As green plants die and fall to the ground or sink to the ocean floor, a small fraction of their organic carbon is buried. It remains there for millions of years after taking the form of substances like oil, coal, and shale.
"The oxygen released to the atmosphere when this buried carbon was photosynthesized hundreds of millions of years ago is why we have so much oxygen in the atmosphere today," Sarmiento said.
Today phytoplankton and terrestrial green plants maintain a steady balance in the amount of the Earth's atmospheric oxygen, which comprises about 20 percent of the mix of gasses, according to Frouin.
A mature forest, for example, takes in carbon dioxide from the atmosphere during photosynthesis and converts it to oxygen to support new growth. But that same forest gives off comparable levels of carbon dioxide when old trees die.
"On average, then, this mature forest has no net flux of carbon dioxide or oxygen to or from the atmosphere, unless we cut it all down for logging," Sarmiento said. "The ocean works the same way. Most of the photosynthesis is counterbalanced by an equal and opposite amount of respiration."
The forests and oceans are not taking in more carbon dioxide or letting off more oxygen. But human activities such as burning oil and coal to drive our cars and heat our homes are increasing the amount of carbon dioxide released into the atmosphere.
Most of the world's scientists agree that these increasing concentrations of carbon dioxide in the atmosphere are causing the Earth to warm. Many researchers believe that this phenomenon could lead to potentially catastrophic consequences.
Some researchers argue that enriching the oceans with iron would stimulate phytoplankton growth, which in turn would capture excess carbon from the Earth's atmosphere. But many ocean and atmospheric scientists debate whether this would indeed provide a quick fix to the problem of global warming.
Research by Frouin and his Scripps Institution of Oceanography colleague Sam Iacobellis suggests an increase in phytoplankton may actually cause the Earth to grow warmer, due to increased solar absorption.
"Our simulations show that by increasing the phytoplankton abundance in the upper oceanic layer, sea surface temperature is increased, as well as air temperature," Frouin said.
As Sarmiento notes, phytoplankton obtains most of its carbon dioxide from the oceans, not the atmosphere.
"Pretty much all of the carbon dioxide taken up by phytoplankton comes from deep down in the ocean, just like nutrients, where bacteria and other organisms have produced it by respiring the organic matter that sank from the surface," Sarmiento said.
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