Within is a country that may have the preprogative over the most pleasant places known; for large and pleasant navigable rivers, heaven and earth never agreed better to frame a place for man's habitation . . . Here are mountains, hills, plains, valleys, rivers, brooks, all running most pleasantly into a fair bay, compassed but for the mouth with fruitful and delightsome land. -- John Smith, The Generall Historie of Virginia

Summer dies hard in Washington. Heat hits asphalt, grows through the summer and lingers into September. The city is as hot now as ever, maybe more so, glass and stone having supplanted vegetation. Plants collect moisture and sap the sun's energy through evaporation. But when sunlight strikes the hard, dry, man-made city, it generates heat. While the verdant rim of the city simmers through summer, its concrete core fries. Climatologists refer to this phenomenon as the "heat island effect."

At opposite ends of the Washington heat island sit two men in tall buildings, thinking different thoughts. What links them, if nothing else, is the fact that each has observed climate for 30 years now. Herb Gilkey, who works for the Air-Conditioning and Refrigeration Institute, has been studying the indoors. Murray Mitchell, a climatologist with the National Oceanic and Atmospheric Administration (NOAA) knows a place that has no walls.

Gilkey's office is in Rosslyn, eight stories up in a cool cube of a building. He is in his element here, for he is a mechanical engineer, knowledgeable about the viscera of man's more advanced structures.

About air conditioning, he explains, it "picks up heat and moves it to another place. The only direction heat moves on its own is 'downhill,' toward cooler air. So you've got to heat the air enough so you can throw it away."

How that translates, Gilkey continues, is that the air given off by an air conditioner must be hotter than the air outside. That creates a thermal spiral of sorts. The hotter the day, the more air conditioners there are pumping still hotter air into the atmosphere.

Thirty years ago people lived behind thick walls and under ceiling fans. Today they move easily about, inside thin-skinned, glassy buildings. Air conditioning has changed -- and insulated -- our lives.

Gilkey nods vaguely at the notion. "One of the great changes in this town that has been brought about by air conditioning is that Congress used to meet in spring and go home in summer."

In the years since World War II the use of air conditioning has become virtually universal in Washington. During many of the early postwar years the nation's capital was the leading "new market" in air conditioner sales, and today 90 percent of the homes in the area have some form of air conditioning. This compares with a national average of 50 percent. In more recent years, air conditioning, powered by cheap and abundant electricity, made possible Sunbelt megacities such as Houston and Atlanta where once there was only backwater.

Behind the venetian blind, which Gilkey now raises, stands Rosslyn. Tall buildings block the horizon, offering only segments of river and city beyond.On the roof of an adjacent building water shoots across the top of an air-conditioning unit, then trickles down through it. Gilkey explains that refrigerant passed through a coil sucks heat from the water. The water is then drawn down through the unit and pumped around the skin of the building in copper pipes. He lowers the blind, returning us to the cool of his office, which could be as easily in Atlanta or Houston.

Gilkey is a genial white-haired man with large hands attached to thick arms emerging from white short sleeves. He grew up in Iowa, a son of the Depression, and he remembers when the climate turned ugly. "During those drought years we were lucky compared to the people in Oklahoma. Some days, though, the wind would come up, and it would get pretty gritty." He also remembers the temperature reaching 100 degrees, but still he would go out to cut the grass. "We were acclimated. We learned to tolerate it because we didn't know anything else. We didn't have air conditioning."

What is the Air-Conditioning and Refrigeration Institute? "A trade group." In Washington a trade group is a lobby. What encroaches on Gilkey's best-of-all possible worlds? He leans forward in his high-backed vinyl chair and selects his words carefully.

"We're concerned about the quality of the electricity supply. Additional generating capacity is not being built; the needs of the people are not being met." He produces a report which shows that from 1973 to 1978 electricity generated by American power plants increased by 18 percent.

Gilkey continues, warming up now. He cites the current difficulties with nuclear licensing and restrictions on the burning of coal, not to mention the vagaries of oil supply. Energy is a deep, wide nerve, newly risen to the surface. Mention it these days and you can generate some easy heat.

Could Washington possibly function without air conditioning? The smile is surer now. "If you look at a lot of the older buildings, all the rooms had outside exposure. The windows opened. There was the possibility for cross ventilation. Now, with city land and other costs -- taxes -- so high, you can't afford to build a building with a courtyard or with an "H" shape.

"Besides, if you have computers you have to have air conditioning. Computers use a lot of electricity, which means that they give off a lot of heat. The components fail if it gets too hot. Then you have electric typewriters and copy machines and a lot of people working in a small place. These are all heat sources. We used to make five or six carbon copies of every letter we wrote. Now we just copy them on the Xerox machine. Of course we still write at the bottom 'cc'." He pauses, beguiled by the idea. "I don't know why we do that."

Murray Mitchell looks out of the sixth-floor window of his office at NOAA. It is open, and the sweep of the view arrests the gaze. To the left, haze-hung trees fill the horizon. Moving right, the vegetation thins, then yields to the severe urban angles of downtown Silver Spring.

Mitchell continues: "What are the consequences if we continue to rely on fossil fuels as we do now? The data suggest that in 50 years the world would warm an average of 3 to 5 degrees Fahrenheit, the polar regions as much as 10 degrees Centigrade [18 degrees Fahrenheit]. World rainfall would increase by 7 percent, and you would get a shift in the storm tracks, producing significant climatic change over various regions. The western part of this country, for example, which is now semi-arid, could become an arid region."

Mitchell emphasizes that these are projections based on a theoretical model of his, which he now illustrates on a blackboard. The scrawled graph correlates time, carbon dioxide and temperature. Its curve is steep. Mitchell sits down and continues on calmly about fertile cropland lost to desert and cities lost to a swollen ocean.

But soon, too, there emerges some fear in the man. His model must wait until the year 2000 to prove itself statistically -- to prove that the carbon dioxide level can be attributed to more than natural shifts in climate. That would not be a problem were it not that the curve on the graph travels at a geometric pace.

"By the time you see this effect appearing in the weather records, it will have already happened. So then you ask, 'Is there enough time to take action?' We think not because we're still stuck with an economy that runs on fossil fuels. We estimate that it would take 40 years to change over to other energy sources. That is based on the fact that it took the world roughly that long to switch from wood to coal and from coal to oil."

The model predicts that by the year 2040 the earth's temperature will be zooming toward a figure 7 degrees Fahrenheit above the current average. "You won't need instruments to tell you what's happening then.

"The excess [of carbon dioxide] will be there for another 500 years. The natural processes that have evolved over millions of years are very slow to correct for these sudden changes."

Mitchell talks on, anchoring his thoughts with facts. "This country gets 95 percent of its energy from fossil fuels. We use a third of the world's energy . . . Only about 6 percent of the energy value in petroleum actually ends up turning the wheels of your car."

He likens us to profligate heirs. Trees and plants, endlessly renewable, might be thought of as earnings. Fossil fuels, plant matter laid down once over millions of years, are capital. We are digging deep into the latter.Mitchell calculates that if the world could cut consumption of fossil fuels by two-thirds, then carbon dioxide would level off at about 50 percent of the pre-Industrial Revolution amount. He thinks that might be climatically "tolerable."

As for what could fill the void, he sees a host of alternatives. "Trees grow like wildfire. You could farm them like crops. You could get alcohol from plants." The same is true of hydrogen, abundant in the biosphere. "You could pipe it around like natural gas." Then too, there are the promises of solar energy and fusion reactors.

But Mitchell, being the macro-thinker that he is, does not look past the political and technological components in all of this. He readily concedes that this scenario is not just around the corner. "I think there's a crunch ahead, and we'll come out of it okay if we don't panic. We have to take the long view and do some planning. We've built this economy on these cheap energy sources and suddenly they aren't so cheap. It really shakes us."

The pause is brief, and now Mitchell continues, less cool, less scientific. He speaks with new intensity, his thoughts suddenly synergetic. "When I was young I used to hold up warfare as the single evil. I thought that if you could eradicate it then you had gone a long way toward ridding the world of all evil. Now that I am older I see that it is only one of a host of related problems. There may be less warfare now -- people actually shooting each other -- but I think the ideological differences are as pronounced as they ever have been. I now find that there are no easy answers."

Somewhere out on the theoretical landscape there is a middle ground. It lies between the impulse to separate the earth from its riches to make today more comfortable and the determination to forsake some of those riches to make tomorrow possible.

Perhaps Hans Johannsen stands on that middle ground. He is a horticulturist; which is: "a practical user of his energy. He burns little of it. His German accent has been softened by a quarter-century of living in America, as has his mood by spending his days with green matter. He is the head of the District of Columbia's Tree Division. Hiw fellow workers call him "Chief."

Does Washington have more trees than other major American cities? Johannsen doesn't know. He doesn't really care. What he does know is that we should be grateful to Pierre L'Enfant for his plan of Washington. The wide streets were meant to accommodate trees. Trees mean shade -- if nothing else, a livable dent in the intense urban heat.

The result is that the city has become a textbook of sorts. Disciples of the relatively new study of Urban Forestry often make pilgrimages here to study the 108 different species and varieties that comprise the 100,000 "street trees" of Washington.

About 3,000 of those trees die yearly. "We try to replace as many as we lose. We want to fill those empty spaces." Johannsen's biggest fear is turning onto a block full of empty spaces -- a "desert street." So his aim is continuity. When a tree goes, replace it. If a whole block starts aging, replace the trees gradually. To achieve this Johannsen travels around the city often, "just to look around," just to sort out his arboreal priorities. And keep the oasis intact.

I went out with Johannsen one day. We stopped occasionally to study dead and dying trees, made detours I sensed Johannsen could not resist, and some time later found ourselves down in Southwest Washington under a big tree.

It is a Japanese kiki tree that casts a wide net of shade over the sidewalk, a sidewalk that runs in clean, straight lines until it meets the tree.

"This is an amazing thing," Johannsen said. "It's probably close to 200 years old."

He went on to recount proudly how, during the redevelopment of this area during the '60s, the tree became his client. He told the gas company where they could and could not put a line. He told a builder what detour to take with a set of front steps.

After awhile we got back in the car and headed down toward Maine Avenue and the waterfront. Johannsen gazed faraway through the windshield and said: "I was born and raised in Hamburg. There is a big hill overlooking the harbor there, on the north side. The captains of the ships lived there, and on that long slope you find the most exotic assortment of trees. The captains would bring them back from voyages and plant them there.

"You see, that tree back there is not native." He motioned back over his shoulder. "I'm inclined to think that somebody brought that tree on a ship from Japan. See, you have the harbor right here. Sometimes I wonder, where did that tree come from?Who planted it? Of course I'll never know, but still I wonder."