The above graph compares predictions made by the IPCC (pretty much the consensus) to reality (black line).

You can see the predictions don't match reality very well.

The reality is much, MUCH worse than the scientific consensus predicted. The arctic ice cap is melting decades faster than expected. DECADES.

The "skeptics" implied there are only two possibilities: either the scientific consensus is right, or global warming is not as bad as the scientists think.

Nature had other ideas, as She often does. In this critical area, global warming is happening far faster, and with far more severity, than most scientists had predicted.

But don't blame the scientists. They are very much aware of the weaknesses of their models and theories. And many have highlighted the risk of the climate suddenly "lurching" to a whole new "normal" state.

Jim Hansen has repeatedly warned that the IPCC may have underestimated ice melt. Once again, he appears to have been vindicated.

Science Writer Fred Pearce wrote an excellent book about this issue: With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change. Here's a passage from the introduction, which is an excellent summary of climate scientists' warnings about their own models.

"Nature is fragile, environmentalists often tell us. But the lesson of this book is that that it is not so. The truth is far more worrying. She is strong and packs a serious counter-punch. Global warming will very probably unleash unstoppable planetary forces. And they will not be gradual. The history of our planet's climate shows that it does not do gradual change. Under pressure, whether from sunspots or orbital wobbles or the depredations of humans, it lurches - virtually overnight."

And the next time someone tells you that we are not yet certain about the reality of climate change, or that the effects might not be as bad as predicted, make one simple point:

Uncertainty cuts both ways.

Update: The above image comes from University of Illinois Polar Research Group via Jeff Masters' blog (scroll down). Greenland's ice is shown in white (virtually no visible change, yet). Arctic ice is shown in magenta; thicker regions in darker shades (scale on left). Masters and UIUC have more discussion on their sites.

Update: Those new to all this who want to learn more are strongly encouraged to read the blog RealClimate: Climate Science from Climate Scientists. Also, you can search their archives for their older coverage of climate issues and controversies.

And please don't forget to check out Energize America.