Increasing the greenhouse effect by cutting down a shed load of trees and putting fields full of farting cows won't help...
As you increase temp through increasing the greenhouse effect (increased concentrations of Carbon Dioxide and Methane), the Greenland Ice Sheet and Arctic ice melt. Because of the geography of the northern hemisphere, this puts a whole load of fresh water into the North Atlantic, effectively cutting off the Gulf Stream (or at least moving it a whole lot further south). This puts the polar front somewhere near Portugal, and makes our weather sh*te.
I could cite all the evidence for this happening in the past, but I won't bore you with details of it. But if you're really curious, next time you're in your local library look out:
An abrupt climate event in a coupled ocean-atmosphere simulation without external forcing
Alex Hall, Ronald J. Stouffer
In the journal Nature Volume 409 Number 6817 Page 171 - 175 (2001)
The knock on effects of this are that the weather just gets sh*tter and sh*tter. A brief melting of the ice preceeds a build up of ice, increases albedo and makes the planet cool down. Bish-bash-bosh, ice age. Within the next 60-70y (maybe).