There is a cool, steady rain falling outside my window as I write this column. This follows on the heels of the record heat we endured last weekend, which other parts of the country have been suffering through for much longer. I was in Manhattan last Friday when the temperature hit 106, and the combination of radiant heat and humidity was oppressive in the extreme. I cannot begin to imagine what it was like for those without recourse to air conditioning.When the history of North America’s urban growth, rural sprawl and loss of open space is written, there may well be an entire chapter devoted to the influence of air conditioning. One could argue that during the years since World War II, no modern invention or engineering marvel — not interstate highways, nor fiber optic cable nor riding mowers — had such an accelerating influence on the expansion of our urban centers and the development of previously undesirable residential property in the nation’s hot and humid regions.An expanding middle class and the globalization of our economy were certainly contributing factors, but who in their right mind would choose to retire to Florida or Arizona without air conditioning? Who would work in a 60-story office building with only window awnings and ceiling fans to keep the summer swelter at bay? There are historic, climactic reasons why the French take the entire month of August off and head for the Mediterranean, and why summer communities for Bostonians and Manhattanites sprang up in the Catskills, White Mountains and the Litchfield Hills in the late 1800s once the railways made these reasonably accessible to urban populations. According to a September 2006 essay by James Fergusson in Prospect Magazine, America devotes about a third of its electricity consumption to air conditioning; that’s 8 percent of global production. There are clear environmental as well as quality-of-life implications from this figure.Thanks to air conditioning, our cities are habitable, our industries more efficient and our most developing and most populous states are now located in the not so sleepy South and desert Southwest. We have more habitat destruction in the biologically rich southeastern United States than ever would have happened otherwise — and far more economic activity there as well. We also have larger cities, greater movement of perishable foodstuffs at a higher cost in fossil fuel, and greater food preservation and therefore greater food security.We have Willis Haviland Carrier to thank for all this. Newton was beaned by an apple, but Carrier got the inspiration for his Apparatus for Treating Air while waiting for a train on a foggy night in 1902. Working out the relationship between temperature, humidity and dew point, he developed the first commercial air conditioner to create a stable environment for the printing plant where he was a $10 per week employee.The industrial and commercial applications of this invention, and those advances that followed in refrigeration and cooling, were sweeping in scale and scope. The skyscrapers of American cities that soared above the steeple tops in the first decades of the 20th century would have been uninhabitable without air conditioning, and this has accelerated the growth of urban cores and a corresponding depopulation of many rural areas. Home air conditioning was available to average Americans after World War II and made all the difference in the growth of suburbia and the development of the Southeast, Gulf Coast, Sunbelt and California.At the close of the 20th century, there was a remarkable exhibition in Washington, D.C., called “Stay Cool! Air Conditioning America,” that compellingly demonstrated how it opened the South for development but killed the small town front porch.“Domestic air conditioning meant that traditional architectural features — wide eaves, deep porches, thick walls, high ceilings, attics and cross ventilation — were no longer needed to promote natural cooling. Also irrelevant was siting or landscaping a house that maximized summer shade and breezes, since mechanical equipment was able to maintain perfect indoor conditions independent of design.“Builders found they could pay for the costs of central cooling systems by deleting elements made unnecessary by the new technology. As air conditioning replaced traditional features, the design of the modern house became fully integrated with—and dependent on— air conditioning. It allowed postwar architects and builders to achieve a new ‘ranch house’ aesthetic of glass picture windows, sliding doors and rectangular forms.”The most important exterior space in our homes became the backyard. Neighbors put up fences, and stayed inside glued to the tube on sultry nights instead of visiting on the porch swing. The same thing has happened in our region, and from walking my neighborhood at dusk I believe the television screens of our air-conditioned houses are responsible for our wholesale retreat from the public spaces of our homes to the private. It is easier to sleep during a heat wave with air conditioning, but it is also easy to get addicted to it.Here we have a society-transforming invention, lauded as one of the top 10 Greatest Achievements of the 20th Century, and we can’t wean ourselves from its comforts. Nor can we seem to do so to save our atmosphere, which receives terrific damage from pollutants when refrigerants are released and from the fossil fuels that provide most of our electrical generation capacity.Nature’s air conditioning has returned this week to the Litchfield Hills in the form of a cool summer rain. When that was unavailable last week, I sought relief in a cool ravine. There is a small window-mounted air conditioner in my bedroom, and I try to use it sparingly. After all, we were a summertime destination in the days before air conditioning, and it is far better to be outside on a hot day in the Northwest Corner than at the corner of 125th Street and Broadway. Tim Abbott is program director of Housatonic Valley Association’s Litchfield Hills Greenprint. His blog is at greensleeves.typepad.com.