I believe this is a serious problem among many well meaning people in the middle regions of the political spectrum. My sense is that honest, but otherwise uncommitted people believe that there are solid reasons for remaining tentative on global warming. Meanwhile, I believe that a number of folks are committed to the notion as much from "tribal" instincts than from an appreciation of the evidence itself (viz., global warming is a "liberal" cause, and they are "liberals" -- not the best scientific reason, but they could pick worse teams to be on).
Furthermore, my sense is also that much of the skeptical discussion centers around legitimate questions concerning some evidences for global warming. For example, if someone points to hurricanes as evidence, or even recent hot years, these can be critiqued as manifestations of normal fluctuations in the weather and climate patterns.
So is it possible to get to the nitty-gritty on the subject, to a fundamental argument, that can be concisely stated, that is subject to a minimum amount of spinning by the skeptics? I don't know that I have the answer, but I've been trying to construct one. My effort in doing so follows. Criticism of it is welcome.
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The physics of CO2 in the atmosphere began to be understood over 100 years ago, and even then scientists were speculating about the effect a carbon based economy would have on the atmosphere and its “greenhouse” mechanism. Check out the web site of the Amerian Institute of Physics ( http://webster-alt.aip.org/history/climate/ ) for a history of the “discovery of global warming”. Here is a physics Op-ed on the subject from about a year ago:
http://webster-alt.aip.org/history/climate/pdf/Weart_APS_News_2-06.pdf
With your indulgence, I’ll present the essence of the argument that leaves me a GW “believer”. By profession I am an experimental physicist, and as such I find data most compelling. Over the last decade or so scientists have cored ice from various glaciers and ice caps which have, like tree rings, preserved climate data but have done so for more than the last half million years. From the analyses of these cores a record of temperature and the constituents of the atmosphere can be obtained. Cores from locations as widely separated as Greenland and Antarctica correlate very well; this correlation leads scientists to the reasonable belief that the data from the cores are representative of global, rather than merely local, climate variation.
The data show that for at least the time covered by the ice cores the climate has oscillated between longer periods of cold (ice ages) and relatively brief warm periods. The earth has been in the latest warm period for about the last 10,000 years. Someone has noted (Al Gore perhaps?) that an indication of what a cold period means can be obtained by noting that the Chicago area would be under an ice sheet a mile or so thick. The average global temperature difference between warm and cold periods is about 8 degrees Centigrade.
These variations in the climate record occur with a periodicity of 100,000 years. The best candidate for the cause of the ice ages is an independently known variation in the orbit of the earth of the same periodicity and phase. There is a consequent change in solar irradiation associated with this orbital variation; however, taken alone it explains only about half of the temperature change between warm and cold periods.
The ice cores also show a corresponding increase CO2 atmospheric content with the warm periods and a decrease accompanying the cold periods. That the environment should “respirate” CO2 with warming and cooling is reasonable. It is also reasonable that this natural CO2 variation is the root of a positive feedback mechanism which amplifies the temperature extremes to the values associated with the data. During this regular, 100,000 year repeating cycle, the CO2 content of the atmosphere varies by 100 ppm (parts per million concentration).
Roughly, then, during the last half million years, about half of the historic, natural temperature variation of the earth can be ascribed to changes in solar input caused by variation of the orbit of the earth (4 degrees C). The remaining temperature change can be ascribed to the 100 ppm change in atmospheric content of CO2.
Here is why we should expect global warming, even if there were no signs such as melting glaciers, permafrost or polar ice caps: Since 1750 or so, that is since the beginning of the industrial revolution, and mainly in the last century, human activity has increased the amount of CO2 in the atmosphere by 100 ppm. This change in CO2 content has been on top of the normal maximum concentration, and is the same as the natural variation that occurs over 100,000 year periods in association with historic climate change.
In what amounts to an instant of geologic time, then, humans have modified the atmosphere by an amount comparable to what normally occurs in 100 millenia. Furthermore, we have done so in a way which takes the atmosphere far outside its normal range of variation. Moreover, with business as usual, the concentration of CO2 will increase by yet another 100 ppm in a mere 50 years. This is without precedent in the ice core record, and probably nothing like it has happened in several tens of millions, if not hundreds of millions of years (I realize the young earth creationists are now leaving the room, if they haven’t already ).
One of the objections of the “skeptics” is that since we have only seen a change in global temperature of a fraction of a degree, and definitely not four degrees, global warming is a myth, or at least highly overblown. The answer to this is that ALL physical systems require a finite (as opposed to instantaneous) amount of time to fully respond to a stimulation. The response of the earth to CO2 increase will be governed by its heat capacity (which is mainly in the oceans). No one knows precisely what this reponse time is, but it is most certainly not months or even years. Reasonable estimates of the response time from known physics range from several decades to a century or more. Observation of a relatively small temperature change up to this point is entirely reasonable, given the data, and it is foolish and naive to scoff as some do at the the small change seen so far. Indeed, some scientists are alarmed at the apparent speed that is observed.
So, we have, as physicists might say, a back-of-the-envelope estimate of a 4 degree temperature change for each 100 ppm of CO2 we add to the atmosphere. We have added 100 ppm already, and are very firmly on track to add another 100 ppm in the lifetimes, presumably, of most readers of Jesuscreed. We are also on track therefore, for let’s say, a 6 to 8 degree C (11 to 14 F) world for our grandchildren. There are reasons this estimate might be high by a degree or two; there are also reasons it might be low. But to bet the future on the unlikely possibility that the true sensitivity of the climate to CO2 is less than a degree per 100 ppm is … well, simply nuts.
In the foregoing I have not mentioned mathematical climate models (which the “skeptics” appear to despise) — or hurricanes, melting glaciers, or the fact that during the last 20 years the bird species populating my back yard have changed. Each of these individual phenomena could be associated with climate change, or with something else. They are not the reasons to believe in global warming. It is the record of climate change in the past, coupled with the fact that we have irrevocably changed the atmosphere already, that is the reason for expecting global warming to occur. The melting glaciers and ice caps are merely leading indicators of how rapidly or slowly the change is happening.
[Illustrations of the ice core data, as well as versions of the foregoing argument, are all over the web. Here is one of Jim Hansen’s many talks: http://www.columbia.edu/~jeh1/threattalk_complete_05Sept2006.pdf
Go to slide 16 for the historic ice core data, and slide 21 to see the last couple of centuries added in.]
