There is wide agreement among climatologists internationally that human-induced climate change is now under way.1 Global climate change is one of various large-scale, unprecedented environmental perturbations occurring in today's world. These environmental changes reflect the rapid increase in human domination of the biosphere as human numbers increase and as economic activities intensify.2,3 This process has, apparently, now passed certain critical points. Indeed, one recent environmental analysis showed that humankind has been operating in ecological deficit since the 1970s: we now consume and deplete Earth's natural environmental capital faster than it is being replenished and restored.4

These changes — climate change, stratospheric ozone depletion, loss of biodiversity, worldwide land degradation, fresh water depletion, disruption of elemental nitrogen and sulfur cycles, and global dissemination of persistent organic pollutants — have enormous potential consequences for the sustainability of ecological systems, food production, economic activities and the health of human populations. This may have sounded farfetched to "linear optimists" a decade ago. With the passage of each year, and continued global environmental trends, it now seems increasingly plausible. This issue of the Journal, meanwhile, features an ensemble of articles highlighting the current health effects of environmental exposures in five key areas: air,5 food,6 soil,7 water8 and ultraviolet radiation.9

Last year the United Nations Intergovernmental Panel on Climate Change (IPCC), in its Third Assessment Report, documented a coherent pattern of recent changes in various physical and biological systems.1 This included glacier retreat, sea-ice diminution, earlier bird-nesting, earlier flowering, altered timing of insect migration, and so on. While one swallow does not make a summer, the IPCC pointed out that the overall pattern points to the incipient impact of global warming.

Where and when, therefore, might we see effects on human health? The answer is complex. First, most health outcomes are multicausal, and inevitably various non-climate causal factors are also changing over time. Second, climate change affects local environments differently, according to characteristics of local geography. Further, the vulnerability of each human population varies as a function of locality, level of material resources, technological assets and type of governance. For example, the small Pacific Island states are likely to feel the effect of climate-induced sea-level rise in the next 20–50 years. The resulting impairment of agriculture and freshwater resources is a particular concern, along with the health consequences of population displacement.

Several recent reports have suggested that we may now be seeing some early impacts of climate change on infectious diseases. For example, tickborne (viral) encephalitis in Sweden appears to have increased in response to a succession of warmer winters over the past two decades,10 and there is some, though still inconclusive, evidence of malaria ascending to higher altitudes in the eastern African highlands in association with local warming.11 Meanwhile, the intensification of the El Niño Southern Oscillation (ENSO) over the past quarter-century, a likely consequence of global climate change, has been accompanied by a strengthening interannual association of the ENSO index with rates of diarrhoeal disease in Bangladesh.12

Of course, the health prospects are not all bad. Some impacts would be beneficial. For example, milder winters would reduce the seasonal winter-time mortality peak in temperate countries, and a further increase in temperatures in currently hot regions might impair mosquito survival. Overall, however, scientists have consistently predicted that most effects of climate change on health would be adverse.13 The impacts mostly entail changes in the frequency or severity of familiar health risks — such as the effects of floods, storms and fires; the mortality toll of heatwaves; the range and seasonality of infectious diseases; changes in local agro-ecosystem productivity and its nutritional consequences; the impact on health of changes in fresh water supplies; and the many repercussions of economic dislocation and population displacement.

Studies in Australia are beginning to give us a better information base for estimating the impact of changes in climatic means and variability. For example, the dependence of Ross River virus disease on climate variation and its viral repercussions is becoming clearer.14,15 Various other studies have been reported on the climatic influences on Murray Valley encephalitis and on mortality from thermal stress.16 Hall et al (page 614) describe the relationship between temperature and the occurrence of food-poisoning.6 This year, the Federal Government funded its first formal assessment of the effects of climate change on health in Australia over the coming decades (soon to be published).

Meanwhile, even as average global surface temperatures gradually rise, it is likely that we will face an increase in climatic variability, including extreme weather events.1 Indeed, many scientists now consider that human health and safety are more endangered by an impending increase in extreme and anomalous weather events than by changed average climatic conditions.17

The human species, because of its social organisation and cultural practices, is better buffered against environmental stressors than many other plant and animal species. Hence, Homo sapiens is likely to be affected less soon and less sensitively than most other species. Not surprisingly, therefore, there is little empirical evidence to date that climate change is already affecting human health. However, invoking the precautionary principle, we can recognise that adverse impacts are both likely and, in many cases, potentially serious. By thinking more ecologically about the large-scale influences on population health and disease, we could apply a more anticipatory approach.3

Society's ultimate objective should not be to generate wealth and increase consumption for its own sake, but to maximise the wellbeing, health and survival of its people. Indeed, the growing recognition of the links between ecological infrastructure, social conditions and health is beginning to highlight population health as a criterion of "sustainable development".18

We and our governments need to move beyond the narrow, short-term vision of the UN World Summit on Sustainable Development, held in Johannesburg in August 2002. The outcome of the Summit was constrained by preoccupations with achieving continuing economic growth, the needs and responsibilities of transnational corporations, and the alleviation of poverty — primarily by the creation, not redistribution, of wealth. Much of the debate was distorted by recriminations between rich and poor countries, and by self-interested US power-play tactics to minimise various international collaborative commitments. There was relatively little recognition of the damage that humankind is now doing to Earth's life-support systems, and its consequences for humans. Considerations of health in relation to ecological sustainability received little attention.

One positive, late-breaking development at Johannesburg was the commitment made by Russia and China to comply with the international Kyoto Protocol on greenhouse gas emissions. Indeed, that initiative left Australia looking even more aberrant on this great modern environmental issue than it did before the Summit.

The topic of climate change and health will evolve rapidly during this decade. Researchers are increasingly coming to grips with this and related issues. The point of such research is to enrich the information base for farsighted decision-making. As Rene Dubos, the eminent microbiologist, might have said if he were alive today, "Think future, act now."