A look at weather extremes and the big-picture climate connections.

2012 Heat Wave is Historic, if not Unprecedented

Now that the heat wave has finally subsided, we can begin to take stock of how unusual it really was. There is no question that it was extraordinary in its intensity and geographical scope for so early in the summer season. But was it unprecedented in American history?

The short answer is no, it wasn’t unprecedented. The longer answer is that it will go down in history as one of the hottest and longest-lasting early summer heat waves in U.S. history, and by at least one measure, it was the record setter in every sense of the phrase.

It was also a clear taste of what’s to come as the climate continues to warm due to manmade global warming.

An illustration from the U.N. Intergovernmental Panel on Climate Change (IPCC), illustrating how a shift in mean temperature makes warm temperature extremes more likely to occur.

To dig deeper into the numbers, you have to compare the 2012 June-July heat wave with the epic U.S. heat waves of the past, two of which stretch all the way back to the Dust Bowl era.

So I compared how the record totals from this heat wave stacked up against data from some of those hottest summers — 1988, 1954, 1936, and 1934.

(Note that my number-crunching doesn't exactly compare one heat wave to another, but rather looks at the number of records set over a slightly longer timespan. It also omits record warm overnight low temperatures, since these records are not available via the NCDC's database for years prior to 1993.)

During June 2012, there were 3,282 daily record highs set or tied across the country, according to preliminary numbers from the National Climatic Data Center (NCDC). These figures are likely to rise in coming weeks as more records filter into NCDC. During the first week of July (July 1-7), there were 2,278 daily record highs set or tied.

These records include the sweltering 105°F high temperature in Washington, D.C. on Saturday, which was its second-warmest high temperature on record. Washington had a record 11 straight days with temperatures of 95°F or higher, and tied the record for four consecutive days of 100°F or higher.

It turns out that the total number of daily records this June were about the same as occurred during the month of June during two of America's hottest summers — 1936 and 1934, both of which occurred during the Dust Bowl.

However, June of 1988 blew this June away, with 9,800 record highs set that year. Not coincidentally, 1988 was the summer when massive wildfires charred Yellowstone National Park, and global warming first made national headlines when NASA climate scientist James Hansen testified before the Senate.

The tally of monthly and all-time record highs shows that June of 2012 was still an extraordinary month. Of the 3,282 daily records this June, 645 qualified as monthly record highs, and 173 were all-time highs. In 1988, there were 963 monthly records set or tied in June, and 163 all-time records.

So June of 2012 tied or broke more all-time records compared to the month of June during those previous hot summers.

During the period from July 1-7, 2012, there were 2,278 record daily highs set or tied in the U.S. This beat the same period during 1988, 1954, 1936, and 1934. That's not surprising since the time period of individual heat waves differ. More monthly record highs were set or tied during this period than during the other hot summers as well. Again, given the varying timeframes, that's not surprising. July 1-7 of 1936 came the closest, with 155 monthly records set or tied during the period.

Jet stream pattern during the height of the heat wave, on July 5, 2012.

Interestingly, in terms of all-time records, July 2012 came in second, with 119 all-time records set or tied, behind the 134 records set or tied during July 1-7, 1936. (Many of the all-time records set or tied this year with records that had been set in 1936.)

Because there is a much longer period of record today than there was during the previous hottest summers — especially when viewing the marks from the Dust Bowl era in the '30s — it is now more difficult to set monthly and all-time records. This argues in favor of viewing the 2012 heat wave as a more impressive event.

On the other hand, the number of records in 2012 may be inflated somewhat, since there are more weather observation stations now than there were during the previous hottest summers.

One thing I’m curious about is whether the atmospheric conditions that led to this heat wave were as unusual as what took place during past hot summers. We already know from studies that global warming makes it more likely that heat extremes will occur.

Given the background warming that has already taken place in the U.S. and elsewhere, it’s reasonable to think that while a record-breaking heat wave used to require a massive, unusually powerful dome of high pressure in order to occur, now the same record could be set with a weaker weather system.

The heat dome that was present during this year’s event was persistent and formidable, but it did not set records for its strength. In fact, it’s possible that with a stronger High Pressure center, temperatures could have shot up even higher than they did.

I'll leave you with Climate Central's Record Temperature Tracker, which allows you to investigate the records as they were set this year. I recommend clicking "play month" to see the heat records sweep across the country during late June into early July.

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