What is the best way to evaluate the strength of methane as a greenhouse gas? Scientists have struggled over this question for a long time, and so far have failed to come up with a good, short answer that is easy to work with for any purpose. What mystifies me is why they have not sought out an answer on the same basis as the one used in evaluating the strength of CO2. Actually there are two answers to the question in the case of CO2. One is short and simple, because it applies to CO2 in isolation. According to the results of an assortment of tests and measurements, if you double the amount of CO2 in the atmosphere, and everything else is either unchanged or separately accounted for, the greenhouse energy effect of the additional CO2 will add about 1.0C to surface temperatures. Because the greenhouse energy effect is expressed logarithmically, by a rule of nature, the next double will raise temperatures by the same amount, one degree, and so on. This one number is easily adapted to all kinds of calculations over any time frame.
The other way to describe CO2’s greenhouse effect makes use of an assumption that certain feedbacks come into existence as a sole result of the heat added by CO2. These feedbacks are known to have their own powers with respect to raising temperatures. In some cases science is confident that it is capable of accurately measuring these powers while in other cases rough estimates must be relied upon. When everything is sorted out and assembled, and given reasonable margins of error, we get a completely new calculation for the consolidated greenhouse power of CO2, often called CO2’s ‘sensitivity.’ The numbers end up well above one degree, but now require that wide brackets be tacked on in order to show the limits of estimated uncertainties. Water vapor’s greenhouse effect is included among these feedbacks, and so is the albedo effect of various classes of cloud cover, with the latter generally considered to be the key source of uncertainty.
So where does methane fit into this picture—or does it? Methane has a greenhouse effect similar to that of CO2 in terms of how it works, but how strong is it? Can we make a comparison with the strength of CO2, using either of the two methods just described? The second method obviously would not work as long as CO2 is treated as the sole source of these feedbacks—to the exclusion of methane and all other well-mixed gases—but what about the first? Why is there no discussion about the potential for calculating the global warming power of methane when its concentration in the atmosphere is doubled and everything else, including CO2, is either unchanged or separately accounted for? We know from studying the radiation bands that it would be less than one degree, meaning less powerful than CO2, but how much less? We are treated to a fairly exact answer in the case of CO2. Why can’t the same process be applied to methane? I have personally fiddled around with a few numbers (as described in earlier letters) and come up with an estimate of about 0.4C for each double of CH4. There should be better ways to seek and find a real answer.
Scientists have habitually taken an entirely different approach to evaluating the greenhouse effect of methane and all of the lesser well-mixed greenhouse gases. It goes by the name of ‘global warming potential,’ providing answers that are intended to demonstrate the relative strength of a current emission of any of these gases compared to the strength of a CO2 emission of similar size, as each unfolds over a relatively short period of time. The relative strength of any gas will always vary according to the expected natural lifetime of the particular emission of that gas in the atmosphere, and also in accord with the period of time that is selected for making the comparison. The variations by time period can be quite large, and in the case of methane, being especially complicated by the unusually short lifetime of its emissions, this has led to considerable controversy. A new study published by Nature dealing with this issue has now come under scrutiny. I invite you to look at the study, in order to appreciate the many complications involved, and then either of two reviews for a little more clarity about what the authors hope to accomplish by this method.
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Carl