I have two goals in mind with regard to the environment:
In some sense, both goals are for improving the environment, but they are different in terms of their risk. A lot of bad things happen because people have grandiose ideas with a large chance of failure. For instance, someone might want to start the next PayPal, become a billionaire, and then start doing philanthropic things, so they end up spending decades without being charitable, investing all of their money into their startup, and not end up with anything to show for it in terms of social impact. I tend to think that if everyone who had a grandiose and deffered idea for social change just did good now, the world would be a much better place. On the other end of the spectrum, prioritizing proven and measured models for improving the world probably wouldn't yield the type of risky, revolutionary ideas that make things 10 times better rather than 10 percent better. In the context of labor conditions, doing no harm means that I don't buy new clothes that might have been made in sweatshops, and doing good means that I donate to poverty reduction nonprofits.
Doing no harm is arguing that people should reliably and verifiably make sure that they aren't making the world worse. Doing good means that, after getting to ground zero, it's probably good to invest some in important but potentially risky projects.
Traditionally, carbon offset nonprofits have been optimized for the former model -- do a lot of verification to make sure that you really are carbon neutral after you give them money. There are a few big organizations doing this, including CarbonFund, TerraPass, and NativeEnergy. Many activist and public policy advocacy organizations such as Greenpeace operate on the other end of the spectrum.
If you have different thoughts on environmental philanthropy, I would love to hear them!