- Is it happening?
- If so, what is causing it?
- Can human activity be causing it?
- Whether or not humans cause it, could we stop or reverse it?
- If we can, should we?
1. Is global warming happening?
YES. The only person who could possibly deny this part is the great Dubya himself. Well, and a few woo-hoos who also believe that the Apollo program was faked.
The actual evidence is very strong. We have direct thermometer measurements of the temperature since the mid-1800's, and the average temperature has increased by a full Celsius degree since then.
Prior to then we need to use indirect methods of calculating the temperature or climate. The most interesting (and least believable) one I've seen is a study of the backgrounds in historical paintings. During what was known as the Little Ice Age, paintings showed more clouds than they did before and after. Like I said, I'm skeptical of it. A better method is tree rings: the thickness of a tree ring indicates how good the growing season was for the tree, and this includes information such as the rainfall and temperature. That only goes back as far as the oldest tree we've cut down, so longer ago than that we need to measure ice core samples - the thickness of ice that's been laid down in glaciers over the last millenia. Graphs of these data show that prior to the 1800's the Earths' temperature was relatively flat. In other words, the temperature has jumped since the 1800's. Global warming IS happening.
2. What is causing global warming?
Probably a runaway greenhouse effect, or maybe increased input of energy (i.e. solar output).
In moderation, the greenhouse effect is a good thing. Without it, we'd be as cold and lifeless as Mars. In an actual greenhouse, light and heat from the sun pass in through the glass, but not all of it makes it back out through the glass. The excess light and heat reflect around inside the greenhouse and heat it up. Eventually the temperature does stabilize at a higher temperature than the outside air.
On the Earth, it's the atmosphere that keeps the heat and light in. You have witnessed this yourself - the easy one to visualize is how on clear winter nights it's colder than on cloudy winter nights. This is because the clouds help trap more heat than the clear air. Same reason why deserts are frigid at night. An everyday example though, is "Why is the sky blue?" Because of the light from the sun that gets scattered in all directions. The blue in the sun's light gets scattered more than the red, so the blue looks like it's coming from all over, while at sunset the blue goes away from the direction of the sun leaving it looking red.
What makes the process "runaway" is when too much heat and light gets trapped. Specifically, we know that chemicals such as carbon dioxide (CO2) are good greenhouse gasses. Like historical temperatures, we have historical records of carbon dioxide only for the past 50 years or so. Those show CO_2 increasing, which seems to confirm the hypothesis that it causes global warming. Another piece of evidence is, well, can you think of something important that happened in the mid-1800's? The industrial revolution and steam power (which releases CO_2). That led to cars (1910's) and even more CO_2. In fact, we can today measure how much CO_2 the world creates, and it matches reasonably well with how much we observe in the air. It is currently the leading theory of the cause of global warming.
However, if you've been reading this blog a while you know that scientists can never be 100% sure of anything. A competing hypothesis is that the sun has been getting brighter. If there's more light coming into the greenhouse, of course it'll get hotter. Again, we only have limited data, which so far is inconclusive, so it is unfalsifiable with current data.
3. Can human activity be causing it?
Yeah. Ditto above. There's evidence pointing that way, but it's "only" a theory.
4. Whether or not humans cause it, could we stop or reverse it?
Yeah again. We know that CO_2 at the very least adds to the greenhouse effect and global warming, so if we reduce CO_2 emissions as per the Kyoto Protocol, we'd help to reduce global warming.
5. Regardless of whether humans caused global warming, if we can stop it, should we?
Probably. Regardless of whether a kid knocked out your window with a baseball or a tree fell on your house, should you replace the window? Probably.
If global warming's a Bad Thing, it doesn't matter where the blame lies, we need to fix it. And yeah, for the most part it IS a Bad Thing: we're losing whole islands in the South Pacific, whole glaciers and the ecosystems they support, and even the winter sports industry is being hurt. :-P
On the other hand, there's probably a few PETA members out there who say "if it's natural, it's not our place to interfere." While I can understand that point of view, I'm not sure I agree. If a killer asteroid were heading towards the Earth, I'd sure as heck wanna find a way to stop it and save us.