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Ships brighten clouds even when you cannot see it

Figure [courtesy: Diamond et al. (2020)]: Increase in cloud optical thickness (measure of reflectivity of a cloud) within the shipping corridor relative to outside the shipping corridor (black curve). The total change in reflectivity was split into two contributing processes (blue: Twomey effect, red: cloud amount adjustments). Their sum was overlaid in purple and is almost identical to the total detected change in cloud optical thickness.

I was involved in this great study lead by Michael Diamond (University of Washington), which demonstrated for the first time that industrial shipping changes the reflectivity of clouds substantially.

It has been known since the 60s that individual ships can alter the cloud structure and reflectivity within narrowly defined tracks that stand out in remote sensing images. These so called ship tracks can be observed from space and are visually not too dissimilar to contrails behind air planes (though their physics is quite different).

Throughout the years people showed that the change in reflectivity inside these detected ship tracks could be large which would imply a substantial, climate-relevant radiative effect generated by industrial ship emissions around the world. Yet we have been unable to detect these changes in observations of the overwhelming number of cloud scenes where numerous ships where present, but no tracks were seen.

Figure [courtesy: Diamond et al. (2020)]: Shipping corridor and prevailing winds in southeast Atlantic.

Due to a chance alignment of the prevailing winds within a dominant shipping corridor in the southeast Atlantic, Diamond et al (2020) was able to detect and attribute the brightening of clouds to ship emissions even when ship tracks could not be observed. The effective radiative forcing of the ships within this region was estimated as -2 W/m^2. This constitutes a substantial surface cooling rate which will likely disappear due to the new MARPOL emission regulations enforced as of this year.

Using the observationally inferred constraint on the efficacy of a change in aerosol concentration on the cloud radiative effect within the shipping corridor, the authors were able to provide an observation-based estimate on the global effective cooling rate generated within low clouds by anthropogenic sulfate emissions as -1 W/m^2 (95% confidence interval -1.6 to -0.4 W/m^2).

Furthermore, the analysis showed that the cloud brightening was predominantly caused by a shrinking of cloud droplets, which increases the overall surface area and thus the cloud's reflectivity (commonly known as the Twomey effect). Changes in cloud amount were found to be small and negative. Thus this paper, like other previous publications, indicates that the radiative forcing contribution by changes in cloud mount is likely small and negative.

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