As a scientist I've learned never to say "never." So human-caused global warming is always a hypothesis to hold, at least until climate science becomes mature. (Climate science is very immature right now: Physicists just don't know how to deal with hypercomplex systems like the earth weather. That's why a recent NASA scientist was wildly wrong when he called anthropogenic warming "just basic physics." Basic physics is what you do in the laboratory. If hypercomplex systems were predictable, NASA would have foolproof space shuttles --- because they are a lot simpler than the climate. So this is just pseudoscientific twaddle from NASA's vaunted Politically Correct Division. It makes me despair when even scientists conveniently forget that little word "hypothesis.")
OK. The human-caused global warming hypothesis is completely model-dependent. We can't directly observe cars and cows turning up the earth thermostat. Whatever the human contribution there may be to climate constitutes just a few signals among many hundreds or thousands.
All our models of the earth climate are incomplete. That's why they keep changing, and that's why climate scientists keep finding surprises. As Rummy used to say, there are a ton of "unknown unknowns" out there. The real world is full of x's, y's and z's, far more than we can write little models about. How do you extract the human contribution from a vast number of unknowns?
That's why constant testing is needed, and why it is so frustrating to do frontier science properly.
Science is difficult because nature always has another surprise in store for us, dammit! Einstein rejected quantum mechanics, and was wrong about that. Newton went wrong on the proof of calculus, a problem that didn't get solved until 1900. Scientists are always wrong --- they are just less wrong now than they were before (if everything is going well). Check out the current issue of Science magazine. It's full of surprises. That's what it's for.
Now there's a basic fact about complexity that helps to understand this. It's a point in probability theory (eek!) about many variables, each one less than 100 percent likely to be true.
If I know that my six-sided die isn't loaded, I'll get a specific number on average one out of six rolls. Two rolls of the die produces 1/6 x 1/6 = 1/36. For n rolls of the die, I get (1/6) multiplied by itself n times, or (1/6) to the nth power. That number becomes small very quickly. The more rolls of the die, the less likely it is that some particular sequence will come up. It's the first thing to know in any game of chance. Don't ever bet serious money if that isn't obvious.
Now imagine that all the variables about global climate are known with less than 100 percent certainty. Let's be wildly and unrealistically optimistic and say that climate scientists know each variable to 99 percent certainty! (No such thing, of course). And let's optimistically suppose there are only one-hundred x's, y's, and z's --- all the variables that can change the climate: like the amount of cloud cover over Antarctica, the changing ocean currents in the South Pacific, Mount Helena venting, sun spots, Chinese factories burning more coal every year, evaporation of ocean water (the biggest "greenhouse" gas), the wobbles of earth orbit around the sun, and yes, the multifarious fartings of billions of living creatures on the face of the earth, minus, of course, all the trillions of plants and algae that gobble up all the CO2, nitrogen-containing molecules, and sulfur-smelling exhalations spewed out by all of us animals. Got that? It all goes into our best math model. So in the best case, the smartest climatologist in the world will know 100 variables, each one to an accuracy of 99 percent. Want to know what the probability of our spiffiest math model would be, if that perfect world existed? Have you ever multiplied (99/100) by itself 100 times? According to the Google calculator, it equals a little more than 36.6 percent.
The Bottom line: our best imaginable model has a total probability of one out of three. How many billions of dollars in Kyoto money are we going to spend on that chance?
Or should we just blow it at the dog races?
So all ye of global warming faith, rejoice in the ambiguity that real life presents to all of us. Neither planetary catastrophe nor paradise on earth are sure bets. Sorry about that. (Consider growing up, instead.)
That's why human-caused global warming is an hypothesis, not a fact. Anybody who says otherwise isn't doing science, but trying to sell you a bill of goods.
James Lewis is the nom de plume of an academic scientist. He blogs at Dangerous Times.