Late Holocene methane rise caused by orbitally controlled increase in tropical sources

Abstract

Considerable debate surrounds the source of the apparently 'anomalous' increase of atmospheric methane concentrations since the mid-Holocene (5,000 years ago) compared to previous interglacial periods as recorded in polar ice core records. Proposed mechanisms for the rise in methane concentrations relate either to methane emissions from anthropogenic early rice cultivation or an increase in natural wetland emissions from tropical or boreal sources. Here we show that our climate and wetland simulations of the global methane cycle over the last glacial cycle (the past 130,000 years) recreate the ice core record and capture the late Holocene increase in methane concentrations. Our analyses indicate that the late Holocene increase results from natural changes in the Earth's orbital configuration, with enhanced emissions in the Southern Hemisphere tropics linked to precession-induced modification of seasonal precipitation. Critically, our simulations capture the declining trend in methane concentrations at the end of the last interglacial period (115,000-130,000 years ago) that was used to diagnose the Holocene methane rise as unique. The difference between the two time periods results from differences in the size and rate of regional insolation changes and the lack of glacial inception in the Holocene. Our findings also suggest that no early agricultural sources are required to account for the increase in methane concentrations in the 5,000 years before the industrial era.

Macroevolutionary consequences of profound climate change on niche evolution in marine molluscs over the past three million years

“…This GCM has been successfully used in a variety of Quaternary and pre-Quaternary modelling studies [46][47][48]. Boundary conditions for the LIG were from [47,49]. Here, atmospheric gas concentrations were derived from ice core records [50][51][52], and orbital parameters were from Berger & Loutre [53].…”

Limited response of peatland CH<sub>4</sub> emissions to abrupt Atlantic Ocean circulation changes in glacial climates

“…Globally the simulated CH 4 changes translated into atmospheric increases ranging from 50 to 110 ppbv, and were considered too small to be reconciled with ice-core observations, especially the changes in emissions from the Northern Hemisphere extratropics. By contrast the model has been used to predict the longer orbital-scale changes in atmospheric CH 4 of the last 120 kyr successfully (Singarayer et al, 2011). The weak response to abrupt changes was thought to result either from deficiencies in the climate scenario or the sensitivity of the CH 4 emission model employed within SDGVM (modified from Cao et al, 1996).…”

“…We used the FAMOUS (Smith et al, 2008) coupled atmosphere-ocean general circulation model (GCM) to simulate the global climate of five time periods during the last glacial period driven by estimates of the major climatic forcings: orography, land ice and sea level, trace gases, insolation and freshwater input, the latter leading to strong AMOC (Atlantic meridional overturning circulation) changes. The simulated climates are then used to drive the dynamic vegetation model LPJ-WHyMe (Wania et al, 2009a(Wania et al, , b, 2010 and for comparison SDGVM to simulate the response of the northern peatlands and permafrost to abrupt climate changes in the North Atlantic region.…”

Evaluation of wetland methane emissions across North America using atmospheric data and inverse modeling

“…2.2). (Singarayer et al, 2011). All flux model outputs used from the WETCHIMP study have a temporal resolution of 1 month, and we regrid all outputs to a spatial resolution of 1 • lat.…”