New Delhi: The oceans have always had a complex relationship with carbon — some are carbon sinks and others release much more carbon than they absorb. But in this exchange of carbon between the seas and the skies, scientists have found that rain plays an understudied role.
A recent study led by researchers at the University of Notre Dame has found that increased rainfall can allow the oceans to absorb up to 13.6 per cent more carbon dioxide.
“When rain falls through the atmosphere, it scoops up atmospheric carbon dioxide that it then injects into the ocean. When the rain hits the ocean surface, the transfer of CO₂ between the atmosphere and ocean increases—like opening a window to ventilate a room,” co-author Wade McGillis said in a university statement.
Rainfall creates a thin layer of freshwater that floats above the salty seas, isolated from the rest of the waters for hours, sometimes for days. While this layer is typically only about 1-3 metres deep, it can absorb surprisingly large amounts of carbon.
Also read: Oil slicks affect ocean area nearly half the size of India each year, satellite study finds
A carbon sink
The science behind what makes this absorption easier is similar to what happens when you add salt to a soda. Higher salinity makes it harder for gases to dissolve, but when rain dilutes these waters they can absorb more carbon.
First raindrops dilute the surface water, then when the raindrops hitting the surface create turbulence the gases can move between the air and the water. The raindrops themself contain dissolved carbon dioxide that they carry into the water.
“We need to better understand what controls rain dilution at the ocean’s surface, how much carbon dioxide rain falling from the atmosphere carries, and how mixing and turbulence at the surface change before, during and after rainfall. Ultimately, we need better ways to measure these effects together and determine how much they influence the exchange of carbon dioxide between ocean and atmosphere,” McGillis added.
According to Sambuddha Misra, a chemical oceanographer at the Indian Institute of Science (IISc) such studies are essential to predict the effects of global warming, the amount of carbon dioxide in the atmosphere and how much of it can be absorbed by the oceans. However, he highlights that the findings cannot be oversimplified to mean — more rain equals more carbon absorption.
In India, the Bay of Bengal provides one such complex example. Heavy monsoon rains and river discharge lead to layers of fresh water over the bay, but tropical summers quickly raise water temperatures and this layer of water, less dense than the rest, floats for days.
“In the short term the fresh water might absorb more carbon dioxide but once the layer gets hotter this absorption is not as much,” Misra told ThePrint. “This study’s findings will have different effects in the tropics and subtropics, and the process might be more efficient in higher latitudes where temperatures don’t go up much,” he added.
However, at Misra’s lab similar research focuses on past measurements of the oceans and not recent ones. According to him, the global community of geologists is yet to devote significant resources to such research.
“The next steps are in-situ measurements, regional and seasonal studies, and ground reports from different parts of the world – the Arctic, the Bay of Bengal, the equatorial Pacific. But this is financially very demanding. You have to get a cruise for every trip. You can go once a year, but going even once a month or once a week is very hard,” said Misra.
The study’s researchers highlighted that carbon-related calculations across the world might be underestimating the power of such rain-driven carbon absorptions by 5 to 15 per cent. While this does not mean that global warming projections are wrong, researchers say that such calculations should factor in these effects of rainfall on the oceans too.