Scripps’ measured an average of 390.19ppm CO2 concentration at their Mauna Loa, Hawai’i’s Observatory during August 2011. This reading is from the Scripps’ dataset, not NOAA’s, from which I previously wrote.
That value is the highest value for August in recorded history. Last year’s 388.54 was the previous highest August value ever recorded. This year’s August reading is 1.65ppm higher than last August’s, which is a significant difference.
The yearly maximum monthly value normally occurs during May. This year was no different: the 394.16 concentration is the highest value reported both this year and all time.
This will likely be the last year that CO2 concentrations will fall below 390ppm during any calendar month. Although it hasn’t happened yet (every month in 2011 has recorded at least 390ppm), the yearly minimum that normally occurs in September is upcoming. Based on the trend from May to September in years past, I expect 2011′s minimum to be ~388ppm.