Aerosols are particles between 0.01 and 10μm that are suspended in the atmosphere, they can be of a solid or liquid form. The largest source of aerosols is the sea, for instance, as water evaporates salt is left in the atmosphere. Other sources include emissions from industry, soot and smog are well known aerosols which are noted under the global dimming topic that I will later return to. Similarly, volcanic ash can be considered an aerosol, or any other sort of ash for that matter, such as biomass burning. Dust similarly can become suspended in the stratosphere that may blow in from Earth’s more arid regions. (Geerts, 2000)
Figure 1
To return to the IPPC radiative forcing diagram (Figure 1), you can see that the they have the largest uncertainty of all factors as denoted by the error bounds. You can also see that they are split in to two different groups “direct effect” and “cloud albedo effect”. The direct effect of aerosols is the scattering of light away from the earth, decreasing the amount that we receive. (Climate Science Report, 2009)
Figure 2 – Aerosols in the atmosphere cross-referenced with the temperature deviation (NASA)
A good example of this is the eruption of Mt Pinatubo in July of 1991, which released 15-30 million tons of sulphur dioxide, this sulphur dioxide mixes with water vapour in the atmosphere to create aerosols of sulphuric acid. The USGS claim that this, on average, decreased temperatures worldwide by 0.5 C until 1993 (TinyScience, 2010) Mt. Tambora of India in 1815 was the largest eruption in 18,000 years. 1816 was known as “the year with no summer”, lakes were freezing in Pennsylvania in July, the ash stayed in the atmosphere for a very long time, scattering the sunlight. (McLeod, 2010)
The indirect “cloud albdeo effect” is lesser so understood. Aerosols act as nuclei, which are required in cloud formation, around these nuclei water vapour condenses forming a cloud. They are effectively the seed to cloud formation. This is part of a natural process, for aerosols are present in the atmosphere without the intervention of man, these aerosols are usually sea salt or sulphate based. A demonstration of this effect can be seen with ship paths, they create ocean spray which acts as cloud nuclei, from above these appear to be very similar to airplane contrails. Contrails themselves are a form of aerosol. (Voiland – NASA, 2010) Conversely to most peoples expectations, pollution usually causes clouds to be brighter , for the pollutant aerosols are generally water soluble, meaning they form smaller droplets, which in turn make said brighter cloud. Brighter clouds have a higher albedo, which means that they reflect more light. Clouds typically reflect 60% of light, aerosols are predicted to be able to increase this by 5% – which could counteract the projected effects of global warming, but similarly a decrease in cloud cover could worsen the situation. (Voiland)
Figure 3 – Present Day Global Aerosol Distribution (NASA)
Aerosols are also not evenly distributed throughout the planet. There is a significant correlation in Asia, fastly growing, industrialising cities such as Beijing are ripe with smog. India has its issues compounded by dust which blows in from the Thar desert. Africa also produces high levels of aerosols thanks to natural fires, an interesting dynamic that would at least counteract some warming for them if the planet were to warm. The high levels in the Amazon basin can be explained by the “slash-and-burn” techniques for land clearance in the area, which also allow winds to whip up to take the soot and dust in to the atmosphere. (NASA)
So far I have discussed that aerosols have a negative radiative effect – meaning that they cool things down. Given that my project is about “the potential alternative causes of global warming” this may seem slightly odd. However if you consider the global environmental movement to cut climate change, the decrease in fossil fuels, the UK Clean Air Act of 1965 and the various worldwide emissions targets under the Kyoto Protocol (for example the US decreasing emissions by 7% by 2012 (Kyoto Protocol, 2007) ) you can clearly see that aerosol emissions are set to fall. Attempts to tackle climate change may simply be compounding it, the level of uncertainty is such that we simply do not know. NASA states that “a long term decrease in cloud cover could have devastating impacts”, furthermore less than 1/3 of the Intergovernmental Panel on Climate Change (IPCC) models include the effects of non sulphate aerosols. (NASA) This seems a terrible oversight, for as we have seen, aerosols have a great potential to influence the climate.