This week, I’m featuring some of the best blog posts written by the students on my first year undergraduate course, PMU199 Climate Change: Software, Science and Society. The first is by Terry, and it first appeared on the course blog on January 28.

A couple of weeks ago, Professor Steve was talking about the extra energy that we are adding to the earth system during one of our sessions (and on his blog). He showed us this chart from the last IPCC report in 2007 that summarizes the various radiative forces from different sources:

Notice how aerosols account for most of the negative radiative forcing. But what are aerosols? What is their direct effect, their contribution in the cloud albedo effect, and do they have any other impact?

What are Aerosols?

Aerosols are tiny particles (solid or liquid) suspended in a gas. The aerosols that are of interest in this post are the ones that occur naturally and ones that occur from human activities (not the ones from hairspray and spray paint cans, which you may have been thinking about). According to NASA, naturally occurring aerosols originate from sources such as volcanoes, dust storms, and forest and grassland fires. The burning of fossil fuels is one human activity that is creating aerosols. Aerosols created by human activities currently make up about 10% of the total amount of aerosols in our atmosphere with most of that concentrated in the Northern Hemisphere. Aerosols play an important role in the Earth’s environment as they have direct and indirect effects on the climate.

Aerosols are largely responsible for hazy skies, as in this photo of Shenzen, China. (Image from pseudo-san via NASA.)

Direct Effects of Aerosols

Aerosols can have a “direct” cooling effect by reflecting radiation from the Sun back to space since most aerosols are brighter than land or ocean.  Thus the amount of solar radiation that reaches the surface of the Earth is reduced.

Although most aerosols reflect radiation, there are some that absorb it. Darker aerosols, such as black carbon, can absorb significant amounts of radiation. This warms up the layer of the atmosphere carrying the black carbon, but also shades the surface below and cools it down.

Aerosols, particularly black carbon, can also dramatically alter the Earth’s surface albedo by depositing a layer of dark residue on ice, snow and other bright surfaces. Long-term buildup of black carbon aerosols in the Arctic and the Himalaya is resulting in increased melting of snow. Scientists, however, believe the cooling from reflective aerosols overcomes the warming effect of black carbon and other absorbing aerosols over Earth.

(Source: NASA)

Indirect Effects of Aerosols

Aerosols can also have an “indirect” effect on climate by changing the properties of clouds.

Clouds with low aerosol concentration on the left and with high aerosol concentration on the right. (Image from NASA by Robert Simmon.)

Clouds in clean air have low aerosol concentration and are composed of a small number of large droplets. They do not scatter light well and are somewhat dark and translucent. Much of the Sun’s radiation passes through these clouds and reaches the surface.

Clouds in polluted air have much higher aerosol concentration.  As aerosol concentration increases within a cloud, the water in the cloud is divided into a larger number of smaller droplets, scattering more light and becoming more reflective. These clouds appear brighter which blocks sunlight from reaching the Earth’s surface, shading the planet and producing net cooling. This is the “cloud albedo effect.” These clouds also last longer since smaller cloud droplets are less likely to fall out of the atmosphere as rain. Both effects increase the amount of radiation from the Sun that is reflected to space without reaching the surface.

According to NASA, aerosols and the clouds seeded by them reflect about 25% of the Sun’s radiation back to space. An increase of just 5% in cloud albedo could compensate for the entire increase in greenhouse gases from the modern industrial era.

(Source: NASA)

But there’s more to aerosols than just net cooling.

Some artificial aerosols that contain iron, nitrogen and phosphorus are fertilizers that settle on the planet and stimulate plant growth on land and phytoplankton in the ocean. Those plants and phytoplankton consume carbon dioxide which can help lessen the radiative forcing due to carbon dioxide.

According to the New York Times, a study at the University of Maryland indicates that increases in aerosols due to air pollution from human activities can affect cloud formation in a way that causes drier regions to experience even less rain and wetter regions to experience more rainfall, snow and severe weather. Clouds in dirty regions can be twice as thick compared to clouds in very clean regions, and the probability of heavy rain in those dirty regions is doubled.

It’s even worse when many aerosols are also making people sick with heart and lung disease, according to this NPR article. Some countries are now in the process of trying to clean up aerosols. Natalie Mahowald, a climate researcher at Cornell University, says:

As we clean up the aerosols, which we really want to do for public health reasons, we are going to be perhaps causing ourselves more trouble in terms of the climate situation.

Now I think this makes a good discussion question. Should we be cleaning up the aerosols for health reasons if it could cause ourselves more trouble in terms of the climate situation or should we just leave them alone?

We should be cleaning them up, in my opinion, but at a slow rate. Looking back at the IPCC figure at the beginning of this post, the radiative forcing from aerosols reduces the total net anthropogenic forcing by roughly 40%. Quickly cleaning up the aerosols created by human activities would result in the total net forcing increasing quickly. Since we are currently also trying to reduce the emission of carbon dioxide from human activities, the radiative forcing from carbon dioxide would slowly decrease which could counteract the increase in the total net forcing if we were to slowly reduce the amount of aerosols released into the atmosphere. Although the total net anthropogenic would barely change in the meantime, until we reached a point at which we could no longer reduce any more aerosols, the energy imbalance of the earth would not worsen. At the same time, we would have cleaner air in our atmosphere resulting in less diseases caused by air pollution, such as lung disease, and less deaths from such diseases. Drier regions would no longer experience the less-than-average rainfall due to increases in aerosols from human activities, which is important for agriculture, and wetter regions would experience less severe weather.

What are your thoughts?

Update (February 10, 2012):
I would like to thank Kate for bringing up the topic of geoengineering using aerosols in the comments (please check out her blog).

Geoengineering using aerosols is the process of deliberately releasing aerosols into the atmosphere to counteract the effects of global warming, as mentioned by Kate. But is this strategy a good idea to follow? In an article by Alan Robock, he gives his 20 reasons why geoengineering may be a bad idea. Here are some of his reasons that I think stand out:

  • Continued ocean acidification: If we start to use geoengineering as a solution to global warming without imposing any restrictions on continued carbon emissions, the ocean would continue to become more acidic which threatens marine life.
  • Ozone depletion: Aerosols in the stratosphere serve as surfaces for chemical reactions that destroy ozone.
  • Less sun for solar power: The reduction in incoming solar radiation due to aerosols reflecting them back to space would have a significant impact in the radiation available for solar power systems generating clean energy.
  • There’s no going back: We cannot remove the aerosols once we put them into the atmosphere.
  • Human error: Humans can make mistakes in the design, manufacturing, and operation of complex mechanical systems.
  • Unexpected consequences: Scientists cannot possibly account for all of the complex climate interactions or predict all of the impacts of geoengineering.

Since the goal of this strategy is to counteract the effects of global warming due to increasing carbon emissions and there are a lot of risks and consequences as a result of adopting this strategy, then this should not be the direction that we take, in my opinion. We need to find other solutions to global warming and clean up the aerosols created by human activities instead.

Join the discussion: