Tuesday, September 18, 2007

Record summer temperatures.

The title of the NOAA report says it all to me: Sixth warmest summer on record ends with record heat in South. Some highlights from the report:

  • August 2007 was 1.2F (1.0C) warmer than the 20th century mean August temperature, and 6th warmest summer since recording such things as summer temperatures began in 1895.
    • Within the 48 contiguous states, August 2007 was 2.7F (1.5C) warmer than average.
    • Globally, the combined land and ocean temperatures for August 2007 was the 8th warmest on record, 0.85F (0.47C) above average.
    • Global land temperatures for August 2007 were the 3rd warmest on record.
  • Of the 50 states, only Texas and Oklahoma were slightly cooler than average.
    • The warmest August in 113 years occurred in West Virginia, Kentucky, Tennessee, North Carolina, South Carolina, Georgia, Alabama, Florida, and Utah.
  • Increased temperatures had concomitant increased energy requirements in the SW and SE.
  • Rainfall was below average in the Southest, mid-Atlantic, Ohio River Valley, northern Plains, and northern Rocky Mountains.
    • Forest fires in Georgia, Florida, and the Rocky Mountain states were attributed to lowered rainfalls.
  • Rainfall was above average in Texas (the wettest on record) and Oklahoma (the 4th wettest on record).
    • Heavy monsoons affected regions of South Asia, affecting millions of people.
  • Hurricane Dean - the first major hurricane of the Atlantic Hurricane Season - was the first storm to make landfall as a Category 5 storm since Hurricane Andrew in 1992.
All of that is (to me) news that doesn't do me good. However, I am wondering how long it will take for people in general come to realize that their concerns shouldn't be limited to record temperatures (i.e., the hottest summer on record, the hottest day on record, etc). Rather, people need to think about how a season's temperature relates to longer-term temperature trends. One method by which to do this is to look at temperature trends in a similar fashion as how hydrologists and fluvial ecologists investigate river water discharge patterns.

A hydrologist (or fluvial ecologist) will use what he/she might consider a representative sample of discharge data for the purposes of the problem. In this case, I used roughly 80 years of discharge data measured by the USGS for the Huron River as it flows through Ann Arbor, MI.

Unless you are a much better person than average at seeing underlying trends, all you will see in such a case will be spikes of high discharge, with most discharge occurring between ~100cfs and 1000cfs. However, there are a lot of times when discharge is higher than 1000cfs and lower than 100cfs. If I was to say that something was an event that was extremely rare (i.e., occurred only one time out of twenty), I would be at a loss to tell someone what that event would be.

For the reason of being able to assess a measured discharge against long-term trends, a ranked discharge curve is created. In this curve, the data are ranked in increasing (if one is a hydrologist) or decreasing (if one is an aquatic ecologist) order.

Looking at this ranked discharge curve (note that I used a logarithmic scale on the y-axis), you can see that events less than 90cfs and events greater than 1200cfs are extremely rare (based on the above definition). This would mean (among other things) that any event of greater than 1200cfs is a very significant discharge event; even a flow over 930cfs could be judged to be a 90th percentile discharge. If I was to check the most recent discharge value (318cfs, measured at Sep-18-2007 @ 8:45am), I would see that the river was experiencing a 47.5 percentile flow - pretty close to the annual median discharge.

Comparisons can also be made between rivers. If you compare the Huron River with New Mexico's Delaware River (both about the same watershed area), you can tell that there are some fundamental differences between how the hydrologies of each area works.

Now, if temperatures were to be measured like this, I would argue that it would be much better than the current method of comparing against the greatest value on record. You wouldn't be saying things like, "The 12th warmest summer in recorded history." A statement like this (imho) would make people think that the 12th warmest summer in recorded history is not that significant; similar to the logic that no one really cares about the 12th-place finisher of a marathon. However, if the 12th warmest summer was very close in temperature to the 4th warmest summer, a graph of ranked temperature - like the ranked discharge graph above - would more easily show how different ranked summers relate to each other. Something as simple as this is (I believe) important in having people understand the relationship between rather abstract concepts as global temperature trends.

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