April 22, 2005

Change of Direction: Do SO2 Emissions Lead to Warming?

Filed under: Climate Forcings

Many scientists believe that sulfur dioxide emissions, either from un-scrubbed power plants or from large-scale agricultural burning, serve to cool the planet’s surface temperature.

The cooling mechanism is fairly straightforward. Sulfur dioxide is transformed in the atmosphere into sulfate aerosol, a fine particle that reflects away the sun’s radiation. The particles also serve as the condensation nuclei for cloud droplets which also reflect away the sun’s energy.

On the other hand, no one really knows the magnitude of these cooling effects (if any). So we have argued that sulfate cooling is simply a fudge factor put into climate models in order to chill the overly-hot projections they make if left to their own devices.

Now comes evidence that sulfur dioxide actually can enhance global warming.

While this doesn’t mean that sulfates aren’t also cooling things by reflecting away radiation, the parent, sulfur dioxide, can do some other things that make the surface warmer. According to research just published in Geophysical Research Letters by J. Notholt and his co-authors, sulfur dioxide is converted to sulfuric acid (remember “acid rain”?), which leads to more ice crystals in the upper atmosphere. Some of these are eventually lifted upwards into the stable stratosphere where they increase the amount of water vapor found there.

Water vapor in the stratosphere serves as a greenhouse gas and is involved in the destruction of ozone, resulting in a stratospheric cooling and a warming of the lower atmosphere and surface.

And, for once, it’s not from the USA. We’re usually blamed for the lion’s share of warming as a result of our carbon dioxide emissions. But the sulfur dioxide is largely from elsewhere. The authors write:

While anthropogenic SO2 emissions in Europe and North America have been decreasing since around 1980, the anthropogenic SO2 emissions from China, Asia and the tropics have been increasing…For example, van Aardenne et al (2001) report a factor of 12 increase for China and 8 for East Asia, respectively between 1950 and 1990.

The authors propose that their mechanism has been responsible for about one-quarter of the increases in stratospheric water vapor during the period 1950 to 2000. According to a NASA model published by Drew Shindell in 2001, this would account for about 5% of the observed warming.

While that seems small, it is a sign about how little we really know (or have known) about the climatic disposition of sulfur dioxide. Every increment of warming that it causes takes away from its putative cooling. Which means, ironically, that it can serve less and less as an explanation as to why we have only witnessed a very modest global warming to date.

Obviously, this points to something being very wrong. We have been mentioning this for years, and we’re going to mention it again:

With so many non-carbon dioxide factors apparently causing warming (soot, methane, sulfur dioxide…), why isn’t it warmer than heck?

There are two options: Either warming is being countered by a tremendous sulfate cooling (which should be obvious in, say, China (which is warming by the way)) or the warming effect of carbon dioxide itself is overstated. We’ll bet on the latter, but it is going to take decades for science to admit to this error after all the fear and bad policies that is has caused.


Notholt, J., et al., 2005. Influence of tropospheric SO2 emissions on particle formation and the stratospheric humidity. Geophysical Research Letters, 32, L07810, doi:10.1029/2004GL022159.

Shindell, D.T., 2001. Climate and ozone response to increased stratospheric water vapor. Geophysical Research Letters, 28, 1551-1554.

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