This seems to be a key point that is obfuscated or ignored by deniers. That there are gigantic sinks of fossil carbon, in addition to the fossil fuels we bring up, that have been sequestered safely under the conditions prevelant during all of human civilization. Many of these are predicted to, or have already begun being released, as global temperatures rise. Positive feedback means the more it happens, the more it will happen, until the system spirals wildly out of control.
Example of this are the permafrost areas at the edge of the Northern polar region, the Methane Clathrates, reversal of the Amazon rain forest from a carbon sink to a source, and the greater absorption of heat from sunlight as melting ice and snow changes Earth's albedo.
And here is what is meant by irreversible--that these feedback loops will accelerate and cause massive disruption in time scales directly relevant to human civilization--i.e. the next few centuries, at least. Deniers like to bring up crap about there having been massive changes in temp all throughout the billions of years of the fossil record, so we should all relax. That is true, but in each case, these changes caused massive dislocation or extinction of the dominant plant and animal species of that time.
We're talking about what happens to this particular race of animals that is here spending its time reading and writing Slashdot postings. It is disingenuous for deniers to claim some lofty halo of wisdom that transcends mere human-centric concerns, especially when, for the most vociferous of them (such as the paid lackeys of Exxon-Mobile), they are more concerned with making a buck in the next fiscal quarter than they are about the concerns of even the humans alive today, much less those not yet born.