Environment and health: 2. Global climate change and health.

G reenhouse gases, naturally present at low concentrations in the lower atmosphere , keep the Earth's mean surface temperature at around 15°C. Without this trapping of heat (" radiative forcing ") the mean air temperature would be –18°C and the Earth would freeze. The mechanism of the greenhouse effect is illustrated in Fig. 1. The atmospheric concentrations of greenhouse gases have been increasing since the early industrial revolution, owing principally to humankind's rapidly increasing combustion of fossil fuels along with increases in deforestation, irrigated agriculture, animal husbandry and cement manufacture. Table 1 shows the most important anthropogenic greenhouse gases and the annual change in their concentrations. 1 In 1997 and 1998 global temperatures reached their highest levels since record-keeping began last century; 9 of the 11 hottest years in the 20th century occurred within the last 10 years (Fig. 2). 2 The global mean surface temperature has increased by 0.4°C in the past 25 years, and climate scientists are becoming increasingly confident that the anticipated process of global warming has begun. Three studies indicating disproportionate mid-atmospheric warming, 3 disproportionate night-time and winter warming, 4 and increased variability 5 — consistent with projections — all led the Intergovernmental Panel on Climate Change, a major international scientific collaboration established in 1988 by the World Meteorological Organization and the United Nations Environment Program, to conclude that there has been a " discernible " human influence on the climate system. 1,6 More recent studies continue to find a dominance of greenhouse gases over solar and other influences. 7 The Intergovernmental Panel on Climate Change has comprehensively reviewed the science of climate change and its potential impacts. 1,3 It foresees an increase of 1.0ºC–3.5ºC in the global mean temperature by the year 2100, with considerable regional variations. This assessment is derived from projections made by computer-based global climate models 1 that combine, through simultaneous equations within a 3-dimensional global grid, the atmospheric and oceanic processes that occur in response to increased greenhouse gases and the resulting rise in radia-tive forcing in the lower atmosphere. Although the current generation of global climate models cannot forecast the precise spatial and temporal pattern of changes in climate means and variability with global warming, 1 extreme weather events such as drought, floods and storms may become more frequent and intense in the future. Indeed, with warming ocean surfaces 1 and the fact that each increase of 1°C in temperature …