Spraying seawater into clouds may slow global warming
Saving the planet has gotten a lot wetter.
Trying to lower CO2 emissions has been the name of the game for avoiding a catastrophic increase in global temperatures above about 1.5-2 degrees C. Achieving a carbon neutral economy is still the best chance we have, but we are now past the point of no return in terms of avoiding major climate changes. We are already seeing rising sea levels, increases in heatwaves, fires, as well as worse storms. Moreover, the oceans have already absorbed a great deal of the CO2 that has been emitted, and, as our emissions decline, they will continue to out-gas CO2 for a considerable length of time.
It is time to consider global geoengineering now as a way of reducing temperatures. There are two ways to do this. The first is to release more heat into space by removing CO2. This is a daunting prospect, but using algae and CO2 absorbing bacteria as well as planting new forests, we can make an appreciable dent. But this may not be enough even if we stop burning fossil fuels. And some ecological features like coral reefs are already dying, bright colors and life giving way to whiteness and death. These cannot wait decades for plant life to try to restore the balance.
Another option is to prevent light from the sun from reaching the surface at all. Some fanciful ideas like giant mirrors in space or mirrored balloons are unlikely to work because of the sheer size they need to be, but there is a substance that can reflect light back into space before it heats the Earth, and it is extremely plentiful.
Water.
Water vapor to be exact. While a cloudy day is appreciated in summer and hated in winter, you cannot deny its cooling effect. White fluffy clouds seen from space are bright because they are reflecting much of the sunlight they receive into space. Increasing cloud cover is potentially one option, but another bright idea is to make existing clouds brighter and whiter so they reflect more light rather than absorbing it. This is a win-win scenario since it has the potential to cool the Earth without decreasing the amount of sunlight reaching the dry land and the solar panels and crops that need it.
Clouds are made of water droplets and the albedo — the brightness — of any given cloud is a function of the size of those droplets. Large droplets absorb more light and heat than smaller ones, so clouds with larger droplets (like heavy rain clouds) are darker both on the bottom and the top. Meanwhile, friendly looking, bright white clouds have much smaller droplets. The smaller droplets mean that the water in the clouds has more overall surface area for a given amount of water content. That surface area is what reflects sunlight, making the return of light to space much more effective.
It turns out you don’t even need clouds for this since there is always some water vapor in the air. A haze of water droplets is all that is required to reflect heat back into space.
Consider for a moment how much the albedo of the Earth’s surface affects how warm it is. The ocean is very dark because its liquid water has a single surface (except at the crests of waves and in foam where air separates water droplets) and so absorbs a lot of heat. That heat has to go somewhere. Much of it sinks into the depths, but a lot is retained and carried away to the land where it rains, raising the overall temperature. Clouds and ice, meanwhile, are made of droplets or crystals that have a lot of surface area and so reflect a lot more light than the ocean. This had a profound effect on Earth’s history since at one time the Earth was completely covered in ice — snowball Earth. The ice was white and so reflected most sunlight into space. Eventually changes in the Earth’s orbit caused the ice to melt a little and when enough ocean was exposed, the melting accelerated. The same thing happened at the end of the ice ages and is happening now. Losing ice at the poles means that even more heat is being absorbed in the ocean, but if that ice can be replaced with water vapor then all is not lost.
Marine cloud brightening is taking this concept further by proposing that ships spray seawater into the air increasing the salt content in the atmosphere by aerosolizing it. (This happens naturally with waves.) The water would mostly fall back into the ocean but some of it would carry salt into the cloud layer. The salt would seed water droplets increasing the water droplet content of the clouds. This will increase the Cloud Droplet Number Concentration (CDNC) making the clouds denser with smaller droplets. Increasing this number makes the clouds brighter and whiter. Pollution plays a similar role over land, seeding clouds with dust particles, but over the ocean there is less dust and pollution than on land so salt has to serve that function.
Spraying the sea water into the air has both an indirect effect of reflecting light into space itself and a direct effect of brightening marine clouds. And it turns out that the indirect effect may be important, meaning that the clouds aren’t the only source of brightening as researchers thought just ten years ago.
The best places to carry out marine cloud (or sky) brightening are places with significant low cloud cover. Subtropical stratocumulus clouds off the west coasts of North and South America and Africa in a band from 30 degrees north to 30 degrees south are the best ones to try. The approach is relatively low tech compared to orbital mirrors. Solar powered robotic ships could patrol those areas and simply spray finely aerosolized seawater into the clouds from smokestack-like tubes. The salt would fill the air, rising into the clouds and seeding them.
Sea salt injection should begin soon and continue until at least 2070 with a target of about 2 Watts per meter square reduction in global sunlight. By 2070, global temperatures will be at least 0.5 C lower than they would be without this geoengineering campaign. Larger areas would have an even greater effect up to as much as 3 degrees C. While that may not sound like much, it could spell the difference between life continuing and global ecological collapse and an accompanying humanitarian crisis on a scale never before seen. And since it is just seawater, the risk is relatively minimal versus other solutions. About 1500 robotic ships would be needed to cover a large enough area. Yet that is a tiny fraction of the total shipping in the world.
Depending on the target, e.g., 2 W-5W per meter square reduction, this campaign might cost hundreds of millions to several billion USD per year. That is a small price to pay for a global economy.
Another advantage is that it can be used locally as well to protect already melting ice sheets or the Great Barrier Reef off the coast of Australia which is in the process of dying from increased temperatures. Even a small campaign could save these precious resources for future generations.
The Marine Cloud Brightening Project run out of the University of Washington is currently trying to expand scientific understanding of how to carry out the technique on a global scale. The first step is to carry out controlled field studies that will bring the approach out of computer models into the real world. I for one think this ought to be a funding priority. After sufficient data is collected, it would be up to major powers to fund a global effort.
Unlike CO2 emission reductions, this geoengineering campaign could be underwritten by only a few wealthy nations and carried out in international waters. It seems that, given that cutting emissions promises to be a long and painful process with even longer to see results, cutting global sunlight is not optional.
Marine Cloud Brightening Project
Cloud Brightening from Shipping in Subtropical Low Clouds. Diamond, M. S., H. M. Director, R. Eastman, A. Possner and R.…faculty.washington.edu
Latham, John, et al. “Marine cloud brightening.” Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370.1974 (2012): 4217–4262.
Ahlm, Lars, et al. “Marine cloud brightening–as effective without clouds.” Atmospheric Chemistry and Physics 17.21 (2017): 13071–13087.