Marine Cloud Brightening

Planetary warming is concentrated in the world’s oceans, which have absorbed 31% of CO2 emissions from human activity and 93% of the excess heat produced by our emissions, equivalent to the energy of 3.6 billion Hiroshima bombs. In 2019, the highest temperatures ever were recorded for the global ocean.

The top layer of the ocean is warming 24% faster than it was decades ago, causing harm and disruption to the many species that live in this layer, including key species like phytoplankton and coral reefs.

Ocean Warming Trends in 2018. Credit: National Atmospheric and Oceanic Administration

The consequences of ocean warming ripple across the world, from negative impacts to coral, fisheries and other marine ecosystems, to rising sea levels as water expands and polar ice melt is accelerated, and more extreme tropical storms.

Harm to Ocean Life

In the past three decades, the number of days where marine heat waves were occurring globally estimated to have increased by 50%. Marine heat waves are defined as events in which seawater exceeds the 90th percentile of the expected temperature range for 5 or more consecutive days. Marine heat waves can cause severe harm to ocean ecosystems and the species they support, including coral, kelp and commercial fish or shellfish species.

Credit: MarineHeatWaves.org

Coral reefs are particularly vulnerable to harm from marine heat waves, which can cause bleaching events that weaken or kill coral. Due to heat waves and bleaching, the Great Barrier Reef has lost half of its coral between 1995 - 2017. The very survival of this massive reef, visible from space, is threatened in the coming decades due to ocean warming.

Overall, though some commercial fisheries do benefit from warming oceans, the overall effect is a net negative. Marine species have evolved to live within a narrow range of temperatures, and the rate of climate change far exceeds their natural ability to adapt over time.

Melting Sea Ice

The melting of polar sea ice is destabilizing the Arctic and Greenland as a whole, with major consequences for sea level rise, polar vortexes via an chaotic jet stream and planetary increase in temperature as highly reflective ice is lost to dark, light-absorbent ocean waters.

This is occurring due to a combination of warming air above and warming seas below ice. In fact, one study found that warming oceans were responsible for a greater amount of sea ice melt in the eastern Arctic than warmer air temperatures were.

More Extreme Tropical Storms

Hurricanes, typhoons and cyclones, which are all the same natural phenomenon, are the world’s most expensive natural disasters. While it is unclear if global warming affects the frequency of typhoons, it appears that a warming climate - and warming seas - likely fuel greater occurrence of major Category 4 and 5 storms, while displacing weaker Category 1 and 2 systems.

Rapid intensification of tropical storms, when winds increase by 35 miles per hour or more within 24 hours, is also increasing by 4 mph per decade. In 2020, rapid intensification of hurricanes occurred a record 10 times, most notably with Hurricanes Eta & Iota each intensifying wind speeds by 80 mph in 24 hours, only weeks apart. The apparent cause? Overheating surface waters.

Hurricanes Katia, Jose and Irma in September 2017. Credit: NOAA

The warming of Earth’s oceans, therefore, has several far reaching impacts that will negatively affect human civilization. Even if we were to halt carbon emissions, the oceans would continue to warm, rise and melt the ice for a long time, because it has already absorbed so much heat from the atmosphere.

While there are many long term, systemic changes needed to best support the ocean and its creatures, in the near term we can protect against some of the worst impacts in certain regions by brightening marine clouds artificially.

Already, fuel exhaust from large container ships creates artificial cloud tracks as the vessels cross the ocean, causing a short-lived cooling effect.

Unnatural Cloud Tracks Made from Shipping Vessels. Credit: NASA

Marine cloud brightening (MCB) works by spraying sub-micron concentrations of seawater into the sky using water cannons similar to snow making machines used by ski resorts. To be clear, the purpose of MCB is to make existing clouds brighter, not create new clouds.

Once blown into the air, the water evaporates, leaving a residue of tiny salt particles that get distributed by turbulence throughout the marine cloud layer. These salt particles act as “ideal cloud condensation nuclei”, according to a paper by Professor Stephen Salter.

At a certain height, the salt residues would increase the number of cloud droplets and boost reflectivity via the “Twomey effect”, which demonstrates that smaller droplets are whiter and more reflective. Ships conducting cloud brightening would suck up water from the surrounding sea for their high pressure cannons to blast into the air.

The first prototype cannon deployed in Australia. Credit: SavingtheGreatBarrierReef.org

Salter’s work modeling MCB application found that it is far more efficient if brightening occurs gradually over the course of the year, rather than as a sudden response to a hurricane or marine heat wave. Basically, it is less work to marginally cool surface waters over months, instead of trying to sharply drive down temperatures in a short period of time.

The concept behind cloud brightening is steeped in science, but until recently it was largely conceptual. Now, MCB is in the early stages of testing and deployment by a consortium of groups in Australia, whose mission is to protect and preserve the endangered Great Barrier Reef.

Prototype MCB vessel near the Great Barrier Reef. Credit: SavingtheGreatBarrierReef.org

Desperate times call for creative thinking. After a severe bleaching event in 2016 and more certain to occur in the future, a team of Australian scientists and engineers identified MCB as one of the most promising ways to protect the reef. In March 2020, the team conducted its first small scale trial, and lead scientist Daniel Harrison called the results “really, really encouraging”. 

Based on these results, Harrison believes that with MCB “we could reduce the bleaching stress by about 70 percent". If greenhouse gas emissions continue to rise, MCB could protect coral reefs for 20 - 30 years before being overwhelmed by the effect of ocean warming. If paired with decisive emission cuts, MCB could actually expand coral reefs. (Data from October 5, 2020 presentation by Daniel Harrison).

MCB application would be targeted, based on key marine ecosystems under heat stress, such as fisheries in the North Sea or the Great Barrier Reef, or along the Gulf Stream current, whose warming waters both fuel hurricane intensity and contribute to Arctic sea ice melt.

Gulf Stream Current in the Atlantic. Credit: BBC Environment

For ships built specifically for MCB on the open seas, Professor Salter estimates the construction and maintenance costs over 25 years to be $400,000 per vessel. Small fleets could be deployed regionally to brighten clouds over vulnerable areas, working to gradually cool surface waters over the course of the year.

Professor Salter and John Latham, another prominent scholar of cloud brightening, found that 300 advanced MCB vessels could fully offset the warming effect of 520+ ppm CO2 levels (we are currently around 415 ppm) by increasing Earth’s total albedo by 1.1%.

However, there are legitimate concerns that such broad application of MCB could alter regional precipitation patterns. For example, a study from the Journal of the Royal Society showed that cooling the mid-Atlantic causes reduced rainfall and drying in the Amazon, impacting the world’s largest rainforest and adjacent agriculture.

The same study also found that where and how MCB was applied were key factors that determined rainfall impacts. In other words, certain configurations of MCB did result in drying, while others created no change in precipitation.

For these reasons, deploying MCB on a more targeted, localized scale involves less risk of disrupting regional precipitation patterns. Cumulatively, many small scale MCB efforts could still boost Earth’s albedo enough for a measurable cooling effect.

Even if more modest application of MCB caused unacceptable side effects, it is not an irreversible process. We can simply stop brightening marine clouds, and systemic equilibrium would be restored, particularly since the warming climate would overwhelm temporary cooling effects. With so much at stake in our seas, brightening a few clouds doesn’t seem so crazy.