A retired U.S. Geological Survey scientist will speak Sunday on the role sulfur dioxide plays in causing global warming and what humans can do about it. The free lecture by Peter Ward starts at 7:30 p.m. in the theater at Center for the Arts.

Ward argues that sulfur dioxide emitted from volcanoes and from burning fossil fuel is the primary initiator of global climate change.

“Carbon dioxide is a greenhouse gas compounding global warming, but it is not the initiator of climate change,” Ward said in a prepared statement.

In a recently released paper, Ward says sulfur dioxide emissions regulate the ability of the atmosphere to clean itself by oxidizing pollutants. Ward observed that the highest rates of volcanic activity in the past 46,000 years occurred at the same times as the highest rates of global warming.

“When very large volcanic eruptions occur every few months, rapid warming follows,” Ward said. “Too much sulfur dioxide in a short period of time causes warming.”

Large eruptions today typically occur once per century. Yet by 1962, human activities were putting as much sulfur dioxide into the atmosphere every 1.7 years as one of these large eruptions. That was enough to cause world temperatures to climb at their highest rate in the 20th century, Ward said.

Global efforts to reduce acid rain succeeded in decreasing human sulfur dioxide emissions 18 percent by 2000. The rate of increase of methane in the atmosphere began declining in 1990.

By 2000, global temperature and the concentration of methane became nearly constant and have remained so. Current climate theories cannot explain these changes, Ward said.

“These observations make sense, when you realize that sulfur dioxide is changing the oxidizing capacity of the atmosphere,” Ward said. “A dirty atmosphere warms the earth, and a clean atmosphere cools the earth. The atmosphere cleans itself by oxidizing greenhouse gases and other pollutants, causing their molecules to become larger and therefore to fall out or be rained out of the atmosphere.”

Ward said the primary oxidants in the atmosphere are generated by the effects of sunlight on ozone, which is in short supply. Sulfur dioxide reacts swiftly with these oxidants.

“Too much sulfur dioxide uses them all up, causing methane and other pollutants to accumulate. By reducing acid rain, we accidentally reduced global warming,” Ward said. “The problem now is that sulfur dioxide emissions are rapidly increasing again as new power plants are coming on line every week, especially in developing countries. But we know how to reduce sulfur emissions both technically and politically. It is much easier to do than reducing carbon dioxide emissions.”