Over and over at the U.N. climate summit in Glasgow, world leaders have stressed the need to limit global warming to 1.5 degrees Celsius.
The 2015 Paris Agreement commits countries to limit the global average temperature rise to well below 2°C above pre-industrial levels, and to aim for 1.5°C.
Already, the world has heated to around 1.1°C above pre-industrial levels. Each of the last four decades was hotter than any decade since 1850. "Climate change is already affecting every inhabited region across the globe," said climate scientist Rachel Warren at the University of East Anglia."For every increment of global warming, changes in extremes become larger," said climate scientist Sonia Seneviratne at ETH Zurich.An extreme heat event that occurred once per decade in a climate without human influence, would happen 4.1 times a decade at 1.5°C of warming, and 5.6 times at 2°C, according to the U.N.
But blow past 2°C and the ice sheets could collapse, Mann said, with sea levels rising up to 10 metres - though how quickly that could happen is uncertain.