The Ocean’s Carbon Balance

From NASA’s Earth Observatory:

by Holli Riebeek • design by Robert Simmon • June 30, 2008

[WARNING: Beware of booby traps!]

The idea seemed simple enough: the more carbon dioxide that people pumped into the atmosphere by burning fossil fuels, the more the oceans would absorb. The ocean would continue to soak up more and more carbon dioxide until global warming heated the ocean enough to slow down ocean circulation. Water trapped at the surface would become saturated, at which point, the ocean would slow its carbon uptake. To oceanographers of 30 years ago, the question was less, how will human emissions change the ocean carbon cycle, and more, is the ocean carbon cycle changing yet?

One of the largest unknowns in our understanding of the greenhouse effect is the role of the oceans as a carbon sink. Much of the carbon dioxide released into the atmosphere by the burning of fossil fuels is soaked up by the oceans, but changes in the climate are altering this absorption in surprising ways. (Photograph ©2007 *Fede*.)
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The question matters because if the ocean starts to take up less carbon because of global warming, more is left in the atmosphere where it can contribute to additional warming. Scientists wanted to understand how the ocean carbon cycle might change so that they could make more accurate predictions about global warming. Thus motivated, oceanographers began a series of research cruises, trolling across the Pacific from Japan to California, from Alaska to Hawaii, and through the North Atlantic from Europe to North America. On shore, others developed computer models.

For more than 30 years, research ships have cruised the world’s oceans, measuring carbon dioxide concentrations, ocean temperatures, winds, and other properties. The map shows the paths of research cruises conducted as part of the World Climate Research Programme’s Climate Variability and Predictability project. Cruise measurements—along with those from buoys, drifting floats, orbiting satellites, and land-based weather stations—are beginning to reveal long-term trends to ocean researchers. (Map by Robert Simmon, based on data from Dana Greeley, NOAA.)

After 30 years of research, the question itself hasn’t changed, but the reasoning behind it couldn’t be more different. Oceanographers started out wanting to know if the ocean was keeping up with the amount of carbon dioxide people are putting into the atmosphere. Instead, they found that people aren’t the only players changing the ocean carbon cycle. Over decades, natural cycles in weather and ocean currents alter the rate at which the ocean soaks up and vents carbon dioxide. What’s more, scientists are beginning to find evidence that human-induced changes in the atmosphere also change the rate at which the ocean takes up carbon. In other words, it turns out that the world is not a simple place. More …

For eons, the world’s oceans have been sucking carbon dioxide out of the atmosphere and releasing it again in a steady inhale and exhale. The ocean takes up carbon dioxide through photosynthesis by plant-like organisms (phytoplankton), as well as by simple chemistry: carbon dioxide dissolves in water. It reacts with seawater, creating carbonic acid. Carbonic acid releases hydrogen ions, which combine with carbonate in seawater to form bicarbonate, a form of carbon that doesn’t escape the ocean easily.

As we burn fossil fuels and atmospheric carbon dioxide levels go up, the ocean absorbs more carbon dioxide to stay in balance. But this absorption has a price: these reactions lower the water’s pH, meaning it’s more acidic. And the ocean has its limits. As temperatures rise, carbon dioxide leaks out of the ocean like a glass of root beer going flat on a warm day. Carbonate gets used up and has to be re-stocked by upwelling of deeper waters, which are rich in carbonate dissolved from limestone and other rocks.

In the center of the ocean, wind-driven currents bring cool waters and fresh carbonate to the surface. The new water takes up yet more carbon to match the atmosphere, while the old water carries the carbon it has captured into the ocean.

The warmer the surface water becomes, the harder it is for winds to mix the surface layers with the deeper layers. The ocean settles into layers, or stratifies. Without an infusion of fresh carbonate-rich water from below, the surface water saturates with carbon dioxide. The stagnant water also supports fewer phytoplankton, and carbon dioxide uptake from photosynthesis slows. In short, stratification cuts down the amount of carbon the ocean can take up.

The rest of this feature article is available at: The Ocean’s Carbon Balance

Related Links:

• Southern Ocean already losing ability to absorb CO₂
• Oceans, Where Life Started, Are Dying – Part IV :  Researchers found evidence of corrosive water about 20 miles off the west coast of North America from Canada to Mexico.
• Human carbon emissions make oceans corrosive : ‘Carbon dioxide spewed by human activities has made ocean water so acidic that it is eating away at the shells and skeletons of starfish, coral, clams and other sea creatures …’
• The Eight Steps that Help Kill More of Our Fish : How Your Car’s Exhaust Emissions Helps Create Dead Zones and Kill Our Fish.
• Global warming could starve oceans of oxygen: study : Areas of the eastern Atlantic and Pacific Oceans with low amounts of dissolved oxygen have expanded in the past 50 years, apparently in line with rising temperatures.
• The Floating Toxic Garbage Island : A patch of garbage twice as large as the continental United States and dubbed the Great Pacific Garbage Patch floats in the middle of the Pacific Ocean in North Pacific Gyre.
• Oceans, Where Life Started, Are Dying – Part III : Tourism: The Most Destructive Human Activity After Warfare
• Oceans, Where Life Started, Are Dying – Part II : Major Problems: Fertilizer Runoff; Tourism; Coastal Developments [and Ocean Warming due to climate change]
• Oceans, Where Life Started, Are Dying – Part I : Our Oceans Are Now Dying!