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Discussion papers | Copyright
https://doi.org/10.5194/gmd-2018-150
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Model experiment description paper 16 Jul 2018

Model experiment description paper | 16 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Geoscientific Model Development (GMD).

Limitations of the 1 % experiment as the benchmark idealized experiment for carbon cycle intercomparison in C4MIP

Andrew H. MacDougall Andrew H. MacDougall
  • Climate & Environment, St. Francis Xavier University, Antigonish, Nova Scotia, Canada

Abstract. Idealized climate change simulations are used as benchmark experiments to facilitate the comparison of ensembles of climate models. In the Fifth Assessment Report of the IPCC the 1% per yearly compounded change in atmospheric CO2 concentration experiment was used to compare Earth System Models with full representations of the global carbon cycle (C4MIP). However this ``1% experiment'' was never intended for such a purpose and implies a rise in atmospheric CO2 concentration at double the rate of the instrumental record. Here we examine this choice by using an intermediate complexity climate model to compare the 1% experiment to an idealized CO2 pathway derived from a logistic function. The comparison shows that the logistic experiment has three key differences from the 1% experiment. (1) The Logistic experiment exhibits a transition of the land biosphere from a carbon sink to a carbon source, a feature absent from the 1% experiment. (2) The ocean uptake of carbon comes to dominate the carbon cycle as emissions decelerate, a feature that cannot be captured by the 1% experiment as emissions always accelerate in that experiment. (3) The permafrost carbon feedback to climate change in the 1% experiment is less than half the strength of the feedback seen in the logistic experiment. The logistic experiment also allows smooth transition to zero or negative emission states, allowing these states to be examined without sharp discontinuities in CO2 emissions. The protocol for the CMIP6 iteration of C4MIP again sets the 1% experiment as the benchmark experiment for model intercomparison, however clever use of the Tier 2 experiments may alleviate some of the limitations outlined here. Given the limitations of the 1% experiment as the benchmark experiment for carbon cycle intercomparisons, adding a logistic or similar idealized experiment to the protocol of the CMIP7 iteration of C4MIP is recommended.

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Andrew H. MacDougall
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Andrew H. MacDougall
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Latest update: 18 Aug 2018
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Short summary
The 1% per yearly exponential change in CO2 concentration experiment is an idealized climate change scenario that has traditionally been used to facilitate comparison of different climate models and to create benchmark statistics. Here we examine the limitations of this experiment for assessing the global carbon cycle and propose an alternative idealized experiment.
The 1% per yearly exponential change in CO2 concentration experiment is an idealized climate...
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