Simulated exposure of world population to water scarcity (a) and of global endemism richness to severe habitat changes (b), plotted as functions of ΔTg
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Figure 4. Simulated exposure of world population to water scarcity (a) and of global endemism richness to severe habitat changes (b), plotted as functions of ΔTg. Left panel: function for all 8ΔTg 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 2.3.1); (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).
This modelling study demonstrates at what level of global mean temperature rise (ΔTg) 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 ΔTg 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 ΔTg 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 ΔTg 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 ΔTg and highlight regional disparities in impact magnitudes and critical ΔTg levels.