I guess it's just another example of trying to describe a hideously complicated, feedback-driven system, in terms of a very few easily measured parameters.
Economists had to admit their failure on this score back in the mid-70s -- and economies are nowhere near as complicated as the Earth's climate and ecosystems.
Article:
Now it seems that deforestation - that bête noire of the environmentalist movement - may have helped combat the rise of this greenhouse gas. While no one is suggesting chopping down the world's trees to save the planet, the new research highlights the astonishing complexity of environmental science. Measures to combat climate change that once seemed simple common sense are turning out to be anything but. I think we need more roads and parking lots, especially in Atlanta. If 16 lanes can't be made to work, I say let's have 28 lanes. So to counter the above, I will be the first to dispell the "no one" assumption above.