Rapid saturation of cloud water adjustments to shipping emissions

Abstract

Abstract. Human aerosol emissions change cloud properties by providing additional cloud condensation nuclei. This increases cloud droplet numbers, which in turn affects other cloud properties like liquid-water content and ultimately cloud albedo. These adjustments are poorly constrained, making aerosol effects the most uncertain part of anthropogenic climate forcing. Here we show that cloud droplet number and water content react differently to changing emission amounts in shipping exhausts. We use information about ship positions and modeled emission amounts together with reanalysis winds and satellite retrievals of cloud properties. The analysis reveals that cloud droplet numbers respond linearly to emission amount over a large range (1–10 kg h−1) before the response saturates. Liquid water increases in raining clouds, and the anomalies are constant over the emission ranges observed. There is evidence that this independence of emissions is due to compensating effects under drier and more humid conditions, consistent with suppression of rain by enhanced aerosol. This has implications for our understanding of cloud processes and may improve the way clouds are represented in climate models, in particular by changing parameterizations of liquid-water responses to aerosol.

Multi-model effective radiative forcing of the 2020 sulphur cap for shipping

“…Shipping emissions also interact with cloud and change cloud properties even if ship tracks are not visible in satellite images (Glassmeier et al, 2021;Manshausen et al, 2022). Diamond (2023) Using satellite retrievals, reanalysis wind, ship positions and modelled emissions Manshausen et al (2023) showed that the cloud droplet numbers respond linearly to ship emissions. On the other hand, the increase in liquid water content is constant over a wide range of emissions perturbations which are caused by compensating effects of increases and decreases in liquid water path in different regimes.…”

“…This indicates that chemistry models without detailed representation of the liquid water path adjustments do not largely underestimate the ERF of the shipping emission cap. A remaining issue Manshausen et al (2023) highlighted is how the ship emission regulations have impacted the cloud fraction (Chen et al, 2022;Chen et al, 2024).…”

Comment on egusphere-2024-1479

“…This differs from Manshausen et al (2023), who consider a null experiment that uses ship locations and winds from a certain day to predict their ship track locations, but the satellite data from the day before (therefore the winds and satellite data will be uncorrelated). In this study, we retain the correlation between the winds and satellite data, as in the true ship track case this correlation will be present.…”

“…This work predicts ship track locations using a similar method to that of Gryspeerdt et al (2019bGryspeerdt et al ( , 2021, utilising over 35,000 ships from automatic identification system (AIS) transponder data in 2018, filtered to include specific ship types (large container vessels, bulk carriers, oil tankers, cruise ships and general cargo ships; Smith et al, 2015). The region of interest of this work is chosen to be the same as in Manshausen et al (2022Manshausen et al ( , 2023, to enable direct comparison of results. This region in the Atlantic Ocean bounded by (50 S,50º N) and (90º W, 20º E), and contains both stratocumulus and trade cumulus regimes.…”

Large radiative forcing from the 2020 shipping fuel regulation is hard to detect

“…The combustion of sulfur-containing fuel produces sulfate aerosol -an effective source of CCN for marine boundary layer clouds -as a byproduct or via oxidation of the emitted gaseous sulfur dioxide. This regulation leads to a reduction in sulfate aerosol production over global oceans 13 , resulting in lower cloud droplet number concentrations (N d ) that have been detected regionally [14][15][16] . Since IMO2020, a pronounced reduction in ship track occurrence has been documented globally 13,14 , adding to documented impacts from earlier fuel regulations designated for specific emission zones near the Californian and European coasts 17 .…”