# Invisible ship tracks show large cloud sensitivity to aerosol

## TL;DR

The study shows that even when ship tracks are not visible in satellite images, aerosol emissions from ships significantly alter cloud properties, leading to a larger cooling effect on the climate than previously estimated.

## Contribution

The paper introduces a new method to quantify aerosol-cloud interactions in clouds without visible ship tracks, revealing a stronger cooling effect from liquid water path adjustments.

## Key findings

- Aerosol emissions from ships alter cloud properties even when no visible ship tracks are observed.
- Cloud droplet number increases and liquid water response is more positive in the absence of visible tracks.
- The study finds a strong liquid water path response, indicating a larger aerosol cooling effect on climate.

## Abstract

Cloud reflectivity is sensitive to atmospheric aerosol concentrations because aerosols provide the condensation nuclei on which water condenses1. Increased aerosol concentrations due to human activity affect droplet number concentration, liquid water and cloud fraction2, but these changes are subject to large uncertainties3. Ship tracks, long lines of polluted clouds that are visible in satellite images, are one of the main tools for quantifying aerosol–cloud interactions4. However, only a small fraction of the clouds polluted by shipping show ship tracks5,6. Here we show that even when no ship tracks are visible in satellite images, aerosol emissions change cloud properties substantially. We develop a new method to quantify the effect of shipping on all clouds, showing a cloud droplet number increase and a more positive liquid water response when there are no visible tracks. We directly detect shipping-induced cloud property changes in the trade cumulus regions of the Atlantic, which are known to display almost no visible tracks. Our results indicate that previous studies of ship tracks were suffering from selection biases by focusing only on visible tracks from satellite imagery. The strong liquid water path response we find translates to a larger aerosol cooling effect on the climate, potentially masking a higher climate sensitivity than observed temperature trends would otherwise suggest.

Investigations of the effect of aerosol emissions from shipping on cloud properties when both visible and invisible ship tracks are considered implies a large negative radiative effect (and associated cooling) from liquid water path adjustments.