1. Very frequent large explosive volcanic eruptions: a 1991-Pinatubo-sized eruption (17 Megatons (Mt) SO2) every year instead of the historical average of once per century
2. A single large effusive basaltic eruption such as Laki in Iceland in 1783 that erupted 122 Mt SO2 in 5 months
3. Continued and increasing emissions by humans burning fossil fuels that peaked in 1979 at 150 Mt SO2 per year

SO2 is a triatomic, non-linear, asymmetric-top molecule with a permanent dipole moment very similar to ozone (O3) but very different from CO2, a linear molecule without a dipole moment. SO2 has an absorption intensity two orders of magnitude greater than ozone in the near ultraviolet spectrum (350-400 nanometers) where the atmosphere is normally transparent to solar energy. Photons at these wavelengths contain 43 times more energy than infrared photons absorbed most significantly by CO2 . These high energy photons cause electronic transitions that are much more effective at increasing the kinetic energy and thus the temperature of the atmosphere.

The heating potential of solar-energy absorbing gases such as SO2 and O3 in concentrations of tens of parts per billion is well observed in several ways:

1. Ozone absorbs enough solar energy to heat and form the stratosphere.
2. SO2 in the stratosphere forms aerosols that have major effects on the atmosphere by
   reflecting sunlight, cooling the earth ~0.5^oC for ~3 years, and absorbing sunlight, raising the temperature of the lower stratosphere ~3^oC for more than a year
3. SO2 in the troposphere appears to cause greater warming than four orders of magnitude greater concentrations of CO2 .

Between 1979 and 2000, humans decreased SO2 emissions 18% in an effort to reduce acid rain. The rate of increase in global temperatures and concentrations of methane decreased to zero by 1998. Temperatures have been relatively constant for 12 years while concentrations of CO2 have continued to rise at a constant rate. Clearly global mean surface temperatures are not a direct function of CO2 concentrations as is assumed in most atmospheric models.

As SO2 concentrations decreased, hydroxyl radical (OH) concentrations increased, oxidizing methane, causing methane concentrations to stop increasing. This implies that the increase in methane concentrations during the 20th century may have been caused by increasing amounts of SO2 emissions decreasing the oxidizing capacity of the atmosphere rather than increased emissions of methane.

During the 20th century, humans burning fossil fuels warmed the lower atmosphere and therefore the ocean ~0.8^oC, resetting the thermostat for the earth. There is no proven way to cool the ocean similar amounts within decades. Continued melting of ice and snow, most notably in the Arctic is the result of the ocean and atmosphere slowly approaching equilibrium after this sudden change in the earth's thermostat.

Solar intensity decreased (global dimming) in response to increasing SO2 and decreased (global brightening) in response to decreasing SO2. SO2 absorption is greater in polar regions due to increased length of the paths of photons within the atmosphere. This fact helps explain why temperature increases have been so much greater, especially in the Arctic where Arctic haze increased with increasing SO2 emissions.

Increased SO2 emissions primarily in China and India threaten to raise the thermostat even more, but the good news is that we know how to limit SO2 emissions at a reasonable cost.

Only when the effects of solar-energy absorbing gases such as SO2 are built into climate models, will it be possible to determine the effects of the prodigious anthropogenic emissions of CO2 in the past or in the future.

Related Documents

Understanding Volcanoes May Be the Key to Controlling Global Warming Click for PDF 
A paper based on a Plenary talk April 18, 2010 to be published in the SVC Bulletin, Summer, 2010