Climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity.
In general terms a change in climate means a change in those conditions considered average or normal for a given time period. The IPCC use a relatively broad definition of climate change which is considered to mean an identifiable and statistical change in the state of the climate which persists for an extended period of time. This change could be the result of internal processes within the climate system or from external processes. These external processes (or forcing) could be natural, for example volcanoes, or caused by the activities of people, for example emissions of greenhouse gases or changes in land use.
Other bodies, notably the United Nations Framework Convention on Climate Change (UNFCCC), define climate change slightly differently. The UNFCCC makes a distinction between climate change that is directly attributable to human activities and climate variability which is attributable to natural causes.
Peoples' response to climate change is usually thought of as including both and adaptation.
Further explanation is provided in Box 1.1 of the UKCP09 Climate change projections report.