It cannot explain why, before the days of man-made CO2, the world wasCommonly, this sort of argument will be followed with a quip along the lines of "Dinosaurs / Romans / Medieval kings didn't drive 4x4s and fly about in Lear Jets." But the answer is very, very obvious. The science that underpins anthropogenic global warming does not attempt to explain previous variations.
warmer during the Middle Ages, Roman and Minoan warm periods. (1)
Put simply, there is lots of stuff that can change climate. On the macro scale, the biggest influence on the climate is the sun. Variations in the Earth's orbit around Sol are responsible for kick starting ice ages, and for ending them. On a less dramatic scale, solar activity influences climate. So do weather patterns such as el Nino and la Nina. And human activity, resulting in the release iof CO2 into the atmosphere and forcing warming, also contributes.
The coincidence of significant solar activity, el Nino weather patterns and anthropogenic global warming lead to a searing month in 1998. The switch to a cooler la Nina pattern, and reduction in solar activity, means subsequent years haven't been quite as hot. But as they haven't been significantly cooler, the contribution of anthropogenic warming to the sum is obviously considerable.
But, as deniers love to point out, those dinosaurs / Romans / Medieval kings didn't drive 4x4s and fly Lear Jets. So how come warming. Answer simple - all the other stuff that makes up our climate was going on. Sometimes it got hot, sometimes it got cold. But now it is getting hotter and hotter, in a manner that is not consistent with natural climate variation - the stuff that should be heating or cooling the planet is being overshadowed by something else, which is complementing the natural warming tendencies, and countering the natural cooling tendencies (2). There is no reasonable mechanism that explains current warming, other than anthropogenic activity putting more CO2 into the atmosphere.
So the theory that accounts for current warming (and no, not all of it - just the significant part that can't be attributed to natural causes) doesn't account for previous variations. It doesn't try to. It would be outlandish if it did. It's even more outlandish that deniers pu up that sort of arguement and expect it to be taken seriously.
1 - 'Powering our future or wrecking the economy?,' by Brian Leyland in the NZ Herald, 7th of January, 2008. (http://www.nzherald.co.nz/section/3/story.cfm?c_id=3&objectid=10485514)
2 - As described previously on lefthandpalm: http://lefthandpalm.blogspot.com/2008/04/lurgees-paradigm-planet-isnt-warming.html