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Coupled climate-carbon cycle simulation of the Last Glacial Maximum atmospheric CO2 decrease using a large ensemble of modern plausible parameter sets

Research output: Contribution to journalArticle

  • Krista M.S. Kemppinen
  • Philip B. Holden
  • Neil R. Edwards
  • Andy Ridgwell
  • Andrew D. Friend
Original languageEnglish
Pages (from-to)1039-1062
Number of pages24
JournalClimate of the Past
Issue number3
DateAccepted/In press - 10 May 2019
DatePublished (current) - 18 Jun 2019


During the Last Glacial Maximum (LGM), atmospheric <span classCombining double low line"inline-formula">CO2</span> was around 90&thinsp;ppmv lower than during the pre-industrial period. The reasons for this decrease are most often elucidated through factorial experiments testing the impact of individual mechanisms. Due to uncertainty in our understanding of the real system, however, the different models used to conduct the experiments inevitably take on different parameter values and different structures. In this paper, the objective is therefore to take an uncertainty-based approach to investigating the LGM <span classCombining double low line"inline-formula">CO2</span> drop by simulating it with a large ensemble of parameter sets, designed to allow for a wide range of large-scale feedback response strengths. Our aim is not to definitely explain the causes of the <span classCombining double low line"inline-formula">CO2</span> drop but rather explore the range of possible responses. We find that the LGM <span classCombining double low line"inline-formula">CO2</span> decrease tends to predominantly be associated with decreasing sea surface temperatures (SSTs), increasing sea ice area, a weakening of the Atlantic Meridional Overturning Circulation (AMOC), a strengthening of the Antarctic Bottom Water (AABW) cell in the Atlantic Ocean, a decreasing ocean biological productivity, an increasing <span classCombining double low line"inline-formula">CaCO3</span> weathering flux and an increasing deep-sea <span classCombining double low line"inline-formula">CaCO3</span> burial flux. The majority of our simulations also predict an increase in terrestrial carbon, coupled with a decrease in ocean and increase in lithospheric carbon. We attribute the increase in terrestrial carbon to a slower soil respiration rate, as well as the preservation rather than destruction of carbon by the LGM ice sheets. An initial comparison of these dominant changes with observations and paleoproxies other than carbon isotope and oxygen data (not evaluated directly in this study) suggests broad agreement. However, we advise more detailed comparisons in the future, and also note that, conceptually at least, our results can only be reconciled with carbon isotope and oxygen data if additional processes not included in our model are brought into play.

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