Whiter than white: could brightening clouds reduce symptoms of climate change?


Marine cloud brightening: it’s a concept that has been floated in climate engineering discussions for some time. But what are the moral implications of this geoengineering technology, and how likely is it to be implemented?

What is cloud brightening?

Cloud brightening is the idea that we could increase a cloud’s albedo (reflectivity) to reflect a greater amount of radiation away from the earth, thus producing a cooling effect. This is one of several ideas for geoengineering (climate engineering); a means of reducing the symptoms of climate change.

Cloud brightening involves seeding clouds with a fine spray of saltwater, which encourages cloud micro-droplets to form. Unlike cloud seeding, where large droplets form and produce a rain shower, the droplets in cloud brightening are smaller and remain in the sky as “white cloud”. The micro-droplets scatter incoming radiation, and increase the longevity of the cloud.

Cloud brightening produces micro-droplets that reflect more sunlight. NASA image by Robert Simmon http://earthobservatory.nasa.gov/Features/Aerosols/page4.php

Cloud brightening produces micro-droplets that reflect more sunlight. NASA image by Robert Simmon http://earthobservatory.nasa.gov/Features/Aerosols/page4.php

This method would be most effective on clouds over the ocean. Clouds over land already contain small particles of dust and pollution, so the introduction of saltwater aerosol would have little effect.

Cloud brightening falls into the category of geoengineering techniques known as solar radiation management (SRM), together with stratospheric aerosols, space mirrors, and painting roofs white. All of these techniques focus on reducing the amount of radiation reaching the earth’s surface. The other category of geoengineering techniques is carbon dioxide removal (CDR), which involves taking carbon dioxide out of the atmosphere and trapping it in storage.

The cause vs the symptoms

Climate engineering techniques do not address the true cause of the problem of climate change – carbon dioxide emissions. They only have the potential to partially manage some of the symptoms at best. For example, solar radiation management methods do nothing to address the symptom of ocean acidification, and the maximum cooling effect that cloud brightening can achieve is limited.

This is one of the arguments against pursuing geoengineering. Surely we should focus our efforts on mitigation, rather than a partial fix?

The answer to this is “absolutely”. Climate engineering is risky and full of uncertainty, and will impact the entire world’s population.

But then what is the motivation for the research that is occurring in the USA and UK into geoengineering methods?

This answer is more nuanced, and is based on the timescales that are involved in mitigation and climate change. Even if we stopped increasing our emissions from today, CO₂ levels in the atmosphere would continue to rise for hundreds of years. Reaching a ‘tipping point’ in the climate may be unavoidable. How likely is it that we will be able to reduce our emissions quickly enough to avoid the worst effects of climate change?

We base our future predictions on climate modelling, which is validated against past observations. There is much scientific uncertainty, but current predictions indicate that we will see significant climate impacts in the future.

Research into the feasibility, risks and impacts of climate engineering has begun. This research may lead us to conclude that some (or even all) geoengineering techniques are simply not viable. The development times for the most-promising technologies are currently estimated to be between decades and centuries.

At some stage in the future, climate engineering, with all its shortcomings, may offer a better way forward than living with the effects of extreme climate change.

Get-out-of-jail-free card

Some argue that climate engineering is a “get-out-of-jail-free card” to carbon emitters. Others prefer to call it an “insurance policy” for the earth. Will consideration of climate engineering lessen political will for reducing CO₂ emissions? (This is known as the “moral hazard” argument.)

Climate engineering is clearly not a desirable course of action. It offers no freebies or rewards.

There are few proponents of geoengineering at present. Discussion of this technology often acts as to raise awareness of the urgency of the climate situation, thus increasing resolve to reduce emissions.

What is clear however, is that the first publicised outdoor test of a climate-engineering technology will place a marker in the political and scientific landscape.

Testing cloud brightening

In a recent paper, University of Washington atmospheric physicist Rob Wood describes a possible outdoor test of cloud brightening. This test is yet to receive funding support.

The first stage of Wood’s experiment involves testing the seawater-spraying technology, by examining the properties of the sprayed particles and their dispersion in the wind. The second stage of the test measures the effect of an aerosol plume on a cloud, and the third stage would look at the effect of 5-10 plumes arranged in a line.

Much like the SPICE balloon test, the experiment would test the feasibility of the spraying technology. Unlike SPICE, the experiment would also involve monitoring the effect of introduced particles on the atmosphere.

The same governance and intellectual property issues that led to the cancellation of the SPICE test also apply to cloud brightening. In view of this funding of a cloud brightening test would be a controversial and symbolic move.


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