22 Apr 2021

22 Apr 2021

Review status: a revised version of this preprint is currently under review for the journal ESD.

23rd Century surprises: Long-term dynamics of the climate and carbon cycle under both high and net negative emissions scenarios

Charles Koven1, Vivek K. Arora2, Patricia Cadule3, Rosie A. Fisher4,5,6, Chris D. Jones7, David M. Lawrence4, Jared Lewis8, Keith Lindsey4, Sabine Mathesius9, Malte Meinshausen8,10, Michael Mills4, Zebedee Nicholls8,10, Benjamin M. Sanderson4,5, Neil C. Swart2, William R. Wieder4,11, and Kirsten Zickfeld9 Charles Koven et al.
  • 1Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
  • 2Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, University of Victoria, Victoria, British Columbia, Canada
  • 3IPSL, CNRS, Sorbonne Université, Paris, France
  • 4Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, Colorado, USA
  • 5Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, (CERFACS), Toulouse, France
  • 6Évolution & Diversité Biologique, University of Toulouse Paul Sabatier III, Toulouse, France
  • 7Met Office Hadley Centre, Exeter, UK
  • 8Climate and Energy College, School of Geography, Earth and Atmospheric Sciences, The University of Melbourne, Parkville, Victoria, Australia
  • 9Department of Geography, Simon Fraser University, Burnaby, British Columbia, Canada
  • 10Climate Resource, Victoria, Australia
  • 11Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado, USA

Abstract. Future climate projections from Earth system models (ESMs) typically focus on the timescale of this century. We use a set of four ESMs and one Earth system model of intermediate complexity (EMIC) to explore the dynamics of the Earth’s climate and carbon cycles under contrasting emissions trajectories beyond this century, to the year 2300. The trajectories include a very high emissions, unmitigated fossil-fuel driven scenario, as well as a second mitigation scenario that diverges from the first scenario after 2040 and features an “overshoot”, followed by stabilization of atmospheric CO2 concentrations by means of large net-negative CO2 emissions. In both scenarios, and for all models considered here, the terrestrial system switches from being a net sink to either a neutral state or a net source of carbon, though for different reasons and centered in different geographic regions, depending on both the model and the scenario. The ocean carbon system remains a sink, albeit weakened by climate-carbon feedbacks, in all models under the high emissions scenario, and switches from sink to source in the overshoot scenario. The global mean temperature anomaly generally follows the trajectories of cumulative carbon emissions, except that 23rd-century warming continues after the cessation of carbon emissions in several models, both in the high emissions scenario and in one model in the overshoot scenario. While ocean carbon cycle responses qualitatively agree both in globally integrated and zonal-mean dynamics in both scenarios, the land models qualitatively disagree in zonal-mean dynamics, in the relative roles of vegetation and soil in driving C fluxes, in the response of the sink to CO2, and in the timing of the sink-source transition, particularly in the high emissions scenario. The lack of agreement among land models on the mechanisms and geographic patterns of carbon cycle feedbacks, alongside the potential for lagged physical climate dynamics to cause warming long after CO2 concentrations have stabilized, point to the possibility of surprises in the climate system beyond the 21st century time horizon, even under relatively mitigated global warming scenarios, which should be taken into consideration when setting global climate policy.

Charles Koven et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Review of "23rd Century surprises: Long-term dynamics of the climate and carbon cycle under both high and net negative emissions scenarios" by Charles D. Koven et al.', Jörg Schwinger, 20 May 2021
    • AC1: 'Reply on RC1', Charles Koven, 25 Jun 2021
  • RC2: 'Comment on esd-2021-23', Victor Brovkin, 26 May 2021
    • AC2: 'Reply on RC2', Charles Koven, 25 Jun 2021

Charles Koven et al.

Charles Koven et al.


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Short summary
We explore the long-term dynamics of Earth's climate and carbon cycles under a pair of contrasting scenarios to the year 2300, using five models that include both climate and carbon cycle dynamics. One scenario assumes very high emissions, while the second assumes a peak in emissions, followed by rapid declines to net negative emissions. We show that the models generally agree that warming is roughly proportional to carbon emissions, but that many other aspects of the model projections differ.