Oct. 6 () –
Policies against pollution must be accompanied by decisive action against global warming, to compensate for the loss of the cooling effect of human aerosols.
That’s the conclusion of a group of researchers based at the Climate Processes Group at the University of Oxford, who have used novel methods of analyzing satellite data to more accurately quantify the effect of human aerosol emissions on climate change. The results are published in Nature.
Human aerosol emissions have a cooling effect on the planet, because they can make clouds brighter by providing additional condensation nuclei in which cloud droplets form. The brightest clouds reflect more of the sunlight that reaches them, deflecting it from the surface of the earth.
However, it is currently unclear how large this cooling effect is, particularly if the change in cloud brightness cannot be seen on satellite images. This could be when the emissions are diffuse, such as from city traffic, or when there are winds that scatter them. The cooling effect offsets some of the warming effect of greenhouse gases and provides the greatest uncertainty in human disturbances to the climate system.
SHIP TRACKS
To investigate this, the research team analyzed data on ship emissions as a model system to quantify the climate effect of human aerosol emissions in general. Sometimes when a ship passes under a cloud, its aerosol emissions illuminate the cloud in a long line, similar to a wake.
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 its cooling effect.
As a result of the study, it was appreciated that the invisible shipping footprints had a clear impact on the properties of the clouds they polluted. Surprisingly, the specific effects were different from the visible shipping footprints.
Likewise, the tracks of invisible ships showed a smaller increase (approximately 50% less) in the number of droplets in the clouds, but the amount of water increased more, compared to the effect of the visible traces. This implies that for a given increase in droplets, the increase in water is greater than previously thought, which equates to a greater cooling effect.
The same may be true for aerosol emissions in general: clouds may react to air pollution more strongly than previously thought, becoming brighter and having a stronger cooling effect.
Ship emissions often occur in remote ocean environments and thus provide unique opportunities to study the effects of aerosols in isolation from other human-induced factors that affect climate. This new study, led by student Peter Manshausen, used a global database of ship routes containing the location of almost all ships at any given time: more than two million ship routes for six years.
Combining this with historical weather observations, the researchers simulated where all emissions from these ships were carried by the wind and entered the cloud. Studying these locations on satellite data allowed them to measure the number of droplets and the amount of water in the polluted and unpolluted clouds. Importantly, this method does not depend on ship emissions being visible on satellite images.
According to the research team, the findings indicate that human health policies to reduce air pollution should be carefully considered when forecasting future climate change scenarios.
In a recent study, the Climate Processes Group also found that ship footprints dropped by around 25% almost immediately after the International Maritime Organization introduced tough new fuel regulations in 2020 to reduce air pollution. caused by global shipping. This analysis used a machine learning approach to automatically measure more than a million visible ship tracks from satellite images over a 20-year period.
Professor Philip Stier (who heads the Climate Processes Group in the Department of Physics at the University of Oxford), co-author of the study, said it’s a statement: “These techniques show the value of combining new data science approaches with the enormous amount of Earth observation data.” Now available. They will allow us to transform the analysis of climate processes in Earth observations from case studies to global monitoring, providing entirely new observational constraints on our understanding of the climate system and future climate models.”
