Poor things are getting blamed for global warming! The reality is this: the grass they eat would turn into methane anyway - with or without cows.
What is damaging isn't the natural cycle of carbon ie. grass grows, cows eat it, methane etc goes into the atmosphere, it eventually gets absorbed by plankton and other plants, and gets eaten again. The real problem is that humans are digging out the excess carbon that is locked away in the ground ie. coal, oil, gas etc. By burning those materials we are returning the earth to a state it was last in a few hundred million years ago.
Then again, when you think about it, the world used to be very different, and the climate we are experiencing is just one of many global climate phases. And if it is getting warmer now, well, that could just be old mother earth heading into another phase.
A much more important consideration is this: resources. We're experiencing exponential population growth - yet our natural resources are finite. One day we are going to run out of oil - and that could be catastrophic. So we do need to cut back on consumption, and in that respect, I fully support the green movements ideals. Even if I differ in opinion on why we need to do it.