the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Mar 2025
A normalised framework for the Zero Emissions Commitment
Abstract. The Zero Emissions Commitment (ZEC) measures the transient climate response after carbon emissions cease, defined by whether there is a continued rise or decrease in global surface temperature. A normalised framework for the ZEC connects the surface temperature response post emissions to carbon, radiative and thermal processes, involving changes in carbon inventories, radiative forcing, planetary heat uptake and climate feedback. The normalised ZEC, defined by the surface temperature change since the pre industrial divided by the temperature change at the time of net zero, is controlled by opposing-signed contributions: (i) a cooling contribution from a weakening in radiative forcing due to a decrease in atmospheric CO2 from carbon uptake by the land and ocean versus (ii) surface warming contributions from a decline in the fraction of radiative forcing used for planetary heat uptake augmented by possible amplification by climate feedbacks. From a set of 9 CMIP6 Earth system models following an idealised atmospheric CO2 scenario, inter-model differences in the post-emission climate response are primarily determined by differences in the ocean heat uptake and the land and ocean uptake of carbon. These inferences as to the controls of the ZEC broadly carry over for diagnostics of a large ensemble, observationally-constrained efficient Earth system model using two different emission scenarios to reach net zero. The large ensembles reveal a partial compensation between the changes in landborne and oceanborne fractions, as well as revealing ensembles with greater range in amplification of warming by climate feedbacks.
- Preprint
(2867 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 16 May 2025)
-
RC1: 'Comment on egusphere-2025-800', Anonymous Referee #1, 28 Apr 2025
reply
Review of “A normalised framework for the Zero Emissions Commitment”
This article explores the contributions of thermal, carbon cycle, and radiative components to the temperature response after net zero CO₂ emissions. The analysis uses nine ESMs from the ZECMIP A1 protocol and an efficient Earth system model (WASP), which includes 1138 posterior ensemble simulations. A normalised ZEC framework is applied, where the ZEC is the temperature change relative to pre-industrial compared to the change at net zero. The ZEC response is determined by a competition between cooling from declining atmospheric CO₂ and warming from strengthening thermal contributions, as ocean heat uptake declines and climate feedbacks amplify surface warming. Different models achieve positive or negative ZEC through varying strengths of thermal and carbon contributions; for example, a strong thermal contribution drives positive ZEC in CNRM-ESM2, while large land and ocean carbon uptake leads to negative ZEC in NorESM2-LM. Inter-model differences are mainly driven by variations in ocean heat uptake and land carbon uptake, and diagnostics from WASP suggest that current model ensembles may underestimate the range of possible climate feedback responses.Overall, this article is of excellent quality and makes an important contribution by clarifying the physical mechanisms that govern model behaviour after net zero emissions. The revisions suggested below are minor and primarily aimed at improving clarity.
General
It would enhance clarity by explaining the interpretation of some of the values once calculated. For example, in lines 210–212 the values for the normalised ZEC is given. I know that the interpretation of the normalised ZEC is provided in line 92. However, because there are many different variables used in this article, an explanation also in lines 210-212 here would improve understandability. This goes for many of the values throughout.
Line Specific
Line 91: “A positive ZEC corresponds to ∆T (t)/∆T (tZE) > 0 and a negative ZEC to ∆T (t)/∆T (tZE) < 0.”
Should this threshold be one rather than zero? A normalised ZEC less than one would indicate cooling relative to the net zero temperature, and greater than one would imply warming. Additionally, moving this explanation to immediately follow the definition of the normalised ZEC would make more sense (line 85).
Line 217: Values such as ΔT(t)/ΔF(t) are given without units. Some values are given with units (such as ZEC), and some are given without units. This occurs throughout with several other values that I believe should also have units.
Line 220: The explanation in this section would benefit from additional clarification. It appears that contributions are inferred from changes between years 50 and 90, but this is not explicitly stated. For example:
“The normalised carbon contribution, ΔIA(t), decreases from 0.70 ± 0.06 at year 50 to 0.63 ± 0.06 at year 90.”
Clarifying the methodology used to calculate these contributions would make the reasoning more transparent.
Figures 2 and 3
The lines in these figures are difficult to distinguish, and the legend overlaps with the x-axis. Using more distinct colours or varied line styles would improve figure readability.
Citation: https://doi.org/10.5194/egusphere-2025-800-RC1
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 90 | 15 | 5 | 110 | 6 | 5 |
- HTML: 90
- PDF: 15
- XML: 5
- Total: 110
- BibTeX: 6
- EndNote: 5
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1