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white1.GIF (834 bytes) A Climate Gift from Rothamsted

By Robert C. Balling Jr., Ph.D.
Arizona State University

Long-running and continuous climate records from pristine sites are difficult to find—especially for temperature. But those few records that do exist have a special place in the ongoing debate about global warming and the greenhouse effect. One such climate record comes from a famous agricultural experiment station located in southeastern England. Nearly 150 years ago at this site, pioneering agricultural scientist Sir John Lawes began measuring precipitation to determine its effect on crop growth and the leaching of nutrients from the soil. The site would become the home of the Rothamsted Experimental Station, which today is a part of the United Kingdom’s Institute of Arable Crops Research.

As the years passed, Lawes and Sir Henry Gilbert began to look more comprehensively at plant growth, including a focus on the thermal environment of the crops. Accordingly, they began to carefully measure daily air temperatures in 1878 with thermometers very similar to those in use today. Observations of daily maximum and minimum temperatures, along with a variety of other climate elements, continue to this day every morning at 9 a.m.

Over the entire 121-year record, there are no missing data, giving Rothamsted the distinction of having one of the longest continuous sets of weather recordings in the world. And because Rothamsted has been a center of agricultural research over the past 150 years, the landscape around the meteorological station has remained relatively unchanged over the past century and a half. Major microclimatic contaminations to the record, such as effects of urbanization, should be minimal at the site, though the town is near Harpenden, a town that has seen some growth over the past century.

Figure 1 shows the mean annual temperatures from Rothamsted —remarkably similar to temperature time series presented for the entire globe. Warming occurs from 1878 to 1950, cooling from 1950 through the 1980s; the most recent decade has been dominated by high temperatures. The coldest year by far occurred in 1879, while the warmest year on record occurred in 1997.

Figure 1.  Mean annual temperatures at Rothamsted, 1878 to 1998.

Over the entire 121-year record, Rothamsted warmed by 0.71C (1.28F). The results from the agricultural experiment station are similar to the temperature trends reported recently from other nonurban sites in Europe. Suggested causes of this significant rise in temperature have included a natural recovery from the Little Ice Age; natural fluctuations in the ocean-atmospheric system; an increase in solar output; and of course, the exponential rise in the atmospheric concentration of greenhouse gases.

Four other features of the Rothamsted record are of interest in this regard as well:

1.)  As Figure 2 shows, the partitioning of the warming over the past 121 years into maximum and minimum temperatures by six-month seasons reveals a highly unequal pattern. Winter warming of minimum temperatures accounts for more than 40 percent of all warming observed at Rothamsted; summer maximum temperatures account for only 10 percent of the warming. Once again, a mid-to-high latitude station (52N) observes the bulk of the warming, which occurs basically at night and in winter.

2.)  While the station warmed 0.71C (1.28F) over the past 121 years, it is noteworthy that Rothamsted warmed by 0.66C (1.19F) from 1878 to 1950. Literally 92.5 percent of all warming at Rothamsted had already occurred by 1950!

3.)  With minimum temperatures rising more quickly than maximum temperatures, the diurnal temperature range should be decreasing. Indeed, the daily range has fallen 0.50C (0.90F) over the past 121 years. This decrease in the diurnal temperature range is evident throughout the world and may represent a climate signal directly related to the increase in greenhouse gas concentrations.

4.)  From 1915 to present, the scientists at the experiment station have systematically collected data on sunshine and cloud cover to go along with the ongoing temperature and precipitation records. Both sunshine and rainfall have increased slightly over this 84-year period (not statistically significant), while cloud cover has decreased at a significant rate.

Figure 2. More than 120 years of Rothamsted warming, divided into maximum and minimum temperatures by six-month seasons.

The agricultural experiments at Rothamsted have been conducted for nearly a century and a half, leading to outstanding scientific achievements in crop nutrition, soil science, insecticides, virology, and biometrics. The unwavering commitment to collect climate records at this site provides us with a reasonably accurate account of climate trends and variations for more than a century.

How have plants and crops at Rothamsted fared given these ongoing changes in climate. Plants at Rothamsted must now cope with a slight increase in sunshine and precipitation and warmer nights in the winter; plants also are now enjoying the many biological benefits associated with atmospheric carbon dioxide levels’ increasing from 300 ppm to 360 ppm. And crops there now perform better than their ancestors in the days of Lawes and Gilbert.

Acknowledgment:  We are grateful for the cooperation of Robin Thompson and others at IACR-Rothamsted who kindly supplied us with the climate data from the experiment station.