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An analysis of the global atmospheric methane budget under different climates
von Abhijit BasuMethane is the second most important gas after CO2 in the atmosphere in terms of radiative
forcing. It also plays an important role in tropospheric chemistry and influences the oxidation
capacity of atmosphere and amount of CO, O3 and water vapour. Various biogenic and
anthropogenic sectors including gas and oil extraction, wetlands, animal ruminants emit methane
in the atmosphere while it is mainly OH which displaces it. At present, the mean global methane
concentration is balanced approximately at 1780 ppb after undergoing several changes over the
past decades. The sources and sinks currently contribute between 450 and 510 Tg per year
although the strength of each source components suffers from uncertainty. Methane is also
assumed to be a key player in past climatic changes and its global abundance underwent several
transitions which were recorded in the ice cores. One of the drastic changes in methane mixing
ratio is observed during the last glacial-interglacial transition, as it shows an increasing trend
from 350 ppb till it reaches 700 ppb at the pre-industrial Holocene. The post industrial increase
in global methane concentration is also unprecedented.
In this study, methane distribution at present climate as well as at Last Glacial Maximum (LGM)
and pre-industrial era is simulated with a simplified global tropospheric model ECHAM MOZ.
For this simulation, methane emissions from various inventories have been used. A new
parameterisation method is developed to estimate wetland methane emission for present day
which is later adapted for LGM and pre-industrial time. Wetlands are the largest natural source
of methane, still suffers from huge uncertainties. Contrary to the other hydrological models, the
present wetland parameterisation follows a simplified approach based on a handful of soil
parameters from CARAIB vegetation model. This method is easily adaptable to past climate
simulations. The model result for present day from ECHAM MOZ chemistry simulation has
been validated with station observation data across the globe and a set of sensitivity analysis with
the modified sources are carried out to optimize the global methane budget. One of the major
findings from this study is the optimized wetland methane strength which falls in the lower range
of IPCC AR4 report. The ECHAM MOZ transient simulation could produce the recent methane
trend and inter annual variability between 1990 and 2006 reasonably well although shows an
underestimation in a range of 20-40 ppb for the first eight years. This is perhaps caused due to
the underestimation of the oil and gas extracted methane source used in the model. For LGM and
pre-industrial period, the model, using my wetland methane source successfully reproduces the
ice core methane records. Compared to previous studies, the present LGM model source strength
is weaker which raises the possibility of a less deviated sink than present. This is supported by
some recent studies on the tropospheric oxidative chemistry which found less OH variability
than previously assumed. The important aspect of the present study is that contrary to previous
studies where sinks are often hold responsible to explain atmospheric methane variability, here
the emphasis has been given to the role of changing source based on these recent findings.
forcing. It also plays an important role in tropospheric chemistry and influences the oxidation
capacity of atmosphere and amount of CO, O3 and water vapour. Various biogenic and
anthropogenic sectors including gas and oil extraction, wetlands, animal ruminants emit methane
in the atmosphere while it is mainly OH which displaces it. At present, the mean global methane
concentration is balanced approximately at 1780 ppb after undergoing several changes over the
past decades. The sources and sinks currently contribute between 450 and 510 Tg per year
although the strength of each source components suffers from uncertainty. Methane is also
assumed to be a key player in past climatic changes and its global abundance underwent several
transitions which were recorded in the ice cores. One of the drastic changes in methane mixing
ratio is observed during the last glacial-interglacial transition, as it shows an increasing trend
from 350 ppb till it reaches 700 ppb at the pre-industrial Holocene. The post industrial increase
in global methane concentration is also unprecedented.
In this study, methane distribution at present climate as well as at Last Glacial Maximum (LGM)
and pre-industrial era is simulated with a simplified global tropospheric model ECHAM MOZ.
For this simulation, methane emissions from various inventories have been used. A new
parameterisation method is developed to estimate wetland methane emission for present day
which is later adapted for LGM and pre-industrial time. Wetlands are the largest natural source
of methane, still suffers from huge uncertainties. Contrary to the other hydrological models, the
present wetland parameterisation follows a simplified approach based on a handful of soil
parameters from CARAIB vegetation model. This method is easily adaptable to past climate
simulations. The model result for present day from ECHAM MOZ chemistry simulation has
been validated with station observation data across the globe and a set of sensitivity analysis with
the modified sources are carried out to optimize the global methane budget. One of the major
findings from this study is the optimized wetland methane strength which falls in the lower range
of IPCC AR4 report. The ECHAM MOZ transient simulation could produce the recent methane
trend and inter annual variability between 1990 and 2006 reasonably well although shows an
underestimation in a range of 20-40 ppb for the first eight years. This is perhaps caused due to
the underestimation of the oil and gas extracted methane source used in the model. For LGM and
pre-industrial period, the model, using my wetland methane source successfully reproduces the
ice core methane records. Compared to previous studies, the present LGM model source strength
is weaker which raises the possibility of a less deviated sink than present. This is supported by
some recent studies on the tropospheric oxidative chemistry which found less OH variability
than previously assumed. The important aspect of the present study is that contrary to previous
studies where sinks are often hold responsible to explain atmospheric methane variability, here
the emphasis has been given to the role of changing source based on these recent findings.