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»Eine Erderwärmung um vier Grad Celsius muss verhindert werden«

Bericht des Potsdam-Instituts für Klimafolgenforschung im Auftrag der Weltbank, November 2012 (engl. Originalfassung)

This report provides a snapshot of recent scientific literature and new analyses of likely impacts and risks that would be associated with a 4° Celsius warming within this century. It is a rigorous attempt to outline a range of risks, focusing on developing countries and especially the poor. A 4°C world would be one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on ecosystems and associated services. But with action, a 4°C world can be avoided and we can likely hold warming below 2°C.

Without further commitments and action to reduce greenhouse gas emissions, the world is likely to warm by more than 3°C above the preindustrial climate. Even with the current mitigation commitments and pledges fully implemented, there is roughly a 20 percent likelihood of exceeding 4°C by 2100. If they are not met, a warming of 4°C could occur as early as the 2060s. Such a warming level and associated sea-level rise of 0.5 to 1 meter, or more, by 2100 would not be the end point: a further warming to levels over 6°C, with several meters of sea-level rise, would likely occur over the following centuries.

Thus, while the global community has committed itself to holding warming below 2°C to prevent “dangerous” climate change, and Small Island Developing states (SIDS) and Least Developed Countries (LDCs) have identified global warming of 1.5°C as warming above which there would be serious threats to their own development and, in some cases, survival, the sum total of current policies—in place and pledged—will very likely lead to warming far in excess of these levels. Indeed, present emission trends put the world plausibly on a path toward 4°C warming within the century.

This report is not a comprehensive scientific assessment, as will be forthcoming from the Intergovernmental Panel on Climate Change (IPCC) in 2013–14 in its Fifth Assessment Report. It is focused on developing countries, while recognizing that developed countries are also vulnerable and at serious risk of major damages from climate change. A series of recent extreme events worldwide continue to highlight the vulnerability of not only the developing world but even wealthy industrialized countries.

Uncertainties remain in projecting the extent of both climate change and its impacts. We take a risk-based approach in which risk is defined as impact multiplied by probability: an event with low probability can still pose a high risk if it implies serious consequences.

No nation will be immune to the impacts of climate change. However, the distribution of impacts is likely to be inherently unequal and tilted against many of the world’s poorest regions, which have the least economic, institutional, scientific, and technical capacity to cope and adapt. For example:

• Eventhough absolute warming will be largest in high latitudes, the warming that will occur in the tropics is larger when compared to the historical range of temperature and extremes to which human and natural ecosystems have adapted and coped. The projected emergence of unprecedented high-temperature extremes in the tropics will consequently lead to significantly larger impacts on agriculture and ecosystems.

• Sea-level rise is likely to be 15 to 20 percent larger in the tropics than the global mean.

• Increases in tropical cyclone intensity are likely to be felt disproportionately in low-latitude regions.

• Increasing aridity and drought are likely to increase substantially in many developing country regions located in tropical and subtropical areas.

A world in which warming reaches 4°C above preindustrial levels (hereafter referred to as a 4°C world), would be one of unprecedented heat waves, severe drought, and major floods in many regions, with serious impacts on human systems, ecosystems, and associated services.

Warming of 4°C can still be avoided: numerous studies show that there are technically and economically feasible emissions pathways to hold warming likely below 2°C. Thus the level of impacts that developing countries and the rest of the world experience will be a result of government, private sector, and civil society decisions and choices, including, unfortunately, inaction.

Observed Impacts and Changes to the Climate System

The unequivocal effects of greenhouse gas emission–induced change on the climate system, reported by IPCC’s Fourth Assessment Report (AR4) in 2007, have continued to intensify, more or less unabated:

• The concentration of the main greenhouse gas, carbon dioxide (CO2), has continued to increase from its preindustrial concentration of approximately 278 parts per million (ppm) to over 391 ppm in September 2012, with the rate of rise now at 1.8 ppm per year.

• The present CO2 concentration is higher than paleoclimatic and geologic evidence indicates has occurred at any time in the last 15 million years.

• Emissions of CO2 are, at present, about 35,000 million metric tons per year (including land-use change) and, absent further policies, are projected to rise to 41,000 million metric tons of CO2 per year in 2020.

• Global mean temperature has continued to increase and is now about 0.8°C above preindustrial levels.

A global warming of 0.8°C may not seem large, but many climate change impacts have already started to emerge, and the shift from 0.8°C to 2°C warming or beyond will pose even greater challenges. It is also useful to recall that a global mean temperature increase of 4°C approaches the difference between temperatures today and those of the last ice age, when much of central Europe and the northern United States were covered with kilometers of ice and global mean temperatures were about 4.5°C to 7°C lower. And this magnitude of climate change—human induced—is occurring over a century, not millennia.

The global oceans have continued to warm, with about 90 percent of the excess heat energy trapped by the increased greenhouse gas concentrations since 1955 stored in the oceans as heat. The average increase in sea levels around the world over the 20th century has been about 15 to 20 centimeters. Over the last decade the average rate of sea-level rise has increased to about 3.2 cm per decade. Should this rate remain unchanged, this would mean over 30 cm of additional sea-level rise in the 21st century.

The warming of the atmosphere and oceans is leading to an accelerating loss of ice from the Greenland and Antarctic ice sheets, and this melting could add substantially to sea-level rise in the future. Overall, the rate of loss of ice has more than tripled since the 1993–2003 period as reported in the IPCC AR4, reaching 1.3 cm per decade over 2004–08; the 2009 loss rate is equivalent to about 1.7 cm per decade. If ice sheet loss continues at these rates, without acceleration, the increase in global average sea level due to this source would be about 15 cm by the end of the 21st century. A clear illustration of the Greenland ice sheet’s increasing vulnerability to warming is the rapid growth in melt area observed since the 1970s. As for Arctic sea ice, it reached a record minimum in September 2012, halving the area of ice covering the Arctic Ocean in summers over the last 30 years.

The effects of global warming are also leading to observed changes in many other climate and environmental aspects of the Earth system. The last decade has seen an exceptional number of extreme heat waves around the world with consequential severe impacts. Human-induced climate change since the 1960s has increased the frequency and intensity of heat waves and thus also likely exacerbated their societal impacts. In some climatic regions, extreme precipitation and drought have increased in intensity and/ or frequency with a likely human influence. An example of a recent extreme heat wave is the Russian heat wave of 2010, which had very significant adverse consequences. Preliminary estimates for the 2010 heat wave in Russia put the death toll at 55,000, annual crop failure at about 25 percent, burned areas at more than 1 million hectares, and economic losses at about US$15 billion (1 percent gross domestic product (GDP)).

In the absence of climate change, extreme heat waves in Europe, Russia, and the United States, for example, would be expected to occur only once every several hundred years. Observations indicate a tenfold increase in the surface area of the planet experiencing extreme heat since the 1950s.

The area of the Earth’s land surface affected by drought has also likely increased substantially over the last 50 years, somewhat faster than projected by climate models. The 2012 drought in the United States impacted about 80 percent of agricultural land, making it the most severe drought since the 1950s.

Negative effects of higher temperatures have been observed on agricultural production, with recent studies indicating that since the 1980s global maize and wheat production may have been reduced significantly compared to a case without climate change.

Effects of higher temperatures on the economic growth of poor countries have also been observed over recent decades, suggesting a significant risk of further reductions in the economic growth in poor countries in the future due to global warming. An MIT study1 used historical fluctuations in temperature within countries

to identify its effects on aggregate economic outcomes. It reported that higher temperatures substantially reduce economic growth in poor countries and have wide-ranging effects, reducing agricultural output, industrial output, and political stability. These findings inform debates over the climate’s role in economic development and suggest the possibility of substantial negative impacts of higher temperatures on poor countries.

Projected Climate Change Impacts in a 4°C World

The effects of 4°C warming will not be evenly distributed around the world, nor would the consequences be simply an extension of those felt at 2°C warming. The largest warming will occur over land and range from 4°C to 10°C. Increases of 6°C or more in average monthly summer temperatures would be expected in large regions of the world, including the Mediterranean, North Africa, the Middle East, and the contiguous United States

Projections for a 4°C world show a dramatic increase in the intensity and frequency of high-temperature extremes. Recent extreme heat waves such as in Russia in 2010 are likely to become the new normal summer in a 4°C world. Tropical South America, central Africa, and all tropical islands in the Pacific are likely to regularly experience heat waves of unprecedented magnitude and duration. In this new high-temperature climate regime, the coolest months are likely to be substantially warmer than the warmest months at the end of the 20th century. In regions such as the Mediterranean, North Africa, the Middle East, and the Tibetan plateau, almost all summer months are likely to be warmer than the most extreme heat waves presently experienced. For example, the warmest July in the Mediterranean region could be 9°C warmer than today’s warmest July.

Extreme heat waves in recent years have had severe impacts, causing heat-related deaths, forest fires, and harvest losses. The impacts of the extreme heat waves projected for a 4°C world have not been evaluated, but they could be expected to vastly exceed the consequences experienced to date and potentially exceed the adaptive capacities of many societies and natural systems.

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