Elsevier

Atmospheric Research

Volume 82, Issues 1–2, November 2006, Pages 328-336
Atmospheric Research

Computational assessment of a proposed technique for global warming mitigation via albedo-enhancement of marine stratocumulus clouds

https://doi.org/10.1016/j.atmosres.2005.11.013Get rights and content

Abstract

A simplified version of the model of marine stratocumulus clouds developed by Bower, Jones and Choularton [Bower, K.N., Jones, A., and Choularton, T.W., 1999. A modeling study of aerosol processing by stratocumulus clouds and its impact on GCM parameterisations of cloud and aerosol. Atmospheric Research, Vol. 50, Nos. 3–4, The Great Dun Fell Experiment, 1995-special issue, 317–344.] was used to examine the sensitivity of the albedo-enhancement global warming mitigation scheme proposed by Latham [Latham, J., 1990. Control of global warming? Nature 347, 339–340; Latham, J., 2002. Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds. Atmos. Sci. Letters (doi:10.1006/Asle.2002.0048).] to the cloud and environmental aerosol characteristics, as well as those of the seawater aerosol of salt-mass ms and number concentration ΔN, which–under the scheme–are advertently introduced into the clouds. Values of albedo-change ΔA and droplet number concentration Nd were calculated for a wide range of values of ms, ΔN, updraught speed W, cloud thickness ΔZ and cloud-base temperature TB: for three measured aerosol spectra, corresponding to ambient air of negligible, moderate and high levels of pollution. Our choices of parameter value ranges were determined by the extent of their applicability to the mitigation scheme, whose current formulation is still somewhat preliminary, thus rendering unwarranted in this study the utilisation of refinements incorporated into other stratocumulus models.

In agreement with earlier studies: (1) ΔA was found to be very sensitive to ΔN and (within certain constraints) insensitive to changes in ms, W, ΔZ and TB; (2) ΔA was greatest for clouds formed in pure air and least for highly polluted air. In many situations considered to be within the ambit of the mitigation scheme, the calculated ΔA values exceeded those estimated by earlier workers as being necessary to produce a cooling sufficient to compensate, globally, for the warming resulting from a doubling of the atmospheric carbon dioxide concentration.

Our calculations provide quantitative support for the physical viability of the mitigation scheme and offer new insights into its technological requirements.

Introduction

Low-level marine stratocumulus clouds cover about a third of the oceanic surface and characteristically possess albedos, A, in the range 0.3 to 0.7 (Schwartz and Slingo, 1996). They therefore make a significant (cooling) contribution to the radiative balance of the Earth. Latham, 1990, Latham, 2002 proposed a possible technique for ameliorating global warming by means of controlled enhancement of the droplet concentrations Nd in such clouds, with a corresponding increase ΔA in their albedo (and also possibly in their longevity), thereby producing a cooling effect. The technique involves dissemination–at or close to the ocean surface–of seawater (NaCl) droplets around 1 μm in size, which are sufficiently large to act as cloud condensation nuclei (CCN) when they rise into the bases of these clouds. The central physics behind this scheme, which have been authoritatively treated–both theoretically and experimentally–in a considerable number of studies (for example, Twomey, 1977, Twomey, 1991, Charlson et al., 1987, Albrecht, 1989, Wigley, 1989, Slingo, 1990, Ackerman et al., 1993, Pincus and Baker, 1994, Rosenfeld, 2000, Brenguier et al., 2000, Peng et al., 2002, Stevens et al., 2005), is that an increase in droplet concentration Nd causes the cloud albedo to increase because the overall droplet surface area is enhanced and can increase cloud longevity (tantamount to increasing cloudiness) because the growth of cloud droplets by coalescence to form drizzle or raindrops–which often initiates cloud dissipation–is slowed down, since the droplets are smaller and the clouds correspondingly more stable. In some circumstances, increasing Nd may completely suppress the production of precipitation. Ship-tracks are, of course, examples of inadvertent and uncontrolled albedo increase in such clouds. Calculations by above-mentioned workers indicate that a 50–100% increase in Nd, in marine stratocumulus clouds, on a global basis, would be sufficient to produce a cooling, which would more than compensate for the warming (roughly 4 W m 2) associated with a doubling of the atmospheric carbon dioxide concentration. The studies of above-mentioned and other workers indicate that the associated changes ΔA in planetary and top-of-cloud (marine stratocumulus) albedo are about 0.005 (1.5%) and 0.02, respectively These calculations of required values of ΔA take no account of possible further cooling resulting from increased cloud longevity/cover and may thus be regarded as upper limits.

Advertent generation of sea-salt CCN at the ocean surface, thereby enhancing Nd, is, of course, basically a version of a process that happens naturally. Latham showed that the seawater dissemination rates required to produce the above-mentioned value of top-of-cloud ΔA were modest (around 108 to 109 kg h 1 (globally) or 0.1 to 1.0 kg h 1 km 2). This is because the required values of droplet size and associated salt-mass ms are so small: ms values as low as 10 17 kg (corresponding to a droplet diameter d of about 0.8 μm) being adequate for activation as CCN. Two advantages of this proposed global warming mitigation technique, if it were to be deployed operationally, are that: (1) albedo-control could be exercised by measuring A from satellites and switching seawater droplet disseminators on or off as required; (2) the only raw material needed is seawater, which would largely fall back into the oceans within a few days.

Further support for the quantitative validity of this scheme for global warming mitigation is provided by the computations of Jones et al. (submitted for publication) using the Hadley Centre's (UK Meteorological Office) HadGAM1 general circulation model (GCM). These indicate that warming due to a doubling of the carbon dioxide content of the atmosphere could be roughly compensated for–when taking account of the negative forcing due to the production of anthropogenic aerosol to date–by a doubling of the droplet number concentration Nd in three extensive regions of maritime stratocumulus clouds (off the West coasts of Africa and North and South America), which together cover about 3% of the global surface. If the anthropogenic aerosol factor is discounted, Nd would need to be roughly quadrupled. If only clouds covering this specially selected 3% of the Earth's surface were modified, instead of all marine stratocumulus clouds, the critical value of top-of-cloud albedo-change required to compensate for a doubling of carbon dioxide concentration would rise from 0.02 (mentioned earlier) to about 0.16. The associated values of enhanced A are within natural bounds.

The principal goal of the study described in this paper was to employ a simple model of marine stratocumulus to establish–over wide ranges of parameter values–the conditions under which this mitigation technique might be able to produce a cooling comparable with global warming. More specifically, we explore the sensitivity of albedo-change ΔA to the properties of the natural aerosol, the clouds and the advertently created seawater aerosol introduced into them. (The calculations of Latham, 1990, Latham, 2002 were much more limited and did not include estimates of ΔA, while the GCM calculations of Jones et al. (submitted for publication) did not lend themselves to sensitivity calculations of this kind.)

It is important to stress that our primary objective was to assess the quantitative validity of the proposed global cooling technique, not to advance our understanding of the development and properties of marine stratocumulus clouds. A worthwhile study of the latter topic would have required a fully developed dynamical and chemical model with treatment of entrainment, drizzle formation, the possible interrelationship between the advertently added aerosol and cloud thickness, and other refinements. However, in our case, with our focus on the former topic, it was deemed adequate to utilise our much simpler model (with prescribed dynamics and no entrainment, droplet coalescence–and thus drizzle formation–or chemistry), on the grounds that the mitigation scheme is currently still at an early stage of development, and further sophistication in the modelling would not add significantly to our understanding. On the question of possible drizzle formation, we note that, since the adulterated cloud droplet size distributions are significantly narrower than the natural ones, the calculated values of ΔA are likely to err on the side of being too low.

In Section 2, we present a brief description of our model, together with the ranges of parameter values employed, and details of the three (experimentally observed) aerosol spectra utilised. The results of our computations are presented and discussed in Section 3. In Section 4, we discuss the implications of these calculations with respect to the proposed albedo-enhancement global cooling scheme, and outline the current status of important technological and meteorological questions regarding it.

Section snippets

The model, input parameter values and aerosol spectra

In our modelling study, marine stratocumulus clouds (assumed to be adiabatic, of infinite width and in a steady-state condition) are formed in air whose natural aerosol characteristics are one of three specified alternatives, based on observation and corresponding, in turn, to negligible, modest or significant levels of pollution. In some further calculations, these clouds are inoculated with additional NaCl aerosol of prescribed concentration ΔN and constant salt-mass ms, which are added to

Results of the computations: discussion

Table 1 presents, for all three aerosol spectra, calculated values of cloud droplet concentration Nd, maximum supersaturation S and top-of-cloud albedo-change ΔA for the full range of values of additional aerosol concentration ΔN and salt-mass ms employed. These calculations are for the specimen values of TB, ΔZ and W displayed in the table or its legend. Table 2 presents further results for spectrum B, with larger salt-mass values.

As mentioned earlier, the focus of this study is on our

Application of calculations to proposed mitigation scheme: discussion

The calculations presented in Section 3 yield some significant implications with respect to technological aspects of the proposed global warming mitigation technique:

  • a.

    Since–as long as activation of the added CCN occurs–the calculated albedo-changes are highly insensitive to the values of salt-mass ms–and therefore seawater droplet size–for the range considered, the choice of ms can, to a considerable extent, be dictated by technological convenience. It would probably be optimum for effective

Acknowledgements

Part of this work was conducted while JL was in receipt of a Tyndall Fellowship.

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