We’ve all heard of “climate change,” a term that feels so charged, but what does it really mean, and what is actually happening? Let’s break down the scientific definitions to help us really understand what researchers and climate professionals are talking about when they say “climate change.” 

Scientifically, the term “climate change” describes long-term changes in the average weather, which we might experience as changes in temperature, precipitation, and other variables. Climate change can result from natural variations, but it can also be caused by human activity. So, let’s focus on what’s causing the changes we are seeing right now, and what that means for residents of Illinois.  

Energy movement in Earth’s atmosphere 

Let’s take a look at how energy moves in and around our planet Earth and its atmosphere. How is energy gained? We gain energy through the sun, with the amount of energy arriving depending on the time of day, time of year, cloud cover, air quality, and other factors. When sunlight reaches the earth’s surface, a large portion is absorbed, leading to a warming of the surface. Now, let’s consider how energy moves out of our system. Interestingly, the climate system doesn’t lose energy in the form of light, but rather in the form of heat. A large determining factor of how much is lost depends on the composition of our atmosphere, a subset of which is greenhouse gases.  

Greenhouse gases: What are they, and how much is there in the atmosphere? 

Greenhouse gases are a group of atmospheric gases that naturally occur in our atmosphere, like water vapor, carbon dioxide, methane, and others. These gases allow solar energy in the form of light to come into the atmosphere and warm Earth’s surface, but they absorb and re-emit outgoing terrestrial energy in the form of heat. There are many natural sources of these gases in the earth’s atmosphere and natural processes through which the earth’s surface and the atmosphere exchange these gases, like volcanic eruptions. But some greenhouse gases, like carbon dioxide, are also a byproduct of fossil fuel combustion. That means that when we drive our gas vehicles or use electricity derived from fossil fuel sources (coal, oil, etc.), we are contributing additional carbon dioxide, a greenhouse gas, to the atmosphere.  

As a result of human activity, the atmospheric concentration of carbon dioxide has increased from the pre-industrial value of around 285 parts per million to the current concentration of around 425 ppm. It is this increasing concentration of carbon dioxide and other greenhouse gases that is responsible for most of the climate change that has been observed in recent decades. Climate scientists also anticipate additional warming this century as greenhouse gas emissions continue. 

How greenhouse gases change our climate 

At the global scale, near-surface air temperature has increased by more than 1°C (1.8°F) since the late 19th century. This may seem like a trivial amount of warming, but the impacts of 1°C are already widespread and cause a cascade effect beyond temperature. For example, warming also affects precipitation intensity (more extreme), changes in rain/snow distributions, and how long precipitation is on the surface before melting or evaporating. Increasing the temperature also leads to a loss of ice/snow on land and the expansion of ocean water, which are the largest contributors to the rise of global sea levels. 

Illinois’ changing climate

Like nearly all the world’s land areas, Illinois has warmed slightly faster than the planet overall. One interesting aspect of warming in Illinois, and many other areas, is that the minimum (nighttime) temperatures are increasing faster than the maximum (daytime) temperatures. Remembering back to how greenhouse gases work, this faster nighttime warming tells us that greenhouse gases are responsible for the warming. The increase in minimum temperature has also led to a large increase in the length of the frost-free season in Illinois. Illinois has also experienced more warming during the winter and transition seasons and very little warming during the summer. Despite summer not “warming,” humidity has increased in the Midwestern U.S., leading to an increase in human heat stress.  

A warmer atmosphere can increase rainfall

In a warmer atmosphere, the air can hold more water vapor. The implication is that, under warm conditions, we should get more precipitation when it rains. Details vary across data sets, but there is agreement regarding the large increase in statewide precipitation. According to the Illinois State Climatologist, total annual precipitation has increased by around 5 inches, 12%-15%, in the last 120 years, with most of the increase occurring during spring in southern Illinois and during the summer in central and northern Illinois. There has also been an increase in extreme precipitation events such as flash floods and other risks related to infrastructure (e.g., reservoirs). 

Looking to the future: What climate models say

The significance of historical warming, both globally and in Illinois, raises concerns about our climate future and the choices that we must collectively make. For that, we rely on climate models. We start with complex computer models of the climate system that include the major components (atmosphere, oceans, land surface, ice, vegetation) and the exchanges that take place among them. Then there is the unknown factor: future greenhouse gas emissions - a factor dependent on society’s response to the threat of climate change. Researchers produce scenarios based on the different levels of greenhouse gas emissions. On one hand, if we quickly reduce our emissions, we can limit atmospheric greenhouse gas concentrations and the amount of warming that occurs. On the other hand, if we continue to rely heavily on fossil-derived energy, we could experience as much as 4°C (>7°F) of warming during this century. By looking at many different climate models, across many different scenarios and inputs, we can start to understand different possible climate futures. 

While projecting the future climate at the regional scale is challenging, several general statements can be made about the 21st-century climate of Illinois:

• Illinois is likely to continue warming in the coming decades as large-scale warming continues. The rate of warming, and therefore the nature of warming-related impacts, is highly dependent on global greenhouse gas emissions. 

• It is also very likely that the increase in precipitation will continue accompanied by more precipitation variability (e.g., drought). 

• We expect to see an increase in extreme events, particularly precipitation events, driven by the increase in atmospheric moisture. 

This article was written by Dr. Justin Schoof, Director of the School of Earth Systems and Sustainability at Southern Illinois University, Carbondale

Thank you for reading!