George Will recently had a column in the Washington Post about global warming. Actually this was the most recent of several on the subject he has published over the years. He is a skeptic. One could go on all day about the various points he brought up in the article, but possibly the main one was toward the end where he has a paragraph about "global cooling" that was predicted in the 70s. If 30 years ago scientists were saying the world was getting colder, and these things happen over periods that are long compared to human lifetimes, why should they be believed today when they say the opposite is occurring?
Some smart people talk about that here, and point out that "global cooling" excitement was not a part of the mainstream science of the time.Suppose that global cooling was a major concern in the 1970s. I remember people reading about it myself. Question. What was the scientific evidence for cooling at the time? Probably the main reason was the knowledge that the earth goes through periods of warming and cooling; that time intervals between successive cooling or warming are several tens of thousands of years; that these periods are driven by long term, predictable, and well understood changes in the orbit of the earth that result in slight changes in the amount of sunlight reaching the earth; and that the last warming period in this cycle occurred about ten thousand years ago. If this is all you know (and I think that might have been approximately the case for the 1970s), then it is not unreasonable to hypothesize that the long term prospects were for the earth to get cooler. So even if people were claiming that the earth was cooling off, it is reasonable to state that they may have been doing the best with what they had.
Now it turns out that we're about 30 years down the road from the 70s and we know a lot more about the subject of climate than we did then. If climate scientists are making different predictions today than they were then, it is appropriate to ask whether they have reasons for having changed their tune. And it turns out that they have. Huge amounts of data, primarily from ice cores, have been gathered, primarily in the last decade. These data have provided voluminous detailed information about climate variation for almost the last million years. This information was simply unavailable in the 70s. To criticize the scientific community for changing its point of view since the 70s, without taking into account the fact that a great deal has been learned since then, is to be ignorant, or worse, dishonest. (George Will may in fact be both; I don't know.)
Here is a sketch of the situation as it now stands, as far as understanding of climate history is concerned. Ice cores (from Greenland and Antarctica, primarily) provide climate information on an annual basis going back more than half a million years. Ice caps are in layers, where each layer corresponds to a year, much like tree rings. Gas and other chemicals in the atmosphere are trapped in the ice, and by drilling down in the ice caps, ice cores can be obtained and analyzed. Analysis can inform about warm and dry periods, chemical content of the atmosphere, variations in average annual temperature, and so forth.
The level of detail provided in the ice cores is truly astounding. One feature of interest is that the cores show that climate can be very stable over long periods of time -- hundreds, or even thousands of years -- and then change quite suddenly, in just a few years. This has happened quite often and there are fascinating explanations for why these changes occur (but that is not the main point I want to make here).
This is the main point I want to make: The ice cores contain a record of greenhouse gas (GHG) content of the atmosphere (for this discussion greenhouse gases are primarily carbon dioxide and methane). What is found is that there are long term, but periodic, variations in the amount of GHG content of the atmosphere, and these changes line up with similar long term periodic variations in temperature as observed in the ice cores. From knowledge of the solar system and the orbit of the earth, the variation in the solar radiation reaching the earth can be computed over the same period of time that is covered by the ice cores. Lo and behold the warm periods and cold periods line up with the periods of relatively high and low solar radiation.
What does this say? Well it must mean that somehow the variation in solar radiation drives the correlating changes in the climate, and not the other way around. One doesn't have to understand the details of all the climate mechanisms involved (and many details in fact are not understood) to realize that, having observed the correlation, this must be true. There is another aspect to this that is not quite so easily appreciated however, and that is that the actual variation in solar radiation is very slight and appears incapable on its own at providing a physical explanation of the actual variation in climate -- that is to say, the observed long term temperature swings in the climate are too great to be explained solely by increases or decrease in solar radiation.
A clue is that the greenhouse gas content of the atmosphere is also observed to vary during these times. Specifically, the GHG content is relatively high during warm periods and then it is significantly lower during colder times. Scientists admit to not understanding exactly how the GHG concentration changes, but the changes themselves provide a feedback mechanism that can explain the observed temperature variation. The typical scenario goes like this: If the earth is in a cool period, and the variation in the orbit results in additional radiation, warming the earth, things can happen -- slight warming of the ocean, increased biological processes -- which will over decades and centuries release extra CO2 into the environment. This extra CO2 causes the earth to warm up more than it other wise would with the solar radiation that it is experiencing. The earth warms, oceans release more CO2, more biology happens, and it gets even warmer. A positive feedback loop. Eventually, though, the orbit changes enough where the solar radiation starts to decrease and the feedback loop starts to work in reverse.
Now the current controversy is between those who say we are affecting the climate and those who doubt it. In all this discussion so far there is not much that the skeptics haven't said already and often -- that natural processes drive variation in the climate. Always have, always will.
Well, here's the rub. First: For the last half million years the GHG content of the atmosphere has been varying in synch, more or less, with the warm and cool periods, which are in turn driven by the variation in the orbit of the earth. In warm periods the maximum CO2 content has always been about 275 parts per million (ppm), in cool periods it has been maybe 175 (I'm running from memory here -- I may be somewhat mistaken in this number, and I need to go check, but not now. The point will stand, no matter what).
Second: As was pointed out earlier, considerations on the orbit of the earth would indicate that we are heading into a cool period. The maximum of the most recent warm period was about ten thousand years ago, and we should in fact be well into a cooling period. In fact if you look at the data for past cooling epochs, for similar points in the repeating cycle, the GHG content of the atmosphere has alwasy been below the maximum of 275 ppm, sometimes well below that mark.
So, what is the GHG content of the atmosphere? It turns out that today the CO2 concentration is not below 275 ppm, it is in fact much higher, more than 100 ppm higher. This is not only unusual, it is unprecedented. The ice core measurements, which go back the better part of a million years, simply do no show a concentration of GHG gases as high as we are experiencing today. But there is more (the facts in this discussion come mainly from the publications of William Ruddiman; here's one of his
books.).
One can compare the ice core data for the period since the last warming with the data following similar previous warm periods. What one finds is that in previous periods the GHG content of the atmosphere decreased substantially during post warming intervals similar to our own. However, although after the end of the last warming period the GHG content of the atmosphere initially began to decrease -- about 10,000 years ago -- it then started to gradually increase again, after about 2,000 years. It has been increasing slowly ever since. Up to about 200 years ago, the beginning of the industrial age, it had gradually increased to where it was somewhat greater than the 275 ppm maximum of most warming trends.
So, the ice cores have given us some long term information about climate variation, solar radiation, and greenhouse gases. What is the picture? The solar radiation variation would suggest that the earth should be headed into an ice age, as predicted by some in the 1970s. And the record at similar points in previous climate cycles would indicate that in fact the temperature of the earth should be cooler than what is today -- that is we should be well on our way to the next ice age. To say the least, that does not appear to have happened. What is different about our current era from similar ones in the past? Well, the GHG content of the atmosphere from 8,000 years ago through the beginning of the industrial age 200 years ago did not decrease as it had in the past; rather it actually slowly increased. Why, when in other cooling eras it has decreased.
This Ruddiman's thesis, and I believe he is correct. 8,000 years ago is coincidentally the beginning of human agriculture. Civilizations grew. There is reason to believe that significant deforestation has occurred across regions where humans have lived. Ruddiman asserts, then, that humans are not only affecting the environment now in the industrial age, but the results of civilization can be seen in the climate record for the last 8,000 years. The bottom line is that the climate record since the last warming period has been anomalous, and that is precisely the time that humans have been working at the environment. So during the last 8,000 years, when the earth should have been cooling and the GHG content of the atmosphere should have been dropping by about 100 ppm, humans have through their activities added enough GHG's to increase the concentration in the atmosphere by 1-2 ppm every century. Ruddiman concludes, then, that the climate, which has been fairly stable and accommodating to humans over the last few thousand years, has been so in large part because humans inadvertently made it so.
So, astronomy and the record of past climate variations say that the earth should be colder than what it is. On the other hand, up until 200 years ago, the GHG content of the atmosphere had not decreased as it should have; instead it very gradually increased. The extra GHG provided the insulation, the blanket if you will, that was needed to keep the earth warmer than it otherwise would have been in the cooling period that should be occurring. Where did the GHG come from? The answer is the deforestation that occurred around the world as civilization grew, the growth in the cattle industry (methane from cattle farts is a significant GHG, it turns out -- this may be humorous but it is a serious contribution to the problem), etc.
The fact that this hypothesis is novel is not a valid reason for discounting it; in fact, it takes into account a vast amount of relatively new data, as well as provides an explanation for the false predictions that climate science has (supposedly) made in the past. Interestingly, there are some significant temporary dips in the upward trend of the GHG content during the last couple of millenia that support Ruddiman's thesis. The Black Death, for example, wiped out about 30% of the population of Europe (and possibly the whole world; apparently there is not much information on the subject from the Eastern hemisphere). It is known that this severely set back the economy and agricultural activity. There appears to be a corresponding, statistically significant dip in the rate of increase of GHG content of the atmosphere at the same time. Q.E.D.?
O.k. We're up to 200 years ago. More later.
One can compare the ice core data for the period since the last warming with the data following similar previous warm periods. What one finds is that in previous periods the GHG content of the atmosphere decreased substantially during post warming intervals similar to our own. However, although after the end of the last warming period the GHG content of the atmosphere initially began to decrease -- about 10,000 years ago -- it then started to gradually increase again, after about 2,000 years. It has been increasing slowly ever since. Up to about 200 years ago, the beginning of the industrial age, it had gradually increased to where it was somewhat greater than the 275 ppm maximum of most warming trends.
