Water Vapour Absorption and its Role in the Earth’s Energy Budget

By: Jon Elsey

I’m Jon, a postdoc working with Prof. Keith Shine in the Atmospheric Radiation, Composition and Climate (ACRC) group. My work is very much in the “R” of ACRC, and specifically on the role of water vapour on atmospheric radiation.

When we think of “radiation”, you might think of the Hulk, or conjure up images of nuclear meltdown, or some other catastrophe. What I actually mean here by radiation is light emitted by the Sun (visible light, ultraviolet and high frequency infrared) which we call shortwave radiation, or by the Earth and its atmosphere (mostly lower-frequency infrared) which we call longwave radiation.

 Figure 1: The longwave and shortwave atmospheric absorption processes are circled. All units in W m-2. (from Stephens et al. (2012), Nature Geoscience).

This radiation is part of the Earth’s energy budget shown in Figure 1; shortwave radiation comes in from the Sun, which is then absorbed by the atmosphere and surface, with some being reflected back to space. The absorbed radiation heats up the surface and atmosphere, which then emit radiation in the longwave. The radiation emitted up from the surface in the longwave is then absorbed again by the atmosphere, trapping this energy within the Earth system. This is the well-known greenhouse effect, the main contributors to which are water vapour (H2O) and CO2.

The energy emitted by the Sun and Earth is in the form of a spectrum, made up of the visible, infrared and ultraviolet light I mentioned earlier. This radiation is absorbed by atmospheric gases at specific frequencies. For example, CO2 and water vapour absorb radiation very differently – CO2 absorbs very strongly in only very specific parts of the spectrum, whereas H2O absorbs radiation very strongly across most of the shortwave and longwave, with opaque regions (the so-called absorption bands) interspersed with much more transparent regions (the atmospheric windows).

Figure 2: Plot of the optical depth from CO2 (blue), H2O (orange) and H2O continuum (green) for the entire depth of a real atmosphere (measured in Cornwall, UK). The red dotted line shows the demarcation between the longwave and shortwave. Note that the y-axis is on a logarithmic scale – the 105 line is 1000x stronger than the 102 line, etc.

Figure 2 shows the absorption in a real atmosphere measured during my PhD, in units of optical depth, which is just a measure of how opaque the atmosphere is. So, looking at Figure 2, if water vapour absorbs more radiation than CO2, then why are we worried so much about CO2? The issue here is to do with what we call climate forcings and climate feedbacks. A forcing can be thought of as due to something outside of the climate system causing a change in its properties, while a feedback is the result of the climate system responding to a forcing.

In this context, CO2 contributes to the forcing (we emit CO2, which causes the atmosphere to heat up) and water vapour is a feedback (higher temperature leads to more water vapour in the atmosphere from evaporation of the oceans). This means that as CO2 heats up the Earth, it also causes an increase in the water vapour absorption, trapping much more radiation than the CO2 would alone, amplifying the overall effect on the climate by a factor of 2-3.

My own work focuses on what we call atmospheric spectroscopy, the study of how water vapour (and other molecules!) absorb radiation. Specifically, I study the continuum absorption, the green line in Figure 2. Like the spectral lines of water vapour, this continuum absorption occurs both in the shortwave and longwave, but is distinct from the lines in that it is smoothly-varying (in frequency) with no distinguishable features in the atmospheric windows. It is in these windows that the continuum is most significant.

From a climate science point of view, it is the effect on the energy budget that we are most interested in. Previous studies indicate that the shortwave continuum which I study could be significant in this context. However, measuring the continuum in the lab is really rather difficult, since there are no obvious features to look at, leading to difficulty in extracting the real absorption from any kind of instrumental noise.

There are a few groups working on studying this shortwave continuum, but with poor agreement between their measurements. My work is a collaboration between Reading and the Rutherford Appleton Laboratory, using state-of-the-art spectroscopic techniques. These measurements are still ongoing, but will hopefully go some way to solving this puzzle and contributing to our understanding of how water vapour absorption affects the climate.

References:

Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang, 2013: Anthropogenic and Natural Radiative Forc-ing. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA

Radel, G., Shine, K. P. and Ptashnik, I. V. (2015) Global radiative and climate effect of the water vapour continuum at visible and near-infrared wavelengths. Quarterly Journal of the Royal Meteorological Society, 141 (688). pp. 727-738. ISSN 1477-870X. https://doi.org/10.1002/qj.2385

Stephens, G., Li, J., Wild, M. et al. An update on Earth’s energy balance in light of the latest global observations. Nature Geoscience, 5, 691–696 (2012). https://doi.org/10.1038/ngeo1580

Shine, K.P., Campargue, A., Mondelain, D., McPheat, R.A., Ptashnik, I.V. and Weidmann, D., The water vapour continuum in near-infrared windows–current understanding and prospects for its inclusion in spectroscopic databases. Journal of Molecular Spectroscopy, 327, pp.193-208. (2016). https://doi.org/10.1016/j.jms.2016.04.011

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