CLEVELAND - Extreme weather is not unusual in the United States.
This may sound like an obvious statement for many of you. But, based on what we've all been hearing on the news these days, some of you may have the impression that severe weather events in 2012 were more frequent and more dangerous than any other year in history.
Time for a different perspective.
As humans, we have short memories. Our historical perspective on weather events is limited by our age and experience. I remember my grandmother telling me that current winters were "nothing like the terrible winters she experienced as a kid." We as humans tend to remember that "big" storm from our childhood that disrupted our lives in a way we will never forget. That memory colors our weather world forever. Today's headlines, however, are trying to convince us that current weather extremes are worse than anything grandma lived through.
According to National Oceanic & Atmospheric Administration (NOAA), 2012 was the “second most extreme year on record” in the US. (behind 1998). NOAA officials write:
"2012 was a historic year for extreme weather that included drought, wildfires, hurricanes and storms; however, tornado activity was below average, according to an analysis released today by NOAA’s National Climatic Data Center."
A friend of mine, Paul Homewood, writes a popular climate blog called "Not A lot of People Know That." He loves to crunch the numbers. Homewood actually devised a system that would categorize extreme weather events by year and rank each year. He doesn't think the weather extremes of 2012 were all that remarkable.
"With global warming failing to materialize as planned," he wrote, "NOAA and others have been desperate to show that extreme weather is on the increase, and that mankind is responsible. In recent years, they have been running a ' US Climate Extremes Index ', and it is this index, (Picture 1, above), that they have based their claims around."
But Homewood has real problems with this index.
"Let’s start by looking at how they define and measure extreme weather,"" he said. The index is based on 6 different indicators:
1. Daily maximum temperature.
2. Daily minimum temperature.
3. The Palmer Drought Severity Index (PDSI).
4. Extreme 1-day precipitation events.
5. Days with/without rainfall.
6. Landfalling tropical storms and hurricanes.
(It is worth noting that tornadoes are not included, partly because of any meaningful data before around 1970. As last year was an extremely quiet one for tornadoes, their inclusion would have made a big difference to the index.)
"Now, let’s take a closer look at these indicators," Homewood said.
1) Temperatures – extremes in maximum temperatures, (and similarly with minimums) are defined as the sum of percentage of the United States with maximum temperatures much below normal and percentage of the United States with maximum temperatures much above normal.
According to Homewood, this immediately creates a couple of problems.
According to this definition, mild winters would be classed as “extreme”, "which is clearly nonsense." What is “normal”? For instance, an average of rainfall, based on the dust bowl years, would tell us that higher levels in later years were “excessive”.
"The index calculates 'normal' on the full record back to 1910, but in what way was weather in 1910 any more normal than weather today? Indeed, if we took recent years as the 'normal', most of the 20thC would look pretty extreme."
Most people would associate “extreme weather” with something undesirable and harmful. As both minimums and maximums are measured separately, it could be argued there is double counting here.
2) PDSI – As this index is essentially based on a combination of rainfall and temperature data, it is utterly superfluous, as these are already factored in.
3) Extreme daily rainfall – again, rainfall is factored in under 5). In any event, the lack of recording stations in earlier years must make this an unreliable indicator.
4) Days with/without rainfall – again, this is in comparison with a “normal”.
Instead, Homewood has devised his own way to categorize years with extreme weather events. His results are much different than NOAAs.
"We often hear today’s climate described as 'post-normal', but what was so normal about climate 50 or 100 years ago?" he said. "The bottom line is that the climate of the last couple of decades or so is what we have all lived through, and adapted to."
Homewood's index is based on:
1. Cold temperatures in winter – the colder it is, the more “extreme”.
2. Hot temperatures in summer – the hotter it is, the more “extreme”. (I appreciate Minnesota might like a warmer summer, but I have to draw the line somewhere!)
3. Annual precipitation variation, compared with the 1981-2010 mean, (both higher and lower).
4. Tropical storms/hurricanes, as calculated in NOAA’s index.
Here is the key to the index: Rather than using the “percent of area affected” system that NOAA have adopted.