I have always felt that a rise in global temperatures was the least of our worries about increasing CO2 in the atmosphere. I’m much more concerned about how it could perturb the delicate balance that allows mammals to live, i.e. us. One of the things that could be trouble is that CO2 dissolved in water can make the oceans more acidic by forming more carbonic acid, which could make it harder for marine creatures to make shells through calcification, which in turn could have a large impact on the coral reefs and the ocean food chain.
Another thing I worry about is that our oxygen supply could decrease. Although the direct effect of converting oxygen to water and CO2 through increased combustion of fossil fuels is small, the effect on photosynthetic organisms that make our oxygen is largely unknown. I’ve actually been somewhat optimistic on this account thinking that since we are introducing more nutrients into the oceans and CO2 is increasing then perhaps phytoplankton, which make much of our oxygen and is a blanket term for photosynthetic microscopic sea organisms like cyanobacteria and dynoflagellates, might increase. However, a paper in Nature this week, says otherwise.