Population growth may be a significant factor contributing to depletion of Earth's protective ozone layer, two atmospheric chemists have found.
Reporting in today's issue of Science, the researchers trace a complex linkage between the steadily growing amount of methane released into the air and destruction of ozone in the stratosphere. Many experts tie the amount of methane increase to population growth.
The linkage includes chlorofluorocarbons (CFC), the industrial chemicals used as refrigerants and aerosol propellants that break down ozone molecules. It also includes chemical and physical processes that contribute to the "greenhouse effect" that may be causing Earth's climate to warm slowly.
"It's a very complex network of chemicals and phenomena," said Donald R. Blake, one of the scientists. "We're getting a handle on parts of it, and we can make some guesses. But the exact details and the sizes of the effects are still hard to predict."
Blake and his collaborator, F. Sherwood Rowland, one of the first to warn of CFC effects on ozone, are atmospheric chemists at the University of California at Irvine. Ozone, an irritant when in contact with the body, is useful in the upper atmosphere because it filters much of the sun's harmful ultraviolet radiation.
The study began with measurements of methane in the atmosphere. The main component of natural gas, methane is produced chiefly by bacterial decomposition of organic materials. Most of it comes from the bottoms of flooded rice paddies, swamps and the guts of various plant-eaters, from cattle to termites.
Blake and Rowland found that the concentration of methane in the atmosphere has been rising steadily, growing 11 percent since 1978, when their measurements began. Other studies, dating to the 1950s, agree with the rate. Analysis of air trapped in bubbles in ice that froze 200 to 3,000 years ago suggests that methane levels changed little until modern times.
The recent rate of increase is believed tied to population growth. As Asia's population grows, for example, more rice is being grown. For the same reason, the number of cattle in Africa and in North and South America is growing.
In most of the world, forest clearing is making new grassland habitat available to soil-dwelling termites. Digestion of plant matter in their guts, carried out by bacteria, accounts for as much as 20 percent of world methane production.
The link to ozone arises when methane reaches the stratosphere and reacts with molecules called hydroxyls to form water vapor. About half of the water vapor in the stratosphere comes from this process. The stratosphere is well above the air layers in which rain clouds form.
The methane-derived vapor exists in the stratosphere all over Earth, but only during the six-month Antarctic night does the stratosphere become cold enough (135 degrees below zero Fahrenheit) for the vapor to freeze into clouds of ice crystals. The crystalline surfaces greatly speed the chemical reaction in which CFC breaks down ozone.
Ironically, methane alone breaks down CFC, preventing it from destroying ozone. Thus, Blake and Rowland said they think that, outside the Antarctic region, methane is actually retarding ozone destruction.
They noted, however, that the net effect is global ozone loss. This occurs because, over the years, most of the world's air travels over Antarctica, loses ozone there and then moves on to dilute the ozone concentration elsewhere.
Blake and Rowland said the methane buildup also is contributing to the greenhouse effect, which results when the atmosphere lets sunlight reach Earth but blocks the resulting heat from radiating into space.
A molecule of methane is about 20 times more effective in causing climatic warming than a molecule of carbon dioxide, the better-known greenhouse gas produced chiefly through burning fossil fuels. Water vapor and ozone also are greenhouse gases.