Global Warming

I've written before, briefly, about global warming. With Al Gore's movie coming out soon, a lot of people are talking about it, so let me join the fray. I understand this is a political issue, but I want to focus purely on the science side. I won't argue a case, but rather express some questions in the hope that some reader will provide links to additional information.

As I wrote before, the primary evidence I have seen for global warming (hereafter GW) are plots of temperature over time or measurements that show glaciers receding, or similar. These basically show the planet is getting warmer. Wait a minute, though. GW is not a theory that says the globe is warming. Rather it is a theory which attempts to explain why and how the globe is warming. So a plot that shows the earth's temperature rising is not proof of GW, but is instead a statement of the phenomenon GW purports to explain.

Evidence for GW would be to superimpose the predictions of GW on the observed data to show that the theory tracks the observations well. This plot would be accompanied by a statistical analysis to quantify just how well the two match. My understanding is that this is, in fact, a problem because the predictions don't track with observations. (I can't provide a link to prove this, but it's something I remember reading. My recollection is that the theory predicts a sharper rise in temperature than is observed.) As a scientist, what should be the response to a theory that doesn't match nature? Skepticism.

And what does it mean to say the earth is getting warmer, that the temperature is rising? What is this quantity that is being called "temperature"? I know what temperature is. It is a scalar valued function of four variables: space and time. If I measure temperature at a specific spot in my backyard at 3 AM and 3 PM today, the values will most likely be quite different. If I measure at the same spot at 3 PM on Christmas and 3 PM on July 4, the values will almost certainly be dramatically different. So temperature varies in time, quite a bit in fact. If I measure temperature at 3 PM GST today in Nome, AK, Paris, France, and Baghdad, Iraq, I will get different values. So temperature varies according to position on the globe. And finally, if I measure the temperature at 3 PM GST in Baghdad, both on the ground and at 50,000 feet elevation, I will get very different answers. So temperature varies according to altitude as well.

Hence, variability in 3 spatial dimensions as well as time. Then how does one reduce this 4 dimensional function to a simple value varying in time? How does one eliminate the variability in spatial dimensions, as well as the short term temporal variability, which can be much larger than the long term temporal variability (swings of up to tens of degrees in a single day versus one or two degrees over the years)?

And why is it that the climatic theory du jour 30 years ago was that the earth was headed into an ice age, and today it's that we're getting warmer? If the warming trend just started in the last couple of decades, doesn't that pretty much invalidate GW which attributes warming to the build up of greenhouse gasses, which have been accumulating for over a century?

Update 5/19/2006 I found this page which talks about global warming models and matches to prediction. Interesting comments.

Can we match the observation of temperature trends with the model predictions? The temperature record of the past hundred years does show a warming trend, by approximately 0.5 degrees C. However, the observed warming trend is not entirely consistent with the carbon dioxide change. Most of the temperature increase occurred before 1940, after which Earth started to cool until the early seventies, when warming resumed. Carbon dioxide, on the other hand, has been increasing steadily throughout the past century. Other factors that could have affected climate during this period include the possible change in the solar energy reaching Earth, the cooling effects of volcanic aerosols, and the possibility that sulfur dioxide and other pollutants might be affecting the amount of solar radiation that is reflected back to space. Some of these effects can cause a cooling that could counteract the warming due to carbon dioxide and other greenhouse gases. All of these effects would have to be taken into account and appropriately modeled in order to predict the changes that one might expect in the next century.

This shows one of the big problems I would expect to find in this area. There are numerous natural phenomena which can cause climatic variation, contributions which have to be modeled, a difficult task. He also makes the comment that he earth was, in fact, cooling until the 70's, which might explain the fact that climatologists a few decades were worried about an ice age, not global warming.