the Southern Hemisphere accounts for more than 95% of the recent decline in global water availability, according to a new study. Global land water availability has varied due to climate change and increased human water use. Although this crucial resource underpins livelihoods, socioeconomic development, and ecosystems worldwide, it remains unclear how water availability has changed in recent decades and what is driving these changes at a global scale.
Yongqiang Zhang and colleagues combine various data, including streamflow observations of large river basins of the world with terrestrial precipitation data and satellite measurements of evaporation and water storage to calculate global land water availability for the last 20 years. Overall, global land water availability declined from 2001 to 2020, according to the findings. However, global water availability was dominated by variability and a negative trend in water availability in the Southern Hemisphere, which contributed to more than 95% of the global variance in land water availability despite only accounting for ~26% of the global land surface.
The water availability across the Southern Hemisphere decreased significantly across the study period – roughly 3.55 millimeters per year over the last 20 years. Conversely, a complex mix of positive and negative trends in different regions of the Northern Hemisphere have largely canceled each other out, resulting in a negligible change in land water availability over the same period. Zhang et al. show that the variability and trend in water availability observed in the Southern Hemisphere are driven mainly by changes in large-scale atmospheric climate modes, particularly the El Niño-Southern Oscillation. In a related Perspective, Günter Blöschi and Pedro Chaffe discuss the study in greater detail.