Very good question. I totally believe that, as a developer we can contribute to this day or not just this day but everyday in many ways. Like the idea of smart light switches, saving energy. As a developer, I can think of creating an application which will keep us updated about our contribution to global warming. Example: if we use a car for shorter distances which could be easily replaced by a cycle/walk, it will give us an overall estimate of the air pollution (we have caused), same goes with electricity, water, etc. If we can connect our consumption to an application and see the graph and analysis, I believe on every individual level we will have a good realization and might reduce our contribution to global warming.
This is just an idea, might be useful, might not. I look out for things to do, which helps me in keeping our environment clean and green. But, this isn't daily, sometimes, I do things (accidentally) which will harm our environment but, if we could get a daily statistic report on our app, I guess there will be a decent improvement.
Again, this is just a simple idea (might work out).