Rain helps the ocean trap more carbon

Three decades ago, David Ho set up two pink, dinosaur-patterned children's pools in the parking lot of a NOAA building in Miami, where afternoon thunderstorms were common. He was 22, freshly done with his undergraduate studies, and working as a technician at NOAA.

He filled both pools with water, added a gas tracer, and put a canopy over one pool as a control. Then, every day for several months, he waited for the downpour, getting drenched as he pulled up samples from each pool in glass syringes.

"It was pretty miserable," he said, "but I got some interesting results."

These early experiments showed that rain enhances the transfer velocity of carbon dioxide (CO2), or the efficiency with which it is transferred from air to water. Ho, now an oceanographer at the University of Hawai'i at Manoa, has pursued the research topic ever since, scrutinizing the effect at a NASA rain simulator and during research voyages in the Pacific.

His latest study is the culmination of this work, delivering the first comprehensive and global estimate of what happens to CO2 fluxes when rain hits the ocean. The global ocean takes up about a quarter of the CO2 emissions from human activities, and this research shows that rainfall raises this uptake by 140–190 million metric tons, or 5%–7%, per year.

"It may be surprising that it should take so long to quantify this process, but partly it's because this is a hard problem to examine," Ho said. Most measurements of gas concentrations in the ocean come from ships, which collect water samples at a depth of 5–7 meters. But because rain hits the surface, its effects are invisible at those depths. "This has been ignored because we don't have the data."

Turbulence, dilution, and wet deposition

Carbon dioxide uptake in the ocean is not uniform. Some regions act as sinks, drawing the gas down, while other areas release it.

When a raindrop falls into the ocean, it temporarily changes the physics and chemistry of the seawater around it. The study identified three main means by which rain increases the ocean's carbon uptake: turbulence, dilution, and wet deposition.

When a drop strikes the surface, it generates turbulence that brings more water in contact with the atmosphere and the carbon it contains. Each drop is also a splash of relatively fresh water, which dilutes the seawater and changes the air-sea concentration gradient of CO2, enabling higher uptake. And finally, wet deposition refers to how each drop absorbs CO2 as it falls through the atmosphere, then injects that gas directly into the ocean.

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Wet deposition is a one-way flux, said Rik Wanninkhof, an oceanographer at NOAA who pioneered the use of inert chemical tracers to study gas exchanges between air and water. Wanninkhof was not involved in the latest study but worked with Ho on the pool experiments. "We didn't think about this in early studies, but on the global scale, this research shows that this is probably one of the single largest effects of rain on carbon dioxide fluxes," he said.

The study was led by Laetitia Parc as part of her doctoral studies at Sorbonne Université. For her, the most significant finding is that the size of this rain effect on ocean carbon uptake is comparable to the size of the global carbon budget imbalance, or the estimated difference between total carbon emissions and total carbon absorption by the ocean and land. Being able to quantify these small-scale processes at the air-sea interface should improve scientists' ability to model carbon flows between the atmosphere and the ocean, she said.

Tracking rain patterns across the ocean

A pivotal part of the research is a model, developed by coauthor Hugo Bellenger, that tracks rain-induced salinity changes at the ocean surface. "Physical models for the ocean skin were first developed in the 1960s," said Bellenger, a climate modeler at the Laboratoire de Météorologie Dynamique at the French National Centre for Scientific Research (CNRS). "But while they tracked temperature changes well, there [was previously] no measurement of its salinity counterpart."

This model allowed the researchers to delineate where rain has the strongest impact. Turbulence and dilution play prominent roles in the tropics, which are characterized by heavy rains and weak winds. The effects of wet deposition are also significant in tropical regions, as well as in other areas with heavy precipitation, such as storm tracks and the Southern Ocean.

This came as a surprise.

"We were expecting the rain to mainly increase carbon uptake in the tropics. To find that it could also have a noticeable impact in regions of higher latitudes was unexpected," Parc said.

Tatiana Ilyina, an Earth scientist and carbon cycle modeler at the Universität Hamburg who was not involved in the research, said the study quantified an effect that so far has not been considered in observational or model-based estimates of the global carbon budget. "It sends a strong signal that we have no excuse not to consider these effects in our global carbon budget estimates."

The effect could grow stronger as the world warms and precipitation increases: The team found that wet deposition rates scale with the rate of rainfall. Storms are already dumping more rain, and scientists expect precipitation patterns over the ocean to shift with climate change. In their analysis, based on satellite observations and reanalysis of global weather datasets from 2008 to 2018, the researchers saw a slight growing trend in the effect rain has on the ocean's carbon uptake.

The work brings attention to yet another previously unconsidered carbon-climate feedback, Ilyina said. "Getting precipitation patterns right has been a long-standing issue in global models. Now we know that this has direct implications for quantifying the ocean carbon sink."

This article was originally published on Eos.org. Read the original article.