Until humans, the general gaseous air composition was controlled almost exclusively by bacteria, with plants (mostly algae) having a lesser effect on carbon and oxygen levels. Animals didn't really get much of a look in until humans started releasing all the locked up carbon in fossil fuels.
Bacteria that are currently contributing to global warming are the methanogens, most notably those in the gut of ruminant mammals (i.e cows, sheep and other edible things). Cows and sheep can't break down cellulose in the plant material that they eat, so they have bacteria that do it for them. Unfortunately this process releases huge amounts of methane, and methane is around 20 more planet-warming than carbon dioxide.
When I went to Copenhagen last year (I didn't go for the conference, in fact I didn't realise it was on until I started wondering why it was so hard to find a hostel room!) someone handed me a leaflet saying that climate change could be prevented if everyone in the world became a vegetarian. It was an ... interesting point of view, but you could see where the idea came from. Cows are little methane factories.
However bacteria are also heavily involved in keeping climate change under control with photosynthesis, which uses up carbon and releases oxygen into the environment. Despite being very leafy and green, forests (even rainforests) tend not to be huge carbon sinks, they take up carbon during the day certainly, but at night they respire and use most of it up again, and anything they've stored tends to be released once they die and decompose. Marine cyanobacteria, however, take in carbon like its going out of fashion, and when they die they sink down to the bottom of the ocean and lock it all away in calcified rocks. One of the most prolific carbon-eating bacteria is Prochlorococcus. Around 100 million Prochlorococcus can be found in every litre of seawater and, along with fellow bacteria Synechococcus it removes about 10 billion tons of carbon from the air every year.
In terms of helping to moderate climate change, there are plenty of ideas floating around as too how bacteria could be useful, but one of the more helpful ones is trying to make a bacterial-based carbon neutral biofuel. The idea is that if you find bacteria that take up as much carbon for their growth as they release while being used as fuel they are technically 'carbon-neutral'. You can grow pretty much anything in bacteria, up to and including oils that can drive cars, it's just currently not very efficient.
Whether or not anything can be done to stop climate change (or, more importantly, whether or not people can agree to do anything) may be an unresolved issue, but its becoming clear that the issue of how the worlds climates are changing is a subject for microbiologists and plants-scientists as much as for meterologists. Whatever happens to the climate in the future, bacteria will still have a large part to play.
Exams and course-work are all over, so from now on I am hoping to keep this blog purely for the prokaryotes: