Extremes Of Temperature Decreasing In The US

Guest essay by Paul Homewood

WUWT carried the story yesterday of the paper by Kodra & Ganguly, forecasting a wider range of temperature extremes in the future.

According to the Northeastern University press release, using climate models and reanalysis datasets, the authors found that

While global tem­per­a­ture is indeed increasing, so too is the vari­ability in tem­per­a­ture extremes. For instance, while each year’s average hottest and coldest tem­per­a­tures will likely rise, those aver­ages will also tend to fall within a wider range of poten­tial high and low tem­perate extremes than are cur­rently being observed.

But is there any evidence that this has been happening? We can check what’s been happening in the US, by using the US Climate Extremes Index, produced by NOAA.

Of course, the US only accounts for 2% of the Earth’s surface, (except when there is a polar vortex, a mild winter or a drought in California), but it seems a sensible place to start. We also know that climate models often bear very little resemblance to reality!

Just to recap, the US Climate Extremes Index, or CEI, is based on an aggregate set of conventional climate extreme indicators which, at the present time, include the following types of data:

1. monthly maximum and minimum temperature
2. daily precipitation
3. monthly Palmer Drought Severity Index (PDSI)
4. landfalling tropical storm and hurricane wind velocity.

In terms of temperature, the CEI is

1. The sum of (a) percentage of the United States with maximum temperatures much below normal and (b) percentage of the United States with maximum temperatures much above normal.
2. The sum of (a) percentage of the United States with minimum temperatures much below normal and (b) percentage of the United States with minimum temperatures much above normal.

So, for instance, we can plot maximum temperatures during summer months:

http://www.ncdc.noaa.gov/extremes/cei/graph/1/06-08

And, minimum temperatures in winter:

http://www.ncdc.noaa.gov/extremes/cei/graph/2/12-02

The reds indicate the percentage of the US, which were “much above normal”, and the blues “much below normal”. The CEI also lists the actual percentages, so we can plot the “much aboves” in summer, and the “much belows” in winter, thus:

The trend is to an increasing percentage with above average summer temperatures, although recent years seem to be at similar levels to the 1930’s. (The CEI is based upon adjusted temperatures, before anyone asks).

In winter, though, the trend is decreasing.

We can now combine the summer and winter sets together.

[I have simply added together the percentages, although of course some areas could have experienced both hot and cold – think of it as an index].

Clearly, the overall trend is to extreme temperatures reducing. In other words, the area of the US experiencing unusually high or low temperatures is tending to grow smaller. (Although it is interesting to note the relative absence of such extremes in the years around 1970).

Of course, although this analysis tells us about the area of the country affected, it does not say anything about how extreme the temperatures are. But we can check this very simply, using the NCDC Climate At A Glance datasets.

The graph below shows the difference each year between winter and summer temperatures, for the country as a whole, along with a 10-Year average. As can be seen, the variation from winter to summer has been getting smaller in recent years.

The most extreme year was 1936, when the hottest summer on record (even after adjustments) followed the second coldest winter. I wonder how their models account for that?

http://www.ncdc.noaa.gov/cag/

FOOTNOTE

NOAA offer this definition of how they calculate their index:

The U.S. CEI is based on an aggregate set of conventional climate extreme indicators which, at the present time, include the following types of data:

1. monthly maximum and minimum temperature
2. daily precipitation
3. monthly Palmer Drought Severity Index (PDSI)
4. landfalling tropical storm and hurricane wind velocity

* experimental (not used with the Regional CEI)

Each indicator has been selected based on its reliability, length of record, availability, and its relevance to changes in climate extremes.

Mean maximum and minimum temperature stations were selected from the U.S. Historical Climatology Network (USHCN) (Karl et al. 1990). Stations chosen for use in the CEI must have a low percentage of missing data within each year as well as for the entire period of record. Data used were adjusted for inhomogeneities: a priori adjustments included observing time biases (Karl et al. 1986), urban heat island effects (Karl et al. 1988), and the bias introduced by the introduction of the maximum-minimum thermistor and its instrument shelter (Quayle et al. 1991); a posteriori adjustments included station and instrumentation changes (Karl and Williams 1987). In April 2008, maximum and minimum temperature data from the USHCN were replaced by the revised USHCN version 2 dataset. In October 2012, a refined USHCN v

ersion 2.5 was released and replaced version 2 data for maximum and minimum temperature indicators.

The U.S. CEI is the arithmetic average of the following five or six^# indicators of the percentage of the conterminous U.S. area:

• The sum of (a) percentage of the United States with maximum temperatures much below normal and (b) percentage of the United States with maximum temperatures much above normal.
• The sum of (a) percentage of the United States with minimum temperatures much below normal and (b) percentage of the United States with minimum temperatures much above normal.

In each case, we define much above (below) normal or extreme conditions as those falling in the upper (lower) tenth percentile of the local, period of record. In any given year, each of the five indicators has an expected value of 20%, in that 10% of all observed values should fall, in the long-term average, in each tenth percentile, and there are two such sets in each indicator.

A value of 0% for the CEI, the lower limit, indicates that no portion of the period of record was subject to any of the extremes of temperature or precipitation considered in the index. In contrast, a value of 100% would mean that the entire country had extreme conditions throughout the year for each of the five/six indicators, a virtually impossible scenario. The long-term variation or change of this index represents the tendency for extremes of climate to either decrease, increase, or remain the same.

The index is built up from the Climate Divisional Database, and therefore reflects the area of the US, rather than a simple percentage of stations.