The low-carbon sector employees a lot of software developers. For example, wind farm operators use SCADA systems to provide data on performance and to inform proactive maintenance regimes; and wind-farm developers use modelling software to design the optimum layout of the windfarms.
If you want to dedicate your career to it, and you don't have success at first when applying to such posts, then I'd recommend a 1-year Masters' course in one of the environmental subjects that most interests you, as a way to make your CV more attractive to employers in the sector. For example, University College London has several Masters that might interest you (disclosure: I have a little involvement with these), such as the MSC in Economics and Policy of Energy and the Environment and the MRes in Energy Demand; Imperial College (also in London) has an MSc in Sustainable Energy Futures, and the well-established and well-respected MSc in Environmental Technology. And the Danish Technical University offers MScs (in English) in Wind Energy and Sustainable Energy, among other sustainable topics.
Voluntary, with possible routes into professional.
There are various open-source environmental projects. One I'm a little involved with is the OpenMod Initiative, which is building an international network of open-source clean-energy modellers - here's a list of some live open-source energy models, including one of my own (NB they do cover quite a broad definition of "open").