the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2025
Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations
Abstract. Starting in November 2023, the Houthi militia occupying northeastern Yemen has attacked ships passing through the Bab al-Mandab Strait, a chokepoint on the Europe-Asia route via the Suez Canal. Cargo ship traffic through the Red Sea has since plummeted, with ships instead taking the longer route around the Cape of Good Hope. The increase in traffic in the southeastern Atlantic Ocean is readily apparent in satellite retrievals of nitrogen dioxide. Within the stratocumulus deck covering much of the southeastern Atlantic, a previously detectible cloud microphysical perturbation due to ship pollution had largely disappeared following the International Maritime Organization's sulfur-limiting regulations in 2020 but returns during 2024 due to the increase in ship traffic despite the lower cloud brightening efficacy per ship. Because nitrogen dioxide pollution per unit of fuel oil burned is not affected by switching to low-sulfur fuel, quantifying the ratio of shipping-enhanced cloud droplet number and nitrogen dioxide concentrations before and after the fuel sulfur limits went into effect provides a constraint on the cloud changes from the regulations. We find that the ~80 % reduction in sulfur emissions leads to a ~66 % reduction in the increase in cloud droplet number concentration per unit marine fuel oil burned.
- Preprint
(2010 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2025-3735', Anonymous Referee #1, 04 Sep 2025
- AC1: 'Author response', Michael Diamond, 15 Oct 2025
-
RC2: 'Comment on egusphere-2025-3735', Gerald Mace, 06 Oct 2025
The study by Diamond and Boss examining the change in cloud droplet number (Nd) in the SE Atlantic region following the increase in shipping there during 2024 relative to the years since the fuel change and relative to the pre 2020 results shows that Nd increased in proportion to the increase in traffic. This allows the authors to quantify the efficiency with which Nd is influenced by shipping exhaust from before and after the 2020 change in sulfur content. Overall, I find the paper to be straightforward, concise, and compelling. I suggest only minor revisions.
The authors acknowledge the fact that examining a single year (2024) for the increase in traffic in their study region is somewhat fraught. They make a compelling and convincing argument that their findings are significant. However, I think they need to at least examine the large-scale atmosphere during 2024 to see if the months they consider are typical or perhaps anomalous relative to other years. The reason I think this is necessary is due to the role of drizzle in modulating Nd. While their data do not constrain drizzle occurrence or rate, were, perhaps, the marine inversion different from other years, drizzle may be more or less common, etc. While I think their results will hold up against this examination, it would be at least useful to examine this and report upon it
Lines 125-130 (first paragraph in the methods): It took me a bit to digest this and realize that the delta values in figure 2 were relative to the regression they discuss (do I have that right?). I think an illustrative figure would be helpful here to show their method. I think the approach is sound, but understanding their text would benefit greatly from a figure.
Caption to Figure 2: Note that the region displayed is from SE Atlantic. This is somewhat obvious but it would still be helpful to casual reader who might just scan the figures.
With compliments to the authors,
Jay Mace
Citation: https://doi.org/10.5194/egusphere-2025-3735-RC2 - AC1: 'Author response', Michael Diamond, 15 Oct 2025
- AC1: 'Author response', Michael Diamond, 15 Oct 2025
- AC2: 'Track changes (for reference)', Michael Diamond, 15 Oct 2025
- AC3: 'Track changes (for reference)', Michael Diamond, 15 Oct 2025
Data sets
Data for Diamond & Boss, "Conflict-induced ship traffic disruptions constrain cloud sensitivity to stricter marine pollution regulations" Michael S. Diamond and Lili F. Boss https://doi.org/10.5281/zenodo.15738910
Tropospheric NO2 from satellites Tropospheric Emission Monitoring Internet Service https://www.temis.nl/airpollution/no2.php
CLDPROP_M3_VIIRS_NOAA20 NASA VIIRS Atmosphere SIPS https://doi.org/10.5067/VIIRS/CLDPROP_M3_VIIRS_NOAA20.011
CLDPROP_M3_MODIS_AQUA NASA LAADS DAAC https://doi.org/10.5067/MODIS/CLDPROP_M3_MODIS_Aqua.011
Emissions Database for Global Atmospheric Research European Commission, Joint Research Centre https://doi.org/10.2904/JRC_DATASET_EDGAR
NOAA Extended Reconstructed SST V5 Boyin Huang et al. https://doi.org/10.7289/V5T72FNM
Datetimes and Locations of ship-tracks Hua Song https://doi.org/10.7910/DVN/JII4DN
Interactive computing environment
michael-s-diamond/ConflictClouds Michael S. Diamond https://doi.org/10.5281/zenodo.16637711
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 3,777 | 37 | 13 | 3,827 | 27 | 35 |
- HTML: 3,777
- PDF: 37
- XML: 13
- Total: 3,827
- BibTeX: 27
- EndNote: 35
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
The paper uses the change in cloud drop number concentration ratioed to the change in NO2 to assess how the IMO’s implementation of sulfur-limiting regulations in 2020 affected cloud brightening due to ship tracks. The SE Atlantic is the area of focus due to recent increases in ship traffic as ships transit around the Cape of Good Hope rather than through the Suez Canal to avoid Houthi militia attacks. Comparing years 2019 and 2024, a large decrease in the ability of ship emissions to impact Nd is found. The paper is concise and well-written. The figures are of high quality and effectively display the results reported in the paper. I only have a few minor comments.
Line 35: “the estimated magnitude of the IMO 2020 effect ranges from ~15-75%”. Is this based on the reduction in detectible ship tracks? Please clarify.
Lines 67 – 68: “Because the 2018 NO2 values have a particularly low tail and 𝑁d values have a high tail…” I could be interpreting this statement incorrectly but the 2018 high tail of the Nd values is not evident in Figure 2b.
Line 88: Does the “binary detection threshold metric” refer to relying on only two years to assess the cloud response to the decrease in ship sulfur emissions?