New World Bank Report Warns That “A 4°C Warmer World Must Be Avoided”

By Carol Dreibelbis

The global mean temperature is now approximately 0.8 degrees Celsius (°C) above pre-industrial levels, and it continues to rise. In a recent report prepared for the World Bank, Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided, researchers with the Potsdam Institute for Climate Impact Research and Climate Analytics examine what the world would look like if it warmed another 3.2 degrees Celsius. The report discusses the effects of warming that are already being seen around the world and explores the likely impacts and risks of future warming.

Turn Down the Heat outlines likely consequences of climate change. Here, Greenland surface melt measurements vary drastically from July 8 (left) to July 12 (right), 2012. (Photo credit: NASA)

The researchers project that a warming of 4°C would bring potentially devastating consequences, including “the inundation of coastal cities; increasing risks for food production potentially leading to higher under and malnutrition rates; many dry regions becoming dryer, wet regions wetter; unprecedented heat waves in many regions, especially in the tropics; substantially exacerbated water scarcity in many regions; increased intensity of tropical cyclones; and irreversible loss of biodiversity, including coral reef systems.”

Like similar previous studies, the report predicts that many of these effects would have the largest impact in the poorest regions of the world, particularly in Asia and Africa. The authors’ warnings about projected impacts on agriculture are particularly concerning given that, irrespective of climate change, a growing human population is projected to put unprecedented strain on the global food system.

Heat events will be more extreme and more frequent in a 4°C-warmer world, putting crops and the people who grow them at risk, the report notes. Some of the most extreme warming is projected to occur in the sub-tropical Mediterranean, the Middle East, northern Africa, and the contiguous United States. In these locations, most summer months are likely to be warmer than today’s most extreme heat waves.

In the Mediterranean, for example, temperatures during the warmest July between 2080 and 2100 are expected to approach 35°C—about 9 degrees warmer than today’s warmest July. Such intense heat would threaten agriculture: one study cited in the report finds that crop yields decrease by 1 percent for each “growing degree day” spent above 30°C. While there is potential for the carbon dioxide fertilization effect to boost crop yields in some regions, the report concludes that these gains will likely be offset in the face of higher temperatures.

In addition to heat stress, the global food system will face increasing water scarcity in a 4°C-warmer world. The report finds that northern and eastern Africa, the Middle East, and South Asia face severe risks related to reduced water availability, noting that the Ganges, Nile, and other river basins are particularly vulnerable. One study cited in the report projects that 43 to 50 percent of the global population will be living in water-scarce countries by 2080.

Meanwhile, wetter conditions in northern North America, northern Europe, Siberia, and some monsoon regions, may reduce water stress, the report notes. Still, because of expected sub-seasonal and sub-regional changes to the hydrological cycle, flooding and drought may increase significantly even if annual averages undergo little change.

The report also predicts that hotter weather and water shortages will contribute to both lower agricultural yields and concerns about food security. Drought, which already affects 15.4 percent of global cropland, may affect around 44 percent of cropland by 2100. The most severe impacts will be felt in southern Africa, the United States, southern Europe, and Southeast Asia over the next 30 to 90 years. Together with sea-level rise, which will cause contamination of coastal aquifers used for irrigation, the changes associated with a 4°C-warmer world “could substantially undermine food security,” the report concludes.

Despite these serious warnings, the authors remain optimistic that a 4°C-warmer world can be avoided with swift, cooperative, and sustained policy action. In the report’s Foreword, World Bank President Jim Yong Kim issues a call for international action, writing, “It is my hope that this report shocks us into action. Even for those of us already committed to fighting climate change, I hope it causes us to work with much more urgency.” The report calls for numerous preventative and adaptive initiatives, including investment in crop adaptation, which could “play a major role in ensuring food security in a changing climate.”

Click here to read more about Turn Down the Heat: Why a 4°C Warmer World Must Be Avoided.

Carol Dreibelbis is a research intern with the Nourishing the Planet project.

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