Done deal: Washington, D.C. to complete warmest year on record in 2012 - The Washington Post
What was driving this durably warm pattern? The main culprit appears to be a hyper-charged response to what is known as a negative Pacific Decadal Oscillation or - PDO.
In addition to the weather pattern that favored these warm temperatures, a long-term warming trend is also obvious in Washington, D.C.’s data record. The warming trend is thought to be a result of three primary factors: 1) the move of D.C.’s observing station from 24th and M St. to the warmer Reagan National Airport in the 1940s 2) urbanization 3) climate warming from increasing greenhouse gas emissions. The relative contribution of these three factors to the warming is complicated and not known with certainty.