A new study has provided compelling evidence as to how the level of atmospheric CO2 rose significantly as the Earth came out of its last ice age.
Long before humans started burning fossil fuels, the level of atmospheric CO2 rose around the end of the last ice age. Many scientists have long suspected that the source of that carbon was from the deep sea, but until now researchers haven’t been able to document just how the carbon made it out of the ocean and into the atmosphere.
“The Pacific Ocean is big and you can store a lot of stuff down there – it’s kind of like Grandma’s cellar – stuff accumulates there and sometimes doesn’t get cleaned out,” said Professor 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.”
The new study, published 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 team say 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 almost 1,000 years for the water’s round trip journey in the abyss. 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 and brought the CO2 to the surface near Antarctica where it was released into the atmosphere.
Dr Brian Haley, co-author on the study, said: “Up near the surface, plankton grow, but when they die they sink and decompose. That is a biological pump that is always sending carbon to the bottom. The slower the circulation, the more time the water spends down there, and carbon can build up.”
What concerns the researchers is that it could happen again as the climate continues to warm.
“We don’t know that the circulation will speed up and bring that carbon to the surface, but it seems like a reasonable thing to think about,” Du said. “Our evidence that this actually happened in the past will help the people who run climate models figure out whether it is a real risk for the future.”