Satellite snaps show sea food shortage
8 December 2006
Climate change is causing trouble for Earth's ocean life, new satellite evidence has revealed. Scientists say their snaps show warming waters are making marine life's main food supply - phytoplankton - plummet.
Antenna finds out more...
This story was published in Nature on 7 December 2006.
Image: NASA
Phytoplankton are tiny free-floating plants that thrive in sunlit water near the ocean's surface. They're at the bottom of the ocean's food chain, so without them bigger sea creatures, such as fish and birds, would starve.
Individual phytoplankton plants are microscopically small - sometimes only a few micrometres across.
Image: NASA
Now scientists have pulled together ten years of satellite pictures to see how climate change is affecting phytoplankton growth. Worryingly, they've discovered that when the ocean's temperature increases, phytoplankton numbers dwindle.
'When we compared changes in phytoplankton with simultaneous changes in climate conditions, the agreement between the two records was simply astonishing,' says Michael Behrenfeld, a marine scientist from Oregon State University.
Lead scientist Michael Behrenfeld from Oregon, USA
Image: NASA/Mike Behrenfeld
So why is this happening? Even though phytoplankton live near the ocean's surface, they rely on rising currents from cold deeper waters to deliver the nutrients they need to survive.
But when Earth's climate warms, the sea's surface temperature goes up too. This layer of warm, light water traps the colder, heavier water underneath. So there's less mixing with the nutrient-rich water below, which cuts off the phytoplankton's food supply.
This is the first time that scientists have been able to study the ocean's food source on such a large scale. Ships are too slow to map the world's oceans, but satellite sensors can measure phytoplankton variations worldwide. How?
Phytoplankton contain a green pigment called chlorophyll, just as land plants do. So ocean areas with more phytoplankton appear greener and satellites can pick up this colour difference.
This is how satellites see the ocean's phytoplankton: green means lots of phytoplankton, and blue means less.
Image: NASA
What will fewer phytoplankton mean in future? It's not just marine animals that could suffer if numbers continue to fall.
These tiny plants are responsible for soaking up carbon dioxide - one of the greenhouse gases responsible for global warning. Every day they absorb over a hundred million tonnes of carbon - that's as much as all the vegetation on land.
So falling phytoplankton numbers could further contribute to climate change as Michael Behrenfeld explains: 'As the climate warms, phytoplankton levels go down and along with them the amount of carbon dioxide these plants consume. That means more carbon dioxide in the atmosphere, which produces even more warming.'
Green phytoplankton near the ocean's surface absorbs bubbles of carbon dioxide from the atmosphere.
Image: NASA
But marine scientist Scott Doney thinks we still need to do more research to predict the effects of climate change. 'We have theories about how ocean life might respond to global warming, but so far not enough data to test these ideas.'
He says this research is an important step towards finding out more: 'This study provides strong support for the idea that human-driven climate warming may lead to reduced ocean food supplies in warm waters.'
Scott Doney, Woods Hole Oceanographic Institution, USA
Image: Scott Doney