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Droughts, Floods, and Climate ChangeBack
 
Precipitation affects the availability of water resources, and periods of deficient and excessive precipitation often result in droughts and floods. One of the biggest concerns regarding future water supplies is that potential global warming will change the frequency and/or intensity of severe floods and droughts.

In order to understand what may occur in the future, it is first necessary to examine historical records of past precipitation extremes that led to droughts and floods, as well as the climate conditions that contributed to these events. The following tables provide data of years, seasons, and months in which such extremes occurred.

Precipitation amounts in the five wettest and five driest years vary by almost a factor of two (~51 inches compared to ~26 inches). Massive flooding on the Illinois and Mississippi Rivers during 1993, the wettest year, caused widespread damage after 51.15 inches of rainfall. This is a sharp contrast to 26.32 inches in 1901, the driest year. Other dry years were in 1930 (beginning of the Dust Bowl era), 1963 (record-breaking low water levels on the Great Lakes), and 1953 (during a multi-year period of drought).

Precipitation amounts in the five wettest and five driest summers vary by more than a factor of three (~18 inches compared to ~6 inches). The wettest summer, also in 1993, had 18.34 inches, while the driest summer, 1936, had 5.64 inches and was the final year of a 7-year sequence of very dry, hot conditions in Illinois. One of the most serious recent droughts in Illinois and across much of the nation occurred in 1988, the second driest summer. Dry springs in both 1936 and 1988 exacerbated the agricultural yield reductions from the dry summer conditions. A very wet spring preceded the third driest summer, 1991, and impacts were limited primarily to decreased agricultural yields.

Precipitation amounts in the five wettest and five driest falls ranged from 4.07 inches in 1953, the driest fall, to 17.29 inches in 1926, the wettest fall. Flooding continued across parts of the state when the fifth wettest fall followed the wettest summer on record in 1993. Impacts of extremes are often less in fall than in spring and summer. Certainly, very wet falls can cause flooding, but soils are often dry at the beginning of this season and have some capacity to absorb excess precipitation. Dry falls can cause water shortages, but agricultural impacts are usually minimal because the growing season has ended. In fact, agriculture often benefits from dry falls because harvest and tillage can advance with minimal weather delays.

Precipitation amounts in the five wettest and five driest winters ranged from 15.4 inches in 1949–1950, the wettest winter, to 2.35 inches in 1962–1963, the driest winter. The amount during 1949–1950 is notable because it was 4 inches more than the amount for 1948–1949, the second wettest winter. The driest winters also tend to be cold, with temperatures in four of the five winters averaging 4 to 8°F below normal. This is because the main source of moisture is the Gulf of Mexico, but cold winters have more frequent northerly flow than normal and less frequent southerly flow from the Gulf. However, winter 1930–1931, second driest on record, was one of the warmer winters with temperatures averaging 4°F above normal, which indicates that cold conditions are not exclusively linked with dry winters.

Precipitation amounts in the five wettest and five driest springs ranged from 18.5 inches in 1927, the wettest spring, to 5.12 inches in 1934, the driest spring. Three of the five driest springs occurred during the Dust Bowl era (1934, 1936, and 1930). These dry springs exacerbated the agricultural impacts of the hot, dry summers that followed and began with deficient soil moisture. The year of the wettest spring, 1927, is a notable one because the worst flooding in the history of lower Mississippi River basin occurred. The very wet spring in Illinois contributed to that devastating event.

Precipitation amounts in the wettest and driest months in Illinois are shown. Five of the wettest months occurred during 1941–1960, and five of the driest months occurred during 1971–1990. The driest months during the heart of the growing season, May–August, all occurred during severe droughts and were part of a longer dry period.

The threat of global warming stems from changes in the composition of the atmosphere caused by human activities, such as the burning of fossil fuels, the raising of livestock, and the production of certain chemicals. These changes in the composition are, in turn, changing the balance of energy in the climate system. Key atmospheric constituents that are increasing due to human activities include carbon dioxide, methane, chlorofluorocarbons, and nitrous oxide. These compounds absorb infrared radiation emitted by the earth’s surface and re-emit the energy back to the earth’s surface, causing warming. This natural effect accounts, in part, for earth’s generally moderate climate.

The concern is that the magnitude of this effect is increasing due to increasing concentrations of these gaseous compounds, and this could cause large, possibly unfavorable changes to our climate. Human activities also have increased the amount of smoke and dust in the atmosphere; these constituents are believed to have a cooling effect on climate because they reflect incoming solar radiation back to space and possibly increase cloud cover. There are large uncertainties about the magnitude of this cooling, which could be as large as the magnitude of warming due to increased greenhouse gas concentrations, and further research is necessary. Although smoke and dust released by human activities are removed quickly from the atmosphere by settling and rainout, greenhouse gases remain in the atmosphere for years or centuries. Thus, any offsetting cooling can be maintained only by continued emissions of smoke and dust, but there are adverse health effects associated with such emissions.

Natural causes also affect climate. Two important natural causes are changes in incoming solar energy and volcanic activity that can release large amounts of dust into the stratosphere. Actual climate change will result from a balance between the warming and cooling effects of human activities and natural climate fluctuations.

Scientists have been studying the climate change problem intensively for years by using global climate models to estimate future changes in climate. These sophisticated models use supercomputers to simulate the circulation patterns of the atmosphere and their evolution over the next century.

Several research groups around the world have developed and run such models, which project a 1-5°C (2-9°F) rise in temperatures over the next century. This wide range is a result of differences in how the models represent certain physical processes and of uncertainties in estimating future emissions of pollutants. Many of these physical processes are very complex and must be simplified in the models, and others are poorly understood. The models also project a global increase in precipitation ranging from 1 percent to 9 percent.

What does this mean for future climate change in Illinois? Although there is considerable uncertainty about the magnitude of change on a global scale, the uncertainties are even greater on a regional scale. The wide range of projected conditions makes it impossible to estimate potential impacts on water resources with any degree of certainty. The most unfavorable projections would have huge negative impacts, such as widespread water shortages and crop failures.

The past evolution of Illinois’ climate, both in the historical record and from indirect evidence, indicates that Illinois has experienced substantial climate changes since the last ice age. This shows that the climate system is not stable and unchanging in its natural state. It can be assumed that the ongoing human-induced changes in energy distribution within the atmosphere likely will affect the climate in important, but as-yet uncertain, ways.


Table 1. Wettest and Driest Years in Illinois

Rank

Wettest year

Amount (in.)

Driest year

Amount (in.)

1

1993

51.15

1901

26.32

2

1990

50.34

1930

27.85

3

1927

49.52

1963

27.94

4

1973

48.23

1953

28.08

5

1898

47.31

1914

28.63

 
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Table 2. Wettest and Driest Summers in Illinois

Rank

Wettest summer

Amount (in.)

Driest summer

Amount (in.)

1

1993

18.34

1936

5.64

2

1915

17.80

1988

6.16

3

1981

17.79

1991

6.32

4

1902

17.55

1933

6.36

5

1958

17.41

1930

6.46

 
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Table 3. Wettest and Driest Falls in Illinois

Rank

Wettest fall

Amount (in.)

Driest fall

Amount (in.)

1

1926

17.29

1953

4.07

2

1941

17.12

1956

4.21

3

1911

15.00

1939

4.27

4

1985

14.64

1999

4.49

5

1993

14.03

1908

4.53

 
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Table 4. Wettest and Driest Winters in Illinois

Rank

Wettest winter

Amount (in.)

Driest winter

Amount (in.)

1

1949-1950

15.40

1962-1963

2.35

2

1948-1949

11.34

1930-1931

2.45

3

1915-1916

10.60

1919-1920

2.80

4

1936-1937

10.26

1976-1977

2.93

5

1973-1974

  9.90

1969-1970

3.45

 
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Table 5. Wettest and Driest Springs in Illinois

Rank

Wettest spring

Amount (in.)

Driest spring

Amount (in.)

1

1927

18.50

1934

5.12

2

1945

17.25

1895

5.97

3

1973

17.24

1936

5.98

4

1898

16.75

1930

6.09

5

1933

15.76

1971

6.26

 
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Table 6. Wettest and Driest Months in Illinois

Month

Wettest month

Amount (in.)

Driest month

Amount (in.)

   January

1950

6.85

1986

0.25

   February

1908

4.44

1947

0.18

   March

1898

7.46

1910

0.27

   April

1957

7.12

1971

1.25

   May

1943

8.80

1934

1.02

   June

1902

8.36

1988

1.05

   July

1958

7.99

1930

1.01

   August

1977

6.92

1953

1.13

   September

1926

9.71

1979

0.49

   October

1941

9.27

1964

0.19

   November

1985

8.91

1904

0.28

   December

1982

7.17

1976

0.45

 
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