Abstract
Abstract. Methane (CH4) variations on orbital timescales are often associated with variations in wetland coverage, most notably in the summer monsoon areas of the Northern Hemisphere. Here we test this assumption by simulating orbitally forced variations in global wetland emissions, using a simple wetland distribution and CH4 emissions model that has been run on the output of a climate model (CLIMBER-2) containing atmosphere, ocean and vegetation components. The transient climate modeling simulation extends over the last 650 000 yr and includes variations in land-ice distribution and greenhouse gases. Tropical temperature and global vegetation are found to be the dominant controls for global CH4 emissions and therefore atmospheric concentrations. The relative importance of wetland coverage, vegetation coverage, and emission temperatures depends on the specific climatic zone (boreal, tropics and Indian/Asian monsoon area) and timescale (precession, obliquity and glacial-interglacial timescales). Despite the low spatial resolution of the climate model and crude parameterizations for methane production and release, simulated variations in CH4 emissions agree well with those in measured concentrations, both in their time series and spectra. The simulated lags between emissions and orbital forcing also show close agreement with those found in measured data, both on the precession and obliquity timescale. We find causal links between atmospheric CH4 concentrations and tropical temperatures and global vegetation, but only covariance between monsoon precipitation and CH4 concentrations. The primary importance of the first two factors explains the lags found in the CH4 record from ice cores. Simulation of the dynamical vegetation response to climate variation on orbital timescales would be needed to reduce the uncertainty in these preliminary attributions.
Highlights
Methane (CH4) is a forcing factor for climate as a greenhouse gas (Wang et al, 1996)
Global wetland area during the Last Glacial Maximum (LGM) is at its maximum in June with 5.3 106 km2 and at its minimum in December with 3.3 106 km2, a decrease of 17 % in the annual-mean compared to the Pre-Industrial Holocene (PIH)
In this study we find strong indications that global emissions are dominated by emissions from the tropics
Summary
Methane (CH4) is a forcing factor for climate as a greenhouse gas (Wang et al, 1996). Whether the oscillations in CH4 observed in ice core data originate from the tropics, the boreal zone and/or from the Indian/Asian monsoon area; second, which climatic parameters play an important role in each of these regions; and third, how we can interpret the lags that have been found in the measured data with respect to the orbital forcing. The climate model has coarse spatial resolution It adequately simulates large-scale climatic features like the seasonal migration of the Inter Tropical Convergence Zone (ITCZ), monsoon systems and the freezing and thawing of the northern land masses, which are of importance for wetland formation and global CH4 emissions.
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