Flash back to the last deglaciation, some 19,000–9,000 years ago. During this time, atmospheric CO2 increased significantly by approximately 80 ppm. But why? This was long before the age of the Anthropocene, when humans started pumping carbon dioxide into the atmosphere – so where did that CO2 rise come from? New research from a study published in the journal Nature Geoscience suggests that it occurred as a result of a "flushing" of the deep Pacific Ocean that was itself driven by the acceleration of water circulation patterns around Antarctica. In other words, it came from the depths of the ocean.
But let’s back up a minute. Why does it even matter that we understand why the sudden burst of CO2 so long ago? Well, as the authors of the study explain, this research falls into the category of palaeoclimatology, and “understanding the mechanisms of this change is relevant for predicting future CO2 transfers in a warming world.” The thought is that if this sort of ocean-to-atmosphere CO2 transfer event happened, it could happen again – and in our already wayyy-too-high CO2 ppm existence, that would be an unwelcome change. So yeah, kind of imperative to understand what caused it.
"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, 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."
What the researchers figured out, through analysis of neodymium isotopes in North Pacific sediment cores, was that a specific circulation pattern in the Pacific, beginning near Antarctica, was responsible for mixing up deep ocean waters and flushing CO2 to the surface near Antarctica, where 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," said lead author, Jianghui Du.
The process of that circulation takes nearly 1,000 years – which is key. "The slower the circulation," noted co-author Brian Haley said, "the more time the water spends down there, and carbon can build up." Slow circulation happens when there is a glacial maximum. What happens when there’s not a glacial maximum, you ask? "When the Earth began warming [during the deglaciation period], the water movement sped up by about a factor of three," Du elaborated, "and that carbon came back to the surface." Noticing any patterns?
Cautioning, Mix explains the full significance of the team’s findings. "So far the ocean has absorbed about a third of the total carbon emitted from fossil fuels," Mix said. "That has helped slow down warming. The Paris Climate Agreement has set goals of containing warming to 1.5 to 2 degrees (Celsius) and we know pretty well how much carbon can be released to the atmosphere while keeping to that level. But if the ocean stops absorbing the excess CO2, and instead releases more from the deep sea, that spells trouble. Ocean release would subtract from our remaining emissions budget and that means we're going to have to get our emissions down a heck of a lot faster. We need to figure out how much."
Though the researchers say that their research cannot predict the future for our planet, they do consider their work to play an important role in policymaking. "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."