This is the average amount of methane (CH4) that is in the atmosphere in a given year. The yearly average highlights the long-term trends and hides the seasonal variation.
Methane has a large warming effect, considering its much higher Global Warming Potential (GWP) than CO2, but it acts for a relatively brief period compared to CO2. Methane breaks down quickly in the atmosphere, and a reduction in emissions can cause a rapid decline in its levels and impact.
The primary unit here is parts per billion (ppb), which describes the average concentration of atmospheric methane per year. The secondary unit here is megatonnes, which describes the average weight of atmospheric methane per year. We show this to be able to relate to emissions, which are commonly expressed in megatonnes.Wikipedia: Parts-per notation
In 1983 humans started collecting atmospheric methane concentration measurements, and since then the concentration has risen from around 1645 ppb to more than 1900 ppb. To realize how high these concentrations are compared to pre-industrial values, we compare them to long-term historical concentrations revealed by ice core studies.
When we look at the last 1,000 years, we can clearly see that the methane concentration started rising at the end of the 18th century, coinciding with the industrial revolution.
Throughout the last glacial period, roughly 100,000 to 12,000 years ago, we can see a drastic variation in methane levels. It was also a drastic period! Ice sheets reached the northern parts of the US, the UK, Germany and Northeast Siberia. There were sea-level changes up to 120 meters. The changes in methane levels were smaller and much slower compared to what we have been experiencing over the last 200 years.
The last 800,000 years give perspective on the methane levels during recurring glacial periods and the unusually high methane level of our time. Current day level of over 1900 ppb was unheard of in the very long period with values between 350 ppb and 700 ppb.Wikipedia: Methane in Earth's Atmosphere
The data is from two subgroups of NOAA, the Global Monitoring Laboratory for recent data and the National Centers for Environmental Information for paleoclimatology data.
The Global Monitoring Laboratory has an annual mean dataset starting in 1984.
National Centers for Environmental Information holds datasets that contain reconstructions of historic methane levels based on Antarctic ice cores.
Globally averaged marine surface annual mean dataCredits: Ed Dlugokencky, NOAA/GML (gml.noaa.gov/ccgg/trends/trends_ch4/)Update cycle: yearlyDelay: ~ 3 months
Globally averaged marine surface monthly mean dataCredits: Ed Dlugokencky and Pieter Tans, NOAA/GML (gml.noaa.gov/ccgg/trends/)Update cycle: monthlyDelay: ~ 3 months
EPICA Dome C - 800KYr Methane DataCredits: Loulergue, L., A. Schilt, R. Spahni, V. Masson-Delmotte, T. Blunier, B. Lemieux, J.-M. Barnola, D. Raynaud, T.F. Stocker, and J. Chappellaz. 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature, Vol. 453, pp. 383-386, 15 May 2008.
Law Dome Ice Core 2000-Year CO2, CH4, and N2O DataCredits: MacFarling Meure, C., D. Etheridge, C. Trudinger, P. Steele, R. Langenfelds, T. van Ommen, A. Smith, and J. Elkins. 2006. The Law Dome CO2, CH4 and N2O Ice Core Records Extended to 2000 years BP. Geophysical Research Letters, Vol. 33, No. 14, L14810 10.1029/2006GL026152.
ReferencesReference: Original references for Law Dome Ice Core 2000-Year CO2, CH4, and N2O Data
Reference: Law Dome CO2 and CH4 records of the last 1000 years first published in Etheridge et al., 1996 and 1998. Newer results which fill in gaps, extend record to 2000 BP and include N2O, were published and explained in detail in MacFarling Meure et al. 2006 and MacFarling Meure 2004. Some new CH4 results were also published in Ferretti et al. 2005.
Reference: Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. Journal of Geophysical Research, 101, 4115-4128.
Reference: Etheridge, D.M., L.P. Steele, R.J. Francey, and R.L. Langenfelds. 1998. Atmospheric methane between 1000 A.D. and present: evidence of anthropogenic emissions and climatic variability. Journal of Geophysical Research, 103, 15979-15996.
Reference: MacFarling Meure, C., D. Etheridge, C. Trudinger, P. Steele, R. Langenfelds, T. van Ommen, A. Smith, and J. Elkins. 2006. The Law Dome CO2, CH4 and N2O Ice Core Records Extended to 2000 years BP. Geophysical Research Letters, Vol. 33, No. 14, L14810 10.1029/2006GL026152.
Reference: MacFarling Meure, C. 2004. The natural and anthropogenic variations of carbon dioxide, methane and nitrous oxide during the Holocene from ice core analysis. PhD thesis, University of Melbourne.
Reference: Ferretti, D.F., J.B. Miller, J.W.C. White, D.M. Etheridge, K.R. Lassey, D.C. Lowe, C.M. MacFarling Meure, M.F. Dreier, C.M. Trudinger, and T.D. van Ommen. 2005. Unexpected Changes to the Global Methane Budget over the Last 2,000 Years. Science, 309 (5741): 1714-1717.