Threshold level of Δ<em>T</em><sub>g</sub> leading to significant local changes in water resources (a) and terrestrial ecosystems (b)

<p><strong>Figure 1.</strong> Threshold level of Δ<em>T</em><sub>g</sub> leading to significant local changes in water resources (a) and terrestrial ecosystems (b). (a) Coloured areas: river basins with new water scarcity or aggravation of existing scarcity (cases (1) and (2), see section <a href="" target="_blank">2.3.1</a>); greyish areas: basins experiencing lower water availability but remaining above scarcity levels (case (3)); black areas: basins remaining water-scarce but without significant aggravation of scarcity even at Δ<em>T</em><sub>g</sub> = 5 °C (case (4)). No population change is assumed here (see figure S5 available at <a href="" target="_blank"></a>for maps including population scenarios). Basins with an average runoff <10 mm yr<sup>−1</sup> per grid cell are masked out. (b) Regions with severe (coloured) or moderate (greyish) ecosystem transformation; delineation refers to the 90 biogeographic regions. All values denote changes found in >50% of the simulations.</p> <p><strong>Abstract</strong></p> <p>This modelling study demonstrates at what level of global mean temperature rise (Δ<em>T</em><sub>g</sub>) regions will be exposed to significant decreases of freshwater availability and changes to terrestrial ecosystems. Projections are based on a new, consistent set of 152 climate scenarios (eight Δ<em>T</em><sub>g</sub> trajectories reaching 1.5–5 ° C above pre-industrial levels by 2100, each scaled with spatial patterns from 19 general circulation models). The results suggest that already at a Δ<em>T</em><sub>g</sub> of 2 ° C and mainly in the subtropics, higher water scarcity would occur in >50% out of the 19 climate scenarios. Substantial biogeochemical and vegetation structural changes would also occur at 2 ° C, but mainly in subpolar and semiarid ecosystems. Other regions would be affected at higher Δ<em>T</em><sub>g</sub> levels, with lower intensity or with lower confidence. In total, mean global warming levels of 2 ° C, 3.5 ° C and 5 ° C are simulated to expose an additional 8%, 11% and 13% of the world population to new or aggravated water scarcity, respectively, with >50% confidence (while ~1.3 billion people already live in water-scarce regions). Concurrently, substantial habitat transformations would occur in biogeographic regions that contain 1% (in zones affected at 2 ° C), 10% (3.5 ° C) and 74% (5 ° C) of present endemism-weighted vascular plant species, respectively. The results suggest nonlinear growth of impacts along with Δ<em>T</em><sub>g</sub> and highlight regional disparities in impact magnitudes and critical Δ<em>T</em><sub>g</sub> levels.</p>