Hydrological droughts in the 21st century, hotspots and uncertainties from a global multimodel ensemble experiment

Increasing concentrations of greenhouse gases in the atmosphere are expected to modify the global water cycle with significant consequences for terrestrial hydrology. We assess the impact of climate change on hydrological droughts in a multimodel experiment including seven global impact models (GIMs) driven by biascorrected climate from five global climate models under four representative concentration pathways (RCPs). Drought severity is defined as the fraction of land under drought conditions. Results show a likely increase in the global severity of hydrological drought at the end of the 21st century, with systematically greater increases for RCPs describing stronger radiative forcings. Under RCP8.5, droughts exceeding 40% of analyzed land area are projected by nearly half of the simulations. This increase in drought severity has a strong signal-to-noise ratio at the global scale, and Southern Europe, the Middle East, the Southeast United States, Chile, and South West Australia are identified as possible hotspots for future water security issues. The uncertainty due to GIMs is greater than that from global climate models, particularly if including a GIM that accounts for the dynamic response of plants to CO2 and climate, as this model simulates little or no increase in drought frequency. Our study demonstrates that different representations of terrestrial water-cycle processes in GIMs are responsible for a much larger uncertainty in the response of hydrological drought to climate change than previously thought. When assessing the impact of climate change on hydrology, it is therefore critical to consider a diverse range of GIMs to better capture the uncertainty

Temporal and spatial scales of water temperature variability as an indicator for mixing in a polymictic lake

<p>We applied coarse spectral analysis to more than 2 decades of daily near-surface water temperature (WT) measurements from Müggelsee, a shallow polymictic lake in Germany, to systematically characterize patterns in WT variability from daily to yearly temporal scales. Comparison of WT with local air temperature indicates that the WT variability patterns are likely attributable to both meteorological forcing and internal lake dynamics. We identified seasonal patterns of WT variability and showed that WT variability increases with increasing Schmidt stability, decreasing Lake number and decreasing ice cover duration, and is higher near the shore than in open water. We introduced the slope of WT spectra as an indicator for the degree of lake mixing to help explain the identified temporal and spatial scales of WT variability. The explanatory power of this indicator in other lakes with different mixing regimes remains to be established.</p

Continental-scale effects of selected Δ<em>T</em>_g levels (2 ° C, left bars; 3.5 ° C, middle bars; 5 ° C, right bars), simulated under >50% of the climate change patterns

<p><strong>Figure 3.</strong> Continental-scale effects of selected Δ<em>T</em>_g 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 <a href="http://iopscience.iop.org/1748-9326/8/3/034032/article#erl472982s2" target="_blank">2</a> and figure <a href="http://iopscience.iop.org/1748-9326/8/3/034032/article#erl472982fig1" target="_blank">1</a>). EUR, Europe; ASI, Asia; AFR, Africa; NAM, North America; SAM, South America; AUS, Australasia.</p> <p><strong>Abstract</strong></p> <p>This modelling study demonstrates at what level of global mean temperature rise (Δ<em>T</em>_g) 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>_g 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>_g 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>_g 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>_g and highlight regional disparities in impact magnitudes and critical Δ<em>T</em>_g levels.</p

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

<p><b>Table 1.</b>  Continental and global effects of different Δ<em>T</em>_g levels. Top: millions of people living in river basins characterized by chronic water scarcity (<1000 m³ cap⁻¹ yr⁻¹) (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="http://iopscience.iop.org/1748-9326/8/3/034032/article#erl472982fig3" 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>_g) 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>_g 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>_g 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>_g 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>_g and highlight regional disparities in impact magnitudes and critical Δ<em>T</em>_g levels.</p

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>_g

<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>_g. Left panel: function for all 8Δ<em>T</em>_g 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>_g) 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>_g 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>_g 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>_g 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>_g and highlight regional disparities in impact magnitudes and critical Δ<em>T</em>_g levels.</p

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>_g 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>_g 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⁻¹ 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>_g levels.</p> <p><strong>Abstract</strong></p> <p>This modelling study demonstrates at what level of global mean temperature rise (Δ<em>T</em>_g) 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>_g 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>_g 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>_g 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>_g and highlight regional disparities in impact magnitudes and critical Δ<em>T</em>_g levels.</p