I was wondering if there really was some natural variation that keeps getting cited by skeptics that predicts a warming? The only such variation I can think of would be the milankovitch cycles, however this predicts cooling not warming at present (as far as I recall).
First off, let me preface this answer with a disclaimer: I do not deny climate change.
From what I've read, the skeptics for the most part don't claim to have an alternative explanation for the recent warming. Their arguments (not mine!) are fourfold:
Those are not my arguments.
With regard to point #1,
The above image speaks for itself. The global temperature change over the last forty years is very real, and is not noise.
That the farmer's field might turn into a mountain or be washed out to sea several millions of years from now is irrelevant. What's relevant is that his good or bad practices have an impact on the world food supply. That nature can do far worse does not negate those bad farming practices. Bad farming practices are bad for humanity.
Getting back to climate change, if higher global temperatures are bad for humanity, it doesn't matter matter one bit how close ice came to the equator or how far north dinosaurs roamed in the past. What matters is that modern humanity is sensitive to climate change, be it natural or induced by humans.
With regard to point #4, it is CO2. It was amusing to follow the skeptic response to Richard Muller's Berkeley Earth Surface Temperature project. Muller was a self-proclaimed skeptic, and he was going to prove those leftist climatologists wrong. The skeptic community cheered at the start. "He's going to prove them wrong!" A funny thing happened on the way to proving them wrong: He proved them to be correct. Muller was an honest scientist in this regard. His own work caused him to switch from being a skeptic to ascribing to AGW.
Many natural cycles are known and understood even though "skeptics" like to point to them. For example, we can predict Milankovitch cycles, and we have good solar records for over 200 years. Although we might not necessarily know the magnitude of these effects a priori, we can look for these patterns in the climate record. eg. Does the Earth get warmer when the sun becomes more energetic, and get cooler when it becomes less energetic.
We do see these effects in the Earth's climate (let's face it, it would a big surprise if the Earth did not get warmer when the Sun got warmer!), but they cannot account for all of these changes. For example, when the Sun might become less energetic the Earth does not cool as much as it warmed when the sun became more energetic.