Nitrous oxide is a greenhouse gas with about 300 times the warming potential of carbon dioxide. Emissions from the ocean comprise about one quarter of the total nitrous oxide emissions to the atmosphere.
This is not at all an insignificant source of emissions, yet despite the potency of nitrous oxide, there are enduring gaps in knowledge regarding the timing and distribution of its emissions. Consequently, this has made research into the atmospheric nitrous oxide budget difficult. Now, however, some of these gaps are being filled.
Alyson Santoro, a professor with UC Santa Barbara’s Department of Ecology, Evolution, and Marine Biology has been working on the subject of oceanic nitrous oxide for more than a decade, most recently contributing to a global reconstruction of nitrous oxide emissions.
This piece, written in collaboration with a smorgasbord of other researchers from across the world, was published in the Proceeding of the National Academy of Sciences.
In the past, Santoro has worked to determine the primary source of oceanic nitrous oxide, researched the abundance, diversity and activity of organisms which contribute to nitrous oxide emissions in the central California current and investigated the ebb and flow of nitrous oxide in eutrophic estuaries like Chesapeake Bay.
Santoro and her collaborators also formerly measured nitrous oxide on monthly expeditions in the Santa Barbara Channel, a project which initially began with funding from the UCSB Coastal Fund. However, just like other long-term monitoring projects, these local measurements have been put on hold due to the pandemic.
Nitrous oxide emissions in the ocean result from biological and physical processes. Much of it comes from microbes in water who convert ammonium into nitrate. This process, called nitrification, generates nitrous oxide as a by-product.
In the global ocean, one finds that nitrous oxide is distributed unevenly, with tropical regions comprising the bulk of emissions. The reason for this disproportionate contribution to emissions has to do largely with upwelling.
“[Nitrous oxide] is produced in the dark regions of the ocean, below where sunlight reaches. Its main pathway to the atmosphere is when deep water is brought to the surface by physical processes, known generally as upwelling,” Santoro wrote in email correspondence with the Nexus.
“The equator is a major upwelling region in the ocean.”
Those in California might be more familiar with the effect that upwelling has on sea surface temperatures. The Golden State, bordering on the edge of the Pacific, experiences upwelling in the springtime. This water, rich in nitrous oxide, is hoisted up from the depths of the ocean and contributes to cooler surf.
This phenomenon, aptly named coastal upwelling, also applies in equatorial regions, wherein winds blow towards the equator following the “[eastern] edges of the ocean basins,” according to Santoro.
Armed with 158 thousand measurements of nitrous oxide from the surface ocean — the researchers trained a supervised learning algorithm to reconstruct nitrous oxide emissions, capturing both latitudinal variation as well as coastal “hot spots.”
Regarding the source of these measurements, Santoro stressed the necessity of drawing from particular locations in order to best understand how nitrous oxide emissions can change, and that places like the tropical and subtropical open ocean, where measurements are generally focused, do not necessarily lead to the most representative results.
“The coasts are also really important. But, they are also “messy,” meaning that the way water moves around the coast is complex and things change [rapidly], so you may have to make a lot of measurements before you understand how things work,” Santoro said.
With this, they were able to reduce the uncertainty plaguing previous estimates more than fivefold, and discerned an average nitrous oxide flux of 4.2 trillion grams of nitrous oxide emissions per year, equating to one billion metric tons of carbon dioxide.
“The amount of [nitrous oxide] naturally coming out of the ocean has a global warming potential about 20% of the annual US [carbon dioxide] emissions,” Santoro said.
However, just as illuminating to Santoro and her collaborators was the cycle nature of emissions which revealed itself to them.
Nitrous oxide emissions peaked in the boreal summer. Conversely, emissions reached their lowest point in the spring; Santoro attributed both of these to the seasonality of the processes which govern emissions, namely “seasonal upwelling in the tropical ocean and winter mixing in the Southern Ocean.”
“Just as biological and physical processes on land are seasonal, the equivalent processes are seasonal in the ocean.”
In the wake of climate change, the source of this seasonal variation, namely tropical upwelling, becomes concerning.
“As the ocean warms due to human-induced… climate change, the difference between the surface ocean temperature and the deep ocean temperature gets bigger,” Santoro said.
If nitrous oxide emissions from the ocean are sensitive to changes through this “anthropogenic stratification,” and thus respond to changes in climate, as the reconstruction suggests, the impact to the ocean from climate change will be greater than merely warming seas.
“[Climate] change doesn’t just mean that the ocean is getting warmer. It means that fundamental properties of the ocean — like the way water moves around — are also changing,” Santoro said.
“Cycles we are just barely beginning to understand, like El Nino, could be changing as a result of these circulation changes, which can have ripple effects for all kinds of things like the biology and chemistry of the ocean.”
“Our story about [nitrous oxide] is just one small example of this.”