t looks as though this interview might end before it has even started. Ed Mazria doesn't want to talk about all the energy-saving features of the home he has designed for himself in the hills overlooking Santa Fe, New Mexico. There will be no discussion of how he warms his spacious house on a near-freezing November day without a heater, no tips about installing solar panels. Mazria, a maverick architect widely respected for his pioneering designs, wants to talk about a much larger issue. He is convinced that the leading cause of global warming has been overlooked by both scientists and environmentalists. Architects like himself, he argues, and the buildings they design and construct are responsible for nearly 50 percent of the world's greenhouse gas emissions.

It's a startling assertion, and my first reaction is that it must be wrong. What about all the SUVs and trucks, the coal-fired power plants, the deregulated industries, all eructing tons of carbon dioxide into the air?

Hunching his 6-foot-6 frame over a laptop, Mazria points to a diagram on the screen, a pie chart he made using Department of Energy statistics that slices energy consumption in the United States into four neat wedges. The chart holds no surprises and in fact contradicts Mazria's claim: There's no slice devoted solely to architecture or buildings. But there's a fat one -- 35 percent of the pie -- for industry, which includes manufacturing, mining, agriculture, forestry, fisheries, construction, and the operation of industrial buildings. The next largest chunk, transportation, accounts for 27 percent of the total. The residential sector, which comprises energy used at home for heating, air-conditioning, cooking, and electricity, accounts for 21 percent; the commercial sector, which includes energy used in office buildings, hospitals, and government facilities, makes up 17 percent and is the smallest piece. Divvied up this way, the worst offenders are the usual suspects -- industry and transportation. All attempts to rein in greenhouse gases focus on them.

But two years ago, Mazria, who is 64, realized that this way of looking at energy use was fundamentally flawed. His insight came shortly after the younger architects at his firm, Mazria Odems Dzurec, asked him to give a seminar on green design. Mazria has been interested in energy conservation for more than 30 years. In 1979, he wrote The Passive Solar Energy Book, a guide to designing homes that harness the sun's energy to heat and cool living spaces; it has sold more than 500,000 copies. But until he began to prepare for his talk, Mazria had never tried to quantify how the actions of architects affect the most serious environmental problem: global warming.

So he reapportioned the Energy Department data, shifting processes that had been lumped under industry -- the manufacture of construction materials, for example -- into a new category that included all buildings: commercial, residential, institutional, and industrial. When he finished adding up the numbers, he was stunned. The built environment eclipsed the transportation and industry sectors.