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Health risks of climate change: act now or pay later
There is growing scientific consensus that climate change is happening, is largely human induced, and will have serious consequences for human health. The impact of climate change on global health is probably not yet large, compared with major risk factors, but will become greater later in this century, especially if the world follows one of the so-called high-end emission pathways, such as Representative Concentration Pathway (RCP) 8·5, outlined in the UN Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report.1, 2
This emission pathway assumes that present trends of relatively unrestrained use of fossil fuels and high population growth will continue. According to this emission pathway, by 2100 the global average temperature will probably be more than 4°C above preindustrial levels (figure), with higher average temperatures over land. Inertia in climate systems would mean that projected global temperatures would continue to rise for several hundred years; the mean estimate for this emission pathway in 2300 would be at least 8°C above preindustrial levels. Long-term global average warming of 12°C or more is possible from unconstrained fossil fuel burning. An increase of 12°C in global average temperature would create conditions in which physiological limits for outdoor activity would be reached during certain times of the year in many heavily populated parts of the world.3 One study estimates that under RCP 8·5 there would be about a 40% reduction in global labour capacity during the hottest months by 2100.4
The effects of climate change on humanity under such high-end emission scenarios are difficult to estimate. No epidemiology is possible, models cannot be validated, and baseline world health and socioeconomic conditions that far into the future are a matter close to conjecture. However, such extreme temperature trends might cross what has been called the “afterlife” threshold—ie, where the impact on humanity is so great as to be a discontinuity in the long-term progression of humanity.5
The climate implications of different emission trajectories will not strongly diverge until the middle of the 21st century, whatever progress is achieved in the next few decades in reducing greenhouse-gas emissions. Therefore, the near-term and medium-term (up to about 2050) is a time to promote and fund adaptation measures that will reduce the damage to health caused by climate change, to which the world is already committed. Society also needs to prepare for the possibility of more serious changes later in the century. For example, in the next 20—30 years, net global food production might be maintained as a result of increased crop yields in cooler, high latitudes. However, continued warming after 2050 would adversely affect crop yields in most parts of the world, at a time when demand for food will be rising strongly due to projected population growth and economic development.6
Both a high absolute level and a rapid rate of global climate change will severely test biological and social mechanisms for adaptation, with limits to adaptation becoming increasingly evident. Examples range from emergency preparedness in the face of shortening return periods after potentially catastrophic floods, to saltwater intrusion into freshwater lenses in small islands which would potentially compromise water quality to such an extent that migration might be the only option.7 Furthermore, limits to adaptation will arise when hazards associated with a changing climate interact with inappropriate development pathways—eg, where building occurs on flood plains. Adaptation limits might also occur when health systems fail to proactively design and implement adaptation actions needed to address a particular health risk. The need to strengthen health-system capacity in low-income countries to prepare for, cope with, and recover as far as possible from the health effects of climate change that do occur could be partly addressed by equitable partnerships between high-income and low-income countries.
The health community has a role in the promotion of policies to reduce emissions of all climate-active pollutants, both because many of these pollutants are health hazards now and because all push the climate towards possible extremes within this century. Major reductions in mortality would ensue from measures to reduce combustion of carbon-containing fuel and consequent carbon dioxide and fine particle emissions, for example by curbing coal combustion. Furthermore, there are co-benefits for both health and climate from actions to reduce short-lived climate pollutants, such as black carbon and methane, the second most important greenhouse gas and the primary precursor to ozone in the lower atmosphere.8, 9 Provision of wider access to reproductive health services could also reduce carbon dioxide emissions over the century and improve maternal and child health.10
Wise policies to develop low-emission economies that use the full range of available technological options would provide many societal benefits and only have moderate impacts on economic growth projections.9 Nevertheless, such changes tend to be opposed by those with a stake in current economic pathways.
The burden of disease and injury due to climate change falls mainly on specific vulnerable groups, particularly those living in poverty. Vulnerability to climate change is in many cases inversely related to historical greenhouse gas emissions. Thus, the responsibility for leading policies to reduce these emissions should be shouldered by those nations and commercial interests that have enjoyed the benefits of fossil-fuel-driven development. Since emerging economies are responsible for a growing proportion of emissions, however, they too must move towards development with low greenhouse gas emissions, which some are starting to do.
The timescale is daunting but not a reason for inaction: the health community needs to argue for a long-term perspective on climate change. Article 2 of the United Nations Framework Convention on Climate Change states that the objective is to avoid dangerous anthropogenic interference with the Earth's climate.11 Without decisive action this objective cannot be achieved. The world's carbon budget to keep global increases in temperature below 2°C is likely to be exceeded by 2040.9 Society must, therefore, enhance efforts to adapt to reduce adverse effects on human health, particularly for the most vulnerable. The prospect of extreme climates beyond mid-century, however, should impel the health community to promote deep cuts in emissions of climate-active pollutants now for the long-term protection of human welfare, starting with co-benefit actions that protect both health and climate.
The authors were members of the UN Intergovernmental Panel on Climate Change Fifth Assessment Report and contributed to chapter 11 on human health. This Comment represents their own views and not necessarily those of the UN Intergovernmental Panel on Climate Change.
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a Departments of Social and Environmental Health Research and Population Health, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK
b Department of Global Health, University of Washington, Seattle, WA, USA
c School of Public Health, University of California Berkeley, Berkeley, CA, USA
d School of Population Health, University of Auckland, Auckland, New Zealand