The Alaskan record demonstrates possible magnitudes of some of these systematic errors. Both winter and summer heat island effects are large. Site changes (including documentation problems) may have unexpectedly large effects, and virtually the entire state is affected by a change in time zones in 1983.
This paper deals with three examples of local variations or apparent "climate changes" probably due to urban effects, plus one possible real variation in Alaskan climate.
An apparent long-term summer warming
While looking for differences in the character of summer and winter temperature records across the state, we noticed that virtually all of the large negative summer minimum temperature anomalies at Fairbanks occurred during the early years of the record, while the large positive anomalies were recent. When trend lines were fitted to individual monthly mean minimum and maximum temperatures, the May, June, July, and August minimum temperatures at Fairbanks showed significant upward trends with time
|Slope (degrees F / year)||0.084||0.091||0.102||0.075|
|Slope (degrees F/year)||0.095||0.085||0.143||0.078|
The Fairbanks area is subject to strong urban heat island effects (measured to be as much as 13 °C) when skies are clear in winter (Bowling and Benson, 1978). Summer heat islands have hardly been looked at, but they now appear likely to have more effect on recorded climate than do the large winter ones, which are masked by large year-to-year variability in winter and seem to be more confined to the city core.
Why doesn't Anchorage show a similar effect? The most probable culprit is the Good Friday Earthquake of 1964. The earthquake leveled the Control Tower, which required that the instruments be relocated. The max/min thermometers were at Point Campbell, farther from pavement and nearer the waters of Cook Inlet, by 1971, but there is apparently no published record of their location during the intervening period.
Local variability during January 1989
January of 1989 was a record setter in much of Alaska. Not, however, officially in Fairbanks, where the official temperatures are recorded downwind of the city. Unofficial temperatures on the upwind side of Fairbanks were much lower, as were many at surrounding climatological stations
The Misplaced Move and the Encroaching City
Recorded station moves are not necessarily consistent among different sources, nor are the dates always accurate. Take the case of the downhill move at the University Experiment Station. The station list in Climatological Data, Alaska shows an elevation change from 500 feet to 475 feet in the summer of 1947. Was it real, or the result of re-surveying the area? With known current winter inversion strengths in the University area, such a move could have produced a decrease of a few degrees in recorded winter temperatures. Comparison of the University record for December and January with the Fairbanks record from Weeks Field (near where the Borough Library is now located) did indeed show either a decrease in University temperature or an increase in the Weeks Field temperature, but suggested a change in the summer of 1946 rather than 1947. The actual station history for the University Experiment Station confirms that the move was real, but it took place in 1933, 15 years before it was finally brought up to date in Climatological Data.
Trends of temperature differences (Weeks Field - University), 1943-1951.
|Month||Points||Slope, °F/yr||Correlation Coefficient||Significance|
|February||8||.08||.17||Less than .80|
Some of the large deviations from the trend line can even be explained: December 1946 and January 1947, both of which show large positive deviations from the trend line, are known to include a record-setting cold spell which, from previous studies, is known to be an ideal setting for a large heat island effect. But unanswered questions remain. Why does the heat island appear to weaken during late winter and spring? Why do so many months have large deviations from the trend line in 1947?
The 1976 temperature step: a real change?
The 1976-77 winter in Alaska was astonishingly warm. At Fairbanks, pussy willows bloomed in November (author's observation) and daily average temperatures never reached -30°F. Subsequent winters followed the same trend to a lesser degree, with degree days below -40°F showing a reduction quite noticable to long-term residents. An average of four Alaskan stations with good, continuous records, Anchorage, Barrow, Fairbanks, and Nome, show what appears to be a change in mean annual temperatures at around this time
.All of these stations are in areas that could be affected by urban effects, but a similar step, which there appears to interrupt an overall downward trend, appears at McGrath, with a population of under 1000 people. Is it real? And will it last?
In an attempt to find a somewhat independent measure of climate shift, be it temporary or permanant, we used 700 mb grid point heights at 60°N 150°W, 65°N 160°W, 65°N 140°W, and 70°N 150°W to estimate changes in geostrophic flow. The usual net north-south and east-west flows would have been ambiguous as to whether an increase in net southerly flow, for instance, was caused by an increase in the frequency or intensity of southerly flow or a decrease in northerly flow. To get around this, we summed the positive differences between 160°W and 140°W as northerly flow and the negative differences as southerly flow at 65°N over Alaska, dividing by the actual number of soundings used to compensate for missing data. The differences between 60°N and 70°N at 150°W were similarly treated, with positive differences contributing to westerly flow and negative differences to easterly flow.
The somewhat unexpected result was that all four indices decreased in absolute value over the period from 1965 through 1986, suggesting a general reduction in the intensity of circulation over Alaska. The only significant decrease, however, was that in northerly flow, which showed a fairly consistent high level early in the time series, rather violent oscillations for several years around the apparent temperature change, and a second steady period, at around two-thirds the previous level, in the first half of the '80's
Use of high-latitude instrumental data to deduce long-term trends is risky at best, and should be attempted only with reference to the fullest available station histories and some knowledge of the local topography, settlement history, and microclimates. Changes documented by more than one type of data (e.g., soundings as well as surface temperatures) can be considered better supported than simple temperature measurements, but such data are rarely available for really long time series.
Bowling, S. A., 1986: Climatology of high-latitude air pollution as illustrated by Fairbanks and Anchorage, Alaska. J. of Climate and Appl. Meteor. 25, 22 34.
Bowling, S. A. andC. S. Benson, 1978: Study of the subarctic heat island at Fairbanks, Alaska. EPA Report EPA-600/4-78-027, 149 pp.