Limited response of peatland CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; emissions to abrupt Atlantic Ocean circulation changes in glacial climates

P O Hopcroft,R Wania,P J Valdes,D J Beerling

doi:10.5194/cp-10-137-2014

Abstract

Abstract. Ice-core records show that abrupt Dansgaard–Oeschger (D–O) climatic warming events of the last glacial period were accompanied by large increases in the atmospheric CH4 concentration (up to 200 ppbv). These abrupt changes are generally regarded as arising from the effects of changes in the Atlantic Ocean meridional overturning circulation and the resultant climatic impact on natural CH4 sources, in particular wetlands. We use two different ecosystem models of wetland CH4 emissions to simulate northern CH4 sources forced with coupled general circulation model simulations of five different time periods during the last glacial to investigate the potential influence of abrupt ocean circulation changes on atmospheric CH4 levels during D–O events. The simulated warming over Greenland of 7–9 °C in the different time periods is at the lower end of the range of 11–15 °C derived from ice cores, but is associated with strong impacts on the hydrological cycle, especially over the North Atlantic and Europe during winter. We find that although the sensitivity of CH4 emissions to the imposed climate varies significantly between the two ecosystem emissions models, the model simulations do not reproduce sufficient emission changes to satisfy ice-core observations of CH4 increases during abrupt events. The inclusion of permafrost physics and peatland carbon cycling in one model (LPJ-WHyMe) increases the climatic sensitivity of CH44 emissions relative to the Sheffield Dynamic Global Vegetation Model (SDGVM) model, which does not incorporate these processes. For equilibrium conditions this additional sensitivity is mostly due to differences in carbon cycle processes, whilst the increased sensitivity to the imposed abrupt warmings is also partly due to the effects of freezing on soil thermodynamics. These results suggest that alternative scenarios of climatic change could be required to explain the abrupt glacial CH4 variations, perhaps with a more dominant role for tropical wetland CH4 sources.

Highlights

Dansgaard–Oeschger (D–O) cycles are characterised by a series of 25 incredibly abrupt warming episodes which occurred during the last glacial period
The model EQ Atlantic meridional overturning circulation (AMOC) values are relatively stable across the different time periods at around 20 Sv (1 Sv = 106 m3 s−1), which is consistent with the preindustrial value of 18 Sv, and this is close to the observational estimates of modern overturning strength in the Atlantic of 18 Sv ± 3–5 (Talley et al, 2003)
Using net primary productivities (NPPs) to predict CH4 emissions in the different time periods as a linear function of the ratio of NPP (Whiting and Chanton, 1993) in that time period relative to the pre-industrial, we find that this overpredicts emissions in LPJ-WHyMe by up to 89 %, but the maximum error is only ±15 % for Sheffield Dynamic Global Vegetation Model (SDGVM)

Summary

Introduction

Dansgaard–Oeschger (D–O) cycles are characterised by a series of 25 incredibly abrupt warming episodes which occurred during the last glacial period. These events have been reconstructed from Greenland ice-core data (e.g. NGRIP Project Members, 2004; Wolff et al, 2010) and from an increasing number of palaeoclimate proxies from across the globe Peterson et al, 2000; Hendy and Kennett, 2000; Wang et al, 2001; Kanner et al, 2012). Clement and Peterson, 2008; Liu et al, 2009; Li et al, 2010; Petersen et al, 2013).

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