Since geoengineering has gained a lot of attention from the media recently, it is interesting to look at how aerosols from industry have influenced climate. Strange variations in temperature were experienced in the US from the years 1970-1990 where parts of the country, such as Arkansas and Missouri, were nearly 1C cooler than immediately surrounding regions and 0.7C cooler in parts of the Midwest and Mid-Atlantic.
At first this was thought to have occurred due to natural weather patterns, but recent research has shown that this ‘warming hole’ effect over parts of the US was actually due to the presence of sulphate aerosols in the atmosphere that originated mostly from coal-fired power plants.
Ironically, one of the greatest sources of greenhouse gas emissions also spews out sulphur dioxide (SO2) that is a precursor to sulphate aerosols that reflect solar radiation back out into space.
Researchers found that heightened levels of SO2 and NOx (nitric oxide and nitrogen dioxide) were followed by less warmer temps in the US. SO2 is also well-known for causing acid rainfall. After the Clean Air Act was passed by US congress in 1970, sulphate aerosols were significantly reduced. The amount of SO2 released into the atmosphere in the US decreased by nearly 60 percent between 1980-2010 leading to rises in temperature in parts of the country due to climate change.
This puts geoengineering into perspective in that it shows how humans have already been engineering local climatic conditions, but were unaware at first. There are other large sources of SO2 such as volcanoes, which have been known to affect local climate temporarily. Spraying SO2 into the Earth’s atmosphere intentionally by comparison is also temporary and doesn’t appear to be even remotely close to a lasting solution for alleviating the effects of climate change, although this has been researched and could inadvertently lead to ozone depletion increasing the Earth’s exposure to UV radiation, if it were used.
In these satellite images from NASA you can see the concentrations of cooler temperatures. Dark to light blue indicates cooler temperatures and red – warmer.
These bar charts published in Atmospheric Chemistry and Physics show the rise and fall of the presence of different aerosols that cause radiative forcing, which changes the levels of incoming and outgoing energy in the Earth’s atmosphere. Notice how much sulphate aerosols have reduced since 2000. Other influences on climate in the chart include black carbon (e.g. soot) and organic carbon.
You would think that if the US has created warming holes certainly China, India or other countries undergoing rapid development who are building many dirty, coal-fired power plants must be experiencing something similar. And indeed they have: From 2000 to 2006, SO2 emissions in China increased by 53%. As long as China and other countries continue to rapidly expand their energy infrastructure for burning fossil fuels, it seems likely that this form of climate modification will continue with strange consequences that may be interpreted as beneficial in the short-term, but the generation of sulphate aerosols is merely delaying the inevitable effects of climate change in some parts of the world.
References and Further Reading
Warming Hole Over the Eastern US Due to Air Pollution. NASA Earth Observatory
Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 1: Aerosol trends and radiative forcing. Atmospheric Chemistry and Physics (free paper)
Sulfur dioxide emissions in China and sulfur trends in East Asia since 2000. Atmospheric Chemistry and Physics (free paper)