Yuan et al. (2025) Detectable ship tracks account for just 5% of aerosol indirect forcing from ship emissions

Identification

• Journal: Communications Earth & Environment
• Year: 2025
• Date: 2025-11-14
• Authors: Tianle Yuan, Hua Song, Lili F. Boss, Michael Diamond
• DOI: 10.1038/s43247-025-02825-w

Research Groups

• Earth Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
• GESTAR-II, University of Maryland, Baltimore County, Baltimore, MD, USA
• SSAI Inc., Lanham, MD, USA
• Department of Earth, Ocean, and Atmospheric Science, Florida State University, Tallahassee, FL, USA

Short Summary

This study reconciles a long-standing discrepancy in estimates of aerosol indirect forcing from ship emissions (AIF-ship), revealing that visible ship tracks account for less than 5% of the total forcing, with the majority stemming from diffused, undetected aerosol plumes.

Objective

• To reconcile the large discrepancy in literature estimates of aerosol indirect forcing from ship emissions (AIF-ship) by comparing bottom-up (visible ship-track based) and top-down (large-scale observational/model-based) approaches, and to quantify the contribution of visible ship tracks to total AIF-ship.

Study Configuration

• Spatial Scale: Southeast Atlantic (SEA) shipping lane and broader SEA region.
• Temporal Scale: Climatological mean (2003–2019 for some data), pre-2020 shipping, and post-2020 (IMO2020 regulations), focusing on the September, October, November (SON) season.

Methodology and Data

• Models used:
  • NASA's Global Earth Observing System (GEOS) with Goddard Chemistry Aerosol Radiation and Transport (GOCART) aerosol module.
  • Neural-network models for converting aerosol optical depth to cloud condensation nuclei (CCN) and CCN to cloud droplet number concentration (Nd).
  • Geospatial kriging (for Top-Down method).
• Data sources:
  • MODerate resolution Imaging Spectrometer (MODIS) instrument on Aqua satellite (Level 2 instantaneous and Level 3 monthly mean data): droplet effective radius (Reff), cloud optical depth (τ), total cloud fraction (Cftotal), low cloud fraction (Cf), high cloud fraction (Cfhigh), derived Nd and liquid water path (LWP), visible ship-track masks.
  • Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) version 4.1: Radiative fluxes, solar incoming fluxes, cloud and surface albedo, cloud fraction.
  • MERRA2 data.
  • Ship-track block data (from https://doi.org/10.7910/DVN/JII4DN).

Main Results

• Visible ship tracks account for less than 5% of the total aerosol indirect forcing from ship emissions (AIF-ship) in the Southeast Atlantic.
• The majority of AIF-ship originates from aerosols that do not form readily detectable ship tracks, primarily through increasing cloud droplet number concentration (Nd) in diffused, undetected plumes.
• The bottom-up method using machine-learning detected ship tracks (BU-ST) estimates AIF-ship to be an order of magnitude larger than previous manual detection methods, but still more than an order of magnitude smaller than top-down (TD) or hybrid (BU-TD, BU-GEOS) methods.
• TD and hybrid methods estimate AIF-ship to be approximately -1.5 W⋅m⁻² to -2.5 W⋅m⁻² in the Southeast Atlantic during the SON season.
• The forcing due to IMO2020 regulations (AIF-IMO2020) is estimated around +0.11 W⋅m⁻² by BU-ST, but nearly +1 W⋅m⁻² by BU-GEOS and TD methods, indicating a substantial warming effect from reduced emissions.
• Global AIF-ship and its post-2020 reduction are estimated to be on the order of -0.1 W⋅m⁻², consistent with global model estimates.

Contributions

• Resolves a long-standing, large discrepancy (factor of >1000) between observation-based (visible ship-track) and model-based estimates of AIF-ship.
• Demonstrates that visible ship tracks represent only a small fraction ("tips of the iceberg") of the total aerosol indirect forcing from ship emissions.
• Provides independent validation between top-down and hybrid approaches for estimating AIF-ship and the forcing due to IMO2020 regulations.
• Highlights the critical implication for climate engineering techniques like marine cloud brightening (MCB), suggesting that the total aerosol effect, not just visible tracks, must be considered.
• Offers a framework for scaling insights from small-scale experiments to broader aerosol indirect forcing calculations.

Funding

• NOAA Climate Program Office (CPO) Earth’s Radiation Budget (ERB) (Grant NA23OAR4310298)
• NOAA Climate Program Office (CPO) Atmospheric Chemistry, Carbon Cycle, and Climate (AC4) (Grant NA23OAR4310299)
• NOAA Climate Program Office (CPO) Climate Variability and Predictability (CVP) Programs (Grant NA23OAR4310297)
• NASA (Grant numbers 80NSSC24K0458 and 80NSSC24M0045)
• DOE (Grant DE-SC0024078)

Citation

@article{Yuan2025Detectable,
  author = {Yuan, Tianle and Song, Hua and Boss, Lili F. and Diamond, Michael},
  title = {Detectable ship tracks account for just 5% of aerosol indirect forcing from ship emissions},
  journal = {Communications Earth & Environment},
  year = {2025},
  doi = {10.1038/s43247-025-02825-w},
  url = {https://doi.org/10.1038/s43247-025-02825-w}
}