Number of consecutive days at or above 95 degrees in Washington, D.C. since 1872 (Ian Livingston)

This record is just one of countless extraordinary heat records established over the course of the last three summers.

Unrivaled flurry of summer heat records

Consider all of the following heat records which have been set since 2010 (and it’s possible I’m missing some)...

* The hottest two summers on record (2010 hottest and 2011 second hottest)

* Two of the top four hottest Junes on record (2010-warmest and 2011-tied for 3rd warmest with 1943)

* Hottest June day and tie for second hottest June day (2012 at 104, and 2010 at 102 tied with June 9, 1874)

* Hottest two Julys (2011 and 2010)

* Hottest month (July 2011)

* Most 90+ degree days in a month (July 2011, 25 days)

* Earliest 100-degree reading in a day (July 6 2010, before noon)

* Longest uninterrupted stretch of temperatures above 100 (July 6, 2010, 7 hours)

* Longest uninterrupted stretch of temperatures above 80 (July 21 to 24, 2011 - over four days)

* Most and second most nights above 80 degrees (7 in 2011 and 4 in 2010)

* Warmest low temperature (84 on July 23 and 24, 2011 tied with July 16, 1983)

* Hottest days so early (102 on June 9, 2011, tied with June 9, 1874) and late (99 on September 24, 2010) in the season

* Most 90+ degree days in calendar year (67 in 2010, tied with 1980)

That’s quite the (dirty) laundry list.

Warm in winter, spring and fall, too

But wait. We’re not done. Not only have recent summers been unusually hot, but warm weather records are falling year-round. Consider these additional records in the last 3 years:

* Warmest and second warmest spring (2012 and 2010)

* Warmest first six months of year (2012)

* Third warmest winter (2011-2012)

* Earliest last freeze (February 27, 2010, even after snowiest winter)

* Third latest first freeze (December 7, 2011)

Could all of these warm weather records over such a short time span be a coincidence? I have my doubts.

Urban warming effect

An undisputed local factor in increasing heat records in Washington, D.C. is urbanization. Since records began in the late 1800s, the population has grown, and an expanding radius of land surfaces have been built on and paved over. In other words, an ever increasing portion of the region has been covered by dark, heat-absorbing surfaces expanding and intensifying the so-called urban heat island effect.

The heat island effect has its greatest impact on night time temperatures, essentially trapping the heat in the urban core rather than allowing it to escape into space. So the recent tendency towards more record high nighttime temperatures is strongly related to this phenomenon.

Increasing greenhouse gases

Coinciding with the increasing heat island effect, heat-trapping greenhouse gases (GHGs) from the burning of fossil fuels have risen to levels in the atmosphere unprecedented in at least 800,000 years. Global and U.S. temperatures have warmed over the last 100 years, and most publishing climate scientists believe the increase is largely related to GHG trends, although there is a small minority who dissent.

Attempting to sort out how much of D.C.’s high temperature records are related to the urban heat island, GHG warming, and possible other factors is very involved. Scientists - for the most part - have not tackled this. Not to mention, in Washington, D.C., the official weather observing station has changed location from near downtown (prior to the 1940s) to the airport - which further muddies the water.

Last year, I took a stab at analyzing D.C.’s summer temperature records. To try to get a handle on the GHG warming effect, I examined records in D.C. that had removed the urbanization effect and calculated a summer warming rate of 1.4 degrees F. per century - which one might conclude is related to GHGs (and/or other factors). But - on closer inspection - I noticed little warming (after removing the urban effect) over the period from the late 1970s to present - the time during which we’ve established many of the heat records listed above.

That finding might call into question GHGs are strongly influencing local temperatures. On the other hand, we don’t know how temperatures would have evolved had GHGs not been increasing. In the absence of rising GHGs, perhaps D.C.’s temperatures would have cooled.

Some global warming skeptics are fast to point out D.C.’s hottest recorded temperature of 106 occurred on August 6, 1930 and July 20, 1918 well before GHGs ramped up. They may also mention the record for most 100 degree days in Washington, D.C. occurred in 1930. But - overall - records from first half of the 20th century are vastly outnumbered by the records since 1980.

The bottom line is that we’re just not in position to tease out the size of the observed global warming signal (from GHGs) in Washington, D.C. But the more records we set, the less likely they are happening by chance.

The future

The messy discussions about the recent course of D.C.’s temperatures notwithstanding, the recent onslaught of heat records is eye-opening.

Assuming climate scientists are right, and global, regional and local temperature substantially warm in the coming decades, the kind of summer we’ve recently experienced will occur at greater frequencies and potentially get worse.