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Geological records reveal a number of ancient, large and rapid negative\udexcursions of the carbon-13 isotope. Such excursions can only be explained by\udmassive injections of depleted carbon to the Earth system over a short\udduration. These injections may have forced strong global warming events,\udsometimes accompanied by mass extinctions such as the Triassic-Jurassic\udand end-Permian extinctions 201 and 252 million years ago, respectively. In many\udcases, evidence points to methane as the dominant form of injected carbon, whether\udas thermogenic methane formed by magma intrusions through overlying\udcarbon-rich sediment or from warming-induced dissociation of methane\udhydrate, a solid compound of methane and water found in ocean sediments. As\uda consequence of the ubiquity and importance of methane in major Earth\udevents, Earth system models for addressing such events should include\uda comprehensive treatment of methane cycling but such a treatment has often\udbeen lacking. Here we implement methane cycling in the Danish Center for\udEarth System Science (DCESS) model, a simplified but well-tested Earth system\udmodel of intermediate complexity. We use a generic methane input function\udthat allows variation in input type, size, timescale and ocean–atmosphere\udpartition. To be able to treat such massive inputs more correctly, we extend\udthe model to deal with ocean suboxic/anoxic conditions and with radiative\udforcing and methane lifetimes appropriate for high atmospheric methane\udconcentrations. With this new model version, we carried out an extensive set\udof simulations for methane inputs of various sizes, timescales and\udocean–atmosphere partitions to probe model behavior. We find that larger\udmethane inputs over shorter timescales with more methane dissolving in the\udocean lead to ever-increasing ocean anoxia with consequences for ocean life\udand global carbon cycling. Greater methane input directly to the atmosphere\udleads to more warming and, for example, greater carbon dioxide release from\udland soils. Analysis of synthetic sediment cores from the simulations\udprovides guidelines for the interpretation of real sediment cores spanning\udthe warming events. With this improved DCESS model version and\udpaleo-reconstructions, we are now better armed to gauge the amounts, types,\udtimescales and locations of methane injections driving specific, observed\uddeep-time, global warming events

Implementation of methane cycling for deep-time global warming simulations with the DCESS Earth system model (version 1.2)