I recall a conversation I had with Dr. Bob Carter at a restaurant in Townsville, QLD after our public presentations there in June 2010 where he lamented the fact that many of the AGW proponents and many of his critics, “really don’t integrate the earth’s geologic timeline into their critical thinking”. I’ve had dozens of similar comments posted on WUWT. It only takes one look at this graph from Lorraine Lisiecki’s most recent paper in Geophysical Research Letters to get a handle on the geologic timeline of CO2 in recent Earth history. The title and x axis annotations are mine. Compare the peaks of CO2 and Sea Surface Temperature change over the last 1.5 million years.
Figure 3. Proxy comparison. (top) pCO2 (red) [Petit et al., 1999; Monnin et al., 2001; Siegenthaler et al., 2005; Lüthi et al., 2008 , Dd13CP−NA 2 (blue), alkenone concentration (green dashed) [Martínez‐Garcia et al., 2009″], boron‐based estimates with error bars (black dots [Hönisch et al., 2009]; gray circles [Tripati et al., 2009]; triangles [Seki et al., 2010]), and alkenone d13C estimates (squares) [Seki et al., 2010]. Dd13CP−NA 2 and alkenone proxies are scaled to ppm using the mean and standard deviation of pCO2 from 800–0 ka. (See auxiliary material for ODP 1090 age model.) (bottom) Changes in Dd13CP−NA 2 (blue), WEP SST [Medina‐Elizalde and Lea, 2005], and a tropical SST stack (purple) [Herbert et al., 2010] with trend reduced by 0.29°C/Myr to match the WEP. Dd13CP−NA 2 is scaled to °C using the standard deviation of the SST stack from 500–100 ka. – click for larger image”]
Granted, there’s not enough resolution on this graph to see the present (at far left) clearly, and I’m sure there will be arguments complaining it doesn’t show the current measured CO2 ppm value, at ~390ppm, but I’m not posting this to try to dispel current measurements, only to help others gain an understanding of the longer geologic record. Here’s the abstract and conclusion, along with another graph of interest:
Abstract: (emphasis mine)
A high‐resolution marine proxy for atmospheric pCO2 is needed to clarify the phase lag between pCO2 and marine climate proxies and to provide a record of orbital‐scale
pCO2 variations before the oldest ice core measurement at 800 ka. Benthic d13C data should record deep ocean carbon storage and, thus, atmospheric pCO2. This study finds that a modified d13C gradient between the deep Pacific and intermediate North Atlantic (Dd13CP−NA2) correlates well with pCO2. Dd13CP−NA 2 reproduces characteristic differences between pCO2 and ice volume during Late Pleistocene glaciations and indicates that pCO2 usually leads terminations by 0.2–3.7 kyr but lags by 3–10 kyr during two “failed” terminations at 535 and 745 ka. Dd13CP−NA 2 gradually transitions from 41‐ to 100‐kyr cyclicity from 1.3–0.7 Ma but has no secular trend in mean or amplitude since 1.5 Ma. The minimum pCO2 of the last 1.5 Myr is estimated to be 155 ppm at ∼920 ka. Citation: Lisiecki, L. E. (2010), A benthic d13C based proxy for atmospheric pCO2 over the last 1.5 Myr, Geophys. Res. Lett., 37, L21708, doi:10.1029/2010GL045109.
That minimum pCO2 920,000 years ago of 155ppm comes dangerously close to the value at which photosynthetic function shuts down, said to be around 140-150ppm. Earth came close to losing its plant life then.
Here’s another graph, again annotated by me, showing her data:
Figure 2. Comparison of pCO2 (gray) [Petit et al., 1999; Monnin et al., 2001; Siegenthaler et al., 2005; Lüthi et al., 2008 with (top) benthic d18O (black) [Lisiecki and Raymo, 2005 and (bottom) Dd13CP−NA 2 (black). Glacial stages are labeled by MIS number. In Figure 2 (bottom), pCO2 has been smoothed with a 2‐kyr boxcar filter.
I also found this passage of interest:
An anomalous phase relationship between ice volume and pCO2 may explain why these two warming events [Termination 6 (535 ka) and MIS 18 (745 ka)] are weaker than most Late Pleistocene terminations. During both “failed” terminations, the initial d18O change is approximately half the amplitude of most Late Pleistocene terminations; d18O spends ∼20 kyr at intermediate values of 3.8–4.2‰ and then briefly returns to more glacial values before achieving full interglacial conditions ∼40 kyr after the initial warming. The Dd13CP−NA2 lag during these two failed terminations suggests that full deglaciation requires an early pCO2 response.
This is along the lines of Andrew Lacis CO2 knob idea, but it is clear that CO2 isn’t fully in control, but one of many control knobs for climate. There’s also some discussions about the role of polar ice in climate regulation:
The initial trigger for terminations and the mechanistic link between pCO2 and northern hemisphere ice volume remain controversial [e.g., Huybers, 2009; Denton et al., 2010]. Variability in the phase between d18O and Dd13CP−NA2 supports the hypothesis of Toggweiler  that glacial changes in pCO2 are controlled by southern hemisphere processes only weakly linked to northern hemisphere insolation and ice volume. However, tighter coupling between the hemispheres appears to develop at ∼500 ka, as suggested by smaller phase differences between Dd13CP−NA 2 and d18O (Table S3), an increase in pCO2 amplitude, and the phase lock between Antarctic temperature and northern hemisphere insolation during the last five terminations [Kawamura et al., 2007].
 In conclusion, Dd13CP−NA2 correlates well with ice core pCO2 from 800–0 ka and reproduces many features of the pCO2 record. Comparison of Dd13CP−NA
2 and pCO2 suggests that marine and ice core age models [Lisiecki and Raymo,
2005; Parrenin et al., 2007; Loulergue et al., 2007] differ by ≤2.7 kyr at terminations. Within the marine sedimentary record Dd13CP−NA2 usually leads d18O by 0.2–3.7 kyr at terminations but lags by 3–10 kyr during “failed” terminations at 535 and 745 ka. Thus, an early pCO2 response appears necessary for complete deglaciation, and pCO2 appears less tightly coupled to northern hemisphere ice volume before 500 ka.  Several proxies that correlate with pCO2 (Dd13CP−NA2 , South Atlantic productivity [Martínez‐Garcia et al., 2009], and WEP SST [Medina‐Elizalde and Lea, 2005]) and a carbon
cycle box model [Köhler and Bintanja, 2008] suggest that glacial pCO2 minima do not decrease during the MPT. Moreover, the minimum pCO2 concentration of the last
1.5 Myr is estimated to occur at 920 ka. Dd13CP−NA2 gradually shifts from 41‐kyr cycles to 100‐kyr cycles from 1.3–0.7 Ma but shows no secular trend in mean or amplitude over the last 1.5 Myr, whereas tropical SST records suggest warmer glacial maxima before 1.3 Ma [Herbert et al., 2010]. This likely indicates that at least one of these proxies is affected by factors other than pCO2 before 1.3 Ma; thus, additional high resolution proxies are needed.
The thing to bear in mind is that these are proxies, not empirical measurements, and there’s no error/uncertainty shown. Of course at the present, we have ~ 390ppm of CO2 in the atmosphere, and that is nothing I dispute, not does any other skeptic I know of. What is clear from this study though is that our current period of increased CO2 is riding on the back of natural variability of CO2 concentration, which has been observed to occur with regularity over the past 1.5 million years. Of course the question arises as to how much the present concentrations will affect our slide into the next glaciation, if at all. If we are lucky, our “geoengineering” of the planet with some extra CO2 may very well be a lucky break for humanity. Notice that those peaks in CO2 and SST, the most recent of which is the very brief period of the rise of man, are quite short compared with the much longer periods of cooler temperatures.
h/t to Dr. Leif Svalgaard, who has the full paper here