I really don't care about global warming. I started walking to the grocery store and later riding my bicycle to pollute less. Automobile exhaust stinks and the hydrocarbon pollutants are bad for people. Cars take up a lot of space and resources to manufacture. Manufacturing causes more air pollution.
Cars are great tools. They just aren't used to their full capacity all of the time. Thus they are wasteful.
Using my body to transport myself makes me feel good physically and mentally. Not putting money into a car lets me survive on less income. It also disentangles me from some government regulations.
So I started doing it for the environment and other benefits came along with it which are also appreciated.