Dirty Clouds

In my previous post I gave a short summary of how clouds affect the radiation and heat budget of the Earth, and the difference between low and high clouds. In a world with a changing climate, we all want to know what the effects of CO2, aerosols, and associated global warming will be on clouds and their feedback on the system. 

In truth, clouds are difficult to incorporate in climate models. They are small, the physics behind them are complex and not always well understood, and clouds have strong regional effects, all of which makes them complicated to model. Even so, knowledge of clouds is greatly improving, and so models are becoming more certain (for those interested in climate model certainty, always a hot topic, please see NASA on climate uncertainties and the IPCC on the confidence of model predictions in the 4th Assessment Report). 

First, let's just do a quick recap on what aerosols are. Basically, aerosols are all sorts of types of suspended particulate matter. Some occur natural, such as dust or organic compounds. A famous example of the latter can be found in the Blue Mountains in New South Wales, Australia, where the eucalyptus trees release a sort of organic matter into the mountain air. Incoming bluish ultraviolet light is scattered by the particles, giving the Blue Mountains their etymological characteristic.

Blue Mountains, NSW, Australia

Source: National Geographic

However, humans are putting in a fair share of aerosols into the atmosphere as well. In a previous post I discussed the effect of soot deposition on snow in Antarctica. Other aerosols include sulphates from fossil fuels and other pollutants. In the picture below, Ramanathan and Feng (2009) have indicated the emissions of black carbon across the globe.

Emission of BC in tons per year across the globe. Note the emissions across the ocean along traffic routes of airplanes and ships. 

Source: Ramanathan & Feng 2009.

Aerosols alone can affect the incoming solar radiation by reflecting some of it back into space. In this sense, they increase the albedo. However, some of the radiation is absorbed, warming the atmosphere. As a net effect, the surface is cooled, and the atmosphere warmed (Ramanathan & Carmichael 2008).

In any case, like I said above, I want to investigate the effect of aerosols on clouds. Clouds are complicated, and studying the relation between clouds and aerosols is still not well understood. The IPCC rates the scientific understanding of the interactions generally as low to very low (also see the relevant section here). Even so, I want to discuss some of the basic processes.

Aerosols affecting cloud behaviour. 

Source: UK Met Office

Clouds form around small particles, called Cloud Condensation Nuclei (CCN). More aerosols mean more CCN, meaning more droplets that together form clouds. It also means that clouds become whiter, and thus more reflective. In this sense, aerosols can decrease cloud albedo and have a cooling effect (this is the first effect in the figure above).

Because there are more CCN, the droplets are smaller and it will take longer for clouds to form raindrops. This increases the lifetime of clouds as well as decreasing the number of rainfall events. This secondary effect, also known as the Albrecht effect, is not very well understood.

Lastly, there is the semi-direct effect. The aerosols in the clouds absorb radiation, and re-emit it, warming the clouds and reducing the upward flow of moisture - and the formation of clouds. This is perhaps the least-well understood effect.

I hope that this shows how complicated the relationship between clouds and aerosols is; there is no clear-cut correlation. Some effects increase cloud cover, whereas others reduce them. Clouds are difficult to study and model, which makes it tricky to incorporate them in climate projections.

Despite all this, scientists discover more and more about aerosols and clouds, which greatly improves the models for future climate projections. To finish off, I found this animated video of the cloud-aerosol effects I explained in this post.

Another interesting field related to clouds and aerosols has to do with geo-engineering, something I want to look at in my next blog post. For more information on research that is being done on clouds, I can recommend the Guardian article in the "Further Reading" section below, along with some other interesting websites. Happy browsing!

Further Reading

• "Creating clouds in the lab to better understand climate" via The Guardian. 
• "Climate science: The aerosol effect" - a commentary in Nature on progress in the cloud-aerosol research. 
• "What is global dimming?" - an explanation of air pollution and radiation at the Earth's surface, via The Guardian. 

References

• Ramanathan V. and G. Carmichael (2008), Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221, doi:10.1038/ngeo156
• Ramanathan, V., and Y. Feng (2009), Air pollution, greenhouse gases and climate change: Global and regional perspectives, Atmospheric Environment, 43, 37, doi:10.1016/j.atmosenv.2008.09.063