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| Earth on Jul 12, 2010

Daniel Sigman on ocean algae and global cooling of the past

“Since CO2, a greenhouse gas, warms the Earth, ocean life effectively works to cool the Earth by helping remove CO2 from the atmosphere,” says Sigman.

Ocean plants buried in sediment can help reveal Earth’s temperature thousands of years ago. That’s according to Daniel Sigman of Princeton, winner of a 2009 “genius grant” from the MacArthur Foundation. Dr. Sigman has been trying to piece together how efficiently tiny ocean plants – algae – were using nitrogen about 20,000 years ago.

Daniel Sigman: Algae require nitrogen to grow, and their bodies, in turn, fuel the rest of the ocean food web.

Sigman explained that by measuring algae’s use of nitrogen, he can gauge how much these ancient organisms used another important element – carbon dioxide, or CO2. All earthly plants use CO2 for photosynthesis. CO2 is also a major greenhouse gas in Earth’s atmosphere. Excess CO2 warms the Earth.

Daniel Sigman: The ocean is responsible for lowering carbon dioxide in the atmosphere as much as forests on land are.

That’s because algae take CO2 out of surface water. And, when the algae die, Sigman said, they sink.

Daniel Sigman: After the algae sink, they transfer the CO2 where it cannot escape into the atmosphere.

This helps cool the planet. Dr. Sigman is currently working to determine the role the ocean played in the global cooling that caused the last ice age. His early research indicates that algae in polar oceans might have played a role. This illustrates how the collective activity of many small creatures can exert a big influence on climate, he said.

Daniel Sigman:
If you imagine more organic matter being produced in the world’s ocean surface waters and sinking to the deep ocean, that means more carbon removal from the atmosphere. That also means a lower concentration of carbon dioxide in the atmosphere.

Sigman described the combined activity of many small creatures – the algae – as a “biological pump.” In other words, these living creatures acted to remove carbon dioxide from the atmosphere and bury it in ocean sediments. He said that – because carbon dioxide acts to cool Earth’s atmosphere – this biological pump acting in Earth’s oceans might have helped triggered past ice ages.

Daniel Sigman:
One of the great mysteries in climate science is what causes the cycles of what we know as ice ages and interglacial periods.

Sigman said early research indicates that algae might have been working extra-efficiently at the polar oceans during ice ages, and taking up nitrogen – and carbon dioxide – more completely than usual, thus cooling Earth down.

Daniel Sigman:
And it is frankly an unexpected result, which does not mean that it’s incorrect, it just means that we have more work to do.

Sigman added that theories – such as the presence of more iron dust in the air, or less nutrition upwelling from the ocean depths – have been proposed to explain why algae and marine creatures at both poles may have been acting differently during the ice ages than they do today. He said that understanding how polar oceans influence climate is one way of more accurately predicting what might happen in the 21st century, as global temperatures increase due to global warming.