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A 2000-Year History of Atmospheric Methyl Chloride
Reference
Williams, M.B., Aydin, M., Tatum, C. and Saltzman, E.S. 2007. A 2000 year atmospheric history of methyl chloride from a South Pole ice core: Evidence for climate-controlled variability. Geophysical Research Letters 34: 10.1029/2006GL029142.

Background
In the words of the authors, "methyl chloride (CH3Cl) is the largest natural source of chlorine to the stratosphere and the most abundant halocarbon in the troposphere, with a global average mixing ratio of 550 30 parts per trillion (ppt) (Montzka et al., 2003)."

What was done
Williams et al. present CH3Cl measurements of air extracted from a 300-m ice core that was obtained at the South Pole, Antarctica, covering the time period 160 BC to AD 1860.

What was learned
The researchers determined that "CH3Cl levels were elevated from 900-1300 AD by about 50 ppt relative to the previous 1000 years, coincident with the warm Medieval Climate Anomaly (MCA)," and that they "decreased to a minimum during the Little Ice Age cooling (1650-1800 AD), before rising again to the modern atmospheric level of 550 ppt." Noting that "today, more than 90% of the CH3Cl sources and the majority of CH3Cl sinks lie between 30N and 30S (Khalil and Rasmussen, 1999; Yoshida et al., 2004)," they say "it is likely that climate-controlled variability in CH3Cl reflects changes in tropical and subtropical conditions." In fact, they go so far as to state that "ice core CH3Cl variability over the last two millennia suggests a positive relationship between atmospheric CH3Cl and global [our italics] mean temperature."

What it means
The peak CH3Cl concentration measured by Williams et al. during the MCA is - as best we can determine from the graphical representation of their data - approximately 533 ppt, which is within 3% of its current mean value of 550 ppt and well within the range of 520 to 580 ppt that characterizes methyl chloride's current variability. Hence, we may validly conclude that the mean peak temperature of the MCA (which we refer to as the Medieval Warm Period) over the latitude range 30N to 30S - and possibly over the entire globe - may not have been materially different from the mean peak temperature so far attained during the Current Warm Period, i.e., that of the last few years. And this conclusion, of course, suggests there is nothing unusual, unnatural or unprecedented about the current level of earth's warmth, which further suggests that 20th-century global warming may not have had anything to do with the concomitant historical increase in the atmosphere's CO2 concentration.

References
Khalil, M.A.K. and Rasmussen, R.A. 1999. Atmospheric methyl chloride. Atmospheric Environment 33: 1305-1321.

Montzka, S.A. et al. 2003. Controlled substances and other source gases. In: Scientific Assessment of Ozone Depletion: 2002. Global Ozone Research and Monitoring Project, Report 47, World Meteorological Organization, Geneva, Switzerland, Chapter I, p. 5-83.

Yoshida, Y., Wang, Y.H., Zeng, T. and Yantosea, R. 2004. A three-dimensional global model study of atmospheric methyl chloride budget and distributions. Journal of Geophysical Research 109: 10.1029/2004JD004951.

Reviewed 24 October 2007