According to new research, aerosol particles in the atmosphere have a greater impact on cloud cover but less impact on cloud brightness than previously thought.
Aerosols are tiny particles suspended in the atmosphere that play an important role in cloud formation.
Because aerosols are increasing as a result of human activity, numerous assessments by the Intergovernmental Panel on Climate Change (IPCC) have suggested that they could have a significant impact on climate change because clouds reflect sunlight and thus keep temperatures cooler.
However, the cooling effect of aerosols on clouds is difficult to quantify, resulting in significant uncertainty in climate change projections.
The new study, led by the University of Exeter in collaboration with national and international academic partners as well as the Met Office of the United Kingdom, investigated this using the 2014 Icelandic volcano eruption.
“This massive aerosol plume in an otherwise near-pristine environment provided an ideal natural experiment to quantify cloud responses to aerosol changes, specifically the aerosol’s fingerprint on clouds,” lead author Dr. Ying Chen explained. “Our analysis shows that the eruption’s aerosols increased cloud cover by about 10%.” Based on these findings, we can conclude that increased cloud cover is responsible for more than 60% of the climate cooling effect of cloud-aerosol interactions.
“Volcanic aerosols also brightened clouds by reducing water droplet size, but this had a much smaller impact on reflecting solar radiation than cloud-cover changes.”
Previous models and observations suggested that cloud-aerosol interactions accounted for the majority of the cooling.
Water droplets typically form in the atmosphere around aerosol particles, so a higher concentration of these particles facilitates the formation of cloud droplets. However, because these cloud droplets are smaller and more numerous, the resulting clouds can hold more water before rainfall occurs—as a result, more aerosols in the atmosphere can result in more cloud cover but less rain.
Cloud cover and brightness were studied using satellite data and computer learning. It compared the periods before and after the volcano eruption using 20 years of satellite cloud images from two different satellite platforms in the region. The findings will provide observational evidence of aerosols’ climate impacts, which will help scientists improve the models they use to predict climate change.
“Our previous work had shown that model simulations could be used to disentangle the relative contribution of aerosol-cloud-climate impacts and potentially confounding meteorological variability,” said Jim Haywood, Professor of Atmospheric Science at the University of Exeter and a Met Office Research Fellow.
“This work is radically different because it does not rely on models; instead, it uses cutting-edge machine learning techniques applied to satellite observations to simulate what the cloud would look like if the aerosols were not present.” There are significant differences between predicted and observed cloud properties, which can be used to assess aerosol-cloud-climate impacts.”
The paper, titled “Machine-learning reveals climate forcing from aerosols is dominated by increased cloud cover,” was published in the journal Nature Geoscience.