Continental-scale effects of selected ΔTg levels (2 ° C, left bars; 3.5 ° C, middle bars; 5 ° C, right bars), simulated under >50% of the climate change patterns
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Figure 3. Continental-scale effects of selected ΔTg levels (2 ° C, left bars; 3.5 ° C, middle bars; 5 ° C, right bars), simulated under >50% of the climate change patterns. (a) Percentage of continental population exposed to new or aggravated water scarcity, or lower water availability outside water-scarce river basins, assuming unchanged population. (b) Percentage of continental endemism-weighted species richness of vascular plants in biogeographic regions exposed to substantial habitat shifts (Γ > 0.3 on >33% of the regions' area). The upper panel shows values relative to the continental totals, whereas the bottom panel shows values relative to the global totals. Numbers in brackets refer to the four cases of hydrologic change (see section 2 and figure 1). EUR, Europe; ASI, Asia; AFR, Africa; NAM, North America; SAM, South America; AUS, Australasia.
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.