Relative Roles of CO2 and Water Vapor in Radiative Forcing

In the second edition of our book

“Cotton, W.R. and R.A. Pielke, 2007: Human impacts on weather and climate, Cambridge University Press, 330 pp”,

we present a new analysis completed for our book by Norm Woods and Graeme Stephens at Colorado State University.

This new material discusses the relative role of CO2 and water vapor with respect to their radiative forcings, and provides a quantitative documentation of the dominance of water vapor as a greenhouse gas.

An excerpt from the text reads,

“To examine the impact of changing carbon dioxide and water vapor concentrations on radiative fluxes and heating rates, single column radiative transfer calculations were performed on standard atmospheric profiles for which carbon dioxide and water vapor concentrations were varied. Three profiles, representative of tropical subarctic summer and subarctic winter clear-sky conditions (McClatchey et al., 1972), were used for the calculations. The atmosphere was discretized into nineteen layers and the BUGSrad model (Stephens et al., 2001) was used.

Carbon dioxide was treated as uniformly mixed. Concentrations of 0, 280 (preindustrial), 360 (current), and 560 (doubling of preindustrial) ppmv were used for the calculations. For water vapor, the mixing ratios associated with the standard profiles were scaled by factors of
0, 1.0, 1.05, and 1.1. Figures with the heating rates for the particular scenario in K\/day, and the changes in heating rates attributable to the perturbations in carbon dioxide or water vapor mixing ratio, and a Table presenting the effects on downwelling longwave fluxes at the surface will be presented in the book.

The results suggest that the radiative changes induced by perturbations to carbon dioxide and water vapor are substantially different. For water vapor, modest increases beyond the base profile mixing ratios have minimal impact on longwave heating rates, but cause significant increases in downwelling longwave fluxes to the surface. For carbon dioxide, increasing the concentration beyond the base profile of 280 ppmv contributes to enhanced heating in the lower troposphere and to significantly enhanced cooling in the stratosphere, but causes minimal increases in downwelling longwave flux, particularly for the tropical and subarctic summer profiles. For the subarctic winter profile, this doubling of carbon dioxide produces an increase in downwelling longwave flux similar in magnitude to that for a ten percent increase in water vapor mixing ratio.

A number of factors potentially contribute to these differences in the effects of water vapor and carbon dioxide on heating rates and fluxes. First, water vapor is significantly more prevalent in the lower troposphere than in the upper troposphere and stratosphere. Consequently, the scaling approach used here to perturb the trace gas amounts tend to produce stronger perturbations of water vapor mass mixing ratio in the lower troposphere than at higher altitudes. This effect is less significant for carbon dioxide since this gas is more uniformly mixed.

In terms of radiative factors, the heating rate in a layer of the atmosphere is a function of the spectrally-varying absorption/emission characteristics of the layer, the spectral fluxes incident on the layer, and the layer temperature. The absorption by a layer is a function of the abundances of absorbing gases in the layer. In the longwave spectral region in which carbon dioxide is a significant absorber (for wavelengths of about 12.5 micrometers and longer) water vapor is also radiatively active. For a wavelength at which water vapor is already significantly absorbing, the addition of an amount of carbon dioxide to the layer will cause relatively little increase in the flux absorbed by the layer and thus cause relatively little increase in the radiative heating of the layer.

The spectral fluxes incident on the layer are also a function of the temperatures and emission characteristics of the layer’s surroundings. For a layer with given absorption characteristics, a stronger incident flux will cause more flux to be absorbed by the layer and contribute to heating. In particular in the lower troposphere, the proximity of the warm surface of the Earth contributes to this effect. A warmer surface temperature (as in, for example, the tropical or subarctic summer profiles used here) will contribute to enhanced heating in the lower troposphere as opposed to a cooler surface (as in the subarctic winter profile).

Finally, the temperature of the layer itself influences the amount of flux emitted by the layer. A warmer layer will emit flux more strongly, and thus have a greater tendency for cooling, than will a cooler layer. Profiles with warmer temperatures in the lower troposphere, such as the tropical profile, have stronger cooling in the lower troposphere than will a profile, such as the subarctic winter, which has cooler temperatures in the lower troposphere.

The downwelling flux at the surface is a function of the emission characteristics of the atmosphere, in particular the profile of the derivative of transmission with respect to height, known as the weighting function, and the temperature profile of the atmosphere. The height at which the weighting function peaks generally indicates the level of the atmosphere from which emission from the atmosphere most effectively reaches the surface, and this peak can be either broad (indicating the flux reaching the surface is strongly blended from different levels of the atmosphere) or narrow (indicating the flux reaching the surface is strongly selected from that particular level of the atmosphere). As trace gas concentrations in the lower troposphere increase, the general tendency is for the weighting function to shift lower in the atmosphere. For temperature profiles which decrease with height, this shift leads to increased emission to the surface.

The downwelling fluxes at the surface for the subarctic profile appear less sensitive to changes in carbon dioxide and water vapor concentrations than do the fluxes for the tropical and subarctic summer profiles. The subarctic winter profile has a relatively weak lapse rate in the lowest part of the troposphere, so changes in the position of the weighting function may have had little effect on the downwelling fluxes. In addition, the water vapor amounts in the subarctic winter profile are considerably smaller than those in the two other profiles. Since a scaling factor was used to perturb water vapor amounts for this study, the change in water vapor mixing ratio in the lower troposphere would be considerably smaller for the subarctic winter profile than for the two other profiles. This approach probably contributed to cause a less significant change in the weighting function for the subarctic winter case than that for the other two cases.

There are two tables that Norm Woods prepared that are insightful in terms of the effect of different atmospheric concentrations of CO2 and water vapor. For the tropical sounding, the downwelling longwave flux at the surface when the CO2 concentration changes from 360 ppm to 560 ppm is 0.09 Watts per meter squared, as contrasted with a change of 0.41 Watts per meter squared when the concentration changes to 360 ppm from 0 ppm. The reason for this relative insensitivity to added CO2 in the tropics is due to the high concentrations of water vapor which results in additional long wave flux changes due to CO2 being very muted.

For the subarctic summer sounding, the corresponding values are 2.94 Watts per meter squared when changing the CO2 concentrations to 360 ppm from 0, and 0.47 Watts per meter squared when changing the CO2 concentrations to 560 ppm from 360 ppm. For the subarctic winter sounding, the change is 14.43 Watts per meter squared when the CO2 concentrations are changed to 360 ppm from 0, and 1.09 Watts per meter squared when the CO2 concentrations are changed to 560 ppm from 360 ppm.

For water vapor, with the tropical sounding, the change of the concentration from zero to its current value, results in a 303.84 Watts per meter squared change in the downwelling longwave flux at the surface. Adding 5% more water vapor, results in a 3.88 Watts per meter squared increase in the downwelling longwave flux. In contrast, due to the much lower atmospheric concentrations of water vapor in the subarctic winter sounding, the change from a zero concentration to its current value results in an increase of 116.46 Watts per meter squared, while adding 5% to the current value results in a 0.70 Watts per meter squared increase.

This analysis shows that

1. The effect of even small increases in water vapor content of the atmosphere in the tropics has a much larger effect on the downwelling fluxes, than does a significant increase of the CO2 concentrations. Thus, the monitoring of multi-decadal water vapor trends in the tropics should be a high priority. While the increase in CO2 concentrations, and resulting increase in downwelling longwave flux can result in surface ocean warming, and thus increase evaporation into the atmosphere, it is the atmospheric water vapor signal that should be monitored for long term trends, as it is the dominant greenhouse gas that has the greater climate response.

2. The fractional contribution of the effect of added CO2, relative to a 5% increase of water vapor in the subarctic winter is significantly larger than in the tropical sounding. This is because the subarctic sounding is quite dry. An increase in absolute terms of water vapor similar to a 5% increase in the tropical sounding would, however, dominate the increase of downwelling longwave fluxes. This again indicates that the assessment of long term water vapor atmospheric concentrations needs to be a climate science priority.