Study: Growth of 'greenhouse emissions' slowing

Might reduce 'specter of imminent disaster'

NEW YORK (CNN) -- The growth of so-called "greenhouse gas emissions' in the atmosphere continues, but a new report funded by NASA says the growth rate peaked in 1980, and has slowed ever since.

The outlawing of chlorofluorocarbon (CFC) chemicals, and a decline in the growth rate of methane emissions in the atmosphere are primarily responsible for the slower increase, according to the study.

CFCs and methane are secondary contributors to the greenhouse effect -- theorized by most climate scientists as being responsible for at least part of a noticeable increase in global temperatures in recent decades. The main contributor to the greenhouse effect is carbon dioxide (CO2), which is emitted in the burning of fossil fuels and other sources. The growth rate of CO2 emissions has not slowed, the report says.

The Intergovernmental Panel on Climate Change (IPCC), a group of 2,500 climate scientists, has predicted that if the warming continues, climate change could bring dramatic changes in weather patterns and storm frequency in this century.

CFCs are primarily known for a different environmental threat, the thinning of the Earth's protective ozone layer. But they also contribute to the greenhouse effect, combining with atmospheric CO2, methane, soot and other substances to trap heat near the Earth's surface -- theoretically warming the air and oceans.

While some CFCs are still in illicit use, they largely disappeared after the enactment of the Montreal Protocol, a 1980s treaty that banned the manufacture and sale of the chemical.

Methane gas is produced when bacteria break down plant and animal matter. Garbage dumps, farms and forests all produce methane. The NASA study says it's not clear why the growth rate of methane emissions has declined.

James Hansen: Faint hope

The study was co-authored by Dr. Makiko Sato of Columbia University and NASA's Dr. James Hansen. In 1988, Hansen's dramatic testimony before a United States Senate Committee was credited with sounding the first widespread alarm on potential global warming.

But he says he sees some faint hope in his current report.

Hansen warns that continuing increases in carbon dioxide emissions and in soot -- small particles of unburned matter that are a byproduct of coal or diesel fuel burning -- continue to pose a major threat to the climate.

But, he adds, the current trends of slower growth of emissions of some greenhouse gases could greatly reduce what he calls the "specter of imminent disaster."

The IPCC has estimated that uncontrolled global warming could raise sea levels by up to three feet in the next century, and raise temperatures by up to 10 degrees Fahrenheit. Hansen says the removal of CFCs from the atmosphere, and possible future reductions in methane levels, could cut those risks in half.