Terrestrial biodiversity threatened by increasing global aridity velocity under high-level warming

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 CO₂ effects/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 CO₂ emissions [C. R. Schwalm et al., Proc. Natl. Acad. Sci. U.S.A. 117, 19656-19657 (2020)].

Keywords: aridification; climate velocity; global warming; terrestrial biodiversity.

Fig. 1.

Speed maps of historical and future aridity and temperature velocities. (A, D, G, J) Speed distribution of AI_RC-based aridity velocity in history, under RCP2.6, under RCP6.0, and under RCP8.5, respectively. (B, E, H, K) Speed distribution of AI_CO₂-based aridity velocity in history, under RCP2.6, under RCP6.0, and under RCP8.5, respectively. (C, F, I, L) Speed distribution of temperature velocity in history, under RCP2.6, under RCP6.0, and under RCP8.5, respectively. The negative sign of speed indicates drying/cooling, and the positive sign indicates wetting/warming. The future speed values are the ensemble mean of multiple models. Pixels in each speed map with values outside the 0.5 to 99.5% quantiles are removed. All velocities are calculated by using the spatial gradient during 1979 through 2016. Stippling indicates the agreement in the sign of estimated velocities under RCPs across at least seven of nine models (75% of models). AI_RC refers to the aridity index based on the FAO reference crop PET model and AI_CO₂ to one of its variants considering vegetation physiological responses to elevated CO₂.

Fig. 2.

Coupling between aridity velocity (without considering CO₂ effects) and migration of NDVI isolines at multiple regions during 1982 through 2015. (A) The migration of NDVI isolines (NDVI = 0.30) in northern Australia during 1982 through 2015. (B) The migration of NDVI isolines (NDVI = 0.20) in Sahel during 1982 through 2015. (C) The migration of NDVI isolines (NDVI = 0.20) in southern Africa during 1982 through 2015. (A–C) The black and red lines denote NDVI isolines during 1982 through 1986 and during 2011 through 2015, respectively. The blue arrows indicate the directions of wetting velocity, and the red arrows indicate directions of drying velocity. The length of arrows represents the migration distances of aridity velocity. The aridity velocities are calculated based on the spatial gradient during 1982 through 2015. The pixel values indicate the differences between NDVI during 1982 through 1986 and that during 2011 through 2015. (D) Correlations between migration distances of points along the NDVI isolines and the climate migration distances derived using the aridity velocity of these points. The black line is 1:1 line. All correlations are statistically significant with r = 0.52 and P < 0.002, r = 0.37 and P < 0.001, and r = 0.36 and P < 0.015, respectively.

Fig. 3.

(A–D) Probability density distribution of speeds of AI_RC-based aridity velocity for the globe, protected areas (PA), agricultural areas, and urban areas. (E–H) Probability density distribution of speeds of AI_CO₂-based aridity velocity for the globe, protected areas, agricultural areas, and urban areas. Vertical lines indicate the global mean speeds of aridity velocity under different scenarios. In each land use type, the two-sample Student’s t test is conducted for aridity velocities under different scenarios, and the results show they are all significantly different (P < 0.001). AI_RC refers to the aridity index based on the FAO reference crop PET model and AI_CO₂ to one of its variants considering vegetation physiological responses to elevated CO₂.

Fig. 4.

Aridity velocities for all taxa (amphibians, birds, mammals, and plants) and amphibians under different scenarios. (A–D) The global mean speeds of aridity velocity based on AI_RC or AI_CO₂ for all taxa and amphibians in history, under RCP2.6, under RCP6.0, and under RCP8.5, respectively. The mean speed of aridity velocity for each taxon is weighted by grid area and species richness, in hyper-arid (HA), arid (A), semiarid (SA), subhumid (SH), and humid (H) regions. AI_RC refers to the aridity index based on the FAO reference crop PET model and AI_CO₂ to one of its variants considering vegetation physiological responses to elevated CO₂.