Continental and global effects of different Δ<em>T</em><sub>g</sub> levels

<p><b>Table 1.</b>  Continental and global effects of different Δ<em>T</em><sub>g</sub> levels. Top: millions of people living in river basins characterized by chronic water scarcity (<1000 m<sup>3</sup> cap<sup>−1</sup> yr<sup>−1</sup>) (cases (2) and (4)), either with or without B1 and A2r future population change. People in water-scarce basins that show an aggravation of scarcity according to case (1) (see figure <a href="" target="_blank">3</a>(a)) are not counted here. Numbers in brackets denote the changes (relative to the present) that are solely due to climate change. Bottom: number of unique biogeographic regions (out of 90) exposed to severe biogeochemical or vegetation structural shifts. All values refer to changes with >50% confidence, simulated under at least 10 of the 19 GCM patterns. </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>