the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Mar 2025
ESD Ideas: Extended net zero simulations are critical for informed decision making
Abstract. Climate changes under net zero emissions will take many centuries to play out, particularly in the Southern Hemisphere and in the ocean and cryosphere. New millennial-length Earth System Model simulations are required to better understand these committed changes and their dependence on delays in reaching net zero emissions.
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Status: open (until 04 May 2025)
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RC1: 'Comment on egusphere-2025-903', Andrew MacDougall, 09 Apr 2025
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Review of: "ESD Ideas: Extended net zero simulations are critical for informed decision making"
Overall evaluation:
The paper makes an argument for extending zero emissions simulations to millennial timescales. I strongly agree with the authors that having such simulations would be very useful for our science. However, millennial length simulations of zero emissions do exist for intermediate complexity Earth system models (EMICs), and were in fact part of the original ZECMIP (MacDougall et al, 2020). The authors need to better articulate why simulations with full ESMs are needed at millennial timescales to supplement the results from the EMICs.General Comments:
(1) The reason for using ESMs is given at line 70 of the paper "This is particularly problematic for understanding changes in interannual-to- multidecadal climate variability for which changes are only likely to be detectable over long periods. The same is true for multi-year extremes, such as multi-year droughts (Falster et al., 2024), for which limited information may be gained from shorter simulations." EMICs are unable to quantify such metrics, thus justifying the expense of using ESMs. I suggest articulating this point much earlier in the paper.(2) Early in the paper you should acknowledge the millennial length simulations of zero emissions done with EMICs, summarize what they show then highlight the limitations of such EMIC simulations, thus highlighting the need for longer ESM simulations.
(3) It is also important to acknowledge that ESMs often lack processes the EMICs include, which become more important on long timescales. For millennial length simulations processes that are particularly important are: i) dynamic vegetation, ii) ice sheets, iii) permafrost carbon iv) ocean floor carbonate dissolution dynamics. Combined these feedbacks will strongly affect global CO2 concentration, ocean circulation, and regional climates. Also important to note is that many ESMs do not conserve mass and energy to machine precision and thus are not intended for millennial length simulations. Therefore only ESMs with little to no drift in their zero emissions pi-control simulation should be extended to millennial length (hopefully this will be less of a problem for CMIP7 models).
Specific comments:
Line 18: "Humanity must achieve net zero emissions to slow down climate change" is not really correct. Reducing the rate of emissions should slow down climate change, since warming is roughly proportional to cumulative emissions. To stop global average climate change ZECMIP showed the near-zero emissions are needed. With the ZEC range implying that slightly positive to slightly negative emissions are compatible with zero global temperature change.References:
MacDougall AH, Frölicher TL, Jones CD, Rogelj J, Matthews HD, Zickfeld K, Arora VK, Barrett NJ, Brovkin V, Burger FA, Eby M. Is there warming in the pipeline? A multi-model analysis of the Zero Emissions Commitment from CO 2. Biogeosciences. 2020 Jun 15;17(11):2987-3016.
Citation: https://doi.org/10.5194/egusphere-2025-903-RC1
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