Heat Waves Are Going to Get Worse

Here are a few temperature "highlights" from yesterday:

• The Salt Lake City Airport hit 100ºF, the first triple digit reading of the summer.  Was it a record?  Nope.  June 20, 1936 hit 101ºF.  
• The Salt Lake City Airport also set a record high minimum for the day of 77ºF, trouncing the old record of 69ºF set in 2003
• Alta hit 79ºF, breaking the old record for the day of 75ºF set in 2007
• Numerous daily record maximum and high minimum temperatures were set in SoCal, including:
•  122ºF in Palm Springs, which was 1ºF off their all-time record (123ºF) and also equates to 50.0ºC, precisely half way between freezing and boiling. 
• 111ºF at Bob Hope Airport (Burbank), tying their June record
• 125ºF in Needles, setting a new record for June and tying their all-time high (set in 1925 and 2005)
• 126ºF in Death Valley, a record for the day

People often ask me if such heat waves are what we will be facing with global warming. This heat wave is not the future.  The future is worse.

We are still in the early stages of global warming.  Natural climate variability remains a major driver of extreme events like this (although global warming does tip the scales a bit).  When one looks at projections for the future under a "high-emissions" scenario, things don't get really ugly until the middle to late 21st century.  Gurshunov et al. (2013) provide an illustration of this for the most recent Southwest Climate Change Assessment Report.  They define heat waves as days when the maximum or minimum temperature exceeds that of the hottest 5% of summer days or nights (May–September) in the 1971–2000 climatology.   The heat wave index is the cumulative total number of degrees above the hottest 5% temperature threshold on these heat wave days. 

As shown in the graph below, for either maximum or minimum temperature, a clear long-term upward trend in maximum temperature heat waves over the U.S. Southwest has yet to be clearly detected (brown dashed and solid lines, with the latter representing the 5-year running mean), but there is some upward trend in minimum temperature heat waves.  

Source: Gurshunov et al. (2013)

 Heat waves are, however, projected to increase at an accelerating rate during the 21st century (black lines), with the climb for minimum temperature heat waves stronger than that of maximum temperature heat waves.  Studies examining southern California suggest that today's 100-year event becomes a 10-year (or shorter) event in the latter half of the 21st century.  Minimum temperatures are expected to climb faster than maximum temperature (consistent with the larger heat-wave index above), so the character of heat waves will also change, with less nighttime cooling.  

The graphs above are based on a high-emissions scenario in which we remain welded to fossil fuels for future energy demands.  Heat waves are going to get worse, but how much worse ultimately depends on the energy choices we make today and in the coming decades.