Although I don’t have data to support the following claims, and can’t recall specific papers on the topic (undoubtedly my following thoughts are some sort of compilation of papers I have read), a reasonable guess for the lag has to be based on the biological response of phytoplankton to changing environmental conditions. Offhand, it seems reasonable that as the Milankovitch (orbital) forcings initiate the onset of the glacial termination there may be some sort of decrease in the zonal sea surface temperature gradient which would in turn lead to less zonal winds. Less zonal winds would seem to reduce vertical mixing in the surface ocean and induce a subsequent stratification of surface waters. The surface waters may be additionally stratified by an increase in melt water remaining at the surface (reducing the salinity of surface water, consequently increasing water column stability). These combination of these factors do not bode well for carbon fixers such as phytoplankton who are dependent on intense vertical mixing as a source of upwelled nutrients. Less carbon fixation would result in more CO2 remaining in the atmosphere as the glacial termination proceeds, acting as a positive feedback to the initial forcing. Of course, this sort of large scale response would take a long time to occur, and may in fact not happen until well into the glacial termination. Just some thoughts. Perhaps someone with more study in the field could present the conventional wisdom on this topic.
While the fast glacial-interglacial transitions may hide which leads what and to what extent, the much slower (depending of the length of the interglacial) interglacial-glacial transitions make it clear. When the temperature decreases (7Â°C), CO2 levels remain high. When CO2 levels fall (some 50 ppmv), there is no measurable effect on temperature at all. This contradicts the possibility that CO2 plays an important role in the onset of both glaciations and deglaciations.
Response The above statement is incorrect. The correlation being discussed is between CO2 and deuterium/hydrogen isotope ratios in snow (as archived in an ice core), an imperfect measure of temperature. It has been shown that when the deuterium/hydrogen ratios are corrected for the influence of temperature changes at the ocean surface (from which the water that fell as snow originally evaporated), the correlation becomes even more remarkable, and the apparent drop in temperature thousands of years before the drop in CO2 disappears. See Cuffey and Vimeux, Science, 2002.
Further, even if we assume that the average of current climate models is right, the temperature increase from a CO2 doubling is around 3Â°C, or ~1Â°C for a 100 ppmv rise. Which is 1/10th of the >10Â°C rise seen in the last deglaciation.
Rapid temperature swings like the end of the Younger Dryas (probably less than a few decades) are followed by CO2 changes.
Even the past 1,000 years, showed a lag of ~50 years of CO2 vs. temperature for the Law Dome ice core, but the temperature data disappeared from the Internet…
And since the industrial revolution, sea surface temperature changes, like El NiÃ±o, induce peaks in the CO2 increase rate, some 6 months after the onset of the event…
Thus all together, all historic data point to a lag of CO2 after temperature changes, without much influence of CO2 on temperature when that happens…
Comment by Ferdinand Engelbeen — 12 Dec 2004 @ 2:12 PM
Wow! Are you really saying that we have no idea what starts to warm up our world from an ice age but know with near certainty what has caused the warming of the last three decades?
From my now somewhat distant scientific education I recall that it takes some 80 times more heat to turn the ice to water than to raise its temperature by a single centigrade degree. With sea levels 125m or so lower a significant proportion of the planets water must have been in the form of ice. The â??unknown processâ?? you refer to would have had to supply far more extra heat than the CO2 feedback, which was able to take over some 800 years later.