Frequency and Severity of Drought and Dust Storms E.E. Wheaton

Climatologist, Environment Division, Saskatchewan Research Council, Sashtoon, Saskatchewan.

INTRODUCTION The problems created by climate and weather continue to be the primary variables facing agricultural production and success (Olson 1990). Drought and its associated dust storms are among the foremost of these problems. Although the droughts and dust storms of the 1930's are renowned, the "dirty eighties" were also reminders that these problems associated with arid to semiarid climates continue to result in considerable stress on the environment and economy. The study area is the Canadian Prairies and an emphasis is placed on the droughts and dust storms of the 1940's.

The purposes of this paper are: (1) to examine the connection between drought and dust storms; (2) to examine the potential changes in drought and dust storms with future climatic changes; and (3) to present recommendations.

THE DROUGHT-DUST STORM CONNECTION Dust storms and their intensive wind erosion are thought to be almost synony- mous with droughts, but what is the nature of this relationship? Dust storms are frequent during drought years, but dust storms can also occur during relatively short lived dry conditions. This and other drought- dust storm connections are explored in this section. The relationship between dust storms and droughts deserves attention for several reasons including: the provision of further information regarding these important events; the degree of linkage (i.e., do dust storms occur in the absence of drought?) and the relative intensities and frequencies of these phenomena.

A comparison of the time series of drought as measured by the Palmer Drought Index (PDI) and the frequency of dust storms is useful to demonstrate some qualitative aspects of the relationship. Dust storm data is not available prior to 1977. The worst drought conditions for southern Saskatchewan during the 1977-1988 period, as indicated by the April PDI, occurred during 1981 (Figure 1). Close secondary spring droughts occurred during 1977, 1988, as well as 1980 and 1982.

Spring, especially April, is the season during which the risk of dust storms is greatest on average (Wheaton and Chakravarti 1987 and 1990). Spring data, therefore, are most suitable for the comparison of drought and dust storms. 1981 was also the year with the greatest April dust storm frequencies in the

Can. J. Agric. Econ. 38 (1990) 695-700 695

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Saskatchewan during the 1977 to 1988 time period (Tible 1). Saskatchewan contains the area with the greatest risk of dust storms in the Canadian Prairies. Secondary April peaks ofdust storm totals occurred during 1977, 1987 and 1982.

Bble 1. April dust storm totals and ASpril Palmer drought indices in order of severity, southern Saskatchewan. Year April dust stom totals 1981 57 1977 31

1987 19

1982 18 1984 8

1988 5 1985 3

1986 1 1978 1979

0

0

Year 1981

1977

1988

1980 1982

1978

1984 1987

1983 1979

April PDI -3.31

-2.89

-2.76

-2.73 -2.25

-2.09 -1.50 -1.06

-0.82 -0.56

1980 0 1986 -0.40 1983 0 1985 -0.01

Sources: PDI data, K. Jones, Pers. Comm. (1990) (10 station southern Saskatchewan averages). Dust storm data, Wheaton and Chakravarti (1990), remainder abstracted from

1977 1979 1981 1983 1985 1987 1978 1980 1982 1984 1986 1988

YEAR

__t_

APR DUST __e__

APRIL PDI

Figure 1. April PDI and dust storms southern saskatchewan.

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The time series and intensities of droughts and dust storms appear to show a close correspondence. The year of the highest dust storm total for April, 1981, coincided with the year of the worst April PDI. Several of the secondary droughts and dust storm totals also matched, including 1977,1982 and 1988. 1980 had a low April PDI, indicating a dry spell, but had no recorded April dust storms. However, the annual total did show a moderate number of dust storms that year. 1987 totalled a high number of dust storms in April, but was a near normal spring in terms of PDI. These preliminary assessments must be pursued and quantified to improve the understanding of the relationship between drought and dust storms.

A case study of the drought year of 1988 shows further correspondence between patterns of the PDI and the frequency of dust storms. The drought climatology and dust storm conditions of 1988 are examined in detail in Wheaton and Arthur (1989). The worst PDI conditions during 1988 occurred in June in Saskatchewan with a large wedge of severe drought extending from North Dakota northward into the northern agricultural area of Saskatchewan. The most extensive coverage of dust storm activity in 1988 appeared to occur in June as the greatest number of stations reported dust storms during this month (Wheaton and Arthur 1989). This demonstrates a close correspon- dence between PDI and dust storm frequencies at this time.

The spatial pattern of June PDI and dust storm day totals for 1988 also shows some correspondence; high dust storm totals match very low PDIs (drought to severe drought) from the US border, northwards to the Saskatoon area. However, dust storm activity increases westward, whereas the PDIs decrease slightly westward from the core intensity, (although drought condi- tions prevailed). The reason for this difference is not known at this time and should be explored.

The correspondence between drought, as indicated by the PDI, and dust storms shows some similar patterns both spatially and temporally. However, there are dust storm years during which the PDI indicated close to normal moisture conditions (e.g., 1983 and 1985). This is likely a result of the slow response of the PDI as compared to the faster response of a dust storm event. Also the meteorological conditions that produce droughts and dust storms differ, especially in the dependence of dust storms on atmospheric distur- bances. The reasons for these apparent discrepancies in the relationship between drought and dust storms as well as the reasons for the correspondence should be investigated. Such studies may provide useful clues to improve the understanding of drought and dust storms.

Another connection between drought and dust storms that should be considered is the effect of atmospheric dust on climate, including droughts. Atmospheric dust both scatters incoming short wave solar radiation and absorbs solar and terrestrial radiation. The effect on the radiation budget also depends on the height of the atmospheric dust layers. Idso and Braze1 (1977) conclude that as atmospheric dust concentration in the lower atmosphere

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rises, the net radiation balance initially produces warming of the Earth’s surface, but under conditions of frequent and severe dust stormswith very high atmospheric dust concentrations, the warming influence is reversed.

An increase in atmospheric dust particles may lead to enhanced atmos- pheric subsidence and associated droughts for some areas such as the Sahara (Chakravarti 1978). Bryson and Murray (1977) state that atmospheric dust is implicated in drought and more highly variable weather globally. Therefore it is possible that dust storms act as positive feedback mechanism to enhance the possibility of persistence and increasing the intensity of a drought. This aspect of the relationship of drought and dust storms is complex, but must be further examined to improve the understanding of these phenomena in the Canadian Prairies.

DROUGHT AND DUST STORM OUTLOOK FOR FUTURE CLIMATES

The only work addressing the potential effects of wind erosion as a result of the enhanced greenhouse effect on wind erosion appears to be the series by Wheaton (Wheaton 1984a, Williams et al. 1988 and Williams and Wheaton, In Prep.). There is no published literature which examines the effect of future climatic change on dust storms and their trends. Therefore the effect of climatic warming on wind erosion potential is emphasized in this section. Because of the significance of dust storms, the dynamic nature of climate and the increasing risk of global warming, this activity is recommended.

Results of the Williams et al. (1988) and Williams and Wheaton (In Prep) work demonstrate that wind erosion potential would continue to rise with rising temperatures. The wind erosion climatic factor was calculated for two General Circulation Model (GCM)- based doubled greenhouse gas climatic scenarios; one using the precipitation adjustments projected by the model and the other using normal precipitation. The doubled greenhouse gas scenario without the increase in precipitation resulted in wind erosion potentials ranging from 22 to 47% above normal. This scenario also resulted in decreases in estimated biomass; decreased vegetation cover is known to be a factor in increasing wind erosion amounts.

The wind erosion potentials were estimated to shift north-eastward as well, resulting in greater wind erosion damage in more northern locations than with the present climate.

However, wind erosion potential is sensitive to changes in precipitation and projected increases in precipitation may be sufficient to offset the increases in wind erosion potential. The climatic change scenario using increased amounts of precipitation resulted in lower wind erosion potentials; most of the values were in the range from 80 to 90% of normal. This illustrates the importance of having realistic precipitation scenarios in projections of future climate for use in agricultural impact studies.

Presently there is a greater difference in the amounts of annual rainfall as compared to annual temperatures projected by the General Circulation Mod-

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els being used to explore an enhanced greenhouse effect climate of the future (White 1990). One model projects significant decreases in the annual rainfall of the Great Plains, while others project modest increases as compared to other areas.

One of the first studies to explore the potential changes of frequency and intensity of drought with the enhancement of the greenhouse effect was by Williams et al. (1988). The results indicated that for a doubled greenhouse climate droughts would be more frequent and severe; the return period for a severe drought would be only about half as long as for the current climate. For a climate changed by warming but not by precipitation increases, a several-fold increase in drought frequency could be expected.

The implications of these findings for dust storm frequency and intensity in an enhanced greenhouse climate are significant in light of the preliminary comparisons of this paper. This work has indicated that both the time and spatial patterns of drought and dust storms are related with some noted discrepancies. The Williams et al. (1988) conclusions for drought in the context of this paper imply that the dust storms could become more frequent and severe with climatic warming.

CONCLUSIONS In terms of adaptive responses and policies, anything that can be done now to alleviate problems with drought and dust storms will help considerably in dealing with problems under global warming conditions. Diversification has a key role in mitigating the adverse effects not only of present climate, but also of future climates.

The 1980's should emphasize the warning that the soil still blows in the region of the

Prairie Dust Bowl when drought, wind and inadequate vegetative cover pro- vide the necessary factors for dust storms. Farmers must continue to make major adjustments in their farming operations as changing conditions dictate. Producers, Governments and everyone concerned must realize the value of planting more drought resistant crops, of diversification and of reducing/ managing grazing on pasture lands during dry periods. Given the right con- ditions, the Dust Bowl can return just as certainly as the recurrence of droughts. Adequate preparation and adaptation can mitigate or avoid the damage wrought by both droughts and dust storms.

ACKNOWLEDGEMENT The author wishes to thank Dr. W. Wilson, Forestry Canada and V. Wittrock for their comments.

REFERENCES Bryson, R.A. and T.J. Murray. 1977. Climates ofhlungez University ofWiscon- sin Press, Madison, Wisconsin.

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Chakravarti, A. K. 1978. A Case of Droughts in the Sahara: the Effectof Removal of Natural Vegetation on Rainfall. Alternatives, Perspectives on Society and the Environment. Vol. 755-56. Environment Canada, Atmospheric Environment Service (AES). 1981-88. Monthly Record: Meteorological Observations in Canada. AES, Downsview, Ontario. Idso, S.B. and A.J. Brazel. 1977. Planetary radiation balance as a function of atmospheric dust: climatological consequences. Science 198,731-733. Jones, K. 1990. Personal Communication. K. Jones is a meteorologist with the Atmospheric Environment Service, Regina, Saskatchewan. Olson, A. 1990. Witness Presentation in Minutes of Proceedings and Evidence of the Standing Committee on Environment Respectinga study of Global Warm- ing. Dr. Olson is Assistant Deputy Minister, Research Branch, Agriculture Canada. Issue No. 47, May 15,1990. Wheaton, E.E. 1984. The Impacts of Climatic Variations on Wind Erosionof Soil in the Canadian Prairies: the Case of Saskatchewan. SRC Publication No. E-906-29-D-84. Presented at the a s k Force Meeting, Assessment of Climate Impacts on Agriculture in High Latitude Regions, Oct. 1-5,1984, Laxenburg, Austria. Wheaton, E.E. and L.M. Arthur (eds). 1989. Some Environmental and Eco- nomic Impacts of the 1988 Drought. SRC E-2330-4-E-89. Saskatchewan Re- search Council, Saskatoon, Sask. Wheaton, E.E. and A.K. Chakravarti. 1987. Some Rmporal, Spatial and Climatological Aspects of Dust Storms in Saskatchewan. Climatological Bul- letin, 21(2):5-16. Wheaton, E.E. and A.K. Chakravarti. 1990. Dust Storms in the Canadian Prairies. InternationaZJournal of CZimatology, Vol. 10 (In Press). White, R.M. 1990. The Great Climate Debate. Scientific American. Vol.

Williams, G.D.V., R.A. Fautley, K.H. Jones, R.B. Stewart and E.E. Wheaton. 1988. "Estimating Effects of Climatic Change on Agriculture in Saskatchewan, Canada." In Parry, M.L., Carter, TR. and Konijn, N.T. (eds), u e Impact of Climatic Variations on Agriculture, Vol. 1: Assessments in Cool Temperate and Cold Regions, Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 219-379. Williams, G.D.\! and E.E. Wheaton. In Prep. Estimatinglmpacts on Saskatch- ewan Biomass and Wind Erosion for Several Climatic Scenarios. E-906-44-A- 88. Saskatchewan Research Council, Saskatoon.

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