Limitations of the 1 % experiment as the benchmark idealized experiment for carbon cycle intercomparison in C<sup>4</sup>MIP
<p>Idealized climate change simulations are used as benchmark experiments to facilitate the comparison of ensembles of climate models. In the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the 1 % per yearly compounded change in atmospheric <span class="in...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2019-02-01
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| Series: | Geoscientific Model Development |
| Online Access: | https://www.geosci-model-dev.net/12/597/2019/gmd-12-597-2019.pdf |
| Summary: | <p>Idealized climate change simulations are used as benchmark experiments to facilitate the
comparison of ensembles of climate models. In the fifth phase of the Coupled Model
Intercomparison Project (CMIP5), the 1 % per yearly compounded change in atmospheric
<span class="inline-formula">CO<sub>2</sub></span> concentration experiment was used to compare Earth system models with full
representations of the global carbon cycle in the Coupled Climate–Carbon Cycle Model
Intercomparison Project (C<span class="inline-formula"><sup>4</sup></span>MIP). However, this “1 % experiment” was never
intended for such a purpose and implies a rise in atmospheric <span class="inline-formula">CO<sub>2</sub></span> 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
<span class="inline-formula">CO<sub>2</sub></span> pathway derived from a logistic function. The comparison shows three key
differences in model output when forcing the model with the logistic experiment. (1) The
model forced with the logistic experiment exhibits a transition of the land biosphere
from a carbon sink to a carbon source, a feature absent when forcing the model with the
1 % experiment. (2) The ocean uptake of carbon comes to dominate the carbon cycle as
emissions decline, a feature that cannot be captured when forcing a model with the
1 % experiment, as emissions always increase in that experiment. (3) The permafrost
carbon feedback to climate change under the 1 % experiment forcing is less than half
the strength of the feedback seen under logistic experiment forcing. Using the logistic
experiment also allows smooth transition to zero or negative emissions states, allowing
these states to be examined without sharp discontinuities in <span class="inline-formula">CO<sub>2</sub></span> emissions. The
protocol for the CMIP6 iteration of C<span class="inline-formula"><sup>4</sup></span>MIP 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 C<span class="inline-formula"><sup>4</sup></span>MIP is recommended.</p> |
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| ISSN: | 1991-959X 1991-9603 |