22 June 2017 | By James Conca | Forbes.com |
Seeding clouds as a way to change weather began its history as a serious science during WWII, originally as a way to interrupt severe storms or to produce rain and snow. It never developed in practice. But more recently it has been investigated as a way to reflect back heat into space in order to mitigate global warming. Even before the United States pulled out of the Paris climate accords, there was growing skepticism that the world could, or would, decrease its total greenhouse gas emissions by 2050 enough to actually prevent the worst of global warming and keep the temperature rise to only a few degrees. To succeed, we have to reduce human carbon emissions by a total of about 90% by 2050 (see figure). Averaging about 5% reductions per year beginning in 2015 would have done it. But in 2015, emissions were still growing worldwide. The next deadline is 2020, when we have to start reducing emission by almost 10% per year until midcentury. We are unlikely to meet this target as well.
So many scientists have wondered how to either remove GHGs from the atmosphere or mitigate the effects some other way. Removing GHGs is really difficult and really expensive. Much more expensive than preventing them from entering in the first place. So if we refuse to spend the amount of money and effort needed to reduce emissions, we certainly wouldn’t spend even more trying to get them back out. The other strategy is to mitigate the effects of warming by directly cooling the planet. Cooling occurs naturally when a greater amount of particles such as soot from huge fires, particulates and gasses from huge volcanic eruptions, or excessive amounts of ice crystals, enter the atmosphere and either block incoming solar rays or reflect them back into space. Cooling also occurs when clouds dissipate fast and allow more radiation from the warmer surface below them to escape into space. Changing the Earth’s albedo, or reflectivity, is what these strategies target. Darker albedos capture and hold heat, lighter albedos reflect heat. It’s why snow always melts first on a new asphalt street before most other surfaces around it. We have seen the cooling effects of big fires and big volcanoes in the geologic record going back millions of years, so we know they work. But these processes are uncontrolled and in some cases caused very big climate changes themselves that had their own bad effects. Enter cloud seeding. Rain, snow and ice precipitate out in the atmosphere in one of two ways. Usually, the temperature drops and water begins to precipitate on a particle of something like dust or salt, called atmospheric aerosols. The particle serves as a seed crystal, or a nucleation site, because it provides a surface that makes precipitation possible and easier. It decreases the energy barrier that prevents the gaseous atoms from arranging themselves into the new solid or liquid structure. This is called heterogeneous precipitation. It is possible to precipitate without seed crystals, referred to as homogeneous precipitation, that can occur under special conditions. Homogeneous precipitation may even be the better bet for cloud seeding. After much research, we seem to have settled on a few possible methods for increasing the Earth's albedo. One, inject sulphur dioxide (SO2) into the stratosphere where it oxidizes to form sulfate aerosol particles which scatter incoming sunlight. Two, use automated ships to spray droplets of seawater into the atmosphere above the ocean where they evaporate to form an elevated concentration of sea-salt aerosols which seed higher concentrations of cloud droplets in clouds over the ocean, increasing their reflectivity. Third, fleets of large drones would crisscross Earth’s upper latitudes during winter, sprinkling the skies with tons of extremely fine dust-like materials every year. Various seeding materials have been looked at, including alumina, diamond dust, calcium carbonate, and bismuth tri-iodide, all with a hope of decreasing unwanted side-effects like ozone depletion. The amount of materials needed also doesn’t seem to be that large - thousands of tons and millions of dollars, not billions and billions. But it turns out that where you seed may be as important as what you seed. David Mitchell at the University of Nevada is convinced that cloud seeding could work as long as it’s done in regions where cirrus clouds form primarily without dust particles. In the northern latitudes. Under very cold and humid conditions, near the poles during winter, tiny ice crystals can form on their own, spontaneously, without dust or any other seed crystal. Obviously, much research and testing would need to be performed to really get a handle on this approach. Fortunately, you can make the test small and short, so big bad things don’t occur. Which is an important aspect of cloud seeding. It does not change the CO2 concentrations in the atmosphere, but treats a symptom - heat. Seeding would be done in specific regions so we may not be doing the whole Earth all at once. Because this method treats the symptom, seeding would have to be performed continuously for hundreds of years until the climate system rebalanced itself, assuming we eventually decrease our emissions. The effects of seeding happen fast, so it really could be a last ditch effort if we see things getting as bad as predicted. But these complicated systems are difficult just to predict, let alone control. Recently, Alan Robock at Rutgers discussed the benefits and risksof cloud seeding as a way for society to evaluate whether this approach should be tried at all. Much of the basis for doing anything would depend on how well we think we understand the climate and atmospheric systems. Or how scared we are of doing nothing. We actually have sufficient understanding of these systems to begin evaluating cloud seeding. For example, we know that temperature and precipitation cannot both be controlled at the same time, that summer monsoon precipitation would be reduced with seeding, that even if global average temperature could be kept from increasing, there would be cooling and warming in different places not easily predictable beforehand. Ice sheets melt from the bottom, and changing the amount of solar radiation falling on the planet may not be very effective at slowing their melting. The history of past weather and climate modification attempts provides strong lessons about the difficulty of governance and the dangers of military applications, but we’ve messed with the planet so much already that thinking we can keep it pristine by doing nothing is fiction. In Robock’s summary, the primary benefit of seeding is reducing global warming and many of its negative impacts. This may be so important to society that we may decide to do something radical like seeding. Risks or concerns like unwanted ecological changes, ozone depletion, continued ocean acidification, erratic changes in rainfall patterns, rapid warming if seeding were to be stopped abruptly, airplane effects, to name a few, may just not be bad enough to override the imperative to keep temperatures down. Like many epic questions of the future, it would be wise to look into this method further, so we have something reasonable in case we ever need it. The big question, of course, is whose hand would be on the thermostat? Who would make these decisions? If we can’t move as a planetary group to prevent emissions, how would we pull this off? Should we even try?