Scientists trace atmospheric rise in CO2 during deglaciation to deep Pacific Ocean
CORVALLIS, Ore. – Long before humans started injecting carbon dioxide into the atmosphere by burning fossil fuels like oil, gas, and coal, the level of atmospheric CO2rose significantly as the Earth came out of its last ice age. Many scientists have long suspected that the source of that carbon was from the deep sea.
But researchers haven’t been able to document just how the carbon made it out of the ocean and into the atmosphere. It has remained one of the most important mysteries of science.
A new study, published today in the journal Nature Geoscience, provides some of the most compelling evidence for how it happened – a “flushing” of the deep Pacific Ocean caused by the acceleration of water circulation patterns that begin around Antarctica.
The concern, researchers say, is that it could happen again, potentially magnifying and accelerating human-caused climate change.
“The Pacific Ocean is big and you can store a lot of stuff down there – it’s kind of like Grandma’s root cellar – stuff accumulates there and sometimes doesn’t get cleaned out,” said Alan Mix, an Oregon State University oceanographer and co-author on the study. “We’ve known that CO2 in the atmosphere went up and down in the past, we know that it was part of big climate changes, and we thought it came out of the deep ocean.
“But it has not been clear how the carbon actually got out of the ocean to cause the CO2 rise.”
Lead author Jianghui Du, a doctoral student in oceanography at Oregon State, said there is a circulation pattern in the Pacific that begins with water around Antarctica sinking and moving northward at great depth a few miles below the surface. It continues all the way to Alaska, where it rises, turns back southward, and flows back to Antarctica where it mixes back up to the sea surface.
It takes a long time for the water’s round trip journey in the abyss – almost 1,000 years, Du said. Along with the rest of the OSU team, Du found that flow slowed down during glacial maximums but sped up during deglaciation, as the Earth warmed. This faster flow flushed the carbon from the deep Pacific Ocean – “cleaning out Grandma’s root cellar” – and brought the CO2 to the surface near Antarctica. There it was released into the atmosphere.
“It happened roughly in two steps during the last deglaciation – an initial phase from 18,000 to 15,000 years ago, when CO2 rose by about 50 parts per million, and a second pulse later added another 30 parts per million,” Du said.
That total is just a bit less than the amount CO2 has risen since the industrial revolution. So the ocean can be a powerful source of carbon.