Global warming is reducing the numbers of microscopic plants in the world’s oceans, a new study of satellite data reveals. Scientists say that means there are fewer plants to absorb the greenhouse gas carbon dioxide humans are pumping into the atmosphere, leading to a further increase in global warming.
By comparing nine years of global ocean satellite data with records of Earth’s changing climate, scientists from NASA and five other institutions on the study found that whenever climate temperatures warmed, marine plant life in the form of microscopic phytoplankton declined.
Whenever climate temperatures cooled, marine plant life became more vigorous or productive. These tiny plants form the basis of the ocean food chain.
From space, satellites that record ocean color measure the ocean’s biology as plant productivity. In this visualization, high plant productivity is represented in green, while areas of low productivity remain blue. Data were collected by the Sea-viewing Wide Field-of-view Sensor, SeaWiFS. (Photo courtesy NASA/GeoEye)
“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 in Greenbelt. “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,” he said.
The study, published today in the journal “Nature” predicts that phytoplankton will grow even more slowly in the warmer oceans of the future. This, in turn, will reduce the food available to fish and other organisms, including marine birds and mammals.
“This clearly showed that overall ocean productivity decreases when the climate warms,” said lead author Michael Behrenfeld, an Oregon State University professor of botany and expert on remote sensing of marine biology.
Professor Michael Behrenfeld is an expert in novel optical approaches to understanding the physiological ecology of marine algae, regional and global ecological modeling, climate change and carbon cycling. (Photo courtesy OSU)
“Rising levels of carbon dioxide in the atmosphere play a big part in global warming,” said Behrenfeld. “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.”
The findings are from a NASA funded analysis of data from the Sea-viewing Wide Field-of-view Sensor, SeaWiFS, instrument on the OrbView-2 spacecraft, launched in 1997. SeaWiFS is jointly operated by NASA and GeoEYE of Dulles, Virginia.
The uninterrupted nine year record shows in great detail the ups and downs of marine biological productivity from month to month and year to year.
Scientists made their discovery by comparing the SeaWiFS record of the rise and fall of global ocean plant life to measures of recent global climate change 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.
“We show on a global scale that the growth of these plants, called phytoplankton, is strongly tied to changes in the warming of the ocean,” said David Siegel, co-author and professor of marine science in the Department of Geography at the University of California, Santa Barbara. Siegel is also director of the Institute for Computational Earth System Science, ICESS.
“Phytoplankton grow faster in a cool ocean and slower in a warm one,” said Siegel. “The scary part is that the oceans are warming now � probably caused by our emissions of greenhouse gases like carbon dioxide.”
Captured at the start of this data record was a rapid rebound in ocean biological activity after a major El Ni�o event. El Ni�o and La Ni�a are major warming or cooling events, respectively, that occur about every three to seven years in the eastern Pacific Ocean and are known to change weather patterns around the world.
On April 25, 1998, the SeaWiFS instrument obtained a cloudless image of the entire U.S. east coast. This image shows chlorophyll concentrations in the ocean waters off South Carolina.
Ocean plant growth increased from 1997 to 1999 as the climate cooled during one of the strongest El Ni�o to La Ni�a transitions on record. Since 1999, the climate has been in a period of warming that has seen the health of ocean plants diminish.
“When we compared changes in phytoplankton activity with simultaneous changes in climate conditions, the agreement between the two records was simply astonishing,” Behrenfeld said.
When the ocean surface warms, it essentially becomes “lighter” than the cold, dense water below, which is loaded with nutrients, the scientists explain. This process separates phytoplankton in the surface layer, which need light for photosynthesis, from the nutrients below them, which they also need for growth.
Marine phytoplankton come in many shapes and sizes.
Despite their microscopic size, ocean phytoplankton are responsible for about half of the photosynthesis on Earth, a process that removes carbon dioxide from the atmosphere and converts it into organic carbon to fuel nearly every ocean ecosystem.
Compared to plants on land, phytoplankton use a very small amount of biomass to convert large amounts of carbon, because they are eaten by predators about as quickly as they grow.
The entire global phytoplankton biomass is consumed every two to six days, in contrast to land plants that might live several months or hundreds of years.
“This very fast turnover, along with the fact that phytoplankton are limited to just a thin veneer of the ocean surface where there is enough sunlight to sustain photosynthesis, makes them very responsive to changes in climate,” Behrenfeld said.
“This was why we could relate productivity changes to climate variability in only a 10 year record,” he said. “Such connections would be much harder to detect from space for terrestrial plant biomass.”
The researchers say that better understanding of this feedback mechanism which compounds global warming is a top priority for future study.
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