Scientists studying the "greenhouse effect" causing global warming may be underestimating the warming effect of many other processes such as the release of methane gas from arctic tundra, a government analyst said.

Daniel Lashof, a scientist in the Environmental Protection Agency's office of policy analysis, reported on studies of effects neglected in most computerized climate models at a conference Tuesday organized by the Climate Institute.The Institute, a non-profit body supported by government, foundation and corporate grants, tries to call attention to issues affecting the climate such as the greenhouse effect, or the expected warming of the Earth caused by the accumulation in the atmosphere of gases that trap heat that normally would escape into space.

The most important of these gases is carbon dioxide formed in the burning of fossil fuels.

Such a warming could cause sea levels to rise and flood coastal areas and lead to stronger hurricanes, drastic shifts in rainfall, the northward migration of agricultural zones and more frequent droughts.

A widely used prediction is for an eventual warming of between 2.7 degrees Fahrenheit and 8.1 degrees sometime in the middle of the next century.

Lashof used a computer model that yielded a prediction of 2.7 degrees to 9.9 degrees before he incorporated additional "biogeochemical" processes that increased warming might stimulate.

The augmented model yielded a central prediction for 17.5-degree warming, 11.2 degrees more than the original model. The range was from 4 degrees to what Lashof called "unknown territory" where he declined to report a prediction.

Three "quite speculative" processes accounted for 85 percent of the 11.2-degree increase in Lashof's central prediction:

-The release of large quantities of methane, also a heat-trapping gas, from warming arctic tundra.

-Decreased reflection of sunshine back into space from changes in the vegetative cover of the Earth.

-A sharp decline in the oceans' ability to absorb carbon dioxide.