SO2 Decrease Not Responsible for Global Warming

One of the submissions to the Global Warming Skeptic Challenge was the claim that the Clean Air Act reduced SO2 emissions, leading to global warming. SO2 in the atmosphere is known to reflect sunlight and large emissions of this gas during volcanic eruptions has led to cooling episodes. The submitter, Mr. Burl Henry, posited the Clean Air Act was responsible for removing so much SO2 from the atmosphere that the result was an absence of global cooling. Thus, it was the absence of SO2, not the presence of CO2, that was the cause of the observed global warming.

I showed how Mr. Henry's major error was he only considered U.S. and European emissions in his claim without including emissions from other countries. When those other countries were included, you get periods where SO2 levels go down and the temperature goes up. But, you also get periods where SO2 levels go down and temperature also goes down. Further, there are periods where SO2 levels go down and the temperature can go up or down. In other words, there is no correlation in the overall picture between SO2 emissions over the long term and the temperature record.

However, this person was not satisfied. He not only resubmitted his claim (still wrong), but has continuously appeared with comments to the effect he still thinks his claim is correct. I think the graph below should dispel any thought at all about the correctness of his claim:

Source: Open Mind

This graph shows atmospheric SO2 levels for the last 2000 years. We can see the level sky-rocketed beginning in the mid-1800s and today is three to four times the historical average. Despite efforts to reign in emissions, the atmospheric level continues to be higher than at any other time in the last 2000 years.

Most interestingly, take a look at the time period from around AD 1300 to about 1850. This is referred to as the Little Ice Age, often cited by deniers as some kind of evidence global warming isn't real. SO2 levels during this period were a fraction of emissions today. If global warming, as Mr. Henry claimed, was caused because we are removing the cooling effect of SO2 concentrations in the atmosphere, then it stands to reason we would be significantly cooler today than during the Little Ice Age. The cooling effects of SO2 in the atmosphere today would be so much greater that we would be significantly cooler. And, that means we would be cooler than a period so cold it is known as the Little Ice Age. According to his claim, we would be in the depths of a major ice age by now. So, why aren't we?

The answer to that question is that SO2 emissions do, in fact, cause cooling of the atmosphere by reflecting sunlight, but this effect is small in comparison to other factors, especially the effect of atmospheric CO2 concentrations. It is thought the sharp rise of atmospheric SO2 emissions is at least partly responsible for the drop in global temperatures experienced from the 1940s to the mid-1970s. But, we can see the level of SO2 concentration in the air has not dropped to anything resembling historic levels. Any change in the amount of atmospheric SO2 experienced due to the Clean Air Act is a very small percentage of the total concentration (less than 5% of the total, according to the graph above). And, SO2 concentration levels have been increasing since the year 2000 and we have had 14 of the 15 hottest years ever recorded during that time span. Assuming global warming is due to the removal of SO2 from the atmosphere, we have to ask how is it possible to set new global warming records every year, if the cooling effect of SO2 is increasing during that same time period?

Using the scientific method, we used the hypothesis (global warming is due to falling SO2 levels) to make a prediction (lower SO2 levels will lead to higher temperatures) and put it to the test and we can see the hypothesis failed the test (historically high SO2 levels correspond to historically high temperatures).

If Mr. Henry wants to say atmospheric SO2 levels result in a cooling effect, he is correct. If he wants to say the observed global warming is due to removing SO2 from the atmosphere, he is incorrect. We can see from the graph we have not really removed any significant amounts of SO2 from the atmosphere on a historical basis.

I know this will not satisfy Mr. Henry, but it at least give me a link I can point to every time he repeats his claim.