Perhaps I'll start with water.
As you know the Earth consists mostly of water,but liquid 60% percent of the water.The remaining 40% are in samarasena condition in Antarctica,Alaska, etc...
Now all the buzz about global warming and it's actually a problem and not just fiction in 2012.The whole point of warming is that we kind of raped the protective shell of the earth and the sun's rays without problems fall down.
Now distances and sometimes covering the Earth from the sun.It's not as big and without the "shell" temperature can be 10-60 degrees all the time!And in a more warm places about 100!At least it's not really for life.Now most of the "protection" is still there,but with the pace it will disappear by 2100,and even before.
If the average temperature but will grow 30-40 degrees glaciers completely rostat much faster.Instead of 20,000 years will need a total of about 600 years,which means that 2700 year, the world will be covered with water almost 100%
Of course, the Apocalypse is not in danger,but if something doesn't change ,the end of human existence on Simle remained not so much time.