Datensatz

Zusammenfassung

Methane is an important greenhouse gas contributing about 22Vo to the greenhouse effect at
present. Methane emissions from natural wetlands, which constitute the biggest methane
source at present and the major non-anthropogenic one, depend highly on the climate. In order
to investigate the response of methane emissions from natural wetlands to climate variations, a
1-dimensional process-based climate-sensitive model to derive methane emissions from
natural wetlands is developed. In the model the processes leading to methane emission are
simulated within a 1-dimensional soil column and the three different transport mechanisms
diffusion, plant-mediated transport and ebullition are modeled explicitly. The model forcing
consists of daily values of the soil temperature, the water table and the Net Primary
Productivity. Tests of the methane model using observed data obtained at 15 field stations
within 6 wetlands, representing a large variety of environmental conditions, show good
agreement.

For the use of the 1-dimensional methane model on the global scale, global data sets for all
model parameters are generated, derived from the output of a biosphere model and existing
data sets of vegetation and soil properties. Furthermore, a simple hydrological model is
developed in order to simulate the position of the water table in wetlands. Tests of the 1-
dimensional hydrological model, forced by ECMV/F re-analyses with observed water table
levels from different wetlands, show a good agreement between model results and observations
in general.

A global run of the methane model is performed using high-frequency atmospheric forcing
fields from the ECMWF re-analyses of the period from 1988 to 1993. The comparison of the
simulated global methane emissions from natural wetlands agree su{prisingly well with results
obtained by an inverse modeling approach, considering the uncertainties of both methods. The
modeled methane emissions show high regional, seasonal and interannual variability. The
seasonal cycle of simulated global methane emissions is dominated by the seasonal cycle of
methane emissions from the higher northern latitudes. The anomalies of the simulated methane
emissions are compared to the observed anomalous methane growth rate. This comparison is
limited, because we do not take atmospheric transport and the other methane sources and sinks
into account and because the observational data do not reflect the true zonal average. The
agreement is particulary good in 1992 and 1993 and the results suggest that reduced methane
emissions from northern wetlands in 1992, caused by decreased temperatures due to the
eruption of Mt. Pinatubo in June 1991, contribute considerably to the observed growth rate
minimum ]n 1992.