Comment (2-4):Easy blogging
A commenter (0740.1) states that ice core CO2 measurements are impacted by water contamination, and that there are no other methods of measuring historical CO2 (commenter 3722 also objects to ice core record manipulation). Several commenters (0339, 0714.1, 2210.5, 3722) have cited either Beck (2007) or Jaworowski to support a contention that CO2 was at high concentrations in the recent past immediately before the Mauna Loa record started, or during past interglacials (0655).
We disagree with the assertion by several commenters that estimates of historical CO2 concentrations are incorrect. According to IPCC (Jansen et al., 2007), “it is possible to derive time series of atmospheric trace gases and aerosols for the period from about 650 kyr [thousand years] to the present from air trapped in polar ice and from the ice itself.” This methodology has been “verified against recent (i.e., post-1950) measurements made by direct instrumental sampling.” Additionally, these measurements are consistent with various less accurate methods such as using the size of stomatal pores on tree leaves, boron isotope measurements in plankton buried under the ocean, or carbon isotope ratios in algae buried in the ocean floors, moss samples, and foraminefera carbonate shells. Therefore, there is extremely high confidence in the CO2 values determined from the ice core records, and we disagree that there is any evidence that water contamination or other manipulations reduce the confidence in the ice core estimates.
The commenters cited a theory from Jaworowski that water contamination in the ice core record reduces its reliability, and that the IPCC CO2 historical estimates require shifting the ice core records an arbitrary number of years in order to make them line up with the instrumental record. The critiques of Jaworowski on the shift were addressed by Hans Oeschger (1995), who pointed out that the ice core record shift was done in accordance with theoretical estimates of the rate of diffusion in gases in firn, and that these theoretical estimates were confirmed by isotopic enrichment in line with theory. Güllük et al. (1998) also rebutted Jaworowski on contamination, stating that “Jaworowski et al. [1992, 1994] suggested that CO2 measurements may be subject to fractionation due to clathrate formation and destruction. The good agreement of our CO2 measurements with those made by LGGE using the milling extraction procedure makes this artefact unlikely.” Similarly, Raynaud et al. (1993) found that the objections by Jaworowski were unfounded, demonstrating that the changes in CO2 and methane (CH4) are similar for different interglacial periods, regardless of depth, and that ice cores from different locations give the same values regardless of different “brittle zone” conditions between the different locations.
With respect to the citations of Beck (2007) and Jaworowski (1992, 1994) on pre–Mauna Loa CO2 records, these papers rely on chemical measurements that were taken in many environments which were not far enough away from sources and sinks of CO2 in order to measure the background concentration.
Beck himself (2007) notes that many of his measurements were taken from the “periphery of towns” and shows temporal CO2 plots that have large (210 ppm) variability over a time period of two months. He recognizes that some of these data points need to be corrected by 10 to 70 ppm to take into account nearby cities. This large variability is in contrast to the relatively smooth year after year increase in the Mauna Loa and other modern instrumental records. The pattern of CO2 changes in the Mauna Loa records are much more consistent with the ice core records than with the Beck estimates. Therefore, we find that these historical CO2 estimates by Beck and Jaworowski are not reliable alternatives to the conclusions of the assessment literature on historical background CO2 levels.
Therefore, EPA has determined that the assessment literature estimates of historical CO2 concentrations over the past 800,000 years are of high quality and the most reliable estimates available.