"Second, CO2 levels tend to be a lagging indicator in global warming rather than a leading one, i.e. it's a response to atmospheric warming (resulting in increased plant growth which in turn resequesters the CO2 through photosynthesis) rather than a cause."
How can it be a lagging indicator when it supposed to the cause?
Some relevant citations (emphasis mine):
From Science, Vol 283, Issue 5408, 1712-1714 , 12 March 1999
Ice Core Records of Atmospheric CO2 Around the Last Three Glacial Terminations Hubertus Fischer, Martin Wahlen, Jesse Smith, Derek Mastroianni, Bruce Deck
Air trapped in bubbles in polar ice cores constitutes an archive for the reconstruction of the global carbon cycle and the relation between greenhouse gases and climate in the past. High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 ± 400 years after the warming the last three deglaciations.
Scientists at the Scripps Institution of Oceanography at the University of California, San Diego, reported in the March 12 issue of Science that CO2 levels did not rise until hundreds of years after the warming periods that triggered the end of the last three ice ages.
Said Martin Wahlen, a professor in the Scripps Geosciences Research Division who coauthored the paper. "What we have found is that at these periods when the climate is transitioning from a glacial to an interglacial period, the atmospheric CO2 concentrations lag behind the rise in temperature by about 600 years."
Then Fischer et al. 1999 used ice core records from the period of the last three glacial terminations. They concluded that "the time lag of the rise in CO2 concentrations with respect to temperature change is on the order of 400 to 1000 years during all three glacial-interglacial transitions."
Then Indermuhle et al. (2000) studied the relationship between temperature rise and CO2 levels from 60,000 to 20,000 years ago. They concluded that the rise in the air's CO2 content lagged shifts in air temperature. His mean value was 1,200 years.
Then Monnin et al. (2001). Studied this relationship but used CO2 data from Dome Concordia, Antarctica. They concentrated on the period that includes the most recent transition to interglacial temps Monin found that the CO2 increase lagged the start of the temperature increase by 800 years.
This last report is on the corollary to the rise in CO2 lagging behind temperature rises, in this case the lag is in the response of CO2 to falling temperatures, but its clear, temperatures fall and THEN CO2 concentrations fall.