Abstract
Abstract. Atmospheric methane concentration shows a well-known decrease over the first half of the Holocene following the Northern Hemisphere summer insolation before it started to increase again to preindustrial values. There is a debate about what caused this change in the methane concentration evolution, in particular, whether an early anthropogenic influence or natural emissions led to the reversal of the atmospheric CH4 concentration evolution. Here, we present new methane concentration and stable hydrogen and carbon isotope data measured on ice core samples from both Greenland and Antarctica over the Holocene. With the help of a two-box model and the full suite of CH4 parameters, the new data allow us to quantify the total methane emissions in the Northern Hemisphere and Southern Hemisphere separately as well as their stable isotopic signatures, while interpretation of isotopic records of only one hemisphere may lead to erroneous conclusions. For the first half of the Holocene our results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere CH4 emissions by more than 30 Tg CH4 yr−1 in total, accompanied by a drop in the northern carbon isotopic source signature of about −3 ‰. This cannot be explained by a change in the source mix alone but requires shifts in the isotopic signature of the sources themselves caused by changes in the precursor material for the methane production. In the second half of the Holocene, global CH4 emissions increased by about 30 Tg CH4 yr−1, while preindustrial isotopic emission signatures remained more or less constant. However, our results show that this early increase in methane emissions took place in the Southern Hemisphere, while Northern Hemisphere emissions started to increase only about 2000 years ago. Accordingly, natural emissions in the southern tropics appear to be the main cause of the CH4 increase starting 5000 years before present, not supporting an early anthropogenic influence on the global methane budget by East Asian land use changes.
Highlights
Atmospheric methane (CH4) is a potent greenhouse gas and its concentrations are strongly coupled to the Earth’s climate system
There is a debate about what caused this change in the methane concentration evolution, in particular, whether an early anthropogenic influence or natural emissions led to the reversal of the atmospheric CH4 concentration evolution
For the first half of the Holocene our results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere CH4 emissions by more than 30 Tg CH4 yr−1 in total, accompanied by a drop in the northern carbon isotopic source signature of about −3 ‰
Summary
Atmospheric methane (CH4) is a potent greenhouse gas and its concentrations are strongly coupled to the Earth’s climate system. Due to the human influence on the Earth system, the CH4 concentration ([CH4]) in the atmosphere has increased by a factor of 2.5 (relative to the preindustrial level) over the last centuries and contributes significantly to the human-induced radiative forcing (Etheridge et al, 1998; Dlugokencky et al, 2005). Today both anthropogenic CH4 sources (rice agriculture, livestock, fossil fuel production, anthropogenic biomass burning, and landfills) and natural CH4 sources (natural wetlands, wildfires, geologic emissions, wild animals (including termites), and marine CH4 hydrates) contribute to the global CH4 emissions (Kirschke et al, 2013). Together with the other sink processes (such as stratospheric loss, reaction with Cl radicals in the marine boundary layer, and soil uptake) the OH sink determines the recent atmospheric lifetime of CH4, which is 9.1 ± 0.9 years (Prather et al, 2012).
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