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The reason I picked this particular chart is because it clearly challenges the alarmist belief that record setting heat waves will be hitting our major cities because of global warming. In fact, ironically, this actually comes from a Washington Times blogger who seems to think that this proves that very point.
Since 1975, the average annual temperatures in downtown D.C., have simply meandered sideways in a range of about 57 1/2 degrees to 56 1/4. That's almost thirty years of no significant "warming." The reason for this is simple. By 1975, Washington was literally busting at its seams; all in response to the growth of government generated by the Kennedy/Johnson initiatives such as the Great Society. With those initiatives came new roads, new government buildings, and sidewalks; all of which have materials that are able to convert the sun's intense shortwave energy into hot long wave infrared energy. Thus, in the period from 1963 to 1975, temperatures rose rapidly. But, when the building craze and city expansion slowed, so did the rise in temperatures.
To understand this better, you only need to be aware of the phenomenon of Urban Island Heat effect. For example, I live in Las Vegas and the summer temperatures are almost always above 100 degrees. At that point, concrete can get as hot as 145 degrees and literally burn bare feet. You can just forget about walking barefoot on any asphalt road or driveway. Thus was born the challenge that it is hot enough to fry an egg onthe sidewalk. Where you can walk barefoot is on a nice green lawn; where temps are significantly lower. So, simply, many building materials, actually amplify the air temperature, whereas plant life is able to mitigate the heat. That's why, in the chart above, rural temperatures are much lower than those in the city.
If our cities are getting hotter, it's not because of global warming; which, by the way, stopped some 18 years ago. It is because of increased urban sprawl with further additions of concrete, asphalt, and new and bigger buildings; thus fueling the Urban Island Heat effect. In a city like Washington D.C., there is little room left to grow, and the chance of increasing summer heat is low. In other cities, still expanding, expect hotter summers. There was a lot of truth to that old Lovin' Spoonful hit of 1966.
Study: Many U.S. cities unprepared for future heat waves: http://www.washingtonpost.com/blogs/wonkblog/wp/2012/07/26/study-most-u-s-cities-are-unprepared-for-future-heat-waves/
Bloated Government? Federal Employment at 47-Year Low: http://economix.blogs.nytimes.com/2013/10/22/bloated-government-federal-employment-at-47-year-low/
Urban heat island: http://en.wikipedia.org/wiki/Urban_heat_island
Is it possible to fry and egg on a sidewalk?: http://www.loc.gov/rr/scitech/mysteries/friedegg.html