Global warming/climate change is, in my opinion, a bit of a chicken and egg argument. Climate change fuels different atmospheric conditions that yield different results. Instead of getting hung up solely on the "how much is man contributing" piece, I wanted to focus on the important factor that's driving our air conditioners into a humming frenzy this summer.
The 1930's, which are the gold standard of climate extremes prior to our recent couple of decades of heat, featured multiyear periods of drought in the Plains and Midwest. Those, in turn, fueled two of the hottest months of July on national record and, more importantly, fueled the hottest July in a number of states.
July 2012 only had one state that had a hottest ever month. While it had many states in the top five and top ten of hottest on record, this wasn't the hottest on record anywhere other than Virginia...which is not surprisingly, going through a dry spell of their own this year. However, the number of top five and top ten warmest on records do add up -- and when blended together, the result is our national record for hottest July.
Below are a number of graphics that illustrate this point -- don't focus so much on us locally but on the bigger picture of what's going on in the center of the country. If the Midwest and Plains are under an intense dry spell during the summer, you are going to get extreme heat to develop as they are in the middle of the continent, away from oceanic influence, and with dry soils comes the propensity for temperatures to take off. We've posted the precipitation for 2012, 1934, and 1936 -- followed by the temperatures that resulted in those summers throughout each of the 50 states. States in the brightest red are record warmest for the particular year.
Just look at this year -- you see the dry to hot trend -- it's very dry in the Midwest and Plains (not record dry but drought-worthy) and the result is a very warm US throughout the month -- most every state is among the warmest 20% on record although only one state set a record for warmest July.
In 1934, the dry spell was centered in the Plains (again) and the result was similar -- very high heat in the Plains and also the Ohio Valley -- over a dozen states were warmer in 1934 than they were this year...the extent of warmth was not as great as the jet stream was kinda close to the US-Canadian border.
In 1936, the same issue (drought, this time in the Northern Plains and Midwest) drove eight states to record high temperatures for the month. Texas and New England were cooler than average -- Texas due to rainfall, New England due to a cooler trough that likely dug into Atlantic Canada on the east side of the 1936 summer heat ridge (corresponding cooldown to the east of the heat ridge).
Our hottest July on record was last year, fueled by dry conditions in the Plains and Oklahoma which were blasted northeast as the Bermuda High was in a great position to funnel heat north and northeastward. In 2010, another very hot July, was pushed by drought in the Southeast and record or near record heat throughout the Southeastern US. Drought conditions drive the summer bus on heat to a large extent on a regional and even local scale but a favorable upper level pattern will determine how much heat gets distributed around the country. This summer's heat high was centered over the Midwest and Plains, allowing much of the nation to get in on record warmth.
Putting all of that aside, it's clear that the Midwest/Plains drought is a huge factor in this summer's national heat event. Climate change, and man's impact on it, is probably adding more fuel to that toasty reality, which may be fueling additional drought, rinse and repeat, but the expanse of national drought coverage is providing a large dose of kindling for the summer torch to take place. That said, the 1930's still provided some rather extreme temperatures that for many are still the gold standard. It's just that this year has been awfully hot for an awfully large amount of the country.