The chemical composition of ancient atmospheres: A model study constrained by ice core data

Abstract

A coupled chemistry radiation transport two‐dimensional model of the lower and middle atmosphere was adapted to study the chemical composition of the atmosphere at preindustrial time and last glacial maximum (LGM). The model was constrained by trace gas concentrations (CO2, CH4, and N2O) inferred from polar ice core records. The formulation of tropospheric dynamics and chemistry was improved in order to more accurately simulate the transport and the oxidation processes below the tropopause. Our objectives are to infer the changes in middle‐atmosphere temperature, ozone layer, and oxidation capacity of the atmosphere (e.g., methane lifetime) over the last 18,000 years. A middle‐atmosphere cooling was obtained between LGM and preindustrial Holocene (PIH) as well as between PIH and present time. This is mainly due to changes in the CO2 and chlorofluorocarbon (CFC) concentrations, respectively. CFCs are also the main contributors to the middle‐atmosphere ozone decrease since PIH. Between LGM and PIH the compensating effects of CO2 and N2O lead to little variation in stratospheric ozone. A 17% decrease in tropospheric OH was obtained between LGM and PIH, whereas the model provides a 6% OH increase since PIH. The corresponding changes in the methane sink are too small to have played a dominant role in the past methane concentration changes. Our model derived methane emissions for LGM, PIH, and present time are in good agreement with methane sources evaluated during these three periods.

Response of methane emissions from wetlands to the Last Glacial Maximum and an idealized Dansgaard–Oeschger climate event: insights from two models of different complexity

“…The LGM reduction found by Weber et al (2010) is closer to the range of reduction found in studies based on top-down modelling (e.g. Crutzen and Brühl, 1993;Martinerie et al, 1995;Chappellaz et al, 1997) or as suggested based on atmospheric chemistry simulations (Levine et al, 2011). These studies constrained multi-dimensional chemical transport models with ice core observations and inferred the source terms, finding a reduction in the LGM wetland CH 4 emissions by 40-60 %.…”

Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH 4

“…At the end of the Younger Dryas (Termination 1B), AMC abruptly increased once again, this time to over 700 ppb. Changes in the strength of CH 4 sources are usually advanced as the main drivers of deglacial and early Holocene AMC fluctuations (30), though some modeling efforts suggest that variations in CH 4 sinks may have been important as well (e.g., 31). Various sources have been proposed for the abrupt high-magnitude increases in AMC during Terminations 1A and 1B.…”

Terrestrial biogeochemical feedbacks in the climate system

“…Analyses of interhemispheric gradients of ice-core CH 4 concentrations and carbon isotope composition have been interpreted as indicating that changes in wetland emissions drove glacial-interglacial CH 4 changes 75,76 . However, simulations using simple formulations of wetland extent and emissions have been unable to reduce wetland sources sufficiently to produce the low levels of glacial atmospheric CH 4 concentrations 77,78 .…”