the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Warming effects of reduced sulfur emissions from shipping
Abstract. The regulation introduced in 2020 that limits the sulfur content in shipping fuel has reduced sulfur emissions over global open oceans by about 80 %. This is expected to have reduced aerosols that both reflect solar radiation directly and affect cloud properties, with the latter also changing the solar radiation balance. Here we investigate the impacts of this regulation on aerosols and climate in the HadGEM3-GC3.1 climate model. The global aerosol effective radiative forcing caused by reduced shipping emissions is estimated to be 0.13 W m-2, which is equivalent to about 50 % of the positive forcing caused by the global reduction in all anthropogenic aerosols since late 20th century. Ensembles of global coupled simulations from 2020–2049 predict a global mean warming of 0.04 K averaged over this period. Our simulations are not clear on whether the global impact is yet to emerge or has already emerged because the present-day impact is masked by variability. Nevertheless, the impact of shipping emission reductions will have either already committed us to warming above the 1.5 K Paris target or will represent an important contribution that may help explain part of the rapid jump in global temperatures over the last 12 months. Consistent with previous aerosol perturbation simulations, the warming is greatest in the Arctic, reaching a mean of 0.15 K Arctic-wide and 0.3 K in the Atlantic sector of the Arctic (which represents greater than 10 % increase in the total anthropogenic warming since pre-industrial times).
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RC1: 'Comment on egusphere-2024-1428', Anonymous Referee #1, 17 Jun 2024
Using the HadGEM3-GC3.1 climate model, the authors investigate the impacts of reduced shipping SO₂ emissions. They estimate the aerosol effective radiative forcing caused by this reduction to be 0.13 W/m². Ensembles of global coupled simulations from 2020-2049 predict a global mean warming of 0.04 K averaged over this period. The authors suggest that the impact of shipping emission reductions could represent a significant contribution to the rapid global temperature rise observed from 2022 to 2023. While the authors have conducted extensive simulations and applied various methods to reach their conclusions, there are several concerns regarding the numerical design and data analysis.
My major concern is with the methodology used to adjust variations in temperature from the 1850s to the 2020s using differences in global annual temperature between CMIP6 PD and PI. Given the significant internal biases among models, this approach seems unreasonable. I recommend that the authors run additional PI simulations using the HadGEM3-GC3.1 model to estimate PD-PI differences and then compare these values with the CMIP6 datasets.
Additionally, there are concerns about several other conclusions, some of which appear misleading:
- Aerosol Effective Radiative Forcing: The statement that "the global aerosol effective radiative forcing caused by reduced shipping emissions is estimated to be 0.13 W/m², which is equivalent to about 50% of the positive forcing caused by the global reduction in all anthropogenic aerosols since the late 20th century" is misleading. Reductions in anthropogenic emissions since the late 20th century have contributed to warming primarily due to a decrease in scattering aerosols, and to cooling due to absorbing aerosols like soot. This statement could be misinterpreted to suggest that half of the aerosol warming effect is from shipping reductions alone.
- Temperature and Precipitation Changes (Figures 4 and 5): The authors discuss changes in temperature and precipitation, but only a few areas show statistical significance. As a result, the discussion about the warming trend lacks robustness.
- Walker Circulation (Figure Analysis): The authors note a reduction in LW at the TOA in the western tropical Pacific Ocean and attribute this to a strengthening of the Walker Circulation. However, they do not discuss changes in SW, pressure gradients, or sea surface temperature, which are necessary to support this claim.
- Temperature Comparison (2023 vs. 2016): The comparison of global temperatures in 2023 to 2016 is problematic. Although 2016 was a strong El Niño year, it does not necessarily represent the highest global mean temperature in the past decade. Moreover, the conclusion that "the effect of shipping emission reductions (if they have emerged) and the HTHH eruption could explain up to 0.08 K of the 0.17 K warming since 2016" lacks sufficient evidence and requires further clarification.
Given these issues, I recommend requiring major revisions before considering resubmission.
Citation: https://doi.org/10.5194/egusphere-2024-1428-RC1 - RC2: 'Comment on egusphere-2024-1428', Anonymous Referee #2, 19 Jun 2024
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