While the world's climate has always varied naturally, the vast majority of scientists now believe that rising concentrations of "greenhouse gases" in the earth's atmosphere, resulting from economic and demographic growth over the last two centuries since the industrial revolution, are overriding this natural variability and leading to irreversible climate change. In 1995, the Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) confirmed that "the balance of evidence suggests that there is a discernible human influence on global climate". The Report projected that global mean surface temperatures would increase by between 1 and 3.5 C by 2100, the fastest rate of change since the end of the last ice age, and that global mean sea levels would rise by between 15 and 95 cm by 2100, flooding many low-lying coastal areas. Changes in rainfall patterns are also predicted, increasing the threat of drought, floods or intense storms in many regions.

The climate system is complex, and scientists still need to improve their understanding of the extent, timing and impacts of climate change. However, what we know already alerts us to the potentially dramatic negative impacts of climate change on human health, food security, economic activity, water resources and physical infrastructure. Farming could be seriously disrupted, leading to falling crop yields in many regions. Tropical diseases are expected to spread; the geographical zone of potential malaria transmission, for example, could increase from around 45% of the world population today to approximately 60% by the latter half of this century. Sea level rise and changing weather patterns could also trigger large-scale migration from more seriously affected areas. While no one will be able to escape from climate change, it is the poorer people and countries who are most vulnerable to its negative impacts.