Should we use cloud geoengineering to mitigate climate change?
Guest post by Callum Munday
Clouds are a fundamental mediator of the Earth’s radiative balance. By reflecting incoming solar radiation they exert a negative radiative effect of -50W/m2/yr, which is only partially balanced by their absorption of outgoing longwave radiation (30 W/m2/yr)[Ref 1]. This imbalance of roughly -20 W/m2/yr leads to a net cooling of the Earth surface. The magnitude of this effect is large compared to the total net anthropogenic forcing since 1750 of c. 2.3 W/m, implying that any slight adjustment to cloud properties or their distribution could have a profound influence on the warming of the climate system.
In recognising this, geoengineering proposals have aimed to find ways of enhancing cloud albedo to increase their negative radiative effect. One such scheme, proposed by John Latham in 2002 [Ref 2] is to deliberately seed persistent marine stratocumulus clouds to increase water droplet number and thus enhance cloud albedo. In the context of remote marine environments where cloud droplet number is low, general circulation model (GCM) simulations indicate that this could increase net cloud reflectance by -1 W/m2 [Ref 3]. In future projections, this would induce a rapid reduction in global temperature by roughly 0.6K compared to a scenario without geoengineering. This considered, geoengineering seems like a promising solution to the pressing problem of global warming.
Unintended consequences and ethical issues
However, global warming is only one aspect of climate change and altering cloud properties can have other uncertain effects on the climate system. In particular, changing incoming solar radiation does not influence CO2 driven changes in the hydrological cycle or in ocean acidification. Indeed, GCM studies into the climatological effects of marine cloud brightening (MCB) have highlighted the potential for significant precipitation reductions and drought in the Amazon Basin [Refs 3,4]. Given the importance of The Amazon in terms of the global carbon cycle, biodiversity and agriculture, this represents a serious unintended consequence of geoengineering action.
It also illustrates the point that the climatic impacts of MCB will be regionally variable. While there may not be clear winners or losers, there will almost certainly be differentiated impacts. This raises the question of who gets control of geoengineering technology and to what end, placing MCB research and deployment firmly in the realm of ethics.
Moreover if, after a period of use, MCB were halted there would likely be rapid and dramatic warming of the climate system in response to raised levels of CO2 [Ref 3]. This shifts the responsibility for maintaining such a scheme with future generations, who would be compelled to foot the bill no matter how MCB may influence the climate system as a whole.
Thus, the promise of MCB in reducing surface temperatures should be weighed against the other, uncertain impacts on the climate system. Given the potential severity of those impacts and the ethical issues they entail, it would be wise not to let the heat cloud our judgement.
Callum Munday is a research student in the 2014 cohort
1 IPCC, 2013: 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, 1535 pp.
2 Latham, J. 2002 Amelioration of global warming by controlled enhancement of the albedo and longevity of low-level maritime clouds. Atmos. Sci. Lett. 3, 52–58.
3. Jones, A., J. Haywood, and O. Boucher (2009), Climate impacts of geoengineering marine stratocumulus clouds, J. Geophys. Res., 114, D10106
4 Bala, G., Caldeira, K., Nemani, R., Cao, L., Ban-Weiss, G. & Shin, H.-J. 2010 Albedo enhancement of marine cloud to counteract global warming: impacts on the hydrological cycle. Clim. Dyn. 37, 915–931.