"Regions will be affected asynchronously in terms of the ‘timing’, magnitude, type and confidence of impacts," Dieter Gerten of the Potsdam Institute for Climate Impact Research, Germany, told environmentalresearchweb. "For example, at a mean global warming of 2°C above preindustrial levels, many river basins in the Near and Middle East are likely to face significantly aggravated water scarcity, on top of existing water scarcity. At the same time, severe ecosystem changes are likely to occur in regions such as the tundra and some semi-arid regions, where present biodiversity is rather low."

As warming continues, the area and number of people facing new or aggravated water scarcity will increase gradually, says Gerten. But the area affected by ecosystem changes will strongly increase and reach further into biodiversity- and endemism-rich regions such as the humid tropics, especially if the temperature rise tops 3.5°C, the amount likely to result from current pledges on reductions in greenhouse gas emissions.

A mean warming of 2°C by 2100 would expose an additional 8% of the world’s population at 2000 levels – 486 million people – to new or worsening water scarcity, mainly in the Near and Middle East, the team found, with a 3.5 ° temperature rise bringing water problems to 11% of the population, chiefly in the Middle East, North Africa, Southern Europe and the Southwest US. And five degrees of warming would affect 13% of the population. That’s on top of the 1.3 billion people already living in water-scarce regions. Declining precipitation is seen as the key culprit, but raised temperatures would also increase evapotranspiration, cutting water availability.

On the wildlife front, a temperature rise of 2° would see substantial habitat changes in biogeographic regions that contain 1% of present endemism-weighted vascular plant species, mainly in the tundra and some semi-arid areas, with the figure for 3.5° at 10% and for 5° a whopping 74%.

"At global warming levels beyond 3°C, the area affected by significant ecosystem transformation would significantly increase and encroach into biodiversity-rich regions," said Gerten. "Beyond a mean global warming of 4°C, we show with high confidence that biodiversity hotspots such as parts of the Amazon will be affected."

The study focuses on "what regions would suffer first, i.e. at low warming levels, and what would be the incremental impacts between a mean global warming of 2°C (climate mitigation target), 3.5°C (the likely outcome of nation's current emissions reduction pledges), and 5°C – a business as usual case without mitigation".

He believes the results could be informative for the ongoing academic and political debates on climate mitigation targets. "The unequal spatial pattern of exposure to climate change impacts sheds interesting light on the responsibility of high-emission countries and could have a bearing on both mitigation and adaption burden sharing," he said.

Now the team is involved in the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), which enhances the present analysis by considering more than just one impact model. "We have, for example, an interest in extending that analysis to the full set of newly available CMIP5 climate change simulations," said Gerten, "and also in coupling the model to resource use models for exploring how fast humanity is approaching several ‘planetary boundaries’."

Gerten and colleagues reported the results in Environmental Research Letters (ERL).

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