CO2 Lag and Causation in the Quaternary Glacial Cycles

The View from Wesser Bald
Nantahala National Forest, North Carolina

Considerable research has been given to understanding the processes by which climate has changed to produce the glacial-interglacial cycles. Two studies have examined how climate warmed from the Last Glacial Maximum (LGM) to the Holocene Thermal Maximum (HTM). Shakun et al 2012[1] examined proxies from around the globe to construct a timeline of how and when warming occurred. Beginning with the LGM, the Milankovitch Cycles triggered a global warming of about 0.3 C. This initial warming caused CO2 to degas from the oceans about 17,500 years ago. CO2 in this case doesn’t cause the initial warming; it is a feedback that amplifies warming.

“This early global warming occurs in two phases: a gradual increase between 21.5 and 19 kyr ago followed by a somewhat steeper increase between 19 and 17.5 kyr ago (Fig. 2a). The first increase is associated with mean warming of the northern mid to high latitudes, most prominently in Greenland, as there is little change occurring elsewhere at this time.... The second increase occurs during a pronounced interhemispheric seesaw event..., presumably related to a reduction in AMOC strength… Tropical and Southern Hemisphere warming seem to have more than offset northern extratropical cooling, however, perhaps as a result of an asymmetry in the response of feedbacks such as Southern Ocean sea ice or tropical water vapour, leading to the global mean response.”

Shakun argues that since the Atlantic Meridional Overturning Circulation (AMOC) redistributes heat between the two hemispheres, variability in AMOC can cause a “seesaw” effect as heat gets redistributed between the hemispheres. This explains why Antarctic temperatures lead CO2 while global temperatures lag CO2. The northern high latitudes warmed first, reducing AMOC strength. Then about 19,000 years ago, the SH and tropics warmed as the extratropical NH cooled as heat was transferred to the Southern Hemisphere. Warming in the SH and tropics exceeded cooling in the extratropical NH. The SH warming was significant enough to cause the oceans to degas CO2, and the extratropical NH warmed along with the SH. In this way, global temperatures end up following the increase in CO2.

The following year, Parrenin published a study[2] that reexamined the dating of the CO2 and temperature proxies in the Antarctic ice cores. Parrenin examined the data from five Antarctic ice cores and revised the relative age scale for CO2 and Antarctic temperatures. Then the study examined the phasing between CO2 and Antarctic temperatures when there were abrupt changes in trends. This allowed the study to conclude that Antarctic temperatures (AT) and atmospheric CO2 (aCO2) had “no significant asynchrony between them.” This revised relative time scale between CO2 and Antarctic temperatures eliminated the so-called lag of hundreds of years of CO2 following Antarctic temperatures and the "seasaw" effect caused by AMOC described by Shakun.

“The aCO2 rise could contribute to much of the AT change during TI, even at its onset, accounting for positive feedbacks and polar amplification (21), which magnify the impact of the relatively weak rCO2 change (Fig. 4) that alone accounts for ~0.6°C of global warming during TI (21). Invoking changes in the strength of the Atlantic meridional overturning circulation is no longer required to explain the lead of AT over aCO2 (22).”

This suggests that both aCO2 and AT are the result of either a “common mechanism” or “numerous independent mechanisms” - mechanisms that are independent from each other. Candidates include ocean stratification, sea ice cover, ice-albedo, etc. 

There is significant relevance here to debates about whether the CO2 increase caused the warming that allowed the globe to emerge from glaciation or was an effect of the warming that cause the globe to emerge from glaciation. A common argument is that since temperature increases preceded the CO2 increase, the CO2 increase is an effect of warming, not a cause of warming, and CO2 cannot be responsible for the warming brought us out of glaciation. This argument is misguided on several levels:

1. While it is likely true that an increase in ATs preceded the increase of CO2, the increase in global temperatures followed the increase in CO2.
2. It's also true that increased global temperatures with warmer oceans will cause CO2 to degas from the oceans, since CO2 is less soluble in warmer water. But this does not eliminate CO2 as a cause of warming. It merely argues that CO2 can function as a feedback as well as a driver. 
3. Warming was likely initiated by orbital cycles, and that initial warming triggered a release of CO2 from the oceans to the atmosphere. That additional CO2 amplified the warming signal, increasing warming. As ice melted, the earth's albedo decreased, allowing more light to be absorbed by the earth and radiated back to space as heat to be trapped by the additional CO2. CO2, dust, and ice -albedo all functioned as feedbacks that amplified the weak forcing from the orbital cycles.
4. Multiple lines of evidence demonstrate that increasing atmospheric CO2 pushes climate towards warming. This has been understood since the 1850s with the work of Eunice Foote. The fact that CO2 can function as a feedback does not mean that it can't function as a driver of climate change. CO2 led temperatures in events like the PETM and current warming, for instance.

References:

[1] Shakun et al, “Global Warming Preceded by Increasing Carbon Dioxide Concentrations during the Last Deglaciation” Nature 484(7392):49-54 · April 2012
https://www.researchgate.net/publication/223987444_Global_Warming_Preceded_by_Increasing_Carbon_Dioxide_Concentrations_during_the_Last_Deglaciation

[2] Parrenin, F. et al. “Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming.” Science 339, 1060 (2013).
DOI: 10.1126/science.1226368
https://pdfs.semanticscholar.org/d61d/0fbcb5828af1d434d1bd0282ed36e0f00d2a.pdf