How does climate science account for the greenhouse warming power of water vapor, which is widely recognized as the strongest of all greenhouse gases with reference to its energy-absorbing position on the radiation bands? It is also recognized as having far and away the most erratic presence in the atmosphere, ordinarily ranging from around a quarter of a gram per square meter near the coldest pole up to 70 kilograms or so in random equatorial areas. What’s more, readings of vapor content everywhere have a habit of constantly making sizeable changes. In fact the lower-level readings are capable of doubling or quadrupling, temporarily, over a time period of just a few days, and in one place or another are found to be doing just that at nearly all times. Under ideal conditions, which are fairly often met, every double can be observed expressing the power to raise surface air temperatures by a full 10 degrees C. How do you take a package like that and arrive at an accurate representation of what this particular greenhouse gas contributes to the total warming power of all the greenhouse gases when all the rest of them have a practically constant presence in the atmosphere, practically the same for each one everywhere, with practically no change at all from any one day to the next, and practically no diminishment of warming power because of unfavorable conditions?
With all of these things in mind, how can you then.set up a table of actual calculations that account for the power of each gas? All but one will be acceptably accurate while that one, which happens to be the most powerful of all, has no hope of being accurately measured. You just have to compromise in one way or another. In this case the compromise is delivered not by adding water vapor power calculation as an estimate with long error bars, but rather by keeping it completely off the table by name, yet adding a rough estimate of its powers to the table under a disguise. Someone still has to make an estimate of what those added powers should amount to, and then decide where to place the disguised readout. For the estimate, it is possible to come up with a reasonably good figure by making sense of actual historical changes under all sorts of conditions where good data is available. The picture that emerges shows reported temperature increases averaging out at about twice the figure that would be expected from the combined action of all other temperature-changing factors, negative as well as positive, when taken alone. The named factors are believed to constitute all of the active participants, with all of them being well-suited for calculation. Water vapor power can then be assumed to fill in the missing temperature gap, quite reasonably it seems, as long as there are no missing pieces.
There is still the matter of where to place the disguised powers of water vapor within the table, and as I said yesterday a decision was made to just tack them on to the known powers of carbon dioxide This gas had already been established as the one holding the key to understanding long term changes in climate, more so than any other gas of the same type. Adding more to its powers, in spite of the distortion, should do no harm in the rising fight against climate change, whereas water vapor, which cannot be directly controlled by any kind of human strategy, would never be missed. And so it happened. Here is how the move turned out as seen on the most recent graph from the IPCC depicting the many various drivers of radiative forcing:
The absence of a separate line for water vapor is easy to observe, and you can also appreciate how CO2 has been fattened up by considering what it would have been, in terms of watts per square meter, if shown unadorned like methane and everything else. By my best calculation CO2 would have ended up roughly the same as methane in terms of actual responsibility for increases in global warming recorded between 1750 and 2011, due to the fact that methane, in spite of being relatively on the radiation bands, had recorded a much higher rate of growth. My calculation, after separating out CO2, leaves a gap filled by water vapor with a reading of just over one watt. That same amount of difference would also be represented at the bottom of the graph, within the 2011 line, occupying about one-half of the total net gain due to the warming and cooling of all known factors involving human activity, as otherwise listed throughout the graph.
This existing practice could be challenged, in part because there is such a lack of open and full disclosure. Also, the tendency to divert attention away from the relative importance of methane may not be fully justified in terms of formulating optimum climate action strategies. The practice also inhibits motivations that would favor a deeper level of scientific study involving water vapor activity. Should water vapor really be stuffed into a box this way and then forgotten about? I would argue that it has a life of its own, totally independent from CO2 or any other single agent of control, having poorly recognized powers that could be unleashed upon us as a surprise.
Carl