During the Earth’s ice ages the Pacific Ocean stored large amounts of carbon, which for some reason it released again close to the last glacial period’s end, warming the world and melting most of the icecaps.

That is how the ancient link between temperature and CO2 used to be envisioned.

Three US scientists say they now need a creative mind with a nice new hypothesis to better explain where thousands of years ago such amounts of CO2 came from – because the old one that climatologists have worked with just proved not to have been clever enough. Hoping you’ve finished your homework investigating Earth’s carbon cycle they turn to you…

‘Sun comes up; Sun goes down, never a miscommunication’

Earth permanently bathes in a sea of sunlight, billions of years on end. Sometimes though, it dips a little deeper, and sometimes it creates a little extra local shade.

Such gradual changes in our planet’s insolation, which arise from wiggles in the tilt of the axis and the eccentricity of the orbit around the Sun, are called the ‘Milankovitch cycles’ and fundamentally it is thanks to these that ice ages come – and go.

To use simpler words and be more exact: if the total amount of sunshine on the poles is lowest, we have an ice age – in between (the interglacials) it’s mild and comfortable at high latitudes too.

Nothing gradual about a big freeze

The Milankovitch cycles clearly do not explain the entire picture though. Paleo research shows the shifts from glacial periods to interglacials, and back to glacials weren’t gradual at all (at least not during the second half of the Pleistocene – the Age of ice ages). In fact, more and more studies show things happened suddenly, in some cases perhaps on a timescale of just decades.

Last month a Science publication offered some nice new insights into such mechanisms. It showed how sudden disturbances in the oceans’ thermohaline circulation (or ‘conveyor belt’ – known best for the Atlantic Gulf Stream) may kick-start European ice ages, while creating sudden warm periods around Antarctica through a ‘bipolar seesaw’ link.

That ice age switches are not just about redistributing ocean heat, but also about CO2, a Nature study just a week earlier had shown. Ice ages can lead to massive dust storms it stated – and through ocean iron fertilisation these can make plankton absorb carbon dioxide at a planetary relevant scale, leading to further cooling.

Thaw can strike suddenly too

Anyway, today we try to explain the exact opposite: how northern hemisphere ice ages can quite suddenly weaken – at least in case of the last one, which had its cold peak around 18,000 years ago, after which atmospheric CO2 levels ‘suddenly’ (over a millennium or so) rose by 30 per cent, and temperatures started to climb closer* to our current Holocene values.

[*) Some of you may know it was not yet the true beginning of the Holocene, as there was another short cold spell about 12,000 years ago, known as the ‘Younger Dryas,’ after which the icecaps finally receded. This freeze started when the Gulf Stream suddenly stopped, it disappeared when ocean currents picked up again, about a thousand years later. The Younger Dryas is considered a convulsion of an ice age that [judging by the Milankovitch phase] had already come to an end. We’re ignoring this episode – and assume the last ice age started its decline 18,000 years ago.]

Paleoclimatologists thought the CO2 rise had come from the Pacific Ocean, because that made most sense. Not only is it the largest ocean, it also contains the ‘oldest water’ – that is the water that (due to a giant slow loop in the thermohaline circulation) spends the longest time without contact with the atmosphere, with the best chance of accumulating CO2 (as carbonate) at the bottom.

Let’s carbon date some of that foraminifera stuff!

The location where today the oldest Pacific water is found is many miles off the coast of Oregon. As it is thought during ice ages the ventilation age (the time ocean water spends without atmospheric contact) increased, researchers of three different American universities have looked at sediments there dating back to the time of glacial dawn.

On Sunday, in their publication in Nature Geoscience, they dismissed the Pacific Ocean carbon sink hypothesis. Much to their surprise carbon isotope measurements of foraminifera shells (tiny plankton skeletons) showed the ventilation age had actually decreased and there had been no extra CO2 storage in the deep Pacific during the latest ice age, and no big release towards the end.

“We’re going back to the drawing board. It’s certainly fair to say that we need to have some other working hypotheses at this point,” says paleoclimatologist and lead author David Lund, of Oregon University.

Finding that other source could be important for future climate research. Scientists are confident CO2 was lower during the ice ages and that its rise was coupled to rising temperatures when Earth moved to an interglacial. If that CO2 release was in fact (partly) triggered by warming – a positive climate feedback – we need to know from what store it came and exactly how stable that spot looks today. So, the hunt is on, and below is just one guess by yours truly.

Attempt at being clever: wrong ocean (?)

The researchers checked the North-eastern Pacific. Finding no glacial carbon sink there does not imply none of the oceans stored [and then again released] CO2. Carbon sequestration does not only depend on water currents, also on Biology – the total amount of it, plankton biomass especially. The NE Pacific was cold during the last ice age, perhaps hampering growth. That was probably not the case further south, especially on the southern hemisphere.

Even the Southern Ocean is a good missing sink candidate – as research suggests waters may have stagnated there too, and temperature conditions around Antarctica were at times remarkably mild when blizzards roamed the north.

The one other option for Earth would be increased carbon capture on land, during glacials. We know this cannot have been the case; as such a large portion of the Earth’s land masses lie so far to the north, and what is temperate and boreal forest now was either tundra or glacier back then. So, if it wasn’t the ocean, it still has to be the ocean. Right?

© Rolf Schuttenhelm | www.bitsofscience.org