The short reason:
__________
The longer explanation (at YouTube):
https://www.youtube.com/watch?v=OPRUggIb-NI
__________
In Ward’s view, the two most urgent problems surrounding human caused climate change are the following:
- Mass extinction. When the poles heat up (as has happened in other periods of Earth history), and they more closely match the temperature at the equator, the circulation of wind and water slows down, making for anoxic ocean conditions (water depleted of dissolved oxygen; akin to a standing pool of water in your backward). For multicellular creatures in the oceans to thrive, water has to circulate vigorously.
- Sea level rise. Three to six feet this century. And a complete melting of Antarctica over the next several centuries would raise sea level 240 feet, putting Manhattan (for example) 100 feet underwater.
Prometheus Unbound 
I do not believe there will be mass extinction. Civilisation will be fragmented and pockets of survival will cling on, life is like that it adapts when forced to do so. Our civilisation is very fragile and depends on electricity communications will be decimated. Our friend the rat will prosper well, as he now does. We must remember the problem has been largely caused by technology. The worst invention was the internal combustion engine but progress would have come one way or another.
Magno,
Of course there will be things that survive–including, probably, us–but a mass extinction event suggests that perhaps 50% or more of existing species will go extinct, including mass ocean extinctions from reduced water circulation and the chemical changes that accompany this.
What this shows is that we do not control our own destiny as a species. We can see what is going to happen, we know what to do and yet we are powerless to change our behaviour.