This is a scientific assessment of the present impacts of climate change on natural and human systems, and the capacity of these systems to adapt to vulnerability. It concerns the relationship between observed climate change and recent observed changes in the natural and human environment and is based largely on data sets from 1970. This summary presents the following key policy-relevant findings of the Fourth Assessment of Working Group II of the Intergovernmental Panel on Climate Change (IPCC) building on the past assessments and incorporates the new knowledge.

Observations from all continents and oceans show that natural systems are being affected by regional temperature increases.
Global data shows that anthropogenic warming has had a discernible influence on physical and biological systems and other changes on natural and human environments are emerging.
More specific information a wide range of systems and sectors is now available across the regions of the world concerning projected future impacts.
It is now possible to systematically estimate the magnitudes of impact for a range of possible increases in global average temperature. 
Some large-scale climate events will vary regionally but are likely to impose net annual costs which will increase as global temperatures rise.

The following are projections of the likelihood of future trends in the 21st century:

it is certain that there will be warmer days and fewer cold nights
it is very likely that the frequency of heat waves and heavy precipitation will increase over most land areas
it is likely that the areas affected by drought, intense tropical cyclone activity and the incidence of extreme high sea level will all increase.

The summary presents the following policy implications for adaptation measures:

adaptation is unavoidable in addressing impacts resulting from the warming due to past emissions
there exist a wide array of adaptation options but more extensive adaptation is required to reduce vulnerability to future climate change although there are barriers, limits and costs
vulnerability to climate change can be exacerbated by unsustainable development pathways
sustainable development should be used to reduce vulnerability to climate change
various adaptation and mitigation measures should be used to reduce the risks associated with climate change.