Saturday, July 7, 2012

Extreme Heat: The Odds Are Increasing

A number of people have been asking me during this exceptional heat wave if it was being caused by global warming.  My response has been that “global warming doesn’t cause the heat wave; it causes the heat wave to be hotter.”  Privately I had been telling people that we would likely break the all-time record high temperature in Columbia in the next 20 years.  That was two years before it happened.

The fact that so many record high temperatures are being broken is no surprise.  A 2009 study published in the Geophysical Research Letters showed that daily record-high temperatures were outpacing daily record lows by a 2:1 ratio in the first decade of this century.  If the climate were not warming the ratio would be 1:1.  The study showed that on a business as usual scenario the ratio would expand to 20:1 by 2050 and 50:1 by 2100.

The ratio of high to low temperature records from the 2009 study listed above.  Click on the graphic for a high-definition view of the graph.  Image Credit: Climate Central.

However, this year the ratio has expanded to 7:1 for the nation since the beginning of the year.  Almost 8,000 high temperature records have been set in just the past month.  Some of these records recently have been all-time record highs breaking records from the Dust Bowl Era.  Keep in mind that there have been two record heat events already this year.  The first was the record warm spell in March and now the exceptional heat wave this past month.

The ratio of high to low temperature records for June and the first half of 2012.  Click on the graphic for a larger view.  Image Credit: Climate Central.

So how does global warming increase the odds of extreme heat?
If you have ever played with dice, you know something about odds.  The odds of rolling “snake eyes” are one in thirty six.  Those are about the odds of experiencing a temperature of 101 degrees F or higher in Columbia, South Carolina.  So you would expect to see that temperature about 3 times a summer.  Obviously, this does not happen every year.  There have been summers with no 100-degree temperatures and the record is 16 101-degree days which was in 1954.

A review of the database shows that the odds were about two days per summer prior to 1950.  The heat waves of the 1950s changed that on a climatological basis.  Recently the 30-year moving average pushed this to four days per summer and that is the norm now in 2012.


As heat-trapping pollution is dumped into the atmosphere it is like loading the dice to get an extra one.  An ensemble of climate models project that, on our current rate of greenhouse gas emissions, the odds will be one-in-ten by 2040.  This would mean that on average we could expect to see ten days per summer with temperatures of 101 degrees F or higher versus the three now.

Last summer was the hottest summer on record.  It also saw eleven days of 101-degrees+ to rank number 2 in that category.  Last summer was exceptional by today’s standards, but could be normal in roughly 30 years.

It does not stop there.  The models project that by 2070 the odds for a 101-degree day in the summer would be one-in-four.  It would mean that on average you could expect 23 days to be 101 degrees or higher.  That has never happened in 125 years of record keeping, yet this would be the norm.  One could foresee a summer with temperatures topping 100 degrees each day given the natural variability.

The picture for 2040 is almost cast in stone.  The momentum in the climate system means that anything we do today will not show up for about 25 years.  At this point very little is being done or planned, so I do not think the picture for 2040 will change.  However, we can still do a lot to change the picture for 2070, but we have to start now.

Drastic reductions in the emission of heat-trapping pollution are required.  The longer it takes, the harder it will be.  Some politicians say we can simply adapt.  We are not doing well in that category, so far.  For many the adaptation of the future will be to suffer, then die.  Is this the legacy we want to pass on to our children and grandchildren?  This is why some climate scientists like James Hansen have become activists.

I predict that the new all-time record high temperature of 109 F in Columbia set on Friday, June 29, and tied the next day, will be broken in the next 30 years.  Furthermore, the state record of 113 F was set at the campus of the University of South Carolina in Columbia and it Johnston, South Carolina.  This will likely be broken as well.

Two related videos (which require Flash to view):