Glaciers and the Food Chain

Posted: 08-Aug-2005; Updated: 16-Aug-2007

Scientists can't predict exactly how global warming will affect the ocean. Environmental Defense marine ecologist Rod Fujita explains in his book Heal the Ocean: "The impacts of global warming are likely to vary tremendously in different regions, due to the complexities of ocean circulation, chemistry and biology. Increased temperatures, altered wind patterns and increased carbon dioxide concentrations will interact in surprising ways, no doubt."

What powers the oceans' circulation?

The ocean's global system of currents is propelled by the force of cold, salty (and denser) waters sinking in the North Atlantic. This great volume of water falling downward, sort of a giant underwater waterfall, pulls water at the surface of the Atlantic Ocean north and creates a current that flows along the bottom of the oceans. The movement of ocean water around the globe is called the ocean conveyor. And what powers all that falling water is cold temperatures in the polar region and salinity. (More about how the ocean conveyor works) 

Changes are under way in the mix of the ocean's salt and fresh water that could dramatically affect the ocean's system of currents. The mix of salty and fresher water in the oceans is determined by rainfall and evaporation patterns (or hydrological cycle). Changes in patterns of precipitation and evaporation have been altering the pattern of saltiness and freshness across the oceans.

The ocean's system of currents takes 1,000 years to go full cycle.  Warm water is chilled in the far North Atlantic and sinks. The cold, salty current flows south near the bottom. PHOTO: Argonne National Laboratory.

When melting glaciers meet the ocean

As average temperatures rise worldwide, glaciers and sea ice are melting, and evaporation and precipitation patterns are shifting. With more fresh water pouring in some regions and more evaporation in others, parts of the ocean are becoming fresher, while others are becoming saltier at a visible pace.

Studies show that tropical Atlantic and Pacific waters have become saltier, while Arctic waters have become fresher. In a paper published in the June 17, 2005 issue of Science, authors Ruth Curry (of the Woods Hole Oceanographic Institution) and Dr. Cecilie Mauritzen (of the Norwegian Meteorological Institute) say that by the end of the century the freshening of northern Atlantic waters could slow or disrupt the ocean conveyor.

From 1965 to 1995, a volume of freshwater almost as large as that in the Great Lakes melted from the glaciers in the Arctic and flowed into the normally salty North Atlantic. That's nearly 20,000 cubic km or 4,000 cubic miles of fresh water. By comparison, the entire outflow from the Mississippi each year is about 500 cubic km.

The freshening of northern Atlantic waters from melting glaciers could slow or disrupt the ocean conveyor.

So the water's fresher — what happens?

If the North Atlantic loses too much salinity, one of the primary forces driving ocean circulation could weaken. Fresher water is less dense than salty water, so it does not sink in saltier water. If waters were to stop sinking in the North Atlantic, existing currents could slacken or change course, leading to altered climate patterns. 

A slackening of the conveyor could slow or change the course of the Gulf Stream, a warm current that gives northwestern Europe a milder climate than it would normally have so far north, plunging Europe into a colder era even as the rest of world experiences warmer temperatures, more droughts and flooding. 

Breaking the food chain — starving sea birds

Circulation patterns also deliver nutrient-rich waters to strategic parts of the ocean. A disruption of the ocean conveyor would interfere with this delivery system of nutrient supplies to sea animals and could have dire consequences for the marine web of life. (See The Living Ocean Ecosystem.)   

As surface waters heat up, the vertical layers of sea water could mix less with each other, an effect called vertical stratification. Upwellings of cold, nutrient-rich waters would become less frequent, diminishing blooms of phytoplankton, microscopic plants that anchor the marine food chain.  On top of that, phytoplankton use carbon dioxide for photosynthesis. If plankton become depleted, the oceans could not remove as much carbon dioxide from the atmosphere.

The marine food chain may already be showing signs of breaking. This year on the U.S. West Coast and last year in Britain, hundreds of thousands of seabirds failed to breed. Dead seabirds like cormorants and Cassin's auklets have washed up on West Coast  beaches.  Juvenile rockfish counts are the lowest they've been off California in more than 20 years.  Most alarming, small crustaceans like krill — the base of the ocean's food web — have suffered steep declines.

The culprit for the collapse appears to be slackening upwellings, which have decreased phytoplankton blooms in these coastal areas. Fewer phytoplankton mean fewer fish, leaving the birds to face mass starvation. Scientists speculate that this decrease in their food supply could be an effect of global warming. (See Impacts: Fish and other sea life.) 

With no mixing, nutrients at the surface would be used up in about 50 years. Beyond that, "deep waters would be deprived of oxygen and food from the surface," says Fujita, "and many deep sea animals could be affected." Monitoring of ocean waters off Hawaii over the last 20 years shows that waters in the area are indeed more stratified and upwellings are decreasing.

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Solutions: What we can do »

Sources for this special report.