What is climate change?

Climate change refers to the variation in the Earth's global climate over time.

It describes changes in the variablility or average state of the atmosphere - or average weather - over time scales ranging from decades to millons of years.  These changes may come from processes internal to the Earth, be driven by extrnal forces (e.g. variations in sunlight intensity) or, most recently, be cause by human activities. 

In recent usage, especially in the context of environmental policy, the term 'climate change' often refers only to the ongoing changes in modern climate, including the rise in average surface temperaure known as global warming.  In some cases, the term is also used with a presumption of human causation, as in the United Nations Framework Convention on Climate Change (UNFCCC).  The UNFCCC uses 'climate variability' for non-human caused variations.