A group of researchers based at Oxford University’s Climate Processes Group has used novel methods of analysing satellite data to more accurately quantify the effect of human aerosol emissions on climate change. The results are published today in the journal Nature.
Human aerosol emissions have a cooling effect on the planet, because they can make clouds brighter by providing extra condensation nuclei on which cloud droplets form. Brighter clouds reflect more of the sunlight that strikes them, deflecting it from the earth’s surface. However, it is currently unclear how large this cooling effect is, particularly if the cloud brightness change cannot be seen in satellite images. This could be when the emissions are diffuse, such as from a city’s traffic, or when there are winds that disperse them. The cooling effect offsets some of the warming effect of greenhouse gasses, and provides the largest uncertainty in human perturbations to the climate system.
To investigate this, the research team analysed data on ship emissions as a model system for quantifying the climatic effect of human aerosol emissions in general. Sometimes, when a ship passes underneath a cloud, its aerosol emissions brighten the cloud in a long line, similar to a contrail. These so-called ship tracks have been studied previously, however the vast majority of ships leave no visible tracks. This was the first study to provide a quantitative measure of the impact of invisible ship tracks on cloud properties, and thus their cooling effect.
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