By Hilary, Colin and Daniel de la Calle
On my last post I wrote about a University of Florida Research that might have solved “the mystery of where old carbon was stored during the last glacial period”, the answer being that it “ended up in the icy waters of the Southern Ocean near Antarctica.” I was not sure I completely understood the whole thing, I had doubts about what “old carbon” meant, about whether it was still stored in Antarctica and could be released if ice melted there (as is doing), or in regards to the cycle of carbon exchange between the atmosphere and oceans, so I finished by asking if any scientist reading the blog could explain in words that a mere mortal like me would understand. Fortunately, two kind souls (scientists, I am sure) delivered long, fascinating and extremely clear explanations in the comments section, but the problem is that comments on our site are hidden unless you click on them, so readers like me who never click on such places would never lie eyes on them.
What follows are Hilary’s and Colin’s (unfortunately I do not know their last names) words. Hilary wrote first, right after I posted:
…”this study was talking about changes in ocean circulation between an ice age and today’s climate, which while important for understanding how the carbon cycle works, probably aren’t very similar to the changes we expect from climate change today. So, no, we don’t think there’s a pool of “old carbon” in the Southern Ocean today that is going to be released due to modern climate change. There are potential changes in ocean circulation because of modern climate change, though, which is one of the reasons that studies like this are so important to help us understand the ways ocean circulation has changed in the past. But the study you refer to is actually really interesting, so I’m going to try to explain what it was saying. This article is talking about changes in the ocean’s circulation between glacial and interglacial periods that could have affected the amount of carbon stored in the ocean. Right now, we can think of the ocean’s deep circulation as working like a “conveyor belt” moving water all through the deep oceans. But there is a theory that during the last glacial period, that conveyor belt slowed down and there were pools of water in the deep ocean that stopped being mixed. While it sat in the deep ocean, that water accumulated a lot of dissolved carbon that was kept isolated from the atmosphere because it wasn’t being mixed. When the world warmed up at the end of that ice age, the ocean conveyor belt started moving faster, more like today, and that old water with lots of carbon was mixed to the surface, and lots of that carbon went into the atmosphere. Scientists have found evidence that supports this because at the same time that we see an increase in the amount of carbon dioxide in the atmosphere, we also see a signature of that carbon being much older based on isotopic dating – just what you’d expect if you had a big influx of carbon that had been sitting at the bottom of the ocean for centuries. Scientists have suspected that this pool of old carbon was in the Southern Ocean near Antarctica, and when the conveyor belt started up again, it was mixed north into the Pacific Ocean, where it was eventually brought to the surface to put that carbon into the atmosphere. The authors of this article found evidence that this water did in fact come from the Southern Ocean because of a signature of Southern Ocean water that they found having been brought into the North Pacific at this same time. To answer your question about CO2 moving as an endless cycle between air and water – actually that’s exactly how it works! Individual carbon molecules are always moving back and forth between the air and water at the sea surface. That’s why the oceans are being changed by the CO2 we’re adding to the atmosphere. If the oceans get warmer, though, they are able to dissolve less CO2, and more molecules will move from the ocean into the atmosphere than the other way around.”
And yesterday Colin added:
“The article refers to old carbon that was held isolated in the ocean during the glacial period. There are several different ways the oceans back then probably worked to hold more carbon and keep it from escaping to the atmosphere. Ordinarily, carbon is taken out of the atmosphere by photosynthesis in marine organisms in the surface ocean that die and decay back to CO2 after sinking into the deep ocean. Eventually that CO2 returns to the atmosphere when deep water is mixed back to the surface, which happens a lot in the Southern Ocean. This is the return portion of the ocean circulation “conveyor” that Hilary described, which takes about 1,000 years to do one loop. One theory for lower glacial atmospheric CO2 levels, which the article describes,is that the greater extent of sea ice around Antarctica during the glacial period kept CO2 from escaping by being a physical barrier between the carbon-rich water and the atmosphere. Back then the sea ice was more continuous and stuck way out from the shore around Antarctica. Since sea ice today is not so extensive, warming and melting will not uncover a great deal of ocean, so melting today isn’t expected to release a large pulse of “old carbon”. That said, some other theories that describe changes in circulation that occurred as the climate warmed out of the last glacial period do have the potential to predict changes in response to today’s climate. For instance, there is evidence that when the globe is warmer the winds over the Southern Ocean mix deep, carbon-rich waters to the surface more vigorously (you can imagine this as the conveyor that returns carbon from the deep reservoir speeding up). But, when you bring carbon-rich water up to the surface, the carbon only escapes if it is more concentrated than the carbon in the atmosphere. Today, we have so much carbon in the atmosphere from fossil fuel burning that this will mostly keep the increased carbon that is brought to the ocean surface from escaping, but the faster “conveyor” does mean that the ocean around Antarctica will probably be able to absorb less of our fossil fuel emissions, speeding their buildup in the atmosphere. Some scientists think they can measure this already occurring in the last decade or two in the Southern Ocean. On the scale of molecules, it is totally an endless cycle of CO2 going from air to ocean and back, and it can just keep doing that unless the water it is dissolved in sinks away from the surface. This is basically how the ocean is absorbing our fossil fuel emissions- the CO2 molecules diffuse back and forth across the surface into and out of the ocean, eventually (on the scale of maybe 6 months) evening out the difference in pressure until the amount of CO2 dissolved in the surface ocean balances the amount left in the atmosphere. Since our emissions add CO2 to the combined ocean and atmosphere, both the atmosphere and the ocean will end up with higher concentrations than they had before. However, since we are always adding more fossil fuel CO2 to the atmosphere, the ocean is just constantly absorbing and never catches up to the rising CO2 levels in the atmosphere. Remember, the conveyor takes 1,000 years to mix the whole ocean, so the carbon we have added since the beginning of the industrial revolution is just starting the loop.”
Now, I do not want to replicate the CO2 loop from water to air with further questions to your explanations, but I do have a few more after reading your texts: when you say that the oceans and atmosphere try to reach a balance, to even each other out in their CO2 levels, what is the ratio? 1:1? PPM are not the same around the globe’s oceans, right? For example, they are higher in Alaska than in the Caribbean, isn’t that so? But because colder waters release less CO2 into the air the exchange rate might be be similar in both places?? You mention a period of 6 months to do so (if all emissions stopped now and the oceans could actually catch up, I assume). Does this length of time come from a study? How was it carried out? (by measuring the speed at which carbon is currently being absorbed in the oceans?) But then we have the 1,000 year “loop”, correct? Also, if emissions to the air doubled overnight, would the length of time for the oceans to catch up increase as well? Finally, I was wondering if the transition time from air to water is as fast as from water to air.
Thank you both so much for taking the time to write and enriching the blog, I really appreciate it.