Impacts of climate change on the world's most exceptional ecoregions

Abstract

The current rate of warming due to increases in greenhouse gas (GHG) emissions is very likely unprecedented over the last 10,000 y. Although the majority of countries have adopted the view that global warming must be limited to <2 °C, current GHG emission rates and nonagreement at Copenhagen in December 2009 increase the likelihood of this limit being exceeded by 2100. Extensive evidence has linked major changes in biological systems to 20th century warming. The "Global 200" comprises 238 ecoregions of exceptional biodiversity [Olson DM, Dinerstein E (2002) Ann Mo Bot Gard 89:199-224]. We assess the likelihood that, by 2070, these iconic ecoregions will regularly experience monthly climatic conditions that were extreme in 1961-1990. Using >600 realizations from climate model ensembles, we show that up to 86% of terrestrial and 83% of freshwater ecoregions will be exposed to average monthly temperature patterns >2 SDs (2σ) of the 1961-1990 baseline, including 82% of critically endangered ecoregions. The entire range of 89 ecoregions will experience extreme monthly temperatures with a local warming of <2 °C. Tropical and subtropical ecoregions, and mangroves, face extreme conditions earliest, some with <1 °C warming. In contrast, few ecoregions within Boreal Forests and Tundra biomes will experience such extremes this century. On average, precipitation regimes do not exceed 2σ of the baseline period, although considerable variability exists across the climate realizations. Further, the strength of the correlation between seasonal temperature and precipitation changes over numerous ecoregions. These results suggest many Global 200 ecoregions may be under substantial climatic stress by 2100.

Fig. 1.

Distribution of extreme monthly Tas_μ projected for 2070 across the terrestrial and freshwater components of WWF's Global 200. (A) The distribution of 132 terrestrial and 53 freshwater ecoregions, grouped by biomes. (B) Average distance of 21st century monthly Tas_μ from that of the baseline period (1961–1990), where distance is measured as SDs (σ) from the mean (μ) of the baseline. Extreme monthly Tas_μ is defined as exceeding 2σ of the baseline μ. Results are based on a climate model ensemble (n realizations = 173) for the A2 emission scenario. (C) The coefficient of variation (CoV) shows variation across different realizations within the climate model ensemble. Areas with higher values for CoV indicate greater differences across alternate projections of future Tasμ and, hence, less certainty in distance values.

Fig. 2.

Estimated local Tas anomaly required to produce extreme monthly Tas_μ across the Global 200 terrestrial and freshwater ecoregions. The number of ecoregions within each biome is given in parantheses. For a given biome, the black line shows the range of Tas anomalies required to drive the entire distribution of its ecoregions into extreme conditions. Some ecoregions do not have extremes of monthly Tas_μ projected to occur across their entire range by 2070. For these ecoregions, the black arrow indicates the average maximum Tas anomaly projected to occur within that ecoregion by 2070 (multiple black arrows for a given biome represent values for separate ecoregions). Thus, a Tas anomaly greater than this value is required for the entire range of these ecoregions to have extreme monthly Tas_μ. The gray line shows the range of Tas anomalies required for least one 1° grid cell of each ecoregion within the biome to have extreme monthly Tas_μ. Two ecoregions in the Boreal Forests/Taiga biome and one Large River Delta do not have any grid cells projected to be extreme. The maximum projected Tas anomaly for these regions is indicated by the gray arrows. Note: Values are a conservative estimate only, because they are based on average maximum Tas anomalies from the multimodel ensembles used for this study. It is possible that Tas anomalies smaller than these will result in extreme monthly Tas_μ conditions for the ecoregions. Results for individual ecoregions are in Tables S1 and S2.

Fig. 3.

The number of months by 2070 where monthly Tas_μ is projected to exceed thresholds of 2σ across terrestrial or freshwater ecoregions, under three emission scenarios (B1, A1B, and A2).