Record Daily Temperatures and UHI in the USA

By Paul Homewood

https://www2.ucar.edu/atmosnews/news/1036/record-high-temperatures-far-outpace-record-lows-across-us

Every so often, the hoary old chestnut of record daily temperatures is wheeled out, as evidence of global warming. The above chart is from the NCAR study by Gerard Meehl in 2009, and the NCAR Press Release stated: 

Record high temperatures far outpace record lows across U.S.

November 12, 2009

BOULDER—Spurred by a warming climate, daily record high temperatures occurred twice as often as record lows over the last decade across the continental United States, new research shows. The ratio of record highs to lows is likely to increase dramatically in coming decades if emissions of greenhouse gases continue to climb.

“Climate change is making itself felt in terms of day-to-day weather in the United States,” says Gerald Meehl, the lead author and a senior scientist at the National Center for Atmospheric Research (NCAR). “The ways these records are being broken show how our climate is already shifting.”

This graphic shows the ratio of record daily highs to record daily lows observed at about 1,800 weather stations in the 48 contiguous United States from January 1950 through September 2009. Each bar shows the proportion of record highs (red) to record lows (blue) for each decade. The 1960s and 1970s saw slightly more record daily lows than highs, but in the last 30 years record highs have increasingly predominated, with the ratio now about two-to-one for the 48 states as a whole.

The study, by authors at NCAR, Climate Central, The Weather Channel, and the National Oceanic and Atmospheric Administration (NOAA), has been accepted for publication in Geophysical Research Letters. It was funded by the National Science Foundation, NCAR’s sponsor, the Department of Energy, and Climate Central.

If temperatures were not warming, the number of record daily highs and lows being set each year would be approximately even. Instead, for the period from January 1, 2000, to September 30, 2009, the continental United States set 291,237 record highs and 142,420 record lows, as the country experienced unusually mild winter weather and intense summer heat waves.

A record daily high means that temperatures were warmer on a given day than on that same date throughout a weather station’s history. The authors used a quality control process to ensure the reliability of data from thousands of weather stations across the country, while looking at data over the past six decades to capture longer-term trends.

This decade’s warming was more pronounced in the western United States, where the ratio was more than two to one, than in the eastern United States, where the ratio was about one-and-a-half to one.

The study also found that the two-to-one ratio across the country as a whole could be attributed more to a comparatively small number of record lows than to a large number of record highs. This indicates that much of the nation’s warming is occurring at night, when temperatures are dipping less often to record lows. This finding is consistent with years of climate model research showing that higher overnight lows should be expected with climate change.

Remember that last paragraph,as we move on later. But first, a quick recap of the huge flaw in Meehl’s exercise.

The data starts in 1950, and consequently picks up all of the daily low records set during the much colder 1960’s and 70’s, but fails to pick up the daily highs during the 1930’s and 40’s. This flaw was so significant, and obvious, that I am amazed the paper ever made its way past review.

http://www.ncdc.noaa.gov/cag/time-series/us

Nevertheless, we can now move on. WUWT mentioned the other day that CDIAC, The Carbon Dioxide Information Analysis Center of the Department Energy, have introduced a new tool, which crucially uses data right the way back to 1911. It also only uses USHCN stations, regarded as  high quality, and excludes any that don’t have complete, or nearly so, records back to 1911.

The data only goes up to 2010, so there will no doubt be a batch of record highs in 2012, and record lows last year, not included.

Interestingly, CDIAC introduce their new interface thus:

Like politics, you might say that all climate is local. As researchers seek to help the public better understand climate and climate change, a sensible approach would include helping people know more about changes in their own backyards. High and low temperatures are something that all of us pay attention to each day; when they are extreme (flirting with or setting records) they generate tremendous interest, largely because of the potential for significant impacts on human health, the environment, and built infrastructure.

In other words, they want to “educate” the public, who remain stubbornly sceptical! However, it appears they may have shot themselves in the foot

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Southeast Region

Greg Kent, in his essay on WUWT, put together some graphs for the CONUS, which did not look as scary as the original NCAR study, but I wanted to delve a little deeper, so decided to take a closer look at the thing region by region, starting with the Southeast (encompassing VA,NC,SC,GA,FL & AL). Altogether, there are nine regions in the CONUS.

Within the region, CDIAC list 37 stations, out of the national total of 424. The interface offers these screens for each.

http://cdiac.ornl.gov/cgi-bin/broker?id=013816&_PROGRAM=prog.htxctn_d9k_424.sas&_SERVICE=default&_DEBUG=0

http://cdiac.ornl.gov/cgi-bin/broker?id=013816&_PROGRAM=prog.htxctn_data_d9k_424.sas&_SERVICE=default&_DEBUG=0

So, using the record highs and lows, I have built up a database for the whole region. (The 1910’s start in 1911, and so on).

Figure 1

Figure 2

As far as daily highs go, there are 17730 records/ties, an average of 479 per station (i.e suggesting 113 ties). The total number of records set between 2001 & 2010 was 1222, much lower than the average of 1773. Daily lows were also below average in the last decade, with 1094, against an average of 1604.

We can also look at the ratio of highs and lows, in Figure 3, showing that they have been pretty much in balance since 1991. The imbalance during the warm 1920’s to 50’s, and cold 1960’s to 80’s, is also evident.

Figure 3

I don’t want to make too big an issue of these figures, because the Southeast is generally accepted to have shown the least warming of any regions during the last century. We can only get the full picture when I have worked my way through the other regions.

But, here things started to get a bit interesting.

Urban Heat Island Effect

As I was transferring the data, I noticed that some stations had much higher ratios of highs to lows. I only really cottoned on when I noticed it for Charleston, SC, which you may have realised is a rather large city!

So, I backtracked, and entered the population data, using the GISS database. GISS either show <10,000, or the actual population, so I have split the Southeast database into two, one for “rural”, or less than 10,000, and the other half for urban, or more. The numbers split almost equally, with 18 rural, and 19 urban.

The difference between the two sets in Figures 4 & 5 is startling.

Rural stations now show more lows than highs, with a ratio of 0.86 highs to lows during the last decade. In contrast, urban stations show a ratio of 1.61.

Figure 4

Figure 5

It is clear from analysing the numbers that it is record lows, and not highs, which are making the urban and rural ratios so different, as the Table below shows.

Rural stations are actually showing a slightly higher number of record highs than urban ones, relative to the average. But, with daily lows, rural sites have posted double the number compared with urban.

It is hard to find clearer evidence, that night time temperatures have been biased upwards by the Urban Heat Island effect in recent years.

OK, these are just daily records for one region. What does any of this tell us about annual trends for the country as a whole?

The second part of the question will be answered in due course, once I have run through the other eight regions. But the following comment in the NCAR press release, introducing Meehl’s work is significant:

The study also found that the two-to-one ratio across the country as a whole could be attributed more to a comparatively small number of record lows than to a large number of record highs. This indicates that much of the nation’s warming is occurring at night, when temperatures are dipping less often to record lows. This finding is consistent with years of climate model research showing that higher overnight lows should be expected with climate change.

So NCAR admit that much of the warming has occurred at night, and believe that this is caused by “climate change”. The evidence from the Southeast suggests that this is not the case, and that night time warming is largely the result of UHI.

It is also worth bearing in mind, that many of the stations, which I have labelled as rural, could still be small towns with their own UHI effect, given that GISS lump all sites with populations of less than 10,000 together. Lexington, VA is a good example of this. It has a population of 7000, and a ratio of highs to lows during the last decade of 2.82.

Clearly, a lot more work needs doing, but this exercise suggests a lot of serious questions need to be asked about the true impact of the Urban Heat Island effect.

References

The CDIAC interface is here. Go play!

http://cdiac.ornl.gov/climate/temp/us_recordtemps/dayrec.html