Atmospheric Methane Paul Wennberg Radiative Properties as a greenhouse gas Atmospheric Chemistry contributes to control of OH Budgets what processes determine the concentration of CH4 Climate Change 2001 Working Group I The Scientific Basis Figure 2 Long records of past changes in atmospheric composition provide the context for the influence of anthropogenic emissions a shows changes in the atmospheric concentrations of carbon dioxide CO2 methane CH4 and nitrous oxide N2O over the past 1000 years The ice core and firn data for several sites in Antarctica and Greenland shown by different symbols are supplemented with the data from direct atmospheric samples over the past few decades shown by the line for CO2 and incorporated in the curve representing the global average of CH4 The estimated positive radiative forcing of the climate system from these gases is indicated on the right hand scale Since these gases have atmospheric lifetimes of a decade or more they are well mixed and their concentrations reflect emissions from sources throughout the globe All three records show effects of the large and increasing growth in anthropogenic emissions during the Industrial Era b illustrates the influence of industrial emissions on atmospheric sulphate concentrations which produce negative radiative forcing Shown is the time history of the concentrations of sulphate not in the atmosphere but in ice cores in Greenland shown by lines from which the episodic effects of volcanic eruptions have been removed Such data indicate the local deposition of sulphate aerosols at the site reflecting sulphur dioxide SO2 emissions at mid latitudes in the Northern Hemisphere This record albeit more regional than that of the globally mixed greenhouse gases demonstrates the large growth in anthropogenic SO2 emissions during the Industrial Era The pluses denote the relevant regional estimated SO2 emissions right hand scale Based upon a Chapter 3 Figure 3 2b CO2 Chapter 4 Figure 4 1a and b CH4 and Chapter 4 Figure 4 2 N2O and b Chapter 5 Figure 5 4a The growth in atmospheric methane also contributes to moistening of the stratosphere Air entering the stratosphere is desiccated to 3 4 ppmv by the very cold temperatures present at the tropical tropopause 190 200 K As air remains in the stratosphere the methane is oxidized to CO2 and H2O and the moisture increases to nearly 8 ppmv in the oldest air i e where CH4 0 Because H2O in the lower stratosphere is such a good GHG it is cold and has high extinction this source of H2O must be considered in future climate predictions As illustrated in the IPCC CH4 figure the growth rate of atmospheric methane has been highly variable During the 1980s the trend was fairly constant at 10 ppbv yr During the 1990s the rate has oscillated with some years with essentially no trend e g 1992 1993 1999 2000 There is no consensus view on what causes the variability in the methane growth rate Some research has pointed to variability in the sources driven by changes in the atmospheric hydrological cycle others have pointed to possible variability in OH driven by changes in stratospheric ozone Given the lack of understanding of recent trends prediction of future methane concentration remains highly uncertain Oops IPCC TAR Methane Budget Summary Total Sink Sinks Strat Trop OH Soils Total Source Other Anthropogenic Biomass Burning Fung et al 1991 Rice Agriculture Hein et al 1997 Waste Treatment Lelieveld et al 1998 Ruminants Houweling et al 1999 Sources Mosier et al 1998 Landfills Olivier et al 1999 Energy Cao et al 1998 Hydrates Ocean Natural Termites Wetlands 0 100 200 300 400 Source Tg yr 500 600 700 Photochemistry The primary sink of atmospheric methane Methane is destroyed primarily in the troposphere 90 of the loss when it is oxidized by the hydroxyl radical OH ESE Ch Ge 171 OH CH4 CH3 H2O OH is the premier oxidant in Earth s atmosphere It is formed in the daytime via gas phase photochemistry its major source is O3 h O 1D O2 O 1D H2O OH OH h represents a photon of wavelength 315 nm O 1D is the first electronically excited oxygen atom In the troposphere OH has an average mixing ratio of 1 x 10 6 molecule cm 3 At 273 K the rate coefficient for the reaction of OH with methane is about 3 5 x 10 15 cm3 molecule 1 s 1 Thus CH4 CH4 kOH CH4 OH CH4 1 kOH CH4 OH 1 2 8 x 108 s 1 9 years Because the reaction of OH with CH 4 is an important sink of tropospheric OH the lifetime of methane is not independent of the concentration of methane As the concentration of methane increases the lifetime of methane also increases a positive feedback Prather et al Lifetimes and Eigenstates in atmospheric chemistry GRL 21 801 1994 IPCC TAR Methane Budget Summary Total Sink Sinks Strat Trop OH Soils Total Source Other Anthropogenic Biomass Burning Fung et al 1991 Rice Agriculture Hein et al 1997 Waste Treatment Lelieveld et al 1998 Ruminants Houweling et al 1999 Sources Mosier et al 1998 Landfills Olivier et al 1999 Energy Cao et al 1998 Hydrates Ocean Natural Termites Wetlands 0 100 200 300 400 Source Tg yr 500 600 700 Modeled Methane Emissions Biome4 Present Wetlands 140 Tg yr LGM Wetlands 107 Tg yr Kaplan GRL 2002 Factors Controlling Methane Emissions from Wetlands Temperature Water balance Productivity Bubier and Moore Trends in Ecology and Evolution 1994 Vostok Antarctica Ice Core Orbital Cycles Interglacial Interglacial Interglacial Interglacial Warm 420 D 440 Glacial Glacial Glacial 460 480 280 240 220 200 700 CH4 ppb Cold CO2 ppm 260 600 500 400 0 100 200 300 400 3 GT4 Age x10 Years D H relative to a standard proxy for surface air temperature Petit et al Nature 1999 African Monsoon El Ni o E J Dlugokencky et al GRL 30 19 2003 IPCC TAR Methane Budget Summary Total Sink Sinks Strat Trop OH Soils Total Source Other Anthropogenic Biomass Burning Fung et al 1991 Rice Agriculture Hein et al 1997 Waste Treatment Lelieveld et al 1998 Ruminants Houweling et al 1999 Sources Mosier et al 1998 Landfills Olivier et al 1999 Energy Cao et al 1998 Hydrates Ocean Natural Termites Wetlands 0 100 200 300 400 Source Tg yr 500 600 700 Rice Paddies are Man made Wetlands http www gsfc nasa gov topstory 2002 1204paddies html Engineering Solutions Abstract Decreased methane emissions from paddy rice may have contributed to the decline in the rate of increase of global atmospheric methane CH4 concentration over the last 20 years In China midseason paddy drainage which reduces growing season CH4 fluxes was first implemented in the