If you don't prevent global warming, where are you going to rehouse the 100 million people who currently live in the Philippines (which, because all of the islands are so low-lying,face total destruction)? How are you going to rehome all of the people who currently live in much of south-east England, when much of London, and the surrounding area, is lost to flooding? How are you going to deal with the mass famines that will threaten the lives of around two billion people? How are you going to cope with massive health impacts of global warming? (http://www.psr.org/environment-and-health/climate-change/climate-change-resources.html ). How are you going to deal with all of the wars that will break out as nations fight to get access to limited supplies of water?
Oh, and while you're sorting all that lot out, please remember than we are just one species on this planet amongst millions of others (none of which are responsible for global warming). At least a third of those other species face being forced into extinction through global warming. (
http://www.worldwildlife.org/threats/effects-of-climate-change )
As you state, our planet has always had a constantly changing climate but species (including man) had hundreds of thousands of years of evolution to cope with each major change. Such evolutionary changes can't come about within the (roughly) 50 year safety zone that we've got left.