Extreme Weather: Are High Temperatures the New Normal?

This Ricochet Science video describes how average summer weather temperatures have statistically increased since the 1950s and 1960s.  The video’s narrative,  references and links are included below.


This summer’s extreme weather continues to make headlines throughout the world. July 2012 was the warmest single month ever recorded for the continental U.S. Record warmth and melting have occurred over the Greenland ice sheet, and a new Asian heat record of 53.5°C (128.3°F) was reported in Pakistan in May. This year has already seen more temperature records tied or broken than in all of 2011, a year in which 14 extreme U.S. weather events caused over $14 billion dollars in damages.

So what’s happening with our weather?  Is extreme weather the new normal? Well, let’s take a moment to look at how summer weather in the northern hemisphere has changed statistically since 1950.

Summer temperatures vary a lot, even in a constant climate. But if you take an average of summer temperatures from year to year, and plot them on a chart, you will eventually see a pattern of numbers that resembles the shape of a bell. This is a normal distribution of temperatures, or what’s known as the bell curve.

global warming chart 1

In a constant climate, summer temperatures should fit this bell curve shape- with  average temperatures occurring most often, and extremely cold or extremely hot temperatures occurring less frequently.

global warming chart 2

So statistically speaking, if the average temperature occurs here at zero, anything within 1 standard deviation from zero is considered statistically the same, or pretty much an average summer temperature.  But temperatures occurring 2 and 3 standard deviations away from zero are considered statistically extreme.  So the hottest and coldest recorded temperatures would appear in these areas.

Now let’s take a step back and look at what summer temperatures looked like 50 years ago. We’ll use this as our baseline for temperature comparison. In the 1950s, 60s and 70s summer mean temperatures were close to this type of bell-shaped distribution.

global warming chart 3

But, in each subsequent decade the distribution of temperatures has shifted, with hotter temperatures becoming the new norm.

global warming chart 4

Notice that as the distribution curve shifted to the right, the probability of extreme heat greatly increased.  The decade of 2001-2011 showed record temperatures nearly 5 standard deviations away from the normal temperatures of the 1950s, 60s and 70s.

global warming chart 5

But also note that the decade of 2001-2011 continued to show temperatures of extreme cold. Some of the coldest temperatures were nearly 3 standard deviations away from zero. So even though the temperature pattern showed an increased tendency for unprecedented heat, due to variability in the weather there were also brief bursts   of extreme cold.

Still, the periods of extreme heat today already far outweigh the periods of extreme cold. There are now roughly twice as many days with record highs than days with record lows, according to research from the National Center for Atmospheric Research.

So what’s our takeaway from this?  Well, it’s important to realize that “normal” temperature pattern are shifting. If the temperature distribution curve continues to shift to the right, we will probably see some pretty extreme effects.

  • Warmer summers will occur more often, and cooler summers will occur less often
  • Rising temperatures will lead to more extreme weather patterns
  • The probability of extreme storms, heat waves, droughts and floods will increase.

Most distressing of all are some projections made by climate change predictive models. According to one study, the ratio of daily record highs to daily record lows in the lower 48 states could soar to 20-to-1 by mid-century, and 50-to-1 by 2100.




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