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

Abstract

Abstract. Ice-core records show that abrupt DansgaardOeschger (D-O) climatic warming events of the last glacial period were accompanied by large increases in the atmospheric CH 4 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 CH 4 sources, in particular wetlands. We use two different ecosystem models of wetland CH 4 emissions to simulate northern CH 4 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 CH 4 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 CH 4 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 CH 4 increases during abrupt events. The inclusion of permafrost physics and peatland carbon cycling in one model (LPJ-WHyMe) increases the climatic sensitivity of CH 4 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 CH 4 variations, perhaps with a more dominant role for tropical wetland CH 4 sources.

Bayesian Analysis of the Glacial‐Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling

“…The inferred large reduction in wetland emissions is not simulated by H17 and is at the extreme limit of the uncertainty range of recent model-based estimates of the LGM-PI wetland change, (29% to 67%: Hopcroft et al, 2014;Ringeval et al, 2013;Weber et al, 2010). However, several processes (e.g., wetland carbon cycle processes, nutrient status, and tree-mediated transport) that are not properly represented Pangala et al, 2017;Wania et al, 2013;H17) need to be evaluated.…”

NGRIP CH<sub>4</sub> concentration from 120 to 10 kyr before present and its relation to a δ<sup>15</sup>N temperature reconstruction from the same ice core

“…The deviation from our µ values for the specified events is +29, −10, +57, and −41 %, respectively. Hopcroft et al (2011Hopcroft et al ( , 2014) notice that their model generally underestimates the total CH 4 amplitudes; however, it also underestimates the Greenland temperature variations calculated from a freshwater hosing experiment. The largest deviations are found for DO event 11 and p17, which occur around extreme northern summer insolation states (Fig.…”

“…The range of µ (5.0-25.5 ppb • C −1 ) obtained from our data compares well to an experiment from a wetland CH 4 emission model simulation (6.3-24.1 ppb • C −1 ) by Hopcroft et al (2011), who applied different idealized climate states. Hopcroft et al (2014) performed a similar experiment for the BA, p17, and DO events 8 and 11, resulting in sensitivities of 12.2, 11.5, 9.6, and 8.5 ppb • C −1 , respectively. The deviation from our µ values for the specified events is +29, −10, +57, and −41 %, respectively.…”

“…In comparison, the deviations during DO event 8 and the BA, which occur during moderate northern summer insolation states, are smaller. Although the different insolation states are taken into account by Hopcroft et al (2014), it is possible that the influence of northern summer insolation has to be refined in the model. Figure 7e shows the timing of the fast NGRIP temperature and CH 4 increases for all considered DO events, with the events showing a significant lag displayed in red.…”

The BRIDGE HadCM3 family of climate models: HadCM3@Bristol v1.0

“…Hewitt et al, 2001;Singarayer and Valdes, 2010;Lunt et al, 2016;Marzocchi et al, 2015b) or by performing fully transient simulations for multi-centennial or -millennial timescales (e.g. Tett et al, 2006;Hopcroft et al, 2014).…”