It may be caused by natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions and persistent anthropogenic changes in the composition of the atmosphere or in land use. The United Nations Framework Convention on Climate Change makes a distinction between climate change attributable to human activities altering the composition of the atmosphere and climate variability attributable to natural causes. For more details, see the glossary of terms in IPCC (2014b, annex II).