Significance
Under climate change, a point on a map needs to move in some speed and direction to maintain its current climate niche. We calculated the speeds and directions of aridity shifts across the globe to approximate species migration in natural–human systems driven by changes in water availability. We found historically that the aridity shifts had driven migration of vegetation greenness isolines in multiple regions. Most importantly, global drying would be accelerated for terrestrial taxa without mitigation. This would leave some species unable to adapt quickly enough, especially amphibians, which will suffer the largest aridification speed against plants, birds, and mammals. These findings suggest strong climate mitigation actions are required for the benefit of both terrestrial biodiversity and human well-being.
Abstract
Global aridification is projected to intensify. Yet, our knowledge of its potential impacts on species ranges remains limited. Here, we investigate global aridity velocity and its overlap with three sectors (natural protected areas, agricultural areas, and urban areas) and terrestrial biodiversity in historical (1979 through 2016) and future periods (2050 through 2099), with and without considering vegetation physiological response to rising CO2. Both agricultural and urban areas showed a mean drying velocity in history, although the concurrent global aridity velocity was on average +0.05/+0.20 km/yr−1 (no CO2effects/with CO2 effects; “+” denoting wetting). Moreover, in drylands, the shifts of vegetation greenness isolines were found to be significantly coupled with the tracks of aridity velocity. In the future, the aridity velocity in natural protected areas is projected to change from wetting to drying across RCP (representative concentration pathway) 2.6, RCP6.0, and RCP8.5 scenarios. When accounting for spatial distribution of terrestrial taxa (including plants, mammals, birds, and amphibians), the global aridity velocity would be -0.15/-0.02 km/yr−1 (“-” denoting drying; historical), -0.12/-0.15 km/yr−1 (RCP2.6), -0.36/-0.10 km/yr−1 (RCP6.0), and -0.75/-0.29 km/yr−1 (RCP8.5), with amphibians particularly negatively impacted. Under all scenarios, aridity velocity shows much higher multidirectionality than temperature velocity, which is mainly poleward. These results suggest that aridification risks may significantly influence the distribution of terrestrial species besides warming impacts and further impact the effectiveness of current protected areas in future, especially under RCP8.5, which best matches historical CO2emissions [C. R. Schwalm et al., Proc. Natl. Acad. Sci. U.S.A. 117, 19656–19657 (2020)].
