Home > Environmental issues > Global warming > Article

Climate change reduces ocean food supply, threatening marine ecosystems

8 December 2006

In a NASA study, scientists have concluded that when Earth's climate warms, there is a reduction in the ocean's primary food supply. This poses a potential threat to fisheries and ecosystems.

By comparing nearly a decade of global ocean satellite data with records of Earth's changing climate, scientists found that whenever climate temperatures warmed, marine plant life of microscopic phytoplankton declined.  When temperatures cooled, marine plant life became more productive.  The findings appear in the December 7th issue of the journal Nature.

The results provide a preview of what could happen to ocean biology in the future if Earth's climate warms as the result of increasing levels of greenhouse gases in the atmosphere.

Phytoplankton is plant life in the upper sunlit layer of the ocean, responsible for about the same amount of photosynthesis each year as all land plants combined.

Phytoplankton is the essential plant life at the start of the marine food chain.  Changes in photosynthesis and phytoplankton growth influence fishery yields, marine bird populations and the amount of carbon dioxide the oceans remove from the atmosphere.

A close-up view of phytoplankton, the tiny plants that live in the sunlit upper layer of the ocean.

Image NASA

"The evidence is pretty clear that the Earth's climate is changing dramatically, and in this NASA research we see a specific consequence of that change," said oceanographer and study co-author Gene Carl Feldman of NASA's Goddard Space Flight Center.

"It is only by understanding how climate and life on Earth are linked that we can realistically hope to predict how the Earth will be able to support life in the future."

Above: Changes in phytoplankton change how much carbon dioxide the oceans remove from the atmosphere. The life cycle of phytoplankton in the ocean involves absorbing carbon dioxide from the atmosphere and nutrient-rich waters. That carbon is then passed on to higher life forms that feed on the plants. Some also sinks to the ocean floor.

Image right: Satellite data reveals the ebb and flow of microscopic plant life in the world's ocean. In this late 1990s image of the Pacific, robust plant growth is shown in green; areas of low "productivity" are blue.  Research has now shown how these marine plants are linked to changes in climate.
Images courtesy of NASA

On the Pacific coast of New Zealand, the advance of austral spring returns sunlight to spur phytoplankton blooms. This 2005 image shows a plume extending from the coast near Castlepoint in the southern North Island, and rotating in an offshore eddy. Another broader swath of less-intensely colored plankton appears in the lower part of the image.

Large areas of plankton occur at 40° South latitude along the convergence zone known as the Subtropical Front, between the Antarctic Circumpolar Current and subtropical waters. The converging water masses mix and disperse nutrients with plankton blooms when spring lighting becomes strong enough. The zone extends east-west at the latitude of Cook Strait, and plankton in this image went east, past the Chatham Islands.
Image courtesy ISS Crew Earth Observations & Image Science & Analysis Group, Johnson Space Center, NASA

View larger image

Less phytoplankton growth reduces carbon dioxide absorption from the atmosphere, causing increased climate change ... a vicious cycle

"Rising levels of carbon dioxide in the atmosphere play a big part in global warming," said lead author Michael Behrenfeld of Oregon State University. "This study shows that as the climate warms, phytoplankton growth rates go down and along with them the amount of carbon dioxide these ocean plants consume.  That allows carbon dioxide to accumulate more rapidly in the atmosphere, which would produce more warming."

An uninterrupted nine-year record shows in detail the ups and downs of marine biological activity or productivity from month to month and year to year. Captured at the start of this data record was a major, rapid rebound in ocean biological activity after a major El Nino event.

El Nino and La Nina are major warming or cooling events respectively, that occur about every 3-7 years in the eastern Pacific Ocean and are known to change weather patterns around the world.

Scientists made their discovery by comparing the SeaWiFS record of the rise and fall of global ocean plant life to different measures of recent global climate change. The climate records included several factors that directly effect ocean conditions, such as changes in sea surface temperature and surface winds.

The results support computer model predictions of what could happen to the world's oceans as the result of prolonged future climate warming.

Ocean plant growth increased from 1997 to 1999 as the climate cooled during one of the strongest El Niño to La Niña transitions on record. Since 1999, the climate has been in a period of warming that has seen the health of ocean plants diminish.

The new study also explains why a change in climate produces this effect on ocean plant life. When the climate warms, the temperature of the upper ocean also increases, making it "lighter" than the denser cold water beneath it. This results in a layering or "stratification" of ocean waters that creates an effective barrier between the surface layer and the nutrients below, cutting off phytoplankton's food supply. The scientists confirmed this effect by comparing records of ocean surface water density with the SeaWiFS biological data.