Using Clouds to Fight Climate Change

Guest Post by Kathryn Moore, 2022-2023 Sustainability Leadership Fellow, and Ph.D. Student in the Department of Atmospheric Science at Colorado State University

Most people remember the water cycle they learned in school: water evaporates from lakes, rivers, and the ocean, air carrying this moisture rises, cools, condenses, and forms clouds, and these clouds precipitate water back down to the surface. But this simplified pathway misses a key step in cloud formation–and one that could help us combat climate change.

Cloud formation in the atmosphere requires aerosols

To form a water droplet in the atmosphere, water vapor molecules, floating around as a gas, need to stick together. If enough of these molecules get close enough to each other, they’ll condense, turning from multiple gas molecules into a single liquid cloud droplet. But if only a couple molecules run into each other, the droplet is unstable and will break apart into a gas again. The distance between water vapor molecules is determined by how fast the air carrying them rises. Air rising faster will have higher concentrations of water vapor, meaning the molecules will be closer to each other. In a specially designed cloud chamber, clouds can be formed through exactly this process. But in Earth’s atmosphere, it is impossible for air to rise fast enough to generate the high concentrations of water vapor molecules needed to form a stable liquid droplet. So how do we have clouds if water isn’t condensing?

Cirrus clouds run diagonally across this image captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite off the coast of Chile. Image credit: NASA Earth Observatory, Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

The answer is that microscopic particles floating around in the atmosphere, called atmospheric aerosols, provide a surface for water vapor to attach onto and condense (see animations of this process here). Certain aerosols called cloud condensation nuclei, or CCN, are particularly good at collecting water vapor, allowing clouds to form at the water vapor concentrations present in Earth’s atmosphere. CCN can come from almost anywhere: smoke from fires or volcanoes, sea spray from the ocean, soil or dust, plants, or human activity such as driving a car. When extra CCN are added to a cloud, the available water vapor molecules are split up among the extra particles, forming more, smaller cloud droplets. Clouds with smaller and more numerous drops live longer and usually precipitate less.

Liquid clouds form at low altitudes, but clouds can also form high in the atmosphere. High, or cirrus, clouds form at cold temperatures (colder than -30 °C or -22 °F) and are composed of ice crystals instead of liquid droplets. Cirrus clouds can extend large distances both horizontally and vertically and cover almost 25% of the globe, on average. Unlike liquid drops, if it is cold enough, ice crystals in cirrus clouds form directly in the atmosphere from water vapor molecules. Cirrus clouds formed this way have many, small ice crystals.

Altering cloud properties influences Earth’s climate

On sunny days, it’s clear that clouds leave shadows on the landscape. Climate scientists are interested in clouds for exactly this reason- clouds reflect sunlight, which has a cooling effect on surface temperatures. Liquid clouds with more, smaller droplets reflect more sunlight than clouds with fewer, larger droplets. Using satellites, this effect is visible in regions of the ocean where heavy ship traffic exists. Thin, bright lines of clouds mark ship paths because ships emit many small aerosols from their engine exhaust, which act as CCN and form cloud droplets. Scientists used satellite data of ship tracks and climate models to estimate that, globally, changes in low-level clouds due to human-caused (anthropogenic) pollution has a cooling effect that is equivalent to about 25-33% of the anthropogenic warming caused by greenhouse gases, or approximately 1 Watt of energy per square meter. Without these changes to clouds, the Earth would already be warmer.

Ship tracks observed as thin bright lines over the Atlantic ocean by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite. Image credit: NASA Earth Observatory, Lauren Dauphin.

Although scientists have understood the role of aerosols in altering cloud reflectivity since the 1970s, harnessing it to intentionally reduce surface temperatures was first suggested in the early 1990s. The technique is known as “marine cloud brightening” and involves increasing aerosol concentrations–and thus cloud droplet numbers–in low-altitude clouds over oceans. Marine clouds typically have lower particle numbers than clouds that form over land, making them especially susceptible to changes in brightness caused by adding more aerosols. Researchers have suggested generating salt particles from seawater to add to clouds, since it is plentiful near the clouds of interest, and also avoids negative health effects associated with combustion aerosols such as those that seed ship tracks.

While clouds at low altitudes cool the planet by reflecting sunlight, clouds high in the atmosphere also interact with the heat energy emitted by the Earth’s surface. Like low clouds, if cirrus clouds are deep, they reflect sunlight and cool the Earth. But if they are thin and transparent, they reflect very little sunlight and instead trap a lot of the heat radiating from the Earth, resulting in a net warming effect. At night, this effect is even larger, since there is no sunlight to reflect, but the Earth is still emitting heat the cirrus clouds can capture.

Paradoxically, adding aerosols to cirrus has the opposite effect it does for low clouds: there will be fewer, larger ice crystals instead of many small ones. This is because ice grows quickly, so once crystals form on aerosol particles, they collect all the water vapor around them and don’t leave enough to form any ice crystals directly from water vapor. Although fewer, larger crystals reflect less sunlight (net warming), they also trap less heat (net cooling), and the cloud doesn’t last as long because the larger ice crystals are heavier and fall out of the cloud faster than smaller ones. The cooling effect caused by trapping less heat wins, on a global scale. Scientists used this principle to propose “cirrus cloud thinning” about a decade ago to take advantage of the potential net cooling effect of adding aerosols to cirrus clouds.

Just like ship exhaust forms low marine clouds, particle emissions from aircraft engines create cirrus clouds, and they are even easier to spot. The thin, white “contrails” that mark an aircraft’s path are visible from the ground on clear days. These aircraft-formed cirrus are responsible for 5% of the anthropogenic warming to date, and the amount is expected to triple by 2050.

Climate intervention could be used to cool the Earth

Both low marine cloud brightening and high cirrus cloud thinning are types of solar geoengineering that have the potential to temporarily reduce surface temperatures and offset some greenhouse gas-driven warming. Many scientists, including the US National Academies of Science, Engineering, and Medicine, have endorsed additional research into these and other climate intervention techniques due to concerns that current pledges are not sufficient to meet the Paris Agreement’s 1.5 °C temperature target. The Paris Agreement is an international treaty adopted at the United Nations Climate Change Conference in 2015, with the goal of limiting global warming to 1.5 °C above pre-industrial levels by 2100 to avoid the most severe impacts of climate change.

Although climate intervention methods have been discussed by scientists for decades, research on possible side effects, and particularly how they may vary regionally, has only begun recently. Researchers are starting to assess the safest, most effective, and most equitable ways to apply these techniques, and to understand the technical challenges required to implement them on a global scale. As part of the Marine Cloud Brightening Project, scientists have developed a promising nozzle that could be used to spray saltwater into marine clouds, which they hope to test in a small-scale field campaign.

Marine cloud brightening, cirrus cloud thinning, or other solar geoengineering techniques will not fix or reverse anthropogenic warming, which is primarily caused by greenhouse gas emissions. But they are the only known methods to quickly reduce surface temperatures, and with the right strategy, could be a key part in avoiding some of the worst impacts of anthropogenic climate change.



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