Climate is a statistical longer-term description of average weather patterns, usually measured by variables such as surface temperatures or precipitation over 30-year periods, as defined by the WMO (IPCC 2007b). Climate change refers to significant alterations in climate variables that can, for instance, be identified through statistical tests. These changes can refer to mean values or variability, and have to persist over decades. Notably, there has always been natural climate change and climate variability, and climate change refers in this book to the anthropogenic component, i.e. human activities leading to the conversion of land and changes in GHG concentrations in the atmosphere. This is consistent with the UNFCCC definition (Article 1), according to which climate change is ‘a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods’.