I'm adding this because too many people are completely unfamiliar with the IPCC, who they are, what they do, what it concludes and why they reach their conclusions. Reading this FAQ a couple days ago made me think I should include this particular FAQ because it responds directly to a favorite Republican/libertarian claim, one that AL parroted, namely thinking that recent warming is nothing unusual… so, don't worry about it.
The following is copied directly from the IPCC's pages. The text is complete and unaltered, though I have added paragraph breaks and highlights where I thought appropriate.
You could say that the following explains the considered (and learned) expert opinion, often derided by hostile politically motivated cynics as the "consensus."
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Frequently Asked Question 6.2 - IPCC 2007
Is the Current Climate Change Unusual Compared to Earlier Changes in Earth’s History?
Climate has changed on all time scales throughout Earth’s history.
Some aspects of the current climate change are not unusual, but others are.
The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years, and has done so at an exceptionally fast rate. Current global temperatures are warmer than they have ever been during at least the past five centuries, probably even for more than a millennium.
If warming continues unabated, the resulting climate change within this century would be extremely unusual in geological terms. Another unusual aspect of recent climate change is its cause: past climate changes were natural in origin (see FAQ 6.1), whereas most of the warming of the past 50 years is attributable to human activities.
When comparing the current climate change to earlier, natural ones, three distinctions must be made. 