Your personal action affects only your own carbon footprint, and if you somehow eliminate it entirely, you alone can reduce carbon emissions by 16 tonnes per year.

However, by funding climate research, educating the public, and most importantly: contacting your lawmakers, you can affect the footprints of many thousands or potentially millions of people. If you do even 0.001% of the work required for getting a law passed that cuts Shell's emissions by only 1%, you will have reduced the global carbon footprint by approximately 90 tonnes per year (58000000 * 16 * .01 * .00001 = 92.8). That's more than 5 times as effective a use of your time, assuming you were able to do each with an equivalent amount of effort.

Vote with your wallet, yes, but NEVER underestimate the power of campaigning for change. A person's actions carry further when they affect the actions of others.