Four Degrees

At 4 degrees of warming most of the Nile Delta is threatened by rising seas, as is a third of Bangladesh. Tens of millions more become climate refugees.
West Antarctic ice sheet potentially collapses, pumping five metres of water into global sea levels.
Southern Europe becomes like the Sahara, with deserts spreading in Spain and Portugal.
People move north into temperate refuges in Scandinavia and the British Isles, which become increasingly overcrowded, resulting in further conflict.
All glaciers disappear from the Alps, further reducing water supplies in central Europe.
Permafrost melt in Siberia releases methane and carbon dioxide, meaning that global warming spirals upward.

Notes from Mark Lynas author of Six Degrees: Our Future on a Hotter Planet

Four Degrees video description from National Geographic

For more information on 4 Degrees see also: Download the Climate Action Centre primer "4 degrees hotter"

"Will we live in a world 4 degrees Celsius warmer?" .

If only the present levels of commitments by nations are achieved this amounts to global political failure on greenhouse gas reduction. Scientific imperatives will result in 4 degrees Celsius of global warming by 2100.

In 2008 an influential and controversial paper by Anderson and Bows of the UK Tyndall Centre for Climate Change Research concluded that:

…it is increasingly unlikely any global agreement will deliver the radical reversal in emission trends required for stabilization at 450 parts per million carbon dioxide equivalent (ppm CO2e). Similarly, the current framing of climate change cannot be reconciled with the rates of mitigation necessary to stabilize at 550ppm CO2e and even an optimistic interpretation suggests stabilization much below 650ppm CO2e is improbable.

http://climatecodered.blogspot.com/2011/02/4-degrees-hotter-adaptation-trap.html