Average global air and ocean temperatures are rising, leading to the melting of snow and ice and rising global mean sea level. Ocean acidification results from higher CO2 concentrations. With unabated greenhouse gas emissions, climate change could lead to an increasing risk of irreversible shifts in the climate system with potentially serious consequences. Temperature rises of more than 1.5–2 °C above pre-industrial levels are likely to cause major societal and environmental disruptions in many regions. The atmospheric CO2 concentration needs to be stabilised at 350–400 parts per million (ppm) in order to have a 50 % chance of limiting global mean temperature increase to 2 °C above pre-industrial levels (according to the IPCC in 2007, and confirmed by later scientific insights).