It’s been a busy week for water vapor, and I have two recent papers to discuss. The first is the paper "Anthropogenic greenhouse forcing and strong water vapor feedback increase temperature in Europe" by Rolf Philipona et al. (GRL, 2005, subscription required for full text), which has attracted a certain amount of media attention. The overall goal of the paper is to understand, from a physical standpoint, why European temperatures have been increasing three times faster than the Northern Hemisphere average. It focuses on the changes between 1995 and 2002, over which time good surface radiation budget observations are available. The paper reports some results on the role of large scale circulation changes (which they conclude are minor) but I’ll concentrate on the results relating to water vapor.
The most interesting result may be summarized as follows. Measurements from a network of six Alpine surface budget stations indicate that the primary radiative forcing driving the increase in surface temperature is an increase of downward clear sky infrared from the atmosphere to the surface. The annual average increase in this term is nearly 4 Watts per square meter between 1995 and 2002. Net cloud effects are relatively less important. Moreover, the increase in downward clear sky infrared is correlated with an increase in atmospheric temperature, and also an increase in the water vapor content of the surface layer of the atmosphere. Using a simple radiation model, the authors conclude that about a third of the increase in downwelling infrared is due to the increase in atmospheric temperature,and the rest is due primarily to an increase in the water vapor content of the low level atmosphere. This happens because water vapor is a greenhouse gas, so increasing the water vapor content makes air act more like a perfect blackbody emitter, if the air is not already opaque to infrared. In this case, increasing water vapor content will make the air a better absorber and emitter, even if its temperature doesn’t change. From this result we learn that: (a) observations confirm the expected increase of low level water vapor content with temperature , and (b) the increase in water vapor accounts for the bulk of the increase in downward radiation heating the surface.
The authors then subtract off the part of the downward infrared radiation increase attributable to temperature and water vapor increase, and thus estimate the part due directly (as opposed to via feedbacks) to the increase in anthropogenic greenhouse gases such as CO2. They estimate this to be about one third of a Watt per square meter. This is not in bad agreement with estimates from detailed radiation models run by the authors, which say that the change in surface radiation due to the 12ppm CO2 increase between 1995 and 2002 should be about one fourth of a Watt per square meter. It is striking that the changes in the Earth’s surface radiation budget due to anthropogenic greenhouse gases are so profound that they can be directly observed on a regional scale, over such a short time period. So far, so good. Physics seems to be working as it should, and climate scientists seem to be basing their understanding of climate change on rock-solid physical principles. The authors do not fall into the trap of assuming that water vapor is the root cause of the observed warming. They understand fully well that water vapor acts as a feedback to amplify forcing due to CO2 increase, and make this clear in their paper. This paper does not, however, deal directly with the problem of whether European warming can be attributed to CO2 increase. It only shows that, whatever mechanism is causing the warming of the atmosphere in this region, the surface warming is being amplified by low level water vapor feedbacks.
The accuracy of the media coverage of Phillipona et al. is decidedly mixed. The BBC got the scientific story straight (warming due to water vapor amplifying anthropogenic effects, everything working as it should, no worries about the physics, mate.), but their otherwise sound article was published with the unfortunate sub-header "Water vapour rather than carbon dioxide in the atmosphere is the main reason why Europe’s climate is warming, according to a new study." This gives the casual reader the erroneous impression that the study concludes CO2 is unimportant. It feeds the old, discredited skeptics’ notion that the water vapor greenhouse effect is so dominant that there’s no need to be concerned about CO2. National Geographic is a little breathless: " The latest villain on global warming’s most-wanted list is all wet—and a little surprising. Water vapor, experts say, is the culprit behind Europe’s rapidly rising temperatures." However, they get the basic scientific story straight, quoting Philipona as saying "It is an experiment that clearly shows which factors are driving the higher temperatures. It is not the clouds, not the sun, not the aerosols. It is the increased greenhouse gases and the strong water vapor impact." UPI is probably the worst of the bunch. They state "Swiss scientists say Europe’s recent rapid temperature increase is likely due to an unexpected greenhouse gas: water vapor." Unexpected? If they were readers of RealClimate, they’d know better.
All of this was relatively harmless, but all the coverage missed the boat in the same way. Press reports failed to note that the water vapor feedback discussed in Philipona et al. is not the same water vapor feedback usually discussed in connection with global warming. It is instead a surface water vapor feedback which adds additional surface warming on top of the usual things we talk about. The effect is already incorporated in the climate models used in IPCC forecasts, but the new observational study will be useful as a reality-check.
Phillipona et al. analyzed trends in the energy budget of the Earth’s surface. While this is definitely an aspect of climate change, it comes as a surprise to many that the surface energy budget plays a decidedly secondary role in climate change compared to the top-of-atmosphere energy budget. The fact is, that even if the diligent Swiss authors of this paper had found that increasing CO2 contributed nothing to the changes in the surface budget, this would have in no way contradicted our understanding of the way anthropogenic greenhouse gases influence climate. For the most part, surface temperature changes are determined by perturbations to the top-of-atmosphere budget, and the surface budget is just dragged along, accomodating itself to whatever changes in surface temperature are demanded in order to be able to satisfy the top of atmosphere budget. It is impossible to understand the greenhouse effect without thoroughly understanding this point. Even the authors of Phillipona et al. seem to be a little fuzzy on this matter. They seem to think they are looking at the same water vapor feedback discussed in various review articles on the subject (e.g. Held and Soden (Annu. Rev. Energy Environ., 25, 441– 475. (2000)), Pierrehumbert et al. ("On the Relative Humidity of the Earth’s Atmosphere" in The General Circulation, T. Schneider and A. Sobel, eds. Princeton U. Press 2005,) Pierrehumbert (Subtropical water vapor as a mediator of rapid global climate change. . in Clark PU, Webb RS and Keigwin LD eds. Mechanisms of global change at millennial time scales. American Geophysical Union:Washington, D.C. Geophysical Monograph Series 112, 394 pp1999), and the RealClimate article on the subject). but they are not. I shall try to explain.
In equilibrium, the Earth must lose as much energy out the top of its atmosphere as it gains by absorption of Solar energy. This is the principle of energy balance that controls the climate of all Earthlike planets. Currently our planet is out of equilibrium because the rapid rise of carbon dioxide is more than the slow response time of the oceans can keep up with; even if CO2 increase were halted today, the planet would continue to warm for a while as it comes into equilibrium. Planets only have one way of losing energy, which is by infrared radiation to space, often called "Outgoing Longwave Radiation," or OLR. The next piece of the story is that convection is always lifting air from the ground to high altitudes in the troposphere, causing the air to cool by expansion as it rises. This is the basic reason that temperature goes down with height in the troposphere. Convection and other dynamical heat transport mechanisms link together all the air in the troposphere, so that, to a first approximation, the whole troposphere can be considered to warm and cool as a unit. It doesn’t matter much where you put in or take out heat from the troposphere.. It is mainly the net energy budget of the troposphere that counts. Now, if the atmosphere contains a greenhouse gas, the atmosphere will be partly opaque to infrared trying to escape from the surface. Infrared from the surface will be absorbed before it gets very far. As a result, the infrared that escapes to space comes more from the higher, colder parts of the atmosphere. Since infrared radiation increases like the fourth power of temperature, the radiation from these layers is much feebler than the radiation that would escape from the ground. On the other hand, the radiation into the ground comes predominantly from the warm layers nearest the ground.
This situation is illlustrated in Figure 1, showing actual values of fluxes which I computed for a sounding over Paris during the August heat wave of 2003 (with an idealized water vapor profile having 80% relative humidity near the ground and 50% aloft). The red arrows in this figure originate at the mean altitude from which radiation escapes upward or downward. Because the radiation to space and the radiation to the ground come from different places, increasing the greenhouse gas concentration of the atmosphere would affect the two radiations in different ways.
If we increase the concentration of a greenhouse gas (say, CO2), then that makes more of the atmosphere opaque to infrared, and so the infrared escapes from yet higher and thinner and colder parts of the atmosphere. This would reduce the OLR, if the temperature of the atmosphere were held fixed at its original value. The planet would then be receiving more Solar energy than it gets rid of. Solar energy is primarily absorbed at the surface and communicated to the troposphere by surface heat fluxes. This energy input stays the same, while the reduction in OLR has reduced the rate at which the atmosphere is losing energy. As a result, the troposphere must warm until the top of atmosphere energy budget is brought back into balance. Remember that the whole troposphere warms more or less as a unit. That means that the air near the ground must warm along with the rest. In this way, we see that the warming of the entire troposphere can mostly be inferred just by thinking about the top of atmosphere budget, without bringing the surface budget into the picture in any detail. So far, all we need to know about the surface budget is that all the energy absorbed at the surface eventually makes its way into the atmosphere.
We are not done yet. We still have to say how this change in the tropospheric temperature translates into a change in the temperature of the solid underlying surface on which we live. This is where the surface energy budget comes in. The complication here is that, while the top-of-atmosphere balance has only one loss term (the infrared), the surface has many ways to exchange energy with the overlying atmosphere:
- Sensible heat flux (warming or cooling air in immediated contact with the surface and then mixing it aloft by turbulent motions)
- Latent heat flux (cooling the surface by evaporation)
- Infrared heat flux (cooling by emission of infrared by the surface, and warming by absorption of downelling infrared from the atmosphere)
with latent heat flux tends to be the dominant term, because evaporation is such an effective way of transferring heat. In fact, in warm, wet places like the Tropical Pacific Ocean, the evaporative heat transfer is so effective that all the surface budget tells us is that the surface temperature must stay quite close to the overlying air temperature. In a case like this, we don’t even need a detailed surface heat budget to say what the surface temperature change is — it is just dragged along with the tropospheric temperature increase. Changes in the surface budget instead affect the amount of evaporation needed to close the budget, and hence affect the precipitation rather than the temperature. The buffering of the surface budget by evaporation limits the leverage of the surface budget on surface temperature over much of the rest of the globe, though not to the same extent as in the tropical oceans.
The preceding reasoning does not mean that changes in the surface budget cannot affect the surface temperature. The right way to view the system is that (approximately) the top of atmosphere budget determines the warming of the low level air temperature, while the surface budget determines the difference between the air temperature and the surface temperature. There are many cases where this could further modulate the primary climate change, adding to or decreasing the primary top-of-atmosphere driven warming. This is particularly the case when a formerly wet land surface dries out. For example, the hot Sahara sands are around 10 degrees C warmer than the overlying air in the daytime, because in the absence of moisture the relatively inefficient sensible and radiative heat transfers need to have a pretty large temperature difference to work with in order to get rid of the necessary amount of heat. This is also why a dry sidewalk (pavement, to UK readers) gets very hot on a hot summer day. If the Sahara were made moister (as it was some thousands of years ago) the surface would cool regardless of what CO2 is doing. Conversely, if the moister parts of North America dry out in response to CO2 increase, the reduction in soil moisture will compound the surface temperature increase. Getting back to the implications of Philipona’s results, since Europe is not in a completely evaporation-dominated regime, the downwelling infrared increase could possibly allow the surface temperature to warm more rapidly than the air temperature, compounding the general global warming driven by CO2. Whether or not this happens depends in large measure on how evaporative and sensible heat fluxes adjust. This aspect of the problem was not treated by the paper. Philipona et al find that the observed downward radiation increases by roughly 2.7 Watts per square meter over and above what would be expected from the air temperature increase alone. This would lead to a surface warming of about six tenths of a degree C if it were balanced entirely by an increase in surface infrared cooling. Sensible heat flux would bring the warming down by about a factor of two. Evaporative heat flux would bring the warming down yet more, but at the expense of increasing the evaporation and aggravating the drying of soils. These climate changes are not inconsequential, especially in view of the fact that they have taken place over a relatively short period and come on top of the "normal" global warming driven by the top-of-atmosphere balance.
To see why the anthropogenic greenhouse effect does not, however, rely on the direct perturbation of the surface energy budget by greenhouse gas changes, let’s consider an idealized limiting case. Suppose that the lowest dozen meters or so of the atmosphere is so full of water vapor or cloud water that it acts like a perfect black body. It is as opaque as it can be to infrared. Now suppose that we double the atmosphere’s CO2 content. This doesn’t increase the infrared emission to the ground, because the low level air already has so much greenhouse-substance in it that it is radiating like a perfect blackbody, whose emission is determined by its temperature alone. It is radiating as much as it possibly can, for its given temperature. In radiative transfer-speak, its emission is "saturated." Furthermore, since the low layer is opaque to infrared, the CO2-caused change in downward emission aloft does not reach the ground. Does that mean there can be no further global warming in this case? No! What happens is that the increase in CO2 throws the top-of-atmosphere budget out of kilter, forcing the whole troposphere to warm up to bring the planet back into balance. Convection links the whole troposphere, which means the low level air warms up. The warming of the low level air, in turn, increases the flux of energy into the ground by all three of the mechanisms enumerated previously. In particular, the downward infrared flux increases because the air itself has become warmer — not because it has become more optically thick in the infrared. The increase in downward flux then communicates the warming to the surface. As Phillipona et al. show, the real midlatitude European boundary layer is not perfectly opaque to infrared, so increases in water vapor content or CO2 can directly increase the infrared heating of the surface. This is very interesting, but it is in no way essential to the anthropogenic greenhouse effect.
The water vapor involved in the effect of water vapor on infrared downwelling to the surface is almost a completely separate issue – a different water vapor, as it were – from the water vapor we speak of when talking about the role of "water vapor feedback" in the context of global warming.. Water vapor feedback of the latter sort is a consequence of the effect of water vapor on the top of atmosphere radiation budget. Water vapor near the surface has very little effect on this. Making the surface layer of the atmosphere a more effective infrared absorber/emitter has little influence on the infrared upwelling into the rest of the atmosphere because the temperature of the ground differs little from the temperature of the overlying air; one is just replacing one radiating surface with another radiating surface of practically the same temperature. In contrast, the relatively small quantities of water vapor aloft have a much greater effect on the top-of-atmosphere budget, because they increase the infrared opaqueness of layers of the atmosphere that are much colder than the surface; they block the infrared flowing upward from the warmer parts of the atmosphere, and replace it with "new" infrared emission from the cold layer.
That brings us to the second of the two recent water vapor papers, which is perhaps the more important of the two, though the subject matter it treats is less novel. . This one ( B. J. Soden, D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, X. Huang, Science 310, 841 (2005); October 2005 (10.1126/science.1115602)., subscription required for full text) has been more or less ignored by the media. Soden et al. deal with the aspect of water vapor feedback that affects the top of atmosphere radiation budget. The analysis consists in using various satellite observations to compare the behavior of mid to upper tropospheric water vapor between a general circulation model and reality. The analysis is carried out for the period 1982-2004, corresponding to the period of satellite data availability. The basic technique is the "model to satellite" method, in which the model temperature and humidity are used to directly simulate the brightness of radiation that would be observed by satellites looking at the atmosphere in various wavelength bands. By choosing satellite observations that are sensitive to the higher-altitude water vapor distribution, one can zero in on how well the model is doing in these all-important regions. Because of the relatively short period of the comparison, this exercise should not be regarded as an attempt to detect a trend in atmospheric water vapor and compare it with models. Rather, it is a check on whether the model does the same thing to upper layer water vapor as the real world, under varying year-to-year conditions (which do contain a trend over this period, as well as other things, e.g. El Nino).
By examining infrared satellite data, Soden et al. find that upper-level moisture increases in warmer conditions, in much the same way as predicted by the model. Further, by artificially suppressing moisture changes in the computation of the synthetic satellite data, they decisively reject the hypothesis that the atmospheric upper layer water content stays fixed as temperature changes. Synthetic satellite data computed on the basis of this hypothesis look nothing at all like the real thing. The authors take their analysis even further. Because the radiation measured by the satellites depends both on moisture and temperature,there is the possibility that faults in the climate model’s upper level temperature predictions might be leading to spurious agreement with the infrared satellite data. To rule this out, they make use of microwave satellite data that is sensitive to the mid to upper tropospheric temperature, in order to formulate a diagnostic that is primarily sensitive to upper level moisture changes rather than temperature changes. Again, they find that the data demand that the upper troposphere get moister in warmer conditions. They conclude: "Reproduction of the observed radiance record requires a global moistening of the upper troposphere in response to atmospheric warming that is roughly equivalent in magnitude to that predicted under the assumption of constant relative humidity." This is probably the most direct evidence to date that there is nothing terribly wrong about the way general circulation models handle water vapor feedback. This is quite remarkable, given the potential role of small scale cloud processes in moistening the atmosphere. To be sure, the analysis only deals with clear sky regions, but the moisture in these regions originates in the cloudy convective regions, and so it provides a fair test. In any event, within the cloudy regions themselves, the clouds rather than water vapor have the dominant effect on the radiation budget.
There would appear to be less and less room for skeptics to dismiss climate model predictions on the grounds that we aren’t sure they do water vapor feedback right. The picture is about to become even clearer, as researchers begin analyzing microwave upper level water vapor data, which will allow the analysis to be taken deeper into the convective, cloudy regions. To be sure, there is still a gap in understanding what the models are actually doing, in that it is far from clear why such complex processes boil down to a simple behavior: that the water vapor over a deep region of the troposphere changes in such a way as to keep relative humidity approximately constant. I have some ideas on this myself, but the general picture is still very much a work in progress. Meanwhile, it becomes increasingly clear that whyever the models do what they do to upper level water vapor, there can’t be anything too terribly wrong with what they are doing.