The phrase 'global warming' brings a slight froth to both far-leftists (who I will affectionately call by their not-at-all preferred name, moonbats) and far-rightists (who will likewise be referred to by that timeless endearment, wingnuts). Moonbats, who recently received instructions from their hive mind to start using the phrase 'climate change' instead and will take great exception to any attempt to use the older term, get a bit religiously frothy about it -- there is no god but Al Gore, and I am his Prophet! Raise the specter of doubt to these rabid fans of (certain kinds of politically correct) science and like as not you'll be faced with a self-appointed Clarence Darrow thundering righteously against a frightening amalgamation of George W. Bush, Adolf Hitler, Ned Ludd, and William Jennings Bryan (also known as 'you'). Wingnuts, on the other hand, will egg you on into their pre-prepared verbal mine-field, citing misleading popular sources and asking probing but utterly loaded questions. The worst of these folks are ace debaters who wouldn't know science from scientology if Tom Cruise picked them up and strangled them with it.
But who cares, right? Someone recently enthusiastically recommended I read an anti-global warming tract written by some guy. I forget his name, but I looked it up at the time. He wasn't a climatologist, or even a scientist. He was a lawyer with a conservative advocacy group. I pointed out to my mine laying interlocutor that it seemed a bit odd to try and refute the significant library of peer-reviewed scientific literature on global warming with a non-technical, non-scientific policy tract written by a conservative lawyer. He responded, and I quote:
"This is not about science." (emphasis his)
It is, though. You've absolutely got to design policy from a knowledgeable standpoint of the underlying issue. What should our policies be with regard to climate change? Yes, it's a cost/benefit analysis -- but those costs and benefits can only be determined if you understand the process under consideration.
So, what does the science say? Here's my take:
There's three distinct issues here: first, do we observe warming, second, can we draw a reasonable conclusion that warming is anthropogenic, and third, how much can we trust the general circulation models? So, let me state that I don't trust the GCMs. A lot of my own research involves dynamical simulations, so I'm wary of trying to make forward predictions for such a complex system. With a GCM, you can't build-a-little-test-a-little with controlled experiments, so you're stuck saying, Well, this matched previous data pretty well. But that can just be curve fitting: you bounced around parameter space until you found a set that fit pretty well, but that doesn't guarantee your parameters are physically meaningful. If they're not, will that model work for making forward predictions? Probably not. In particular, I think there's so many external factors that are not taken into consideration in the GCMs, as well as parametrizations for factors that are included but that we know we're not able to model accurately (the effects of cloud cover, surface albedo, etc.), that quantitative in silico predictions about climate change shouldn't be taken too seriously. It's important to differentiate between doubts about GCM accuracy, which can be well-founded, and saying The observed global temperature data is wrong!, which isn't.
That said, back to the empirical question: have we observed statistically significant warming? Yes, and perhaps more importantly, the observed warming is nonlinear: recent years have seen it accelerate, and the per-continent surface temperature average increases are consistent with increased sulfate aerosols and greenhouse gases. Furthermore, although ocean temperature has been increasing at a slower rate (about half) of the land surface warming, it absolutely is increasing, and this observation isn't limited to the ocean surface -- temperature increases are observed down to depths of several thousand feet. The issue of the lack of a warming trend in Antarctica illustrates two important points -- that global and local temperature trends are often conflated, by people who should know better, and also that Antarctica (and parts of the tropics) has substantial gaps in its historical temperature data set. This data has been 'filled in' with data interpolation and averaging techniques, but in any consideration of Antarctic temperature trends, it's important to keep this caveat in mind.
Second, is this observed warming trend anthropogenic? This is tricky, because you need to de-couple it from natural climate forcings -- for example, obviously the Medieval Warm Period wasn't caused by man-made aerosols. So, you're trying to draw a statistical correlation between anthropogenic forcings (GHGs, aerosols) and temperature, and you've got GCMs to make this link -- and as I mentioned before, I'm leery of the predictive power of these models.
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