not too worrisome just yet...
The climate has changed drastically since the dawn of the earth. In the beginning, there were really high carbon dioxide concentrations in the atmosphere, which gave plant life a big boost. As I'm sure most of you already know, plants produce energy for themselves through photosynthesis, i.e. taking in CO2 from the environment and replacing it with oxygen. Every now and then, some plants (or the animals that ate the plants, etc.) would become trapped under layers of earth, and the carbon they took from the atmosphere (which normally would be replenished through animals breathing oxygen and expelling CO2) became trapped in the earth. Each time this happened, there was less carbon being used in the carbon cycle (well, not technically, but it was buried so that it couldn't really participate in it).
As billions of years passed, enough carbon became trapped in the earth so that the atmospheric levels of CO2 declined significantly (we have fossil fuels thanks to this trapped carbon, hence the name). Without CO2, plants cannot live. With less CO2 (as opposed to the greater amount of CO2 available to them earlier on), plants had to adapt to use less CO2 for energy production (or use less energy, whatever). Thus, the tropical rainforests that the earth used to be covered in due to the abundance of CO2 were gradually replaced by grasslands (grass is a new species that has only come about recently due to the lack of CO2) and other such biomes (as CO2 is a greenhouse gas, this also explains why the earth had a much warmer climate back in the days of dinosaurs).
CO2 is not a bad thing as many people would have you believe. The only problem with it is that we are putting too much of it back into the atmosphere over such a small period of time. This causes rapid climate change, which doesn't give life enough time to adapt to it.
As a side note, I stumbled across something interesting a few months ago. Apparently, CO2 levels in the earth's atmosphere are now so low that if it weren't for human intervention (by putting so much CO2 back into the air), plant life wouldn't be able to utilize photosynthesis in about 500 million years. Without plants, almost all life on earth would die.
In conclusion, I think that global warming would be a bad thing in the short-term, but I believe that it would eventually turn earth back into the warm and tropical place that it was many millions of years ago. Either way, we win =)