The Science of Sea Spray

The ocean covers 71 percent of the earth’s surface, and atop all that water is a huge amount of sea spray. That foamy whiteness that so enthralls beachgoers and seascape painters is a collection of tiny air bubbles. Within them, ocean-based chemicals that aren’t particularly attracted to water congregate. When those bubbles burst, those hydrophobic chemicals escape into the atmosphere, where they can affect the planet’s climate.

How do they affect our climate, exactly? That’s one of the biggest questions in climatology—how aerosols like sea spray influence heat transfer and precipitation in the atmosphere. Aerosols are tiny bits of liquids or solids suspended in the air. Depending on their size and chemical makeup, these particles can either warm or cool air by absorbing or reflecting heat, respectively. So, knowing what ocean spray consists of would help us understand the role it plays, but believe it or not, we know very little about these marine projectiles.

The chart below from the Intergovernmental Panel on Climate Change shows most of what we do know about the effect certain substances have on climate. Don’t worry if it’s not immediately clear—I’m going to explain it after you spend a few moments staring blankly.

The line that runs vertically down the middle of the chart—the one marked with a zero—divides airborne compounds between those that have a cooling or warming effect. Take, for example, carbon dioxide, the first substance on the chart. Its brown bar runs to the right of the center line, which means it has positive radiative forcing potential—a fancy way of saying it traps heat and warms the planet. Also take note of the black line at the right end of that brown bar. That’s an error bar, signifying how certain we are that the estimate is correct. It’s relatively small compared to the brown bar, because we’re quite certain that CO2 traps heat.

Now scroll down a few lines to where it mentions aerosols. Along with ocean spray, aerosols include things like soot, smoke, and dust. The warming potential for those compounds are all over the place, and the error bars are large. Scientists aren’t sure whether they warm or cool the planet. For a researcher, those error bars look like a great funding opportunity. For a policymaker, it’s a major worry because there are huge gaps in our knowledge of these tiny compounds.

The varying sizes and chemical compositions of these particles cause much of the uncertainty. Particles that are roughly the same wavelength as incoming solar radiation are very good at scattering light, preventing the sun's heat from reaching the earth by reflecting it back into space. Some particles, such as soot, have the opposite effect—their organic content is really good at absorbing energy. It works like a blanket, trapping the heat radiating off the earth’s surface and preventing it from escaping into the atmosphere.

Aerosols also affect precipitation in complicated ways. Water in the atmosphere condenses on airborne particles, creating clouds. Without aerosols, we’d have no clouds. Without clouds, we’d have no rain or snow. Here, too, the number and nature of the aerosols matters. Clouds of densely packed particles leave little room for water, making for rainless clouds. Clouds with fewer particles allow the droplets to grow big and heavy and eventually fall to the earth. Particles that encourage the formation of ice crystals, rather than water, are especially likely to cause precipitation, as ice tends to absorb surrounding water droplets.

So the contribution of ocean spray to climate change depends on what, exactly, ocean spray is. Recently, a team of international scientists recently got us started on that mystery by building a miniature ocean in a California laboratory. They created a wave-maker, filled it with nutrient-rich ocean water, cultured some tiny marine species, then turned the thing on. Their results came out this week in ACS Central Science.

The researchers found that the diverse array of plankton, bacteria, and viruses living on or near the sea surface gives rise to a complex, constantly changing community of chemicals. Carbohydrates, lipopolysaccharides, proteins, and lipids all comprise sea spray. (There are some salt particles, too, of course, but their shapes change frequently as they react with other types of chemicals.) It’s a big ocean, and the nature of all those substances factor into the heat-trapping potential of the earth’s atmosphere, as well as precipitation levels.

What does it all mean? If we made a new chart above, would the sea spray bar move to the right or the left? We don’t know yet. This study is the beginning of something, not the end. It’s a step forward and inches us closer to better climate models, but for now it raises more questions than it answers. In other words, it’s good science. Stay tuned.