A new hypothesis for deglacial CO2 rise?

It is well recognized that warming from a cold glacial period to a warmer interglacial is accompanied by increases in Carbon Dioxide concentrations, which at least amplified Milankovitch-induced forcing over millennial timescales. Atmospheric CO2 concentrations were around 100 ppmv lower than pre-industrial values during deep glacial climates. A key question in paleoclimate studies that is still not sufficiently addressed is how changes in solar insulation could have forced the feedbacks in atmospheric chemistry observed in proxy records.

There are many parts to the puzzle of how atmospheric CO2 changed between glacial and interglacial variations. A simple start is in ocean temperature, where we know from basic chemistry principles that a colder ocean would be better at “holding” more CO2 (since gas is more soluble in cooler water), thus lowering atmospheric concentrations. However, because so much fresh water was locked up in ice, the glacial oceans were saltier than today by a few percent, which reduces the solubility of CO2 gas in water. Changes in the biosphere contributed to some variations in atmospheric chemistry. Increased stratification of surface waters or increases in sea-ice cover that might have limited the release of CO2 to the atmosphere may be very important. There are many competing factors (explored for instance in Sigman and Boyle 2000 ). Also invoked as explanations are the finer points of the ocean carbon cycle such as the productivity of “the biological pump” (i.e., a process where CO2 fixed in photosynthesis is transferred to the deep ocean resulting in sequestration (storage) of carbon) as well as nutrient utilization of high-latitude oceans. Of particular importance is the large Southern Ocean which is key in the atmosphere/ocean CO2 equilibrium. There is evidence of injection of C14-depleted carbon into warmer times (meaning injection from a reservoir long isolated from the atmosphere, such as the deep sea). Most workers point to the Southern Ocean as a locus of deglacial CO2 release, evidence for this also coming from the correlation between atmospheric CO2 and Antarctic temperature records.

In a recent issue of Science, Anderson et al. 2009 provide insight into what may have caused a change in Southern Ocean circulation that would have altered the ocean-air CO2 exchange so substantially. Their answer involves the wind.

The authors use biological proxies to examine changes in upwelling during the last deglaciation. Maximum production of biogenic silica (opal) corresponds with maximum supply of dissolved nutrients to surface waters by the upwelling of nutrient-rich deep waters. Enhanced upwelling in the Southern Ocean during deglaciation is robust. Increased upwelling in the Southern Ocean coincided with the rise in atmospheric CO2 concentrations and Antarctic temperatures during the last deglaciation.

Supporting examples can be found in Abrupt Climate change events. Decreases in upwelling occur during the Antarctic Cold Reversal (14.5 to 12.5 thousand years ago), a time of anomalous cooling in the Southern Hemisphere just before the Younger Dryas in the North. Upwelling resumed after this event and was accompanied by a rise in CO2. Similar changes in CO2 are seen with the rapid warming of Antarctica at Heinrich event 1, interruption of CO2 rise at the Bølling-Allerød, and then CO2 rise warming occurring with Antarctic warming during the Younger Dryas (I didn’t get this backwards for those familiar with the YD; keep in mind the asymmetry between hemispheres as a result of the “see-saw” effect as a result of re-organizing ocean circulation).

_{Alley, 2007}

I now do a brief detour to do a brief description of the Intertropical Convergence Zone (ITCZ) and the “westerlies” in either hemisphere.

In the Northern Hemisphere, viewed from above, surface winds blow clockwise and outward, away from subtropical highs.

As you can see, the westerlies are the winds north of the Horse latitudes (between about 30 and 35 degrees N and S under subtropical highs) and “underneath” the Polar Regions. In the North, the surface winds blowing from the northeast out of the southern flanks of the anticyclones are the trade winds. The reason air doesn’t flow directly from the equator to the poles, but rather is deflected, is because of the Coriolis effect (i.e., the planet is rotating). And contrary to what your high school science teacher probably said, that has nothing to do with how water spirals down your toilet or bathtub.

The trade winds of the two hemispheres converge in a broad east-west equatorial belt of calm winds, known as doldrums. The most active weather occurs at the ITCZ, a belt with thunderstorms that parallel the equator. The world-wide average location of the ITCZ (the “heat equator”) is located around 10 degrees N, due to the fact the North has more land and is a bit warmer.

So what’s that have to do with anything? The process that exposes the deep ocean water to the atmosphere today is the upwelling around Antarctica that is forced by the westerly winds over the Antarctic Circumpolar Current (ACC). Anderson et al. 2009 propose that during these times of ocean re-organization when the North was cool, the ITCZ shifted closer to the equator and the southern westerlies shifted further toward Antarctica. This image is taken from the “Perspective article” in Science

_{J. TOGGWEILER/NOAA Geophysical
Fluid Dynamics Lab in Princeton}

When the strongest westerlies are relatively close to Antarctica and are aligned with the ACC, they are associated with more upwelling. If the glacial westerlies were further to the equator than now, then there would be a buildup of respired CO2 in the deeper ocean as a result of reduced wind stress near antarctica. In general, glacial times follow this pattern. The interesting thought to take from this paper is that this has been a leading mechanism to cause CO2 changes over glacial-interglacial timeframes.

I personally have a quibble with the focus of the paper on NH circulation anomalies that are associated with the Abrupt climate shifts (e.g. Heinrich events and D-O events) for the simple reason that the full range of CO2 variability between glacial and interglacial cycles is evident over the last 800,000 years, not just the last (de)glacial. And in fact, the full range (~180-280 ppm) is amazingly consistent over each peak and trough, whereas it is not self-evident that abrupt climate anomalies followed similar patterns. Remember that temperature wise, these are Hemispheric-scale and relatively short events super-imposed on a much longer Milankovitch-paced timeframe. The argument needs to be extended over a glacial timeframe, not a YD timeframe. I’m convinced by the concept, but I’m not convinced that they’ve demonstrated a significant mechanism for CO2 fluctuations over the last million years, particularly since the spatial and temporal extent of the temperature anomalies are different between glacial-interglacial and stadial-interstadial variations. This criticism may be ill-founded though.

Conceptually, this follows the format of a positive CO2 feedback (also proposed for instance in Toggweiler et al 2006) since warmer temperatures and higher CO2 levels are associated with a poleward displacement of the westerly winds, which according to this hypothesis, would result in further “mining” of CO2 from the ocean to put into the atmosphere.