Monday, January 9, 2012

Our Local Climate Change

Most people have heard about global warming and climate change as well as the concern about both.  What will the impacts be and how will it affect me?  It is much easier to talk about a global warming, but the computer models have a much harder time with regional climate.

An example of regional climate is the extraordinary dust bowl years of the 1930s.  Much of the central U.S. experienced record high temperatures and exceptional droughts.  In fact many of the all-time temperature records for individual states occurred during this time.  This was at a time when the global climate was cooler.  A regional warming of that magnitude today would be unbearable.

The record for South Carolina is 111 degrees on three different dates, twice in 1925 and once in 1954.  However, it was also quite hot and dry for South Carolina during the 1930s.

These are examples of extremes in weather and can be set at any time, but what about the climate.  How has the local climate changed?  We can examine this by looking at the data set from Columbia, SC.

I looked at changes in the average annual temperature and average annual precipitation by looking at the 30-year moving average with help from meteorologist Scott Ryan.  The National Weather Service calculates the climatological norms every ten years.  However, by looking at the moving average you are looking at how the climate is changing every year and can easily see the trends.


Average Annual Temperature for Columbia, South Carolina.  Image Credit: Scott Ryan.
Note that there is a warming in the local climate from the beginning of the data to 1954 where it reaches a peak.  Then there is a significant cooling from 1954 to 1989.  This is a good example of how local climates can be different from the global climate.  The local climate cooled about 1.4 degrees F during this time.  However, since 1989 the average annual temperature has warmed almost 1 degree F (0.96).  That is a significant warming in just 22 years.

It should be noted that when you are looking to the global data sets you may be looking at 5 or 11-year moving averages and this is a 30-year moving average.  Thus you can not compare the two.

The 30-year moving average smooths out a lot of the variability.  How does it compare to the annual temperature?

The Annual Temperature vs The Average Annual Temperature for Columbia, SC.  The purple line is the average annual temperature while the black line is the annual temperature.  Image Credit: Scott Ryan.
There is a much greater variation in the yearly temperature than the average yearly temperature.  Note that 2011 tied with 1933 for the second hottest year.  The hottest year was 1990.  Although the past summer was the hottest summer on record.


The same type of analysis can be done with the average annual precipitation.

The Average Annual Precipitation for Columbia, SC. Image Credit: Scott Ryan.
There was a drying trend from the start of the data to a low in 1935 when the average annual precipitation was 40.01 inches.  It stayed near that value until 1946 when it began to climb.  It peaked in 1987 with a value of 50.47 inches.  Since that time the trend has been a drier one and in 2011 the value was 44.33 inches.  This represents a 12.2% decrease in the average annual precipitation in the past 24 years.

More variability can be seen in the yearly precipitation.

The Annual Precipitation vs The Average Annual Precipitation for Columbia, South Carolina.  The purple line is the average annual precipitation while the black line is the annual precipitation.  Image Credit: Scott Ryan.
The wettest year on record was 1964 with 70.53 inches.  The trend had been toward a drier climate over the past few decades.  In fact only four years have had normal or above normal precipitation since 1995.

Bottom Line

Our climate is most definitely getting warmer and drier.  No matter how you analyze the data, you get the same conclusion.  Furthermore, we are seeing this warming trend in all seasons of the year (seasonal data is not shown here).  The past two summers have been the hottest on record.

The more disturbing trend is that we are moving toward a drier climate.  We have seen an increasing number of droughts and their intensity is increasing as well.  Yet, we are also seeing an increasing number of extreme rainfall events.  Eight of the heaviest rainfall events have occurred since 1958 even as the climate has been drying.  What is happening is that when we see significant rainfall it tends to be quite heavy, then we can go long periods without much rain.

What might be causing this change in climate?  What does this tell us about the future?  I will cover those topics in a later post.