What changes in the future climate might be expected?
The current atmospheric carbon dioxide concentration is about 385 parts per million (ppm). The world will be very fortunate if mitigation efforts can control emissions sufficiently that the 2050 level will not exceed 550 ppm. An atmospheric carbon dioxide concentration of 450 ppm, corresponding to a temperature rise of 1º to 3°C, would result in the worldwide disappearance of mountain glaciers, threatening water supplies to several areas; and some increase in the intensity of storms, forest fires, droughts and flooding. A concentration of 550 ppm, corresponding to a temperature rise of 2º to 4° C, would lead to failing crop yields in many developing regions; changes in water availability for more than a billion people by 2080; the possible onset of the collapse of the Amazon rainforest; and the possible extinction of many species. Concentrations beyond 650 ppm, corresponding to a temperature rise of 4° C or more, could result in major declines in crop yields in entire regions; a sea le
