Except that - even if you agree that climate is changing, which seems to be indisputably true - it's still rather humanocentric to assume that we "could have changed something".*
* First, you have to assume that global warming is driven largely by humans; considering that about every 120k years there's a sudden spike in temps and CO2 and the last one was...120k years ago, I think the burden of proof (that this one is caused by SUVs and Republicans, when none of the previous dozen were) is still on the AGW folks, depite the politically-driven IPCC reports.
Second, you have to assume that the warming isn't simply intrinsic to how we live in the 21st century, like the concomitant carcinogens that are consumed with cooked meat. Sure, if we killed off half of humanity it might reduce warming significantly, is that worth it? We could all live like hunter gatherers, but is it worth it?