Permafrost carbon - Catalyst for deglaciation

Australia: The Land Where Time Began

Permafrost carbon - Catalyst for deglaciation

It is not clear what the sources were that contributed to the rise of CO₂ during deglaciation. It is suggested by climate model simulations that the thawing of permafrost soils were the initial source, which highlights the vulnerability of modern permafrost soils.

A period that has sometimes been referred to as the Mystery Interval (Walter et al., 2016), between 17,500 BP and 14,500 BP, was aa time when the atmospheric concentrations of CO₂ began to rise following the end of glaciation from about 190 ppm during the glaciation to about 270 ppm by the start of the Holocene. During the Mystery Interval the rise in CO₂ is associated with large negative anomalies in the composition of carbon isotopes (Ehhalt, 1974; Kirschke et al., 2013). It is suggested by these anomalies that a carbon pool that had long been isolated was released to the atmosphere. A large pool of old carbon in the Southern Ocean that was ¹³carbon depleted has been suggested as the source, though there are questions of the timing and magnitude of this release that have remained. Crichton et al. (Crichton et al., 2016) report in Nature Geoscience evidence from numerical simulations suggesting the primary source of deglacial carbon during the Mystery Interval was instead a pool of permafrost carbon. Permafrost is soil and bedrock that have temperatures below 0^oC for more than 2 years (Schuur et al., 2015). An immense quantity of carbon in the form of organic matter that is partly decayed is held by permafrost soils: carbon that has been held in permafrost-affected soils is estimated to comprise about 35% of the total terrestrial carbon pool at the present (Hugelius et al., 2013). Much of this carbon is protected from microbial decay by being held in permafrost soil horizons (Hugelius et al., 2013). As with all organic matter, permafrost carbon has low ¹³C concentration and, as it has been locked in frozen soil for thousands of years, permafrost carbon has very little radiocarbon remaining (Schuur et al., 2015). It is believed there were extensive regions of permafrost during the Last Glacial Maximum (LGM). Though terrestrial productivity under cold climatic conditions was half that of the per-industrial period, the carbon pool contained in soils and vegetation was about 10% lower than in the Late Holocene (Ciais et al., 2012). The inactive portion of the terrestrial carbon pool was about 45% larger than that of the present (Ciais et al., 2012). To estimate directly the size of the permafrost carbon pool during the Last Glacial Maximum there is no palaeoclimate proxy. A large glacial permafrost pool, nevertheless, fits the criteria of a large inert carbon pool in a world of low productivity.

An earth system model of intermediate complexity was used by Crichton et al. to simulate the evolution of the atmospheric concentration of CO₂ from the Last Glacial Maximum until the year 1850. The dissipation of the enhanced carbon pool in the Southern Ocean enlarges the atmospheric carbon pool by more than 100 ppm of CO₂. However, the CO₂ concentration rise occurs about 3,000 years after the rise that is observed in the ice-core record. When a permafrost model is added to the Earth system it was found to narrow the differences between the simulation by the model and the palaeoclimate CO₂ record, with simulated CO₂ and the ¹³C concentration closely matching the data until the beginning of the Holocene.

A simplified storyline for the deglacial rise in atmospheric CO₂ is suggested by simulations. Changes in the orbit of the Earth at the end of the Last Glacial Maximum caused a rise in summertime insolation in the Northern Hemisphere. Permafrost soils were induced to thaw by these warmer conditions in summer which began the release of carbon, that had been sequestered for a long time, as CO₂. The climate was warmed further by the carbon that had been released from the permafrost, which induced deglaciation and the further release of carbon from permafrost soils. Changes in the formation of brine and sinking in the Southern Ocean was triggered by sea level rise and a warming climate, a result of which was the dissipation of the glacial carbon pool in the Southern Ocean. Regrowth of the terrestrial biosphere, however, sequestered more carbon than was released in the terrestrial realm. Therefore in net terms, though the release of permafrost carbon to the atmosphere promoted deglaciation, the ocean carbon pool was the dominant source of the glacial-interglacial rise in atmospheric concentration of CO_2.

The development of models to the stage where they can simulate a full deglaciation by using forcing only from the orbital parameters of the Earth is one of the key goals of intermediate complexity Earth system modelling. MacDougall suggests that as the model of Crichton et al. is forced with various parameters that have been derived from palaeoclimate data their work falls short of this goal. In spite of this MacDougall says the work represents a milestone in the quest for understanding deglacial carbon cycle feedbacks.

An immense pool of inert carbon remains held in soils that are permafrost affected which is vulnerable to microbial decay as permafrost thaw is induced by climate change.

The model of Crichton et al. has also been used by them to model future projections of the release from permafrost Soils of carbon. As their results are well within the range of current simulations they help to validate the growing consensus that only a relatively small fraction of the carbon they hold will be released throughout the remainder of the 21^st century (Schuur et al., 2015). There are nevertheless large uncertainties about the system that persist. There is a need, in particular, to gain a better understanding of the mechanism by which permafrost carbon is stabilised, and to understand how these ecosystems at high-latitudes will be radically changed.

It has been shown by Crichton et al. (Crichton et al., 2016) that substantial amounts of carbon have probably been contributed to the atmosphere from permafrost soils during the last deglaciation, which highlights the importance of understanding the feedbacks from permafrost carbon in a warming climate.

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