Washington: Scientists have singled out and quantified the carbon cycle mechanism of the ocean, a "biological pump" that can be used to better predict future global climate change.
The study was published in the journal Nature.
Oceans have an extraordinary natural ability to pull carbon out of the atmosphere and store it deep in the oceanic waters, making important control over the global climate.
Much of the carbon dioxide emitted when people burn fossil fuels, for example, are taken and stored in the ocean through many processes that make up the oceanic carbon cycle. But the rapid increase in carbon dioxide emissions means that the future of the cycle is uncertain, especially when many of the key processes remain poorly understood.
The biological pump describes the sum of all biological processes that transport carbon dioxide from the atmosphere to the deep ocean. Small sea plants, known as phytoplankton, take carbon dioxide from the surface ocean to produce biomass. Biomass is collected together in particles, which then sink to the deep ocean. In the deep ocean, the particles decay, releasing carbon dioxide. "The net effect is" transporting "CO2 from the atmosphere to the deep ocean," said Tom Weber, a researcher.
The deeper the ocean sinks, the longer the carbon will be taken to return to the surface and back into the atmosphere. For example, the carbon released to hundreds of meters is returned back into the atmosphere at intervals of 10 years or less, but if the particles sink into the deep ocean deeper than 1,000 meters, their carbon can be stored up to 1,000 years before return to the surface.
The researchers previously believed that the transfer of particles from the surface of the deep ocean occurs simply by sinking under the gravitational force – what Weber and his colleagues consider a "biological gravity pump".
However, in recent years, scientists have recognized other processes that are important for transferring carbon from surface waters to the deep ocean. As stated in the paper, they include the physical mixing of the ocean by the wind, large scale ocean and through biological transport through animals, such as the small fish that
Eat the particles of biomass on the surface and cool them in depth.
The researchers called these processes collectively as "Injection Particle Pumps" (PIPs) because they can "inject" particles in very deep depths – in terms of gravitational resolution – before decomposition and release of carbon occur.
"It's a much more efficient way of retrieving carbon from the surface in deep waters," Weber said.
Webber and his colleagues combined the evidence and calculations of the new model to measure for the first time how much carbon was transferred by PIP. They found that PIPs are a much more influential factor than previously thought: collectively, they are responsible for how much carbon storage in the ocean as a biological gravity pump.
Because the ocean carbon cycle is affected by environmental changes in light, temperature and availability of nutrients, researchers can use their new results to improve climate models and better predict how the carbon cycle cycle will respond to future global climate change, Weber said. "If we want to have some predictable power over the biological pump, we need to understand all the mechanisms and equip our global climate models with full coverage."
The climate cycle of the ocean is most affected by climate change due to warming of ocean waters. The deep ocean is filled with cold, dense and rich in nutrient water, while the surface ocean is warmer and lighter.
In order to maintain biological productivity, the wind encourages the ocean waters, stirring them to bring the water rich in nutrients to the surface. However, when ocean temperatures increase due to climate change, the difference in density between water in the surface ocean and water in the depths of the ocean increases, making it difficult for the ocean to interfere, Weber said. "Satellite data show that the overall surface-productivity of the ocean is declining, because the mixing of nutrients becomes less efficient."
Weber's new research adds another "wrinkle to the problem," he said. Previous biological pump views suggested that the reduced ocean interference rate would slow productivity, but "does not really affect other processes in the biological pump: after you make the particles, only gravity will sink and fall apart."
However, the new attitude shows that slowing down the mixing will also reduce PIPs, which are vital for the ocean carbon cycle as "very efficient export mechanisms that get the particles beautiful and deep where carbon can be stored longer" Weber said .
"If carbon dioxide is released into flooded depths, it escapes faster in the atmosphere, which means more carbon dioxide in the atmosphere, where it contributes to global warming."