Less than 2C
Arctic sea icecap disappears, leaving polar bears homeless and changing the Earth's energy balance dramatically as reflective ice is replaced during summer months by darker sea surface. Now expected by 2030 or even earlier.
Tropical coral reefs suffer severe and repeated bleaching episodes due to hotter ocean waters, killing off most coral and delivering a hammer blow to marine biodiversity.
Droughts spread through the sub-tropics, accompanied by heatwaves and intense wildfires. Worst-hit are the Mediterranean, the south-west United States, southern Africa and Australia.
Summer heatwaves such as that in Europe in 2003, which killed 30,000 people, become annual events. Extreme heat sees temperatures reaching the low 40s Celsius in southern England.
Amazon rainforest crosses a "tipping point" where extreme heat and lower rainfall makes the forest unviable - much of it burns and is replaced by desert and savannah.
Dissolved CO2 turns the oceans increasingly acidic, destroying remaining coral reefs and wiping out many species of plankton which are the basis of the marine food chain. Several metres of sea level rise is now inevitable as the Greenland ice sheet disappears.
Glacier and snow-melt in the world's mountain chains depletes freshwater flows to downstream cities and agricultural land. Most affected are California, Peru, Pakistan and China. Global food production is under threat as key breadbaskets in Europe, Asia and the United States suffer drought, and heatwaves outstrip the tolerance of crops.
The Gulf Stream current declines significantly. Cooling in Europe is unlikely due to global warming, but oceanic changes alter weather patterns and lead to higher than average sea level rise in the eastern US and UK.
Another tipping point sees massive amounts of methane - a potent greenhouse gas - released by melting Siberian permafrost, further boosting global warming. Much human habitation in southern Europe, north Africa, the Middle East and other sub-tropical areas is rendered unviable due to excessive heat and drought. The focus of civilisation moves towards the poles, where temperatures remain cool enough for crops, and rainfall - albeit with severe floods - persists. All sea ice is gone from both poles; mountain glaciers are gone from the Andes, Alps and Rockies.
Global average temperatures are now hotter than for 50m years. The Arctic region sees temperatures rise much higher than average - up to 20C - meaning the entire Arctic is now ice-free all year round. Most of the topics, sub-tropics and even lower mid-latitudes are too hot to be inhabitable. Sea level rise is now sufficiently rapid that coastal cities across the world are largely abandoned.
6C and above
Danger of "runaway warming", perhaps spurred by release of oceanic methane hydrates. Could the surface of the Earth become like Venus, entirely uninhabitable? Most sea life is dead. Human refuges now confined entirely to highland areas and the polar regions. Human population is drastically reduced. Perhaps 90% of species become extinct, rivalling the worst mass extinctions in the Earth's 4.5 billion-year history.