A synthesis study of the global methane budget 2000-2012
December 2016 - The Global Carbon Project-Methane (GCP-CH4) provides an update on the global methane (CH4) budget based on current knowledge and the collation of published observations and modelling results.
CH4 is currently the second largest contributor to the anthropogenic greenhouse effect after CO2 and contributes about 20% of the radiative forcing. The global warming potential of CH4 is about 28 times that of CO2 over a 100-year time period (Myhre et al., 2013).
Surface atmospheric CH4 concentrations reached 1810 ppb in 2012 (Saunois et al., 2016), which is about 2.5 times as large as pre-industrial concentrations. In the late 1990s and early 2000s there was a near decade of minimal growth in atmospheric CH4 (Nisbet et al., 2014) followed by a renewed and sustained period of increase.
CH4 is generated by a variety of sources such as natural wetlands, fossil fuels and agriculture. Emissions from some of these sources, such as wetlands, may increase strongly under climate change further enhancing the rate at which warming occurs. Hence isolating the source(s) and mechanism(s) for this recent increase in atmospheric CH4 is of considerable interest to better understand the role of methane to both present day and future climate change.
However, this remains scientifically challenging as there is considerable uncertainty in the global methane budget, mainly caused by the large number of both natural and anthropogenic CH4 sources and the difficulty in estimating its removal from the atmospheric through chemical reactions. Hence an initial goal for the Global Carbon Budget for Methane is to quantify the magnitude of these sources and their regional distribution.
The GCP-CH4 synthesis paper presents CH4 emissions for the 2003-2012 decade. Combining atmospheric observations and models global CH4 emissions are estimated to be about 558 TgCH4 yr-1 (range of 540-568 TgCH4 yr-1) of which about 60% (range of 50-70%) are anthropogenic. Emissions are dominated by those from the tropics (~64%) with ~32% and 4% from the mid and high northern latitudes respectively.
Natural wetland emissions are the largest single CH4 source (and up to approximately 40% of the total). They are also thought to be the cause of much of the yearly variability in atmospheric methane (McNorton et al., 2016) and may produce a positive climate feedback in the future (Melton et al., 2013). However their present day emissions remain highly uncertain with wetland emission estimates ranging 127-227 TgCH4 yr-1.
Attributing the recent increase in CH4 to sources remains a scientific challenge, with some analyses suggesting wetlands could be contributing to this (McNorton et al., 2016). The global CH4 budget is a piece in the jigsaw to understanding the mechanisms and feedbacks controlling atmospheric concentrations of the second largest greenhouse gas.
Saunois M., et al. (2016) Figure 7: Regional CH4 budget in TgCH4 yr-1 (or million tonnes CH4 yr-1) per category and map of the 14 continental regions considered. The CH4 emissions are given for the five categories from left to right (wetlands, biomass burning, fossil fuels, agriculture and waste, and other natural). Top-down estimates (which when input into atmospheric models produce the best-fit against atmospheric observations) are given by the left light-coloured boxes and bottom-up estimates (inventories and process-based models) by the right dark-coloured boxes.
Met Office Hadley Centre
The Met Office Hadley Centre develops and uses complex Earth System Models which include models of wetlands and their CH4 emissions and CH4 atmospheric transport and chemistry. This work includes investigating the potential for CH4 emissions from wetlands to act as a positive feedback under climate change.
Met Office Hadley Centre Scientists, Dr Andy Wiltshire and Dr Nicola Gedney contributed to the study.
The Global Carbon Project is an international programme which aims to develop a complete picture of the carbon cycle.
Andy Wiltshire leads the Terrestrial Carbon Cycle research group.
Nicola Gedney is a land surface modeller specialising in wetland methane emissions.
Saunois M., et al. (2016). The global methane budget 2000–2012. Earth Syst. Sci. Data.
Myhre, G. et al. (2013): Anthropogenic and Natural Radiative Forcing, in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.
Nisbet, E. G., et al. (2014). Methane on the Rise-Again. Science.
McNorton, J., et al. (2016). Role of regional wetland emissions in atmospheric methane variability. Geophysical Research Letters.
Melton, J. R., et al. (2013). Present state of global wetland extent and wetland methane 421 modelling: conclusions from a model inter-comparison project (WETCHIMP), Biogeosciences.