Glacial-interglacial methane cycle, what we know and don’t know

Abstract:

Since the preindustrial era, atmospheric methane (CH₄) concentration has increased by ~2.5 fold and accounted for ~17% of the total change in Earth's radiative forcing. Measurements of paleoatmospheric CH₄ from ice cores can provide historical perspectives on the natural component of the CH₄ budget (which currently accounts for ~40% of total CH₄ emissions). High-resolution CH₄ concentration measurements from ice cores show large millennial- to centennial-scale abrupt CH₄­ variability concurrent with rapid temperature changes in Greenland. The rate of CH₄­ increase over the last decade is comparable to some of the fastest rate of CH₄­ increase observed in the ice core record. Low to mid-latitude wetlands are thought to be the dominant drivers of CH₄ variability during these rapid climate events. Recent analyses of radiomethane (Δ¹⁴CH₄) over Termination 1 strengthen this hypothesis by showing that contributions from old carbon reservoirs such as permafrost and marine hydrates were small. Stable isotopes of CH₄ (δ¹³CH₄ and δD-CH­₄) can be used to further identify the sources of the abrupt CH₄ transitions. δ¹³CH₄ can be used to distinguish between microbial (wetlands, ruminants, etc.) and biomass burning sources. δD-CH­₄ can be used to fingerprint the latitude in which the CH₄ is produced, thus distinguishing emissions from low-latitude tropical wetlands vs. boreal wetlands. However, Antarctic CH₄ stable isotopes were decoupled from CH₄ concentration variability. To the first order, this suggests that during the abrupt CH₄ rise there was no significant change in the relative fraction of one CH₄ source vs. the other (i.e., the same relative mixture of sources increasing equally). The CH₄ stable isotopes instead correlate better with the shape of other paleo records such as atmospheric CO₂. Unfortunately, we still do not have a good understanding of the causal relationship behind the correlation. Recent studies showed that extraneous CH₄ is produced during the melting of dust-rich Greenland ice associated with cold climatic periods. It is challenging to interpret the global CH₄ cycle with measurements from Antarctic ice cores only. Reliable, artifact-free measurements of CH₄ isotopes from cold-period Greenland ice are needed to complement the existing Antarctic record and provide further insights.