Climate change is one of the biggest challenges and threats that humanity has ever faced. It has been acknowledged as "one of the greatest challenges of our time" by many organisations including the United Nations. The United Nations Framework Convention on Climate Change (UNFCCC) defines climate change 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". This definition slightly differs from the definition of the Intergovernmental Panel on Climate Change (IPCC), which refers to climate change as "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". The IPCC's definition therefore refers to any change in climate over time, irrespective of the causes, whether due to natural variability or anthropogenic causes.