The short reason:
The longer explanation (at YouTube):
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.