Water vapor feedback is the most important positive feedback in climate models. It is important in itself, and also because it amplifies the effect of every other feedback and uncertainty in the climate system. Most modeling and observational studies suggest that the water vapor feedback in current climate models has the correct sign and magnitude (Held and Soden, 2000). The magnitude of water vapor feedback is so large, however, that modest uncertainty in water vapor feedback can still have a significant effect on the magnitude of climate change.
It is known from basic physical principles that the vapor pressure in equilibrium with a water surface increases exponentially with temperature at a rate such that a 1 percent change in absolute temperature, a change of about 3°C, is associated with an approximately 20 percent increase in saturation vapor pressure. Because water vapor is the most important greenhouse gas in Earth's atmosphere, the dependence of vapor pressure on temperature forms the basis of one of the strongest positive feedbacks in the climate system. If the relative humidity distribution remains approximately constant as temperature and specific humidity increase, then water vapor greenhouse feedback nearly doubles the sensitivity of climate above what it would be in the absence of water vapor feedback.
On the largest spatial scales, existing data and current climate models are basically consistent with the assumption that on interannual time scales, relative humidity is more or less constant (Soden et al., 2002; Wentz and Schabel, 2000). However, local diurnal and seasonal relative humidity variations are significant, and analysis of climate model simulations of these features is needed. Furthermore, the relationship between temperature and humidity on interannual and longer time scales shows substantial vertical and regional structure, which models are only partly successful in simulating (Bates and Jackson, 1997; Bauer et al., 2002; Ross et al., 2002).
As shown by modeling and observational studies (Del Genio et al., 1991; Harries, 1997; Held and Soden, 2000; Shine and Sinha, 1991; Soden et al., 2002), water vapor variations in the tropical upper troposphere seem to have the strongest effect on outgoing long-wave radiation. However, the relative importance of water vapor in different regions of the atmosphere is sensitive to the assumptions made about clouds and about the variations (or lack thereof) of relative humidity with temperature. In fact, according to Harries (1997), "[Uncertainties of only a few percent in knowledge of the humidity distribution in the atmosphere could produce changes to the outgoing spectrum of similar magnitude to that caused by doubling carbon dioxide in the atmosphere," underscoring the importance of reliable upper tropospheric water vapor observations.
Uncertainty about water vapor feedback rests primarily on the question of whether the relative humidity distribution might change in an altered climate state. Several hypotheses have been put forward describing mechanisms that could alter the relative humidity distribution in a warmed world. The mechanisms that seem most likely to be meaningful are those that may govern the relationship between the area of moist and dry regions in the upper troposphere of the tropics (Lindzen et al., 2001; Pierrehumbert, 1995). In the tropics the greenhouse effect is strong, and large contrasts in upper tropospheric relative humidity are sustained between regions of large-scale ascent and descent. So far, no mechanism has been demonstrated to operate that would provide a significantly more reliable projection than an assumption of constant relative humidity distribution. Nonetheless, the factors that influence water vapor distribution need further study.
One useful metric for evaluating the question of whether relative humidity will change was put forward by Inamdar and Ramanathan (1998). Using Earth Radiation Budget Experiment (ERBE) data they examined the relationship between outgoing long-wave radiation at the top of the clear atmosphere and surface temperature. They found the slope of the regression line between these two variables to be consistent with an assumption of fixed relative humidity (vs. absolute humidity). Using this approach it is possible to compute a gain factor of the clear-sky water vapor feedback. Gain factors determined using this and other observational approaches should be compared with the factors similarly derived from models. This approach is discussed here for to illustrate only one of the many approaches that can be used to assess the ability of models to faithfully represent water vapor feedbacks.
Understanding of the water vapor distribution is being hindered by a lack of accurate measurements of water vapor concentration with sufficient spatial and temporal resolution and global coverage (Kley et al., 2000). Accurate measurements of the water vapor distribution can be used to test understanding of the mechanisms that determine its distribution, and also test to see if the increase of water vapor with time is consistent with models of climate change. An integrated water vapor observing system should be developed, which has sufficient accuracy to measure decadal trends in the water vapor distribution and sufficient spatial resolution to test mechanisms by which that distribution is maintained. It should include a network of in situ sounding systems capable of measuring water vapor throughout the troposphere and lower stratosphere, complemented by ground-based remote sensors (such as have already been deployed at Atmospheric Radiation Measurement Project Cloud and Radiation Testbed (ARM CART) sites). These observations would allow quantification of temporal and vertical water vapor variations and would allow calibration and validation of satellite observations, which would extend the global coverage of the observing system.
The global radiosonde network cannot be relied upon for precise water vapor observations, unless substantial improvements are made to ensure higher quality observations in the upper troposphere and in other cold (and dry) regions, and to ensure the long-term continuity of the observations. Expansion of the program for special water vapor soundings of the troposphere and stratosphere (e.g., Oltmans and Hofmann, 1995) to more sites (currently only Boulder is routinely observed) would be very beneficial. These should include both oceanic and continental regions, at a variety of latitudes. Efforts to consolidate and quality-control water vapor observations from different sources (e.g., the NASA Water Vapor Project [NVaP], Randel et al., 1996) should also be encouraged, so that water vapor variability can be examined in conjunction with variations in other atmospheric variables, particularly temperature and radiation. The water vapor observing system should be closely linked to a global cloud, aerosol, and precipitation observing system. Many of the issues mentioned above are discussed in greater detail in a report by the National Research Council (NRC) on the Global Energy and Water Cycle Experiment (GEWEX) Global Water Vapor Project (NRC, 1999c).
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