Has 2009-2010 been the warmest period ever?

Recently, Canada’s Global National News (Friday, July 16, 2010) carried a story indicating that 2010 was the hottest year on record. A follow-up article in Victoria’s Times-Colonist newspaper (July 18, 2010, p.A12) made a similar claim. The claims remind me of the children’s story “The Emperor has no Clothes,” about a tailor who managed to convince the emperor and his entourage that they were incapable of seeing the wonderful tunic he was sewing. When the emperor paraded in his new suit, a child exposed the scam by asking why the emperor had no clothes on.

Thus it is with climate. Although Victorians experienced a mild winter and spring, the last few months have been colder than usual. The Canadian prairies experienced a miserable winter and things were not much better elsewhere in North America and Europe. However, a few weeks of warm weather in Europe and eastern Canada, and it becomes easy to sell the idea that temperatures are the highest they have ever been and that 2010 is on course for become the warmest year ever! Is this so?
NASA temperature data underlie these stories. Consider the ten warmest years as reported in Table 1. The data in the table are from http://data.giss.nasa.gov/gistemp/graphs/Fig.D.txt, but viewed on three different dates as indicated. Since data collected in August 2007 are available only through 2006, 2007 is not included in the earliest ranking given in Table 1.

Table 1: Ten Warmest Years based on Average Contiguous 48 U.S. Surface Air Temperature Anomalies, 1880 through 2006

Date when data was retrieved from NASA website
August 20, 2007   May 27, 2009   April 26, 2010
1934   1934   1998
1998   1998   2006
1921   1921   1934
2006   2006   1921
1931   1931   1999
1999   1999   1931
1953   1953   1990
1990   1990   2001
1938   1938   2005
1939   1954   2007

NASA scientists have adjusted the data in ways that make more recent years appear warmer – scientists have dropped weather stations and/or used a different method for ‘adjusting’ the data. Thus, the number of years from the past two decades that appear in the top twenty warm years (only the top ten are shown in Table 1) has increased from 7 to 8 and finally to 11. In the May 2009 listing, 2007 is the 14th warmest year in the historical record, but it has moved up to tenth by the April 2010 listing. While it may be true that the latest adjustments are scientifically better than earlier adjustments, it seems odd that the most recent years now show up as among the warmest in the temperature record, contrary to evidence from satellite data and reconstructions by the Climate Research Unit at East Anglia University in the UK – reconstructions that use the same underlying data as NASA.

Other evidence fails to support the NASA story. Using the unadjusted U.S. weather station data, 22 out of 50 states recorded their highest temperature during the 1930s. Likewise, a 2006 study in the Journal of Geophysical Research found that 1941 was the warmest year experienced in Greenland between 1784 and 2005, while the 1930s and 1940s were the warmest decades; 1863 was the coldest year while the coldest decade was the 1810s (although it corresponded to two volcanic eruptions in 1809 and 1815). A 2010 study in Climatic Change reconstructs winter and spring temperatures for Stockholm going back 500 years. The warmest and coldest years are reported in Table 2. Notice that 1863 was the warmest winter/spring, while 1569 was the coldest.

Table 2: Ten coldest/warmest January–April Seasons in the Past 500 Years, Stockholm, Sweden, Temperature Anomalies in oC from 1961–90 Average

Rank Year   Rank Year
Ten coldest years   Ten warmest years
1 1569   1 1863
2 1573   2 1990
3 1557   3 1743
4 1595   4 1525
5 1572   5 1989
6 1942   6 1605
7 1614   7 1822
8 1600   8 1790
9 1574   9 1762
10 1940   10 2008
           

Satellite data are arguably the most reliable data available, but these begin in December 1978. From then until the end of June 2010, temperatures have gradually risen at a rate of 0.005oC per decade. If this trend continues, then we can expect an increase in temperature of only 0.05oC by 2100. This is seen in Figure 1. Notice in the Figure that neither the last six months of 2009 nor the first months of 2010 have been the highest on record (since 1978), and that temperatures since April have actually been falling. Also note that the highest temperatures occurred in 1998, and these coincided with an El Niño event while the recent high temperatures also coincide with (a weaker) El Niño event. Thus, they are the result of non-human factors.

Figure 1: Average Monthly Global Temperature Anomaly, Satellite Temperature Data, December 1978 through June 2010

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