Climate change…refers to a change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. It refers to any change in climate over time whether due to natural variability or as a result of human activity (IPCC 2007, 30)
So, IPCC acknowledged that climate change has both natural and human causes.
UNFCCC (United Nations Framework Convention on Climate Change) narrowly defined as “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” (UN 1992,3)