Hello everyone! Thanks for taking the time to read my first research post. If you follow Nick on Facebook, you may have seen my post about climate change in our forecast area. I explained how over the past 50-100 years, temperatures have been increasing, except for one area that includes Louisiana, Mississippi, and Alabama, where temperatures have stayed the same or even slightly cooled over the past 50-100 years. One possible reason to explain this phenomena is due to sulfate pollution in the air, so I thought I would give some more details on how this phenomena works and why MS/AL/LA’s climate history is so unique. I really enjoyed doing this research, so let’s take a deeper dive into how sulfates have been keeping our area cool for the past 100 years!
100 years in the past – MS/AL/LA’s unique climate history
Our region is very special when it comes to climate, as you can see in the image above, our forecast area has either stayed the same or cooled temperature-wise, where everywhere else has warmed to some degree. This area in the Southeast is so unique it is known to many climatologists as the United States “warming hole”.
There are a few different theories for why we have this warming hole over the Southeast. These theories include: land use changes, natural climate variability, hydrologic changes due to the Pacific and North Atlantic Oscillations, and even aerosols. (Climate Program Office).
More recently, many meteorologists and climatologists have thought of certain air pollutants, otherwise known as aerosols, as one possible reason why this area’s temperature had been unchanged for so long. One of these particular air pollutants are sulfate pollutants, created from burning things like coal and crude oil, often in industrial settings. Historically, our region has used coal to fire power plants.
How sulfates keep the air cool
Sulfates are particles formed from sulfur oxide gases, which are formed from sulfur and oxygen. One of the most common sulfur oxide glasses is sulfur dioxide (SO2). Sulfur is found in many different types of fuel, like coal and crude oil. When these fuel sources are utilized in different types of industry, they are burned and the sulfur in the fuel turns into sulfur gas and is released into the air. These sulfur gases react with water vapor in the humid air and turn into sulfates. (MDEQ). Sulfates are unique; unlike greenhouse gases, such as carbon dioxide, sulfate air pollutants reflect sunlight, in turn cooling the surrounding air.
The influence of sulfates on our historical climate
To find out how exactly sulfates and other aerosols have kept our climate unchanged in the past few decades, I looked at a couple different research studies and found some interesting details:
One study used a general circulation model from NASA to model how anthropogenic (caused by humans) aerosols affected/could affect the climate from 1950-2050. These models help atmospheric scientists see what the climate might look like in the past or future using many different parameters. It was discovered that aerosol concentrations peaked in the United States from 1970 to 1990, and during this time average temperature changes cooled from 0.5-1.0 degrees Celsius in our region. The researchers also found that the temperature changes during this 20 year period had the strongest effect on daytime maximum temperatures during the summer and fall months. (Leibensperger et al). After 1990, they found that aerosol concentrations decreased rapidly.
Another study found that from 1950-1975 anthropogenic aerosols in combination with internal variability patterns such as the NAO (North Atlantic Oscillation) and PDO (Pacific Decadal Oscillation) helped to cool areas of the United States during the summer, most significantly in the South. (Mascioli et al).
Both studies found that this kind of significant summertime cooling in the future is unlikely, due to the decrease in aerosol emissions in recent years.
Why we don’t use sulfates anymore:
It’s important to remember that sulfates are still air pollutants, and have negative effects on human health and the environment. For one, sulfur dioxide can be harmful to the human respiratory system, especially for those in vulnerable groups such as children, the elderly, and those with other respiratory issues, like asthma (EPA). In the environment, sulfur oxides can negatively impact tree growth and foliage, and can even play a part in acid rain, which in turn also negatively affects the environment (EPA).
Because of the dangerous effects of sulfate aerosols, the Clean Air Act was passed in 1970. After this act was passed sulfate emissions began to decrease, and today there are very strict regulations regarding sulfate emissions today. (EPA).
Warming up – the future of MS/AL/LA’s climate
Although over the past 100 years our region’s temperatures have stayed the same or cooled, we are now beginning to warm up – and rapidly. According to the climate modeling done in the study led by Leibensperger, the area encompassing the U.S. “warming hole” has warmed by 0.3 degrees Celsius over the time period spanning from 1980-2010. So, the researchers believe that sulfate emissions won’t have a very large effect on future temperature changes in the region, since there haven’t been much sulfate emissions in past years since regulations have been put in place. (Leibensperger et al).
On the other hand, because of all the negative effects of sulfate emissions, other energy sources are becoming more popular, but emit more greenhouse gases. Common greenhouse gases include chemical compounds such as carbon dioxide, water vapor, and methane. Greenhouse gases work in the opposite way of sulfates. Instead of reflecting the sun’s radiation, greenhouse gases absorb it, warming the surrounding air. These greenhouse gases are very good at warming up the air, and are being emitted at high rates in recent years, which is causing the rapid warming that we are seeing.
This means that in recent years, states that have stayed the same or cooled temperature-wise, such as Mississippi, Louisiana, and Alabama are beginning to warm.
Concluding thoughts:
So while the theory that man-made aerosols like sulfates helped keep the climate cool is a newer theory for climatologists studying the U.S. warming hole, it is likely that it may have had a real effect. Learning more about how sulfate emissions work and analyzing some of the formal research that has been done about the subject definitely supports this theory. Based on my research, I believe that aerosols have played a part in shaping our climate historically, but in combination with some of the other theories that have been proposed and researched. There is definitely a lot more research to be done on this topic, and I have really enjoyed learning about this regions unique weather and climate!
Once again, thank you for taking the time to read my research, and I hope you have learned something new! Please see all references below:
References
2021 Air Quality Data Summary, Mississippi Department of Environmental Quality, www.mdeq.ms.gov/wp-content/uploads/2022/03/2021-Air-Quality-Data-Summary.pdf. Accessed 13 July 2025.
Leibensperger, E. M., Mickley, L. J., Jacob, D. J., Chen, W.-T., Seinfeld, J. H., Nenes, A., Adams, P. J., Streets, D. G., Kumar, N., and Rind, D.: Climatic effects of 1950–2050 changes in US anthropogenic aerosols – Part 2: Climate response, Atmos. Chem. Phys., 12, 3349–3362, https://doi.org/10.5194/acp-12-3349-2012, 2012.
Mascioli, Nora R et al. “Timing and seasonality of the United States ‘warming hole’” vol. 12, no. 3, 2017, https://doi.org/10.1088/1748-9326/aa5ef4
“Sulfur Dioxide Basics.” EPA, Environmental Protection Agency, 10 Jan. 2025, www.epa.gov/so2-pollution/sulfur-dioxide-basics.
“What Climate Change Means for Mississippi.” EPA, Aug. 2016, 19january2017snapshot.epa.gov/sites/production/files/2016-09/documents/climate-change-ms.pdf.
“Why Has the Summertime Central U.S. Warming Hole Not Disappeared? – Climate Program Office.” Climate Program Office, NOAA, 13 Oct. 2013, cpo.noaa.gov/why-has-the-summertime-central-u-s-warming-hole-not-disappeared/.
“‘Warming Hole’ over the Eastern U.S. Due to Air Pollution.” NASA, NASA, 18 May 2012, earthobservatory.nasa.gov/images/77966/warming-hole-over-the-eastern-us-due-to-air-pollution.
