Whitening clouds by spraying them with seawater, proposed as a “technical fix” for climate change, could do more harm than good, according to research.
Whiter clouds reflect more solar energy back into space, cooling the Earth.
But a study presented at the European Geosciences Union meeting found that using water droplets of the wrong size would lead to warming, not cooling.
As science advisor John Holdren resasserted in 2009 of strategies such as space mirrors or aerosol injection, “The ‘geo-engineering’ approaches considered so far appear to be afflicted with some combination of high costs, low leverage, and a high likelihood of serious side effects.”
Two major problems for most of the ‘hard’ geoengineering strategies — aka solar radiation management aka smoke and mirrors — are that they still require aggressive mitigation, and they must meet a very strong test of science.
If you don’t do aggressive greenhouse mitigation starting now, you pretty much take geo-engineering off the table as a very limited (but still dubious) add-on strategy — as even geo-engineering experts like climatologist Ken Caldeira have made clear (see Caldeira calls Lomborg’s vision “a dystopic world out of a science fiction story”).
Also, the nation and the world are not going to pursue an expensive and potentially risky strategy unless they have much higher confidence in climate science than many people seem to have today. You’d have to know with near certainty that doing nothing would make things much worse. Nobody undergoes chemotherapy unless the alternative is pretty darn grim and certain.
But if the entire U.S. political system ever gets that high confidence in the science, mitigation is inevitably going to be the cheaper and safer solution (Intro to climate economics: Why even strong climate action has such a low total cost).
The other problem with geo-engineering is that by the time it might seriously be on the table, say, the 2030s, when humanity has become truly desperate to avert the multiple catastrophes scientists have been warning about for decades, the planet itself will probably be warming faster and extreme weather events will be increasingly commonplace.
Thus it will be very hard to tell if your geo-engineering strategy isn’t actually making things worse. That causes real problems if there is any scientific reason to think that your geoengineering strategy might in fact make things worse — which is certainly the case for the most plausible of all the solar radiation management ideas proposed to date, aerosol injection (see “the definitive killer objection to geoengineering as even a temporary fix”).
Now, as the BBC reports, one of the few other SRM semi-plausible ideas, cloud spraying, may also suffer from the same exact problem.
Cloud whitening was originally proposed back in 1990 by John Latham, now of the University Corporation for Atmospheric Research in Boulder, US.
It has since been developed by a number of other researchers including University of Edinburgh wave energy pioneer Stephen Salter, joining a number of other “geoengineering” techniques that would attempt either to reduce solar radiation reaching earth or absorb carbon dioxide from the air.
One version envisages specially designed ships, powered by wind, operating in areas of the ocean where reflective stratocumulus clouds are scarce.
The ships would continually spray fine jets of seawater droplets into the sky, where tiny salt crystals would act as nuclei around which water vapour would condense, producing clouds or thickening them where they already exist.
It has not yet been trialled in practice, although proponents say it ought to be.
But Kari Alterskjaer from the University of Oslo in Norway came to the European Geosciences Union (EGU) meeting in Vienna with a cautionary tale.
Her study, using observations of clouds and a computer model of the global climate, confirmed earlier findings that if cloud whitening were to be done, the best areas would be just to the west of North and South America, and to the west of Africa.
But it concluded that about 70 times more salt would have to be carried aloft than proponents have calculated.
And using droplets of the wrong size, she found, could reduce cloud cover rather than enhancing it – leading to a net warming, not the desired cooling.
“If the particles are too small, they will not brighten the clouds – instead they will influence particles that are already there, and there will be competition between them,” she told BBC News.
“Obviously the particle size is of crucial importance, not only for whether you get a positive or negative effect, but also whether particles can actually reach the clouds — if they’re too large, they just fall to the sea.”
Yet even today, any given year can be considerably warmer or cooler than the fast-rising mean. It will be difficult if not impossible to know by the 2030s whether some cloud whitening experiment is making things better or worse.
The possibility of this technique having a warming impact has been foreseen by cloud-whitening’s developers.
In a 2002 scientific paper, Dr Latham wrote: “… the overall result could be a reduction in cloud droplet concentration, with concomitant reductions in albedo and cloud longevity, ie a warming effect”.
But, he argued, this possibility could be eliminated by careful design of the spray system.
Contacted after the presentation in Vienna, Professor Salter took the same line.
“I agree that the drop size has to be correct and that the correct value may vary according to local conditions,” he said.
“However, I am confident that we can control drop size by adjusting the frequency of an ultrasonic pressure wave which ejects drop from micro-nozzles etched in silicon.
“We can test this at very small scale in the lab.”
You can test it at a very small scale, but it just wouldn’t mean bloody much given the vastly different scale and circumstances in the real world application to achieve a meaningful impact.
One scientist at Ms Alterskjaer’s presentation, having heard her outline why it might not work, commented that it was the most depressing thing he had heard in a long time.
And Piers Forster from the UK’s University of Leeds, who is leading a major UK project on geoengineering techniques, suggested more research would be needed before cloud whitening could be considered for “prime time” use.
“The trouble is that clouds are very complicated; as soon as you start manipulating them in one way, there are a lot of different interactions,” he said.
“We need real-world data and we need modelling that tries to simulate clouds on more appropriate scales, and that means less than 100m or so, because if you look at a deck of stratocumulus it’s not one big thing, it has pockets and cells and other features.
“Far more uncertain is the idea that you’d inject a particular drop size, because it won’t stay that size for long – it will spread out, and that would be uncertain.”
The more one looks into each individual geo-engineering strategy, the less plausible it appears.
And of course, the SRM ‘solutions’ do nothing to stop the consequences of ocean acidification, which recent studies suggest will be devastating all by itself (see Geological Society: Acidifying oceans spell marine biological meltdown “by end of century”).
Even geoengineering advocate Tom Wigley is only defending “a complementary combined mitigation/geoengineering scenario, an overshoot concentration pathway where atmospheric carbon dioxide reaches 530 ppm before falling back to 450 ppm, coupled with low-intensity geoengineering,” with the goal of stabilizing global temperature rise at 2°C, in case we can’t stabilize at 450 ppm. You can see a good discussion of that at the Bulletin of Atomic Scientists‘ expert roundtable response to Alan Robocks’ excellent piece, “20 reasons why geoengineering may be a bad idea.”
Well, stabilizing at 530 ppm requires doing a massive amount of mitigation starting now “” only 2 or 3 fewer wedges than what is needed for 450 (see “The full global warming solution: How the world can stabilize at 350 to 450 ppm“).
Very aggressive mitigation is the only thing that makes geo-engineering even semi-plausible (and adaptation anything other than a cruel euphemism).