Simulated exposure of world population to water scarcity (a) and of global endemism richness to severe habitat changes (b), plotted as functions of Δ<em>T</em><sub>g</sub>

<p><strong>Figure 4.</strong> Simulated exposure of world population to water scarcity (a) and of global endemism richness to severe habitat changes (b), plotted as functions of Δ<em>T</em><sub>g</sub>. Left panel: function for all 8Δ<em>T</em><sub>g</sub> levels and three confidence levels (stacked plot); right panel: results highlighted for 2, 3.5 and 5 ° C and the >50% case. Specifically, (a) shows the additional percentage of current world population exposed to new or aggravated water scarcity (cases (1) and (2); see section <a href="http://iopscience.iop.org/1748-9326/8/3/034032/article#erl472982s2-3-1" target="_blank">2.3.1</a>); (b) shows the percentage of global vascular plant endemism richness presently residing in regions that will be exposed to substantial habitat shifts (>33% of a region's area with Γ > 0.3). Grey bars in (b) show the corresponding number of affected regions (% out of the 90 regions; plotted on the same axis).</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>