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 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
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
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.