Abstract
Abstract. Atmospheric CO2 levels during interglacials prior to the Mid-Brunhes Event (MBE, ∼ 430 ka BP) were around 40 ppm lower than after the MBE. The reasons for this difference remain unclear. A recent hypothesis proposed that changes in oceanic circulation, in response to different external forcings before and after the MBE, might have increased the ocean carbon storage in pre-MBE interglacials, thus lowering atmospheric CO2. Nevertheless, no quantitative estimate of this hypothesis has been produced up to now. Here we use an intermediate complexity model including the carbon cycle to evaluate the response of the carbon reservoirs in the atmosphere, ocean and land in response to the changes of orbital forcings, ice sheet configurations and atmospheric CO2 concentrations over the last nine interglacials. We show that the ocean takes up more carbon during pre-MBE interglacials in agreement with data, but the impact on atmospheric CO2 is limited to a few parts per million. Terrestrial biosphere is simulated to be less developed in pre-MBE interglacials, which reduces the storage of carbon on land and increases atmospheric CO2. Accounting for different simulated ice sheet extents modifies the vegetation cover and temperature, and thus the carbon reservoir distribution. Overall, atmospheric CO2 levels are lower during these pre-MBE simulated interglacials including all these effects, but the magnitude is still far too small. These results suggest a possible misrepresentation of some key processes in the model, such as the magnitude of ocean circulation changes, or the lack of crucial mechanisms or internal feedbacks, such as those related to permafrost, to fully account for the lower atmospheric CO2 concentrations during pre-MBE interglacials.
Highlights
Ice core data have shown that atmospheric CO2 concentration has been different during interglacials of the last 800 000 years (Lüthi et al, 2008; Bereiter et al, 2015)
We use an intermediate complexity model including the carbon cycle to evaluate the response of the carbon reservoirs in the atmosphere, ocean and land in response to the changes of orbital forcings, ice sheet configurations and atmospheric CO2 concentrations over the last nine interglacials
We show that the ocean takes up more carbon during pre-Mid-Brunhes Event (MBE) interglacials in agreement with data, but the impact on atmospheric CO2 is limited to a few parts per million
Summary
Ice core data have shown that atmospheric CO2 concentration has been different during interglacials of the last 800 000 years (Lüthi et al, 2008; Bereiter et al, 2015). Older interglacials before the Mid-Brunhes Event (MBE) around 430 ka BP, i.e. Marine Isotope Stages (MIS) 13, 15, 17 and 19, are characterised by relatively lower atmospheric CO2, around 240 ppm, compared to more recent interglacials, i.e. MIS 1, 5, 7, 9 and 11, which have a higher CO2 level of around 280 ppm (Fig. 1a). Proxy data such as the marine benthic foraminifera δ18O stack record, embedding both deep-sea temperature and ice sheet volume (Lisiecki and Raymo, 2005), indicate that older interglacials (pre-MBE) experienced a colder climate than the more recent ones (post-MBE). Bouttes et al.: Response of the carbon cycle in an intermediate complexity model
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