Mixed-Phase Cloud Thinning

In 1946, scientists at General Electric were studying how ice formed on the wings of aircraft when they made an interesting discovery. They found that if they seeded a cloud containing supercooled droplets – below the freezing point but still stuck in liquid form – with dry ice (solid carbon dioxide), they could turn some of the water droplets into new ice crystals. As ice crystals are substantially larger than the droplets, they fall out of the cloud sooner. This marked the birth of 80 years of cloud seeding research and deployment.

Primarily used as a method to induce a bit more precipitation from clouds, cloud seeding is now in use in some form in at least 50 countries. It often involves using small amounts of silver iodide as a “nucleation” particle, allowing ice to form around it inside the cloud. That basic concept, of seeding clouds in order to change their properties, may have another use: it could potentially thin out mixed-phase clouds to reduce the amount of heat they trap.

This concept, known as mixed-phase cloud thinning (MCT), might offset some of the warming caused by increasing greenhouse gas concentrations, though only under certain conditions.

Mixed-phase clouds have varying climate effects

Different types of clouds can have different effects on the climate. For example, cirrus clouds are thin and exist high up in the atmosphere and generally act to trap heat below them. Lower down, marine stratocumulus clouds are thicker, and have a cooling effect by reflecting sunlight back into space.

As the name implies, mixed-phase clouds have varied compositions and effects, incorporating a mix of ice and water that can either trap or reflect more heat depending on a variety of factors. For most of the year, they have an overall cooling effect, but during polar winters, they have more of a warming effect.¹

MCT joins a growing list of solar geoengineering approaches that could modify clouds as a way to lower temperatures and offset some of the impacts of warming. Thinning high cirrus clouds could allow more heat to escape, while marine cloud brightening could make low-lying ocean clouds reflect more sunlight back into space.

Researchers have suggested that seeding specifically those mixed-phase clouds that are trapping heat at the poles during the winter months could thin them out and produce a regional cooling effect.² At scale, this might help reduce the loss of sea ice in the Arctic and around Antarctica.

Potential to offset Arctic and Antarctic warming

The polar regions are warming faster than the rest of the world – as much as four times faster in recent decades.³ Melting on the Greenland and Antarctic ice sheets has been accelerating, with the potential to raise global sea levels significantly. The ongoing melting of sea ice, meanwhile, drives a critical feedback loop: the darker ocean absorbs heat instead of reflecting it like the white ice, which means melting creates more warming, which leads to more melting, and so on.

By focusing on mixed-phase clouds over both the Arctic and the Southern Oceans, MCT might be able to limit polar climate change and its impacts. Compared to other solar geoengineering ideas like stratospheric aerosol injection, it could only offset a modest amount of warming, but it would target a key area.

One study estimated that an optimised MCT programme could offset a significant portion of the expected warming in the polar regions, helping to slow sea ice loss.²

Many unknowns remain

The primary challenge of MCT lies in the complexity of the clouds in question, which makes accurate computer models of their responses a critical and difficult issue. Studies have found that the ratio of ice and liquid, crucial to determining the efficacy of MCT, can vary significantly even inside a single cloud.^4–6

Scientists’ understanding of mixed-phase clouds has improved in recent years, with field experiments like CLOUDLAB in Switzerland studying how the clouds respond to seeding. However, they are still more complex than other types of clouds, including the cirrus and low-lying marine clouds that could be targeted by other forms of solar geoengineering.

The variability of these clouds suggests that picking and choosing where to seed would be a critical part of any MCT programme, and more work still needs to be done on the clouds’ responses to seeding.⁷ There has also not yet been a detailed technical analysis of what MCT deployment would look like in practice: how would the particles be delivered into mixed-phase clouds, and at what cost to achieve substantial cooling?

Scientists are also still exploring which particles might be best suited for MCT. While silver iodide is widely used in cloud seeding efforts and could be applied, there are ecological concerns with its widespread application.⁸ Naturally occurring particles including mineral dust could also work, though the optimal particle is not yet clear.²

Even if these scientific and technical challenges can be resolved, MCT would be a relatively limited form of solar geoengineering. The total cooling possible is not nearly as large as other methods and the concept would likely work best only in the polar regions, though it may still play a useful role there.²

Research into MCT is at a relatively early stage and plenty of questions about its feasibility and consequences remain. More computer modelling research, outdoor experiments, observations of natural processes, and engineering assessments will be needed to determine whether MCT is a viable solar geoengineering approach.