This report sets out the key policy-relevant findings of the Fourth Assessment (also known as AR4) of Working Group II of the Intergovernmental Panel on Climate Change (IPCC). The Assessment is of current scientific understanding of impacts of climate change on natural and human systems, how well these systems are able to adapt and their vulnerability. The report addresses issues such as managing freshwater resources, impacts on lake and river systems, food security and agriculture impacts, and human health. AR4 builds on past IPCC assessments and incorporates new knowledge gained since the Third Assessment, completed in 2001.With regard to current observed effects as well as projected future climate change impacts, the report’s findings include:many natural systems are being affected by regional climate changes, particularly temperature increasesit is likely that anthropogenic warming has had an influence on many physical and biological systemssome regional impacts are difficult to identify due to adaptation processes and non-climatic drivers of change.more specific information is now available across a wide range of systems, sectors and regions are now able to be estimated more systematically for a range of possible increases in global average temperature Impacts due to altered frequencies and intensities of extreme weather, climate, and sea level events are very likely to changesome large-scale climate events have the potential to cause very large impacts, especially after the 21st centuryCurrent knowledge with regard to adapting to the impacts of climate change include:some adaptation is occurring now, but on a limited basisadaptation will be necessary to address impacts resulting from the warming which is already unavoidable due to past emissionsa wide array of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change. The barriers, limits and costs, are not yet fully understoodvulnerability to climate change can be exacerbated by the presence of other stressesfuture vulnerability depends not only on climate change but also on development pathwaysustainable development can reduce vulnerability to climate change, and climate change could impede nations’ abilities to achieve sustainable development pathwaysmany impacts can be avoided, reduced or delayed by mitigationa portfolio of adaptation and mitigation measures can diminish the risks associated with climate changeimpacts of climate change will vary regionally but overall they are very likely to imposing costs as global temperatures increase.