COOLING AEROSOLS

COOL AEROSOLS

Aerosols have been mentioned several times in the blog discussion as a way to cool the atmosphere. In fact, volcanic eruptions have provided some of the best atmosphere "experiments" that we have. After Pinatubo erupted in 1991, there were model studies that predicted the surface cooling, and over the next year or two these predictions were largely validated. Since then there have been a number of studies that have revisited the Pinatubo eruption and have helped improve the representation of physics in climate models.
( Here's a summary of some work at NASA's Goddard Institute of Space Studies.) The mechanism of cooling is related to sulfate aerosols which form after the eruption. If these aerosols are injected into the stratosphere, then they can stay there for a long time, more than a year.

Aerosols are small particles in the atmosphere. Colloquially, we usually talk about clouds and aerosols as different from each other, but they are both particles. Aerosols are composed of many different substances, and their composition is important to what they do in the atmosphere. The sulfate aerosols that form after volcanic eruptions, and from industrial emissions, reflect solar energy as well as absorb solar energy in the atmosphere. Both of these processes keep energy away from the Earth's surface. Hence, sulfate aerosols are a source of global cooling.

The figure below shows, simplistically, what aerosols might do in the atmosphere.




Figure 1: The role of aerosols in the energy balance of the Earth.


The aerosol layer reflects solar energy back to space, which contributes to cooling. In addition the aerosol might absorb solar radiation, warming the atmosphere. Both of these processes help to cool the surface. So this is a good way to think about the sulfate aerosols from volcanoes. Other types of aerosols include soil dust from deserts, and sea salt, and soot. Some of these influence clouds-- especially the formation and size of cloud droplets. It is also possible that aerosols can absorb and re-emit the Earth's infrared radiation. This would be a warming effect, and it is sometimes important. So, like clouds, whether it is day or night, and whether the aerosols are high or low in the atmosphere impacts how effectively they add to cooling or warming.

Aerosols are included in climate models. Compared with the greenhouse gases they are short lived - a few days in the troposphere. Because of their short lifetime and their localized sources, they are not evenly distributed in the atmosphere. Often aerosols are associated with polluted air. Two regions of particular interest are South Asia and China. This figure, from NASA's Earth Observatory , shows the aerosols in the atmosphere over South Asia.




Figure 2: Aerosols in the atmosphere over South Asia.


You can see the patchiness of the aerosol distribution. If you follow this link there are figures that show the atmospheric warming, the change in albedo (the reflection of solar radiation), and the surface cooling. Again, this provides the spirit of a laboratory experiment which helps use to validate the climate model.

Okay, we have walked through reflection and absorption. We've touched most of the important processes that are involved in the maintenance of the global energy balance. This also sets the foundation for ideas about geo-engineering ... though not directly linked to those tunnels.

ricky

In case you want to look back at them, here are links to my last few figures: Sun-Earth System, Absorption, Reflection, Cloud feedback.