We have all been treated to endless reps of CO2 is a response to global warming and not a cause, and I know because that is what happened as the Earth came out of the ice age. This, in spite of the fact that we know increasing CO2 can be both a cause of warming as it is today, when the gas is released by burning long buried fossil fuels and a positive feedback as gas is released from warming oceans and from soils. The accepted current guess for the lag is 800 + 600 yrs from Monnin, E., et al. "Atmospheric CO2 concentrations over the last glacial termination, Science, 291, 112–114, 2001. Such measurements are made using ice cores and critically depend on coordinating the age of the ice and the age of the gas in the ice. They differ in position and must be put on the same scale
Now comes a new core, Epica, and a new bunch L. Loulergue, F. Parrenin, T. Blunier, J.-M. Barnola, R. Spahni, A. Schilt, G. Raisbeck, and J. Chappellaz in Climate of the Past, 3, 527–540, 2007 to write on New constraints on the gas age-ice age difference along the EPICA ice cores, 0–50 kyr.
Here we bring new constraints to test a firn densification model applied to the EPICA Dome C (EDC) site for the last 50 kyr, by linking the EDC ice core to the EPICA Dronning Maud Land (EDML) ice core, both in the ice phase (using volcanic horizons) and in the gas phase (using rapid methane variations). We also use the structured 10Be peak, occurring 41 kyr before present (BP) and due to the low geomagnetic field associated with the Laschamp event, to experimentally estimate the 1ag during this event. Our results seem to reveal an overestimate of the lag by the firn densification model during the last glacial period at EDC. Tests with different accumulation rates and temperature scenarios do not entirely resolve this discrepancy. Although the exact reasons for the lag overestimate at the two EPICA sites remain unknown at this stage, we conclude that current densification model simulations have deficits under glacial climatic conditions. Whatever the cause of the 1age overestimate, our finding suggests that the phase relationship between CO2 and EDC temperature previously inferred for the start of the last deglaciation (lag of CO2 by 800±600 yr) seems to be overestimated.They are loath to speculate about where this will leave the lag, even when encouraged to do so by the referees (unheard of behavior), but it is clear that this question is being looked into intensively. If Eli HAD to guess, he would say that there should be a lag based on first principles, but would not be much surprised if it shrunk down from the previous 800 year estimate, substantially.
This paper appears to have evaded everyone's radar.
UPDATE: Fergus informs me that William and he discussed this on Stoat when it appeared, As importantly Steve Bloom points to another interesting paper that Hank Roberts finds scary
IIRC the 800 years had been thought to mesh nicely with the turnover of the oceans, but there's a fresh paper in JGR that claims more like century-scale turnover. If correct that would mesh well with the EPICA result, but unless I'm missing something it would have some rather more important implications for present calculations of ocean heat content, the time needed to reach global warming equilibrium and the time needed for CO2 saturation.Eli has found an open access version of the paper Radiocarbon-based circulation age of the world oceans by Katsumi Matsumoto.