Managing Water Resources in the West Under Conditions of Climate Uncertainty A Proceedings (1991) / Chapter Skim
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5. Climate Change and Climate Variability: The Climate Record
5. Climate Change and Climate Variability: The Climate Record
Pages 47-70
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From page 47... ...
This paper discusses the difference between weather and climate and analyzes examples of recent climate anomalies, including the 1988 North American drought. It also summarizes the climate changes expected from increased greenhouse gas concentrations in light of the observational record.
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From page 48... ...
, and bubbles of air in ice cores reveal that the preindustrial carbon dioxide levels of the last century were around 280 ppm. So concentrations have already increased by 25 percent, largely because of man's burning of fossil fuels and clearing of forests.
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THE 1988 DROUGHT During the summer drought of 1988, there was a proliferation of news media speculations about a possible link between the drought and the greenhouse gas effect. In fact, the best assessment we can make indicates that the drought was, more likely, essentially a natural phenomenon brought about primarily by changes in the tropical sea surface temperatures (SSTs)
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so c:/ \ LL LL En o I z llJ I Cat .
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From page 51... ...
The greenhouse effect may well have made the heat waves hotter and the drought somewhat more severe than it would have been otherwise, but these effects were probably relatively small. The drought would almost certainly have occurred without any enhanced greenhouse effect; droughts are essentially a natural phenomenon and have occurred throughout history.
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Weather arises from internal instabilities in the atmosphere that produce anticyclones and cyclones, and cold and warm fronts, as the equator-topole temperature gradient is continually being eroded by cold polar outbreaks and warm southerlies in these weather systems, even as the incoming solar and outgoing earth radiation help maintain those temperature gradients. Weather, then, is an atmospheric phenomenon, and it has an infinite range of variability.
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For land temperatures, station locations have moved, such as from the central city to the airports in the 1950s, and observation times have changed. Because daily averages sometimes use averages of hourly or six-hourly readings and sometimes simply average the daily maximum and minimum temperatures, changes in observation time can introduce a bias.
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. Figure 5.5 shows uncorrected and corrected SSTs averaged over the Northern Hemisphere as a function of time.
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, keeping in mind the uncertainties discussed above, do reveal a modest warming of approximately 0.5 °C, with the warmest years in the 1980s. Note that in the Southern Hemisphere, the temperature trend is what we might expect the largest temperature increases occurred after World War II when greatest increases in greenhouse gases occurred.
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found in the 1930s, the dust bowl era in which droughts and heat waves prevailed. For comparison, many years have been warmer than 1988, the second-to-last point on Figure 5.7 and the year of our most recent major drought.
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The western states were warmer but the eastern states colder, providing little change for the United States as a whole. Note also the cooling in the North Atlantic and North Pacific, which would not be well captured using only land data.
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Over the North Pacific in winter (November through March) , the Aleutian Low pressure system was more intense than normal from 1977 to 1988, bringing more southerly winds over the West Coast and warmer, moister air into Alaska, while on the back side of the Aleutian Low colder, drier air caused cooling over the North Pacific.
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Small changes in station location and changes in exposure, such as tree growth, commonly corrupt rainfall records, but these changes are generally not detectable unless detailed station history records are kept. Because of the small-scale spatial variability, adjustment of rainfall records for inhomogeneities using an
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In addition, changes in instrumentation through alterations in rain gauge design to address factors such as wind, wetting losses, and evaporation that affect the collection efficiency and measurement of rain, and especially snow, introduce biases with time that must be corrected. National practices for converting frozen precipitation into liquid equivalent vary.
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Time series of precipitation for the three western regions are shown in Figure 5.11. In the Northwest, low-frequency fluctuations lasting several decades are in evidence, with the 1920s appearing as particularly dry.
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. 1 900 1 9 1 0 1 920 1 930 1 940 1 950 1 960 1 970 1 980 YERR ~ 23.6 SOUTHWEST = ~ LONG- TCRn ~:RN- 338.60 nix I hurl- 56 1 .34 newly- t~S.07 ,8 NOT I ONRL CL I MRT I C DBTR CENTER, NORR 63 FIGURE 5.11 Time series of annual mean precipitation amounts in the Northwest, West and Southwest regions in mm (left)
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· In the Southern Hemisphere, the main upward trend has been more steady, beginning after about 1930. · E1 Niffo events influence global mean temperatures by 0.1°C.
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From page 65... ...
as mathematical expressions representing the physical laws that can be solved in a supercomputer. In surveying the modeling results for simulations with increased greenhouse gas con centrations, looking for common features, and assessing the reliability of different models, I have drawn the following conclusions concerning the model results and the observational record: · The models indicate warming of 2 to 5°C for an equilibrium climate with doubled carbon dioxide.
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From page 67... ...
Also, substantial discrepancies exist between the observational temperature record and that expected from models~iscrepancies in the timing of Northern Hemisphere warming in the 1 920s and in spatial patterns between the Northern and Southern hemispheres and between the land and ocean. Increases in water vapor (a greenhouse gas)
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From page 68... ...
Real regional decadal variations appear to exist, but overall trends are quite uncertain. Meaningful comparisons between model results and the observational record are difficult to come by.
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Historic Climatology Series 6.1. Asheville, N.C.: National Climatic Data Center, National Oceanic and Atmospheric Administration.
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1990. Recent observed interdecadal climate changes in the Northern Hemisphere.
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