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Most of North America Fails to Board 20th-Century Climate Catastrophe Train
Volume 6, Number 9: 26 February 2003

If global warming has truly been occurring at an unprecedented rate over the past hundred years, as climate alarmists claim it has, and if global warming truly produces more extreme hot weather events, as climate alarmists claim it does, it should be an easy matter to either verify or refute those claims via a study of extreme hot temperature trends over the past century.  So why doesn't someone perform such a study?  Actually, a pair of climate scientists has recently done just that, as we describe in the following paragraphs.

DeGaetano and Allen (2002b), working with data from the United States Historical Climatology Network (USHCN), first created a Daily Historical Climatology Network for Extreme Temperature (HCN-XT) dataset (DeGaetano and Allen, 2002a), which they then used to determine how both hot and cold temperature extremes - defined in terms of the number of exceedences of the 90th, 95th and 99th percentiles of their respective databases - have varied across the contiguous United States over a number of different time scales.  So what did they find?

Over the period 1960-96, DeGaetano and Allen determined that "a large majority of stations show increases in warm extreme temperature exceedences," which would seem to corroborate the claims of the world's climate alarmists.  They also report that "about 20% of the stations experience significant [our italics] increases in warm maximum temperature occurrence," again in seeming vindication of climate-alarmist claims.  Furthermore, they note that "similar increases in the number of >2 and >3 runs of extreme temperatures occur across the country," apparently substantiating climate-alarmist claims of an increasing frequency of deadly heat waves.  However, when the two scientists extended their analyses further back in time, some quite different results were obtained.

Adding another 30 years of data onto the front ends of their databases, DeGaetano and Allen discovered there were "predominantly decreasing [our italics] warm exceedence trends across the country during the 1930-96 period."  In fact, they found that "in the 1930-96 period 70% of the stations [our italics] exhibit decreasing high extreme maximum temperature trends."  Hence, to paraphrase the title of our Editorial of 1 July 2000, it is clear there has been no net increase in extreme hot weather events for the past 70 years.

But, of course, this is only the United States of which we speak, right?  Right.  But, the United States has one of the best maintained and managed networks of weather stations in the world, which makes them some of the most accurate in the world.  In addition, DeGaetano and Allen note that "Bonsal et al. (2001) find little evidence for a consistent change in extreme maximum temperatures across Canada during the entire or last half of the twentieth century."  Furthermore, Polyakov et al. (2002), who recently developed a new-and-improved Arctic-wide temperature history, report that linear regression trends calculated from the 1920s to the present show a small but statistically significant cooling tendency.  Consequently, from the US-Mexico border all the way to the North Pole, there is no evidence for any net increase in either mean temperatures or extreme warm temperatures or heat waves over the last seven decades of the 20th century, demonstrating that for the vast majority of North America, all of the net increase in temperature experienced since the beginning-of-the-end of the Little Ice Age was essentially complete by the 1930s.

Yet even these findings - as effective as they are in contradicting claims of impending climatic catastrophe - do not describe the full extent of the scientific basis for rejecting the erroneous pronouncements of the world's climate alarmists.  Hence, we bring you next the "Paul Harvey half" of DeGaetano and Allen's results, i.e., the rest of the story, which is a tale of temperatures improperly adjusted for the burgeoning effects of growing urban heat islands.

The opening sentence of DeGaetano and Allen's summary states that "trends in the occurrence of maximum and minimum temperatures greater than the 90th, 95th, and 99th percentile across the United States are strongly influenced by urbanization."  With respect to daily warm minimum temperatures, for example, the slope of the regression line fit to the data of a plot of the annual number of 95th percentile exceedences vs. year over the period 1960-96 was found to be +0.09 exceedences per year for rural stations, +0.16 for suburban stations, and +0.26 for urban stations, making the rate of increase in extreme warm minimum temperatures at urban stations nearly three times greater than the increase at rural stations less affected by growing urban heat islands.  Likewise, the rate of increase in the annual number of daily maximum temperature 95th percentile exceedences per year over the same time period was found to be 50% greater at urban stations than it was at rural stations.  Yet in spite of this vast uncorrected-for-bias, when computed over the much longer 1930-96 period, 70% of all stations in the HCN-XT data set still exhibited "decreasing high extreme maximum temperature trends."

In conclusion, it is clear that when it comes to extreme warm temperature events over most of North America, they are no more prevalent currently than they were in the 1930s.  In fact, they may well be even less prevalent nowadays.  Also, there is strong evidence implicating the growing influence of intensifying urban heat islands as being responsible for the apparent rapid increase in the mean annual temperatures of many locations over the last two decades of the 20th century.  Hence, even for the part of the world that shows little, if any, net warming over the past 70-some years, what little there may or may not be is likely still too much.

Sherwood, Keith and Craig Idso

Bonsal, B.R., Zhang, X., Vincent, L.A. and Hogg, W.D.  2001.  Characteristics of daily extreme temperatures over Canada.  Journal of Climate 14: 1959-1976.

DeGaetano, A.T. and Allen, R.J.  2002a.  A homogenized historical temperature extreme dataset for the United States.  Journal of Atmospheric and Oceanic Technology 19: 1267-1284.

DeGaetano, A.T. and Allen, R.J.  2002b.  Trends in twentieth-century temperature extremes across the United States.  Journal of Climate 15: 3188-3205.

Polyakov, I., Akasofu, S-I., Bhatt, U., Colony, R., Ikeda, M., Makshtas, A., Swingley, C., Walsh, D. and Walsh, J.  2002.  Trends and variations in Arctic climate system.  EOS, Transactions, American Geophysical Union 83: 547-548.