Ok, I'd wanted to save this one till a few weeks out and do Has The Oil Spill Gone Away? I Don't Hear About It Anymore or maybe More Bedbugs first, but I'm a busy graduate student and this one I can do based on a project I did as an undergrad. Anyway, it's not as if it weren't topical....
The great thing about this question is that it's the wrong question. I'll get to why it's the wrong question in a minute (and I'll address some corresponding right questions), but this--what the question is--is important, so we'll stay here for a bit, ok?
See, questions are almost as important than answers. Maybe they're more important. My adviser, who is a lot like Yoda, except with better grammar and smaller ears, and is really good at wise one-liners, says "you might get a great answer to the wrong question." Scientists, who are really good at getting great answers, are prone to this. If you don't get the question right, you can't get the answer right, because the question is like a nozzle pointing the waters of the Why River in a given direction, and if you squirt your curiosity over here you just won't get the flowers of understanding over there watered. Sorry. Or you can just clog up your hose to begin with by asking a question with no answer at all, like if you ask an innocent husband (or an unmarried jackass) "have you stopped beating your wife yet?"
There's no shame in asking the wrong question, unless you do it on purpose, and I have to say this is a particularly fascinating wrong question, because it's a hybrid type. Like Yoda's Warning, it has an answer (the answer is "yes, probably") that fails to satisfy because it doesn't get at the heart of the issue. Like the Accusation of the Innocent Husband, it depends on a context that, being wrong, renders any answer meaningless.
Context is what allows you to answer a question like "where is the cat?" which makes immediate sense if you know what cat I mean, but not otherwise. If the context of the question is I saw movement in the bushes, the context could be wrong if the movement was actually a skunk; you can't tell me where the cat is, because there is no cat in that context.
Here, the context is the well-publicized prediction by climate scientists that the planet will get warmer if we keep adding greenhouse gasses to the atmosphere; every time we get a serious heatwave we ask ourselves if this is global warming or if it is just natural variation. Whether the theory of global warming is right is debated in a variety of public media, with proponents pointing out (correctly) that the vast majority of scientists now agree on this, and opponents pointing out (correctly) that science is not a democracy. All of this is subtly, but definitely, wrong.
The planet has been getting warmer, more or less, for close to two hundred years. That the globe is getting warmer is not a theory at all; it's data. Theories are what explain the data, and predict what will happen next. The current popular debate conflates a number of related, but properly very distinct, ideas; global warming, the greenhouse effect, and anthropogenic (human-made) climate change. As long as these issues are mixed up, any questions that come out of the debate will be impossible to answer, and the debate will go on indefinitely.
Briefly, global warming means the atmospheric temperature averaged over the whole planet is getting warmer. Unless you doubt the capacity of meteorologists to make and analyze measurements in an intelligent way (and I wouldn't; weather predictions, which were once a joke, are getting startlingly accurate) this part isn't contestable. The difficult part is that we don't experience average temperatures, we only experience the specific temperature wherever we are, and that varies all over the place making it difficult for individuals to tell what the average is doing. It is impossible to say whether any particular heat wave, or any particular hurricane, is part of global warming, and in a way it isn't a fair question because global warming is a pattern and as such isn't visible unless you look at the whole picture.
The greenhouse effect is the popular name for a property of several atmospheric gasses, notably carbon dioxide, methane, and water vapor, that trap heat in much the same way the glass or plastic panes of a greenhouse do. This, also, is incontestable; the amount of solar radiation the Earth receives isn't enough to explain the temperatures we experience. The greenhouse effect is also not a new idea; that carbon dioxide does this was discovered in the 1800's. It's just something we know about the gas.
Anthropogenic (human-made) climate change is the real subject of debate--that and what is going to happen next, which nobody really knows, although there are some very educated guesses. Is the warming we have measured the result of something we, as humans, are doing? Or are we watching some other, more natural cycle in play? A more accurate way to phrase the question is how much of what we're seeing is natural, and how much is not, because other cycles are always in play, and the climate is very complex. It's like if you smoke cigarettes and you get respiratory problems sometimes--well, everyone does, everyone catches colds now and then. There's no reason to believe that if you didn't smoke you wouldn't get sick, because there are lots of reasons to get sick. Even if you smoke, you are still subject to all those other reasons for getting sick--but on top of those reasons is smoking. It makes no sense to ask "am I sick because I smoke?" The better question is "how much of the reason for my sickness is my smoking, and how much is other factors?" This question does have an answer, and I even almost know what sort of statistics you'd need to find it, but good luck figuring out what the answer means unless statistics really make sense to you. A better question yet, for most people, is "if I quit smoking, will I be healthier overall?"
Conflating these three issues allows the national conversation to get hung up badly; the real uncertainty over how much of our current situation is explained by the anthropogenic greenhouse effect, as opposed to other factors (and the uncertainty about what is going to happen next) gets mixed up with the question of whether the greenhouse effect or global warming are real to begin with, producing an intractable muddle where the things we do know are treated as doubtful instead of being used as the basis of an investigation of what we don't know.
So how uncertain is this uncertainty? For our purposes, not very. We don't need to know how much of our climate problems are explained by this cause or that cause, nor do we really need to know which of the various dire predictions is most nearly right. We can leave those issues to the professionals who do understand statistics and who are interested in the details for their own sake. All we need to know is whether the benefits to be gained from changing how we live outweigh the costs of making those changes, and to figure that out we need a rough idea of how reliable current climate science is; the real question, the right question is "is what we're doing to the sky really going to cause a disaster if we keep doing it?"
Consider; we know certain gasses cause the greenhouse effect. We know humans are creating large quantities of several of those gasses. We also know the atmospheric concentration of those gasses is increasing, as is the average temperature of the planet. Sure, it's a possibility that something else is causing the warming--but if the increased concentration of carbon dioxide and other gasses isn't doing it, why isn't it? If you pour a gallon of water into a bathtub and the water level rises, sure it's possible the water level rose for some other reason (maybe the tub is outside and it's raining?) but in that case what happened that the water you poured in didn't cause the water to rise? Likewise, some other source of gas could be effecting the atmosphere--volcanoes, for example--but if human industry, which we absolutely know creates this stuff, isn't changing the atmosphere, where is all the greenhouse gas we make going?
Remember when I said not to take my word for anything? Well, don't do it now, either. Check out "Global Warming, the Complete Briefing," by John Houghton. It's getting a bit dated (a lot more information has come in since the book was published), but that book does an excellent job of explaining what kinds of information climate scientists actually look at and how. All the factual statements in this essay can be found therein, and reading it will equip you to sort this stuff out for yourself.
And if things get too hot, don't forget to cool off by taking a dip in the Why River.
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