Whitening the clouds


14 December 2009 by Alan Gadian

Cutting the emissions that cause climate change is vital. But what if we can't do it quickly enough to avoid humanitarian catastrophe? Alan Gadian and colleagues describe an idea that could help keep the Earth cool and buy us precious time.

In The Revenge of Gaia (2009), James Lovelock argued that catastrophe will happen within the next 30 years. Severe storms and droughts will become the norm, carbon offsetting is a joke, and current efforts to promote ethical behaviour are a scam. Is he right? Here, we discuss an alternative approach to dealing with climate change - geoengineering the clouds so they become whiter and reflect more sunlight back into space before it reaches the Earth.

As these techniques could buy us time to implement methods to reduce CO2, it would be very wise to research their viability, in case we need them in an emergency.

Geoengineering is man-made environmental change. Since the industrial revolution, people have been geoengineering the planet - cutting down rainforests, burning fossil fuels, and pumping CO2 and other radiative gases into the atmosphere. Environmental temperature change is now accelerating, not only due to CO2, but also because of the release of other gases such as methane.

Some of this comes from agriculture, but the greater concern is that the Canadian and Siberian permafrost could thaw, allowing the methane held in underground gas fields to escape. Although methane is relatively short-lived in the atmosphere, it is between 20 and 70 times more potent as a greenhouse gas than CO2 and could cause a runaway heating effect, only mitigated by the large amount of latent heat needed to melt the ice caps.

The philosophy of cloud whitening

So-called geoengineering schemes are designed to reverse the harm we have already caused and to provide a breathing space in which to cut greenhouse gas emissions. But we need to understand the science behind them, to avoid the risk of unintended consequences. Several possible schemes were analysed and discussed in the Royal Society report 'Geoengineering the climate', published in September 2009.

A view of the Earth with visible stratocumulus clouds

The light grey stratocumulus clouds are visible off the coast of Chile, and were measured using the NERC BAE 146 and Dornier 128 aircraft on the NERC-funded VOCALS consortium project

The report recommended research into two plans aimed at managing the solar radiation that warms the Earth. One of these was the cloud whitening scheme we discuss here. As these techniques could buy us time to implement methods to reduce CO2, it would be very wise to research their viability, in case we need them in an emergency. The cloud whitening scheme has to operate continuously and produces a one-off effect. But its advantages lie in its low ecological impacts.

Its only ingredients are seawater and air. The energy to run it would come from the wind and be relatively cheap. It could be easily and immediately shut down, with conditions returning to normal within a few days. It would give us precise and rapid control, via satellite measurements of albedo - how reflective the clouds are - and cloudiness fed back through a global model. It would be cheap to implement. And if current small-scale experiments confirm that the theory works, we could put it into action quickly.

Stratocumulus clouds

Oceans cover 70 per cent of the globe, and low level stratocumulus or 'layer' clouds cover 30 per cent of the oceans. These clouds are very important parts of the atmospheric and ocean global heat engine system. In November 2008 a large international field project, based in Arica, Chile, with over 200 scientists, five aircraft and two ships, measured these clouds in situ and with remote sensing. NERC funded a consortium project, VOCALS, with scientists from four UK universities. The image on the previous page, taken by the Geostationary Operational Environmental Satellite (GOES) during the project, shows the extent of these clouds.

The water droplets in clouds reflect sunlight back into space. The numbers of these droplets in clouds depend largely on the number of Cloud Condensation Nuclei (CCN). These are tiny particles of matter like dust or soot, that form a seed around which water droplets can form. Many CCN are produced over the land. This means land-locked clouds contain many hundreds of cloud droplets per cubic centimetre, while clouds that form over the sea contain substantially less: typically only a few hundred per cubic centimetre. Generally, for a given total amount of water in a cloud, the more droplets that are present, the smaller these drops are. And clouds with smaller droplets tend to be whiter, and hence more reflective.

These clouds are maintained by a complex balance of factors. How fast the water droplets collide and coalesce affects whether they precipitate out to form raindrops, or maintain a stable system. There is still a lot we don't know about how these processes interact.

The technology

John Latham has suggested that by increasing the number of droplets in maritime layer clouds, known as stratocumulus, we could significantly increase the amount of solar energy these clouds reflect.

Spray ship design

Stephen Salter's spray ship design

The idea is to inject a fine spray of sea salt from the ocean surface into the clouds. The salt particles would act as CCN, artificially increasing the number of droplets in the cloud, and so reducing their size and making the cloud more reflective - that is, whiter. This would in turn reflect more sunlight before it reaches the Earth and so reduce its rate of warming, and could buy us time - maybe as much as 50 years.

We need further research, including numerical modelling and field experiments, to determine the ideal size of the sea-salt CCN. But preliminary results from climate models show that a modest increase of CCN in marine stratocumulus clouds could produce the desired cooling, and suggest this method would let us compensate for anything up to a doubling of atmospheric CO2 from pre-industrial levels.

These initial results from models also suggest that the biggest cooling from this scheme - as opposed to injection of sulphate into the stratosphere, another proposal entirely - would occur around the poles. This is consistent with what the theory predicts, and is good news, as the poles are precisely where cooling is most needed to stop permafrost from melting. It uses natural sea water spray and can be turned off immediately, if it turns out to produce undesirable consequences.

The idea is to inject a fine spray of sea salt from the ocean surface into the clouds.

Scientists, including Stephen Salter of the University of Edinburgh, have suggested a design for a fleet of about 2,000 wind-powered, unmanned yachts which incorporate a sophisticated spray mechanism. The design would release sea-spray with a diameter of around 0·8 microns, providing CCN for the clouds.

We propose to perform detailed research into the scheme, and to find out whether it is viable within five years. This research has four elements. More work is needed on modelling the physics of clouds; there are still questions about how big the sea-salt CCN should be and how the clouds will respond as CCN numbers increase. We are already collaborating with top US cloud physicists on this. We also need further research on climate modelling, and we need to develop and build Stephen Salter's test yachts. Finally, we need small-scale field experiments in a region of stratocumulus to test whether the idea works in practice.

Developing a test spray system and conducting a field experiment to assess the scheme's viability will cost around £6m. This is an insignificant sum compared with the cost of doing nothing. In five to ten years, we could have an answer to Lovelock's question: "Could we have done anything to slow down the warming and the irreversible change in the Earth system?"

Dr Alan Gadian is senior research scientist at the National Centre for Atmospheric Science (NCAS), at the University of Leeds. Professor Alan Blyth is director of the NCAS Facility for Ground-based Atmospheric Measurements at the University of Leeds. Professor John Latham is an atmospheric physicist at the National Center for Atmospheric Research. Stephen Salter is professor of engineering design at the University of Edinburgh, and Laura Stevens is a PhD student at the University of Leeds.

Further reading
'Control of global warming?' - J Latham, 1990, Nature 347.
'Global temperature stabilization via controlled albedo enhancement of low level maritime clouds' - J Latham et al, 2008, Philosophical Transactions of the Royal Society A, 366, 3969-3987.
'The Revenge of Gaia' - J Lovelock, 2009, Penguin.
'Sea-going hardware for the cloud albedo method of reversing global warming' - S Salter et al, 2008, Philosophical Transactions of the Royal Society A, 366.