Large‐Scale Industrial Cloud Perturbations Confirm Bidirectional Cloud Water Responses to Anthropogenic Aerosols

다국어 초록 (Multilingual Abstract)

Aerosols offset a poorly quantified fraction of anthropogenic greenhouse gas warming, and the aerosol impact on clouds is the most uncertain mechanism of anthropogenic climate forcing. Here observations of a relatively weak average response of liquid cloud water to aerosol perturbations are extended to much larger areas than in previous studies, with the polluted cloud areas covering hundreds by hundreds of kilometers. Polluted clouds detected in satellite images at the global anthropogenic air pollution hot spot of Norilsk, Russia, and at other various major aerosol source regions show close compensation between aerosol‐induced cloud water increases and decreases. In the sampled Norilsk cloud perturbations the decrease in liquid water path offsets 3% of the radiative forcing through the Twomey effect on average. Weak cloud water response on average in large polluted areas is in very good agreement with previous results based on observations of small‐scale ship‐track‐like industrial cloud perturbations, helping to reduce the uncertainty associated with the anthropogenic aerosol impacts on clouds.
It is unknown to what extent the global cooling effect of anthropogenic aerosols offsets the warming effect of anthropogenic greenhouse gases. The strength of the aerosol climate cooling effect exerted through changes in the properties of clouds is especially uncertain. This hinders better estimates of Earth's climate sensitivity to anthropogenic radiative forcing and more reliable projections of future climate. It has been previously found that the average aerosol‐induced cloud water change is relatively weak in smaller‐scale ship‐track‐like polluted cloud tracks. Here we analyze satellite observations of much larger‐scale polluted cloud areas, with the polluted cloud areas covering hundreds by hundreds of kilometers. Polluted clouds over Norilsk, Russia, and over other various major aerosol source regions show close compensation between aerosol‐induced cloud water increases and decreases. Large‐scale polluted cloud areas indicate that the effect that aerosols have on cloud droplet sizes has a stronger impact on Earth's climate than the effect caused by the cloud water changes. These results help to reduce the uncertainty associated with the anthropogenic aerosol impacts on clouds and Earth's climate.


Satellite observations confirm aerosol‐induced decreases in cloud droplet size in large areas compared to nearby less polluted areas
Aerosol‐induced cloud water increases and decreases compensate each other at large spatial scales similarly to ship‐track‐like perturbations
On average, Twomey effect is more important than the effect of cloud water changes for the aerosol radiative forcing of the Earth's climate