Crook, J. A., L. S. Jackson, and P.M. Forster (2016), Can increasing albedo of existing ship wakes reduce climate change?, J. Geophys. Res. Atmos., 121, doi:10.1002/2015JD024201.
Engineering Earth’s albedo
We need a plan to mitigate climate change, but while countries work to reduce CO2 emissions, it might not be enough to avoid the worst effects of warming. An alternative way to combat climate change is through geoengineering: manipulating the planet to retain less heat despite rising CO2. The most popular geoengineering technique involves deflecting heat away from the earth. For example, injecting tiny sulfur particles into the upper atmosphere could redirect enough heat away from the atmosphere to offset the effects of CO2 emission. Another way to deflect heat is to increase the earth’s albedo, or reflectiveness. In general, dark surfaces tend to absorb heat while light surfaces reflect it. Think of how hot asphalt gets in on a sunny day, or how wearing a white t-shirt feels cooler than a black one. Changes in the color of the earth’s surface can also have a significant impact on climate. For example, the melting of glaciers (with very high albedo) is expected to accelerate global warming because it exposes darker colored land, which absorbs more heat.
In this new study, researchers explore a creative new idea for combatting climate change: manipulating the wake of cargo ships to reflect more light and increase the ocean’s albedo. On any given day there are about 30,000 merchant ships out to sea. These propellers kick up tiny bubbles that are lighter in color than the surrounding ocean and therefore have higher albedo and ever so slightly cool the planet. The problem is that these bubbles don’t last long enough to have much effect on climate. They get popped by wave action or submerged by wave action, and no longer reflect light.
Evaluating the potential of ship wake geoengineering
This paper explores the potential of manipulating ship wakes to better reflect light and fight back against climate change. First, propellers can be designed to increase bubble production. Millions of tiny (1/1000 of a centimeter across) bubbles are more efficient at reflecting light than fewer, larger bubbles. Ship engines can be designed specifically to produce these “microbubbles”. Second, the life of the bubbles can be extended by adding surfactants, a mix of organic compounds that promote bubble stability. Algae naturally produce some surfactants, but more could be added behind ships to help their wakes last longer.
The authors used modeling to evaluate the potential effect ship wakes could have on climate change. First, they calculated the increase in microbubbles and their longevity that would be necessary to have a noticeable cooling effect.
Then, they used a climate model to compare how temperature patterns would change with and without an increase in microbubbles from ship wakes.
The models show that manipulating ship wakes to maximize albedo could reduce the energy absorbed by the ocean by up to 4 watts per square meter. For comparison, about 1 watt per square meter is the smallest change that would trigger a detectable difference in global temperatures. At first, increasing the lifespan of bubbles in wakes has a large effect on cooling (Figure 1), but further increasing the bubble lifespan from days to weeks yields diminishing returns because the wakes start to overlap. When bubbles last 10 days, wakes cover about 5% of the ocean surface, but when they persist for 90 days, a full 54% of the surface is covered! Further increasing the lifespan of the bubbles doesn’t help more at this point because shipping lanes are already permanently covered in wake.
The simulated increase in ship wake was successful in cooling the planet, offsetting the warming from CO2 emissions (Figure 2). However, the cooling was not evenly distributed across the globe. Instead, it was concentrated in the northern hemisphere, which contains the most well-traveled shipping routes (Figure 3). In contrast, uniformly increasing the Earth’s albedo with a method like aerosol injection into the upper atmosphere would cool both hemispheres more or less equally. Being able to control area cooled by geoengineering is a potential advantage of this approach over other climate engineering proposals, as it could conceivably target the areas hardest hit by climate change such as the Arctic.
However, manipulating wake trails suffers from many of the risks and drawbacks of other geoengineering schemes. It would have no effect on other consequences of increased CO2 like ocean acidification. Its cooling effect would have to keep increasing indefinitely to offset climbing CO2 emissions. If we stopped the geoengineering, temperatures would come roaring back the same level as if we hadn’t tried it at all, but much quicker, giving ecosystems less time to adapt. In addition, bubbles disrupt gas exchange between the atmosphere and the ocean surface. Right now, the ocean absorbs a large fraction of emitted CO2. If geoengineering slows its uptake, that CO2 will stay in the atmosphere and contribute to additional warming. Finally, the ecological effects of adding surfactants is unknown and could be harmful to algae and marine life in ways we can’t anticipate. Manipulating wake trails is a clever new idea for cooling the planet and it’s a good idea to think about how to avert catastrophic warming if a political solution is not forthcoming. However it remains clear that the only 100% safe and effective way to stop climate change is to stop burning fossil fuels.