Even drastic cutbacks in the release of the chemicals blamed for causing the global "greenhouse effect" are unlikely to halt the heating of the planet, according to a new climate analysis.

A computer model of the Earth's climate was used to calculate future climate changes under three different scenarios: continued rapid release of the chemicals at current levels; release at slower rates; and a drastic cutback in those chemicals.In the first case, the Earth's average temperature was calculated to rise by 1.6 degrees Fahrenheit within 20 years.

In the second model, it would take 25 years for that same increase to occur.

But even with a drastic "and probably unrealistic" cutback in chemical releases, the temperature would rise that much eventually, and probably would climb by 0.8 degrees within 15 years, according to the study published in the Aug. 20 issue of the Journal of Geophysical Research-Atmospheres.

The greenhouse effect has been widely discussed recently, although most meteorologists contend that it is too early to determine if it is responsible for the current drought and hot weather affecting much of the nation.

The effect occurs when carbon dioxide, largely from burning fossil fuels, and other gases increase in the atmosphere. These gases let incoming heat from the sun reach the Earth but they reflect outgoing heat, keeping it from traveling back into space. That traps heat, much like a greenhouse traps heat indoors.

James Hansen of the National Aeronautics and Space Administration, the lead author of the new study, told a congressional subcommittee earlier in the summer that the current hot, dry conditions are an example of things to come.

In particular, he noted in the new report, while the average increase in global warming doesn't sound like a lot, it means increasing frequency for hot summers. And some areas will be affected more than others.

If the summer outlook were a die with six faces, Hansen wrote, the period between 1950 and 1979 would have had two faces representing hot years, two for normal years and two representing cold years.

By the 1990s, he anticipates, the die will be "loaded," with three or four faces coming up hot.

That means the chances of any particular summer being hotter than normal would top 50 percent, compared with the 33 percent chance in the past.

Such a change would be large enough to affect the quality of life, Hansen and his co-authors reported.

For example, he noted that over the past 30 years, Omaha, Neb., experienced a run of five or more consecutive days with a high temperature of 95 degrees or more on an average of three times every 10 years. The climate model predicts this would increase to five times every 10 years in the 1990s and seven times in 10 years by 2020.