JULY WAS DETESTABLY hot, with the temperature in the 90s more days than in any month as far back as the records go. August, so far, is no great improvement. It's not simply that Washington is having a warm summer. The climate worldwide is changing.

The planet's temperature has swung up and down many times over the centuries. Previously the reasons were natural, chiefly the changes in the sun's radiation. But the warming trend that has now been running for the past century coincides with the maturing of the industrial revolution and the accumulation of carbon dioxide in the atmosphere. Norman J. Rosenberg of Resources for the Future, a research organization here in Washington, points out that the warmest three years on record have all occurred in the 1980s. The global mean temperature is now half a degree Celsius higher than in the 1880s -- quite enough to produce a perceptible change in climate -- and there is some reason to think that the trend is accelerating.

Carbon dioxide traps the heat that the Earth absorbs from the sun. That's the way a greenhouse works -- or, to take a more unpleasantly familiar example, what happens to your car when it's been left in the sun with the windows closed. The car's glass is transparent to energy at the high frequencies at which the sun delivers it. But the glass is not transparent to the lower frequencies at which it is re-radiated by the warm interior of the car. It just keeps reflecting the heat back into the car. (That's also why the trunk of the car, with no windows, is not as hot as the passenger compartment.) The Earth's atmosphere acts in that respect like glass. As the carbon dioxide content goes higher, the atmosphere becomes less transparent to the re-radiation that the warm planet would otherwise throw off into space.

There are other gases that also tend to interfere with the normal loss of heat through the atmosphere -- chlorofluorocarbons, ozone, methane and carbon tetrachloride, among others. But at present the most important and intractable is carbon dioxide, for it is produced by the combustion of fuel.

Previous studies of this subject have usually either counseled wait-and-see or declared that nothing can be done. In response, Irving M. Mintzer of the World Resources Institute makes three points. First, waiting is dangerous because climatic change is, for all practical purposes, irreversible. Second, some further heating is inevitable. Third, the extent of the heating depends very much on the way the world generates and uses energy. If everyone continues on the present track, he calculates, by the year 2030 the global temperature might rise by 1 to 3 degrees Centigrade to a range well beyond any in previous historical experience. With greater attention to energy conservation and greater reliance on solar energy, Mr. Mintzer believes, the increase could be held to half as much. The changing climate is inevitably going to affect agriculture, water resources and life in general. But the speed and cost of the change are still subject to a degree of human control