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

Potsdam Institute for Climate Impact Research (PIK) and Climate Analytics for the World Bank

[Green Car Congress]  A new report…provides a snapshot of …risks that would be associated with a 4 °C global warming within this century…

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, the report finds. For example:

    • Even though 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 15–20% 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….
  • Projected climate change impacts in a 4°C world. 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.
  • Rising CO2 concentration and ocean acidification. Apart from a warming of the climate system, one of the most serious consequences of rising carbon dioxide concentration in the atmosphere occurs when it dissolves in the ocean and results in acidification. A warming of 4 °C or more by 2100 would correspond to a CO2 concentration above 800 ppm and an increase of about 150% in acidity of the ocean. The observed and projected rates of change in ocean acidity over the next century appear to be unparalleled in Earth’s history…
  • Rising sea levels, coastal inundation and loss. Warming of 4 °C will likely lead to a sea-level rise of 0.5 to 1 meter, and possibly more, by 2100, with several meters more to be realized in the coming centuries. Limiting warming to 2 °C would likely reduce sea-level rise by about 20 cm by 2100 compared to a 4 °C world…
  • Risks to human support systems: food, water, ecosystems, and human health. Although impact projections for a 4°C world are still preliminary and it is often difficult to make comparisons across individual assessments, this report identifies a number of severe risks for vital human support systems. With extremes of temperature, heat waves, rainfall, and drought are projected to increase with warming; risks will be much higher in a 4 °C world compared to a 2 °C world…

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