For years, scientists have been warning about the "greenhouse effect" - a gradual rise in the world's temperature because of pollutants gathering in the atmosphere. Those warnings, for the most part, have been shrugged off as something remote or unreal.

But as the nation wilts under record-setting heat and drought this summer, and the Mississippi River sinks to its lowest level on record, the warnings suddenly are taking on some urgency.There is no specific evidence that the current dry weather and heat wave drought can be blamed on the greenhouse effect. It may be a normal fluctuation in world climate.

Yet four of the past eight years have seen global records for high temperatures and 1988 is sure to break the average as well. The trend matches - at least in some ways - computer projections of the greenhouse effect. Some scientists are confident it is to blame.

Whether or not the current weather is due to the greenhouse effect, it should serve as a warning of what may be coming in the next three or four decades. Drought could become common. Eventually, the warmer weather would begin to melt the polar ice caps, raising the level of the seas, flooding coastal cities.

What can be done? What is easy, how is harder. The pollution that is trapping the heat in the Earth's atmosphere must be reduced. Scientists say that cutting world-wide pollution by three billion tons - 50 to 60 percent of current levels - would stabilize the atmosphere.

This would require much less burning of coal, a major source of electric power. Yet as supplies of oil and natural gas dwindle, the demand for coal will be greater, not less.

One answer would be more nuclear power, but that industry is dying, ironically enough, because of protests by environmentalists. Another problem is the destruction of tropical forests. Such forests, which absorb carbon dioxide and clean the atmosphere, are disappearing at the rate of more than 25 million acres a year.

The present hot, dry weather may dissipate in future years, but somewhere along the line, the greenhouse effect will change life for everyone if something isn't done to head it off.

Scientists, engineers, and political leaders should start taking steps now deal with the problem. The longer we wait, the more serious the consequences - and the harder to handle.