Recent temperature trends

The oscillatory form of time-series variations portrayed by hydroclimatic variables depicts values that move gradually and smoothly between successive maximum and minimum values. For convenience of description, the dominant characteristics of the undulating pattern are identified as cycles and/or trends. Wave physics provides the basic terminology to describe cyclic changes in terms of the duration and magnitude of the wave pattern. The sinusoidal pattern characteristic of cycles is the product of variable fluctuations about some specified mean value. If there is an increasing or decreasing change in the mean value during the period of record the resultant time-series is time dependent and is described as a trend. Trends in hydroclimate variables are important because they signal a continuing change over time that may be attributed to either natural or human factors. Our ability to detect trends in most hydroclimatic variables is hindered by the relatively short record available for analysis. Record length is a primary factor in the study of trends because an apparent trend in a short record may be part of an oscillation in a long record for a specific variable. In general, research on hydroclimatic trends has been geographically dispersed, sporadic in time, and stimulated by individual events (Hunt, 2001).

8.10.1 Global temperature

A multitude of problems surround efforts to define global temperature, and these problems are fundamental to hydroclimate and the global climate change debate. The annual global temperature anomaly time-series is recommended by the IPCC (2001) because it is widely recognized as a representative

1846 1876 1896 1916 1936 1956 1976 1996 Year

Fig. 8.10. Global temperature anomalies for 1856-2005 relative to the 1961-90 mean. The gray line is the annual value, and the bold line is the 9-year moving average. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/trends/temp/jonescru/jones.html)

1846 1876 1896 1916 1936 1956 1976 1996 Year

Fig. 8.10. Global temperature anomalies for 1856-2005 relative to the 1961-90 mean. The gray line is the annual value, and the bold line is the 9-year moving average. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/trends/temp/jonescru/jones.html)

depiction of global surface conditions (Fig. 8.10). These records have been adjusted to take into account urban heating effects, instrument changes, instrument location changes, and other factors that influence the reliability of the instrument record. This time-series for 1856 to 2005 indicates an increasing global temperature trend of 0.6 °C ± 0.2 °C. The overall trend is composed of short-term increasing and decreasing temperature trend segments that form the complete record. Probably the most commonly recognized segments are the warming trend from 1856 to 1945, the cooling trend from 1946 to 1975, and the warming trend from 1976 to the present. Several intermediate points in the time-series support a perception of trend that is different from the trend evident in the period from 1856 to 2005. Temperature reconstructions indicate that twentieth century global temperatures are approaching the warmest temperature that occurred around AD 990 (Esper et al., 2002), and at least a part of the recent warming cannot be solely related to natural factors (Rybski et al., 2006).

At scales smaller than the global scale, annual temperatures display a complex pattern of changes that elude generalization. The Northern Hemisphere pattern is more similar than the Southern Hemisphere pattern to the global temperature time-series. The annual temperature pattern for land areas displays greater variability than the global time-series, while the SST time-series displays closer similarity with the global temperature. Seasonal temperature anomalies and anomalies calculated for different latitudinal zones reveal temperature trends that are increasing, decreasing, and unchanged (IPCC, 2001).

The year-to-year variability of temperature in the high latitudes is several times larger than that observed in other areas. However, the statistical significance of

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1835 1855 1875 1895 1915 1935 1955 1975 1995 Year

Fig. 8.11. January, July, and annual temperatures for 1835-2002 at Hanover, New Hampshire (44° N). (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

1835 1855 1875 1895 1915 1935 1955 1975 1995 Year

Fig. 8.11. January, July, and annual temperatures for 1835-2002 at Hanover, New Hampshire (44° N). (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

the warming is actually greater in the lower latitudes. Variability as measured by the standard deviation is about 0.04 °C greater in the Northern Hemisphere for all latitudes. There appears to be little relation between interannual variability and the relative warmth or coldness of decadal averages except in winter at high latitudes. Karl et al. (1993) and Easterling et al. (1997) suggest that strong evidence exists for a widespread decrease in the mean monthly diurnal temperature range (DTR) over the past several decades. DTR is derived from an average of daily maximum and minimum temperatures. The rise in minimum temperatures (0.84 °C) occurred at a rate three times that of the maximum temperature (0.28 °C) during 1951-93. The DTR decrease is approximately equal to the increase in global mean temperature, and urbanization alone cannot account for the widespread DTR decrease. Change is detectable in all seasons and most regions, but the magnitude of change is inconsistent. Some regions, such as the British Isles, the Iberian Peninsula, India, central Canada, and certain coastal areas of North America have experienced DTR increases (Durre and Wallace, 2001).

8.10.2 Individual station temperatures

The behavior of individual stations reveals a broad array of annual temperature trends. The diversity found in the station records is a reminder that spatial and temporal averaging may mask important information.

Hanover, New Hampshire (44° N), is in the Upper Connecticut River Valley in the northeastern United States at an elevation of 183 m. It is 153 km inland from the Atlantic Ocean, and it is climatically influenced by continental factors more than marine conditions due to the dominance of westerly atmospheric flow at this latitude. The annual temperature at Hanover (Fig. 8.11) displays an

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1871 1891 1911

1931 1951 Year

1971 1991

Fig. 8.12. Annual temperatures for Davis, California, and Redding, California, for 1871-2002. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

1871 1891 1911

1931 1951 Year

1971 1991

Fig. 8.12. Annual temperatures for Davis, California, and Redding, California, for 1871-2002. (Data courtesy of NOAA's National Climate Data Center and the Oak Ridge National Laboratory, Carbon Dioxide Information Analysis Center from their website at http://cdiac.ornl.gov/epubs/ndp/ushcn/usa_monthly.html.)

increasing trend of 0.01 °Cyr-1 for the period 1835 to 2002 and ranges from a low of 4.3 °C in 1875 to a high of 8.8 °C in 1999. The mean January temperature at Hanover displays no trend from 1835 to 2002 even though the mean January temperature ranges between -14 °C in 1857 and 1888 and -1.4 °C in 1932. The mean July temperature at Hanover for 1835 to 2002 is characterized by an increasing trend of 0.01 °Cyr-1, and it ranges from a low of 16.3 °C in 1844 to a high of 23.2 °C in 1999. These data reveal that the annual warming trend at Hanover is not occurring equally in all months nor is the response of each year similar. In addition, the Hanover temperature time-series show little similarity with the general pattern of the global temperature anomaly time-series in Figure 8.10. In marked contrast to the Hanover data is the temperature time-series for Hohenheim University in Stuttgart, Germany, beginning in 1878, which closely resembles the trends in the global data (Wulfmeyer and Henning-Muller, 2006).

Another complication regarding temperature is illustrated by the temperature trends at two California stations (Fig. 8.12). Redding (41° N) and Davis (39° N) are both inland stations 125 to 150 km from the Pacific Ocean. Redding is 225 km north of Davis and 1736 m higher in elevation. Annual temperatures at Davis from 1871 to 2002 increased 0.01 °Cyr-1, but Redding annual temperatures decreased by 0.01 °Cyr-1 from 1876 to 2002. Both stations are included in the USHCN data set and the records are considered to be reliable. The temperature time-series for these two stations are representative of regional characteristics that extend to approximately halfway between the two sites. Also noteworthy in the Davis and Redding data is the absence of the alternating increasing and decreasing temperature trend segments evident in the global temperature anomaly time-series.

1901 1921 1941 1961 1981 2001

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Fig. 8.13. Global land surface precipitation for 1901-2002. (Data courtesy of NOAA's National Climate Data Center from their website at http://www.ncdc.noaa.gov/gcag/ gcag.html.)

1901 1921 1941 1961 1981 2001

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Fig. 8.13. Global land surface precipitation for 1901-2002. (Data courtesy of NOAA's National Climate Data Center from their website at http://www.ncdc.noaa.gov/gcag/ gcag.html.)

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