One of the very few reasonable points made by climate skeptics is that it’s hard to have a great deal of confidence in computer-model predictions of a system as complex and varied as the global climate system. It is comprised of several subsystems — the ocean, the atmosphere, the cryosphere, and the biosphere — each extremely complex in its own right. There is some reassurance to be had in hindcasts and other modeling successes, not the least being the triumph of model predictions over the contradictory satellite records.

But there are really so many unknowns, both the known unknowns and the unknown unknowns, that it is reasonable to cast a suspicious eye on a forecast of global mean temperature in the year 2100. I think you’d be hard-pressed to find a climate scientist who would not admit it.

But the obfuscators and denialists fail to realize something critical: in decision-making, especially when potential futures are extremely bad, uncertainty is not your friend.

"Look!" they gleefully point out, "even the IPCC says it may only warm 1.5oC, and even that might be too high! Why all the worry?"

Well, the worry is because "even the IPCC" says the warming might be as much as 4.5oC, and that might in fact be too low! The worry is that maybe, just maybe, the computer models are wrong … and it will be worse!

A simple analogy can make the point about regret and decision making more clear. Say you are at an airport on a very important business trip and they announce there may be some trouble with the plane. The mechanic comes back and tells everybody, "I’m not totally sure what the problem is, but in my expert opinion there is only a 50/50 chance the plane won’t make it." Are you telling me you would hop on, thinking "well, this is an important trip, and not only does the expert say everything may well turn out all right, but you know, experts can be wrong and he’s not even sure"? I don’t think so. Most of us would hightail it out of that terminal if there was even a 1% chance of going down in flames. Would you even roll those dice if your odds were one in a thousand?

Make no mistake, 5oC warming in 100 years is going down in flames, on a global scale. I would like to know if Richard Lindzen, Patrick Michaels, Fred Seitz, Bob Carter, or any of that bunch would swear under oath … well scratch that, we’ve seen some of what they will swear under oath.

But I would like to know why anyone in their right mind should think this handful of contrarians, many of whom were wrong about the ozone hole, and about tobacco not causing cancer, should give all the rest of us the 99% confidence the worst will not, can not come to pass, required to justify business as usual.

But I have digressed. What I wanted to say is that leaving aside computer models, there is in fact another source of knowledge about possible futures, and that is actual pasts. There is an event in the geologic history of our lovely planet called the Paleocene-Eocene Thermal Maximum. Wikipedia has an excellent article on it here. The basics: a large and sudden rise in GHG causes a dramatic spike in global temperatures, a sudden acidification of the oceans, and an accompanying mass extinction of mammals and marine life. It took the earth approximately 100,000 years to recover back to its previous balance, and much longer for the negative biodiversity impacts to reverse.

Which finally brings me to what prompted this post in the first place: a lengthy and interesting post on ScienceBlogs by GrrlScientist today about The Great Dying, also know as the Permian Extinction. This was the greatest of the five major extinction events and saw the loss of an incredible 96% of all marine life and 70% of all terrestrial life. There have been many candidate explanations proposed, but some new research lends more weight to GHG-driven climate change (read the post!). The source in this instance was volcanic, but it matters little if the CO2 is natural or anthropogenic — the sobering lesson is its effect. On my own blog, I wrote a shorter piece, mainly about the absurd position that this news is reassuring! (Read it here to see where that bit of illogic comes from.)

Now, there are clearly going to be many other differences between today’s changes and those that took place some 250 million years ago — differences in the duration of the build-up, differences in the amount of carbon thrown into the atmosphere, differences in the configuration of the continents, differences in the polar ice caps. Some of these differences will be mitigating and some will be exacerbating. But the lesson is that such an ultimate calamity is not outside the realm of possible futures. If it happened before, it could happen again.

Are we 100% certain this is not where we are headed now?