The Earth's climate has been stable for around 10,000 years, though it has been very variable in earlier periods and has occasionally changed abruptly through natural processes. Industrialization and population growth have brought an exponential rise in the use of carbonaceous fuels, which is now having an observable impact on the composition of the atmosphere. Carbon dioxide levels are already substantially above pre-industrial levels, and rising appreciably year on year. Climate models suggest that the anthropogenic rise in carbon dioxide and other greenhouse gases will lead to rapid climate change over the twenty-first century, with an increase in global average temperatures in the region of two to five degrees Celsius. This will present problems of adaptation for many natural systems and have largely negative effects on human health through both direct and indirect mechanisms. There is also a possibility of unpredicted catastrophic impacts arising from non-linear effects of climate change, which may have more damaging effects on human and other populations. Policy responses have to be directed towards both adaptation needs and mitigation. Mitigation in particular presents formidable social, political and technological challenges, but it may bring net health benefits in the short as well as the longer term.