[lutorm]

It doesn't matter if 99.9% of greenhouse effect is not caused by humans, if the extra 0.1% is. The gross heat fluxes in and out are enormous, but because they precisely balance they're not noticeable. An extra 0.1%, while very small compared to the total heat flux, is a purely net flux because it's not canceled out (until the Earth has warmed). It's simple conservation of energy.

Think of it this way: If you have a gallon of water and you pour it back and forth between two containers, you will eventually run out of water even if you lose only a few drops each time.

Another example: If you trade your assets back and forth between different stocks (and they don't change value), it doesn't matter how much money you start with, if you do it for long enough you will eventually run out of money due to the transaction cost.

A small effect can have a big impact if it's allowed to accumulate over time, and that's precisely what's happening with the greenhouse effect.

[lilsis]

This is a good argument assuming that the Earth's heat fluxes are in balance, but I don't believe this to be the case, what with ice ages, etc.