Likelihood of a decrease in runoff (a), an increase in runoff (b) and a severe change in ecosystems (c) for selected Δ<em>T</em><sub>g</sub> levels

<p><strong>Figure 2.</strong> Likelihood of a decrease in runoff (a), an increase in runoff (b) and a severe change in ecosystems (c) for selected Δ<em>T</em><sub>g</sub> levels. (a) and (b) show whether the simulated decrease (increase) in average annual runoff exceeds present (1980–2009) standard deviation, or whether monthly runoff is >10% more frequently below (above) its present median. Areas with presently <10 mm yr<sup>−1</sup> are masked out. The likelihoods are derived from the 19 climate change patterns. See figures S1–S4 (available at <a href="http://stacks.iop.org/ERL/8/034032/mmedia" target="_blank">stacks.iop.org/ERL/8/034032/mmedia</a>) in the supplement for all eight Δ<em>T</em><sub>g</sub> levels.</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>