[superluserdo]

>But day after day, over decades, surely the minuscule amount of heat being generated by activity on the surface has SOME cumulative effect.

No it doesn't, because the heat escapes out into space, so the minuscule effect is only an immediate one, not a cumulative one. That's why the greenhouse effect, being cumulative, is much stronger. If I put on an extra coat every day, it's not the heat output from my muscles in putting the coats on that is making me feel hot, it's the increasing number of coats that I'm wearing. Likewise, if I burn a tonne of coal, then that heat will have essentially disappeared overnight, but the global warming-causing CO2 will stick around in the atmosphere for another [very big number] years.