Ocean warming's effect on phytoplankton / NASA satellite data show how global climate change hurts marine food chain
Published 4:00 am, Thursday, December 7, 2006
When the climate warms, there is a drop in the abundance of the ocean's phytoplankton, the tiny plants that feed krill, fish and whales, according to scientists who say new research offers the first clues to the future of marine life under global warming.
Ocean temperatures have generally risen over the last 50 years as the atmosphere warms. And now nine years of NASA satellite data published today in the journal Nature show that the growth rate and abundance of phytoplankton around the world decreases in warm ocean years and increases in cooler ocean years.
The findings are crucial because they show a consequence of the changing global climate at the most fundamental level. Scientists estimate that phytoplankton is responsible for about half of Earth's photosynthesis, a process that removes carbon dioxide from the atmosphere and converts it into organic carbon and oxygen that feeds nearly every ocean ecosystem.
Fewer phytoplankton consume less carbon dioxide, aggravating a cycle that can lead to even more warming.
Over the past decades, California scientists have linked declining plankton numbers to El Niños and other warm-water years in the Pacific Ocean, which set off a domino effect of fewer krill and young fish and eventually failed reproduction of seabirds and even deaths of seals and sea lions.
"What's amazing is this is the first time we see it on a global scale," said Dave Siegel, professor of marine science at UC Santa Barbara and a study author. "We have an inkling now what will happen to the ocean's biology in future climates."
The scientists report that the relationship between warmer ocean waters and fewer plankton holds strongly for three-quarters of the world's oceans.
Oceanic plant growth increased from 1997 to 1999 as the oceans cooled as a result of one of the strongest El Niño-to-La Niña transitions on record, said NASA officials. Since 1999, the climate has been in a period of warming that has seen the health of ocean plants diminish, they said.
When the climate warms, the ocean's upper layer where the phytoplankton proliferate becomes separated from the denser, colder ocean water below in a sort of a stratification effect.
The ocean can't mix as well, and the plankton don't reach the nutrients in the deeper, colder waters that they use as a food supply.
For the first time, the NASA data set "is long enough and good enough so that we can evaluate the global links between climate and productivity," Siegel said.
The NASA satellite data measure the oceans' color and helps scientists determine the amount of phytoplankton and the growth rates, he said.
"We can know the amount of plants in the ocean by looking at the ocean's color. A blue ocean has no phytoplankton in it. The beautiful tropical oceans that you see on postcards have little in it. The green ocean is chock-full of phytoplankton."
There is a consensus among the world's experts that carbon dioxide, methane and other greenhouse gases emitted from the burning of fossil fuels have added to natural carbon dioxide levels. The carbon dioxide concentrations measured in ice cores in Antarctica are the highest in 640,000 years.
Gene Carl Feldman, NASA scientist and a study author, said in a statement Wednesday that only by understanding how climate and life on Earth are linked "can we realistically hope to predict how the Earth will be able to support life in the future."
The research is the result of years of collaboration among UC Santa Barbara, Oregon State University, Princeton University, Rutgers University, University of Maine and NASA's Goddard Space Flight Center in Greenbelt, Md.