Corn crops in New Florence, Missouri, wither in the devastating drought of 2012. Photograph: MCT via Getty Images

By Nafeez Ahmed
17 December 2013

(The Guardian) – An international scientific research project known as the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), run by 30 teams from 12 countries, has attempted to understand the severity and scale of global impacts of climate change. The project compares model projections on water scarcity, crop yields, disease, floods among other issues to see how they could interact.

The series of papers published by the Proceedings for the National Academy of Sciences (PNAS) shows that policymakers might be underestimating the social and economic consequences of climate change due to insufficient attention on how different climate risks are interconnected.

One paper whose lead author is Franziska Piontek of the Potsdam Institute for Climate Impact Research explores impacts related to "water, agriculture, ecosystems, and malaria at different levels of global warming." The study concludes that:

"… uncertainty arising from the impact models is considerable, and larger than that from the climate models. In a low probability-high impact worst-case assessment, almost the whole inhabited world is at risk for multisectoral pressures."

The uncertainties in the model are large enough that they may "mask" the risk of a "worst case" scenario of "multisectoral hotspots," where impacts affecting "water, agriculture, ecosystems, and health" overlap in ways that could affect "all the world's inhabited areas."

In the worst-case analysis, "Almost the entire global population is exposed to multisectoral pressure" at global mean temperatures of around 4C higher, with "roughly 18% of the global population" projected to "experience severe pressure in all four sectors. The affected regions are in Europe, North America, and south-east Asia." […]

Other papers point to significant risks that are much more likely on a business-as-usual emissions trajectory.

A study led by Jacob Schewe of Potsdam finds that "the combination of unmitigated climate change and further population growth will expose a significant fraction of the world population" to "chronic or absolute water scarcity."

About 2.7C above preindustrial temperatures:

"… will confront an additional approximate 15% of the global population with a severe decrease in water resources and will increase the number of people living under absolute water scarcity (<500m3 per capita per year) by another 40% (according to some models, more than 100%) compared with the effect of population growth alone."

The Mediterranean, the Middle East, the southern United States and southern China, for example, could see a "pronounced decrease of available water," while southern India, western China, and parts of eastern Africa could see an increase. […]

Water scarcity in turn will have a dramatic impact on agriculture. Another study in the PNAS collection combining climate, agricultural and hydrological models warns that freshwater shortages could double climate change's debilitation of global food crop yields.

Current agricultural models estimate that climate change will directly reduce food production from maize, soybeans, wheat and rice by as much as 43 percent by the end of the 21st century, encompassing a loss of between 400 and 2600 petacalories of food supply. But incorporating hydrological models reveals that when accounting for the decline of freshwater availability, there would be an additional loss of 600 to 2900 petacalories - potentially wiping out quantities equivalent to the total present-day food supply. [more]

'Whole world' at risk from simultaneous droughts, famines, epidemics: scientists

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