Busy Week for Water Vapor

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.

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