As a regular you will be well aware that some clouds cool the climate and other clouds warm. Determining the exact balance of the cloud-climate feedback will help decrease uncertainty margins for 21^st century warming forecasts.

Unfortunately it’s a complicated picture, as clouds are not only influenced by climatic factors like temperature and evaporation, but also by concentrations of condensation nuclei – aerosols indeed.

That means when it comes to clouds anthropogenic forcing is not limited to GHG emissions, but also includes [other forms of] air pollution. We always thought that – apart of course from soot [15% of climate warming] – such aerosol pollution creates cooling – as in the case of Chinese sulfur pollution and the Asian (Indian) brown cloud – and that air quality measures over recent decades in North America and Europe are now actually a major cause of increased warming speeds there – as the actual temperature catches up on the ‘CO2 baseline’.

However – a group of scientists of the US Department of Energy Pacific Northwest National Laboratory, the University of Maryland and the Hebrew University of Jerusalem now say that aerosol pollution does not necessarily lead to (low-lying) stratus clouds one would appreciate for climatic cooling, but that it can also be a factor in the creation of thunderstorm clouds, clouds that have a complicated climate effect, but that are suspected of being net warmers.

Studying Chinese summer thunderclouds the researchers found that an increase in aerosols led to larger and more persistent convective cloud systems, with larger anvils at several kilometers of altitude that may reflect more sunlight, but trap even more heat – as their high-resolution model showed.

“We found that aerosol indirect effect on deep convective cloud systems could lead to enhanced regional convergence and a strong top-of-atmosphere warming.”

“The positive aerosol radiative forcing on deep clouds could offset the negative aerosol radiative forcing on low clouds to an unknown extent.”

As the warming occurs high up in the troposphere [and can be of significant force: up to +5.6 W m⁻²] it could have relatively large consequences for local weather development.

In their Geophysical Research Letters publication the researchers also write that ‘aerosol invigoration effect occurs mainly in warmed-based convection with weak shear ‘ – as they could not find similar effects in frontal convection weather systems, which have higher wind shear and where air is forced up not by land surface warming, but by a pushing cold air wedge. (As case study for such weather systems they used observations of Oklahoma cold fronts.)

© Rolf Schuttenhelm | www.bitsofscience.org