Monthly estimates of 13C/12C in fossil-fuel CO2 emissions. Originally
at
http://cdiac.esd.ornl.gov/trends/emis_mon/emis_mon_co2.html; now off-line.
At one time:
"An annual cycle, peaking during the winter months and reflecting
natural gas consumption, and a semi-annual cycle of lesser amplitude,
peaking in summer and winter and reflecting coal consumption, comprise
the dominant features of the annual pattern. The relatively constant
emissions until 1987, followed by an increase from 1987-1989, a
decrease in 1990-1991 and record highs during the late 1990s, are also
evident in the annual data of Marland et al. However, emissions have
declined somewhat since 2000."