>As an abstract idea I think there is a reasonable argument to be made that the size of any contribution to a problem should be measured as a relative proportion of total influence.

Right, I think you have responsibility for your 1/<global population>th (arguably considerably more though, for first-worlders) of the problem. What I see is something like refusal to consider swapping out a two-stroke-engine-powered tungsten lightbulb with an LED of equivalent brightness, CRI, and color temperature, because it won't unilaterally solve the problem.