Turn Down the Heat: Why a 4°C Warmer World Must be Avoided

The World Bank commissioned this report from the Potsdam Institute for Climate Impact Research and Climate Analytics to synthesize the science and the potential climate impacts on a world without improved mitigation efforts. This report provides a snapshot of recent scientific literature and analysis of likely impacts and risks that would be associated with a 4° Celsius warming within this century, focusing on developing countries and vulnerable populations. 

Some of the observed changes to the Earth’s climate system and their impacts on human society that are already being observed are presented first. Then a background on the climate scenarios utilized for the study is provided, with a discussion of the likelihood of a 4°C warming scenario. The report also examines projections for the coming century on the process of ocean acidification, changes in precipitation that may lead to droughts or floods, and changes in the incidence of extreme tropical cyclones.  A focal chapter of the report provides an analysis of projected sea-level rise and increases in heat extremes. The section focusing on extreme heat details the shifts in temperatures by region, the frequency of warmer months, and the direct impacts of more frequent heat waves. The implications of projected climate changes for society are discussed - specifically for the sectors of agriculture, water resources, ecosystems, and human health. The report closes with an outlook on the potential risks of nonlinear impacts that are not as easily quantified, and identifies the knowledge gaps in understanding a global increase of 4°C from climate change.

The 4°C scenarios presented in this report are staggering. Some of the projections affirm the inundation of coastal cities; unprecedented heat waves in many regions, especially in the tropics; substantially exacerbated water scarcity in many regions; increased frequency of high-intensity tropical cyclones; and irreversible loss of biodiversity, including all coral reefs.

According to the World Bank, "this report is a stark reminder that climate change affects everything. The solutions don’t lie only in climate finance or climate projects. The solutions lie in effective risk management and ensuring all our work, all our thinking, is designed with the threat of a 4°C degree world in mind."

Publication Date: November 2012

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