Department of Urban and Regional Planning   
University of Illinois at Urbana-Champaign   

Climate Change: Causes and Effects

Key words: The carbon cycle, fossil fuels, anthropogenic, residence time, global warming, climate change, greenhouse effect, carbon sources and sinks, mitigation, adaptation, positive feedback cycles, albedo

What is the issue?

The element carbon (C) is the fundamental chemical building block of all life on earth. It also forms the molecular “backbone” to fossil fuels that we use to generate electricity, power our vehicles, and heat our homes. In a standard human body, only oxygen can claim more mass than carbon (65% vs. 18% total body mass, respectively). Most of our “solid” composition, however, is carbon-based matter. Carbon can be found in molecules that are solid (e.g. coal, diamonds, sugars), liquid (e.g. gasoline, diesel, kerosene), and gas (e.g. carbon dioxide, carbon monoxide), and there is a complex system that transfers carbon to and from these various states. This complex system is called the carbon cycle, and it is one of the most crucial biogeochemical systems on earth. The carbon cycle is considered “complex” because it is actually a combination of many other biological, geological, and chemical systems that move carbon between gaseous, solid, and liquid forms.

You are probably familiar with some of these systems. Photosynthesis, a chemical process that takes places in the leaves of plants, is one crucial step in the carbon cycle. Photosynthesis combines energy from the sun, carbon dioxide (CO2) in the atmosphere, and water (H2O) from the ground, to create carbon-based plant matter, namely cellulose. Other photosynthetic organisms like plankton and algae derive their energy from the sun as well, forming important conceptual first-steps in the carbon cycle.
The Carbon Cycle
Figure 1: A very basic rendering of the carbon cycle shows how sunlight, photosynthesis, and rare geological processes result in lots of stored carbon beneath the earth’s surface. When we extract this carbon-based fuel and burn it for energy  (either in the form of electricity, fuel for automobiles, or heat for homes), we emit carbon into the atmosphere. Over time, this extraction process has overwhelmed the natural carbon cycle on earth.

The carbon-based fuels like coal, petroleum, and methane (CH4, “natural gas”) that we use to generate energy in our daily lives are all products of the carbon cycle as well. Petroleum, our principle energy source for transportation and also the main component of materials like kerosene, plastics, and tar, originates from dead plankton and algae that have been buried in sediment. Under layers and layers of pressure and exposed to heat from the center of the earth, the chemical composition of dead plankton and algae change into organic shale, which eventually turns into a material called kerogen. Under additional pressure and heat and some very special geological conditions, kerogen transforms into oil which we refine and use for numerous purposes. Natural gas forms in a very similar process, but at different geological depths and temperatures.

Coal is formed in a similar way, except that it originates as woody plant material buried in calm swampy areas. Like the oil formation process, plant material is buried under layers of sediment over millions of years. Under very special conditions, this material turns into coal which can be extracted and burned to generated electricity. In fact, the vast majority (85%) of electricity consumed in Illinois, including that used at the University of Illinois, is generated from the burning of coal.

We call coal, oil, and natural gas fossil fuels because they originate as dead organic material. These energy sources are literally the fossils of dead organisms that have undergone chemical transitions. When fossil fuels are burned they yield enormous amounts of kinetic energy, which humans have harnessed with impressive ingenuity since the invention of the steam engine in the late seventeenth century. In addition to kinetic energy, fossil fuel burning yields other chemical compounds. Although the exact yield varies from fuel to fuel, all fossil fuels yield CO2 when burned.  The C0­­2—a colorless, odorless gas—re-enters the atmosphere, similar to the CO­­2 that all humans and animals exhale. We call CO­­2 released by human processes anthropogenic—“created by humans”. Conceptually, this CO2 should be re-absorbed by photosynthetic organisms in the ocean and on the land and re-enter the carbon cycle. Trees, plants, and other photosynthetic ocean organisms are all examples of carbon sinks, or natural pathways that sequester carbon in the ground. Unfortunately, the rapid burning of fossil fuels since the industrial revolution has overwhelmed the carbon cycle to an extent that CO2 cannot be absorbed quickly enough by the worlds disappearing carbon sinks. As a result, the CO­­2 emitted into the atmosphere is left no place to go but and remains suspended as gas in the atmosphere.

Scientists have measured the current concentration of CO­­2 in the atmosphere since the 1950s,  and can estimate the historic concentration by examining gas bubbles in ice core samples. Atmospheric carbon—measured in parts per million (ppm)—has increased consistently over the past fifty years and appears to be increasing at an unprecedented rate. In fact, while concentrations of CO2 always fluctuate with natural climactic cycles of the earth, our current concentrations of nearly 400ppm is estimated to be the highest ever in at least the last 650,000 years. For a fascinating illustration of historic changes in atmospheric CO2 concentrations, visit
Atmospheric CO2 is increasing.
Figure 2:

Throughout nature, there are numerous potential sources of atmospheric CO­2. When volcanoes erupt, for example, they spew enormous amounts of gas, including CO2, into the atmosphere. Some atmospheric carbon may also come from the chemical weathering of rocks. But these sources pale in comparison to the amount of CO­2 ­released into the atmosphere through industrial processes like burning coal, petroleum, and natural gas for energy. With very few exceptions, scientists from all over the globe agree that the historically high levels of CO­2  in the atmosphere is the result of fossil fuel consumption (IPCC, 2007). 

The Greenhouse Effect

Carbon dioxide is one of several “greenhouse” gases (GHGs) that contribute to an atmospheric phenomenon called the greenhouse effect. The greenhouse effect is absolutely essential for human life on earth. Without it, the surface of the earth would remain uncomfortably cold and mostly covered in ice. Greenhouse gases like water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrogen dioxide (NO2), and small amounts of other gases effectively trap solar radiation in the earth’s atmosphere, keeping our planet habitably warm. Without some greenhouse effect, solar radiation would reflect off the surface of the earth and “bounce” back into outer-space, leaving our planet as cold as the moon. Over hundreds of thousands of years, the amount of greenhouse gases in the atmosphere have oscillated with a relatively predictable range.  The water cycle, carbon cycle, and other biogeochemical processes have maintained a consistent balance of heat-trapping gas in the atmosphere. Since the industrial revolution, however, we have forced unprecedented amounts of CO2 into the atmosphere at the same time that we’ve removed vegetation that would naturally extract it. The net effect is an overabundance and growing concentration of greenhouse gases, which can be thought of as an extra-thick blanket surrounding the earth.

Water vapor is by far the most common GHG, accounting for about 70 percent of the greenhouse effect. Water vapor, however, has a very short residence time in the atmosphere.  It remains suspended in the atmosphere for about nine days before rains back down to the earth’s surface. Other GHGs like CO2 and CH4 have residence times of up to hundreds of years. Their molecular structure also make them much more efficient trappers of solar radiation. Therefore, small concentrations of these gases make big contributions to the greenhouse effect. Over time, the growing greenhouse effect has increased the global average temperature of the earth’s surface. Colloquially, this effect has been called “global warming,” but for reasons that will be illustrated below, the phenomenon is better called “global climate change” or “global climate destabilization.”


Major Greenhouse Gases

Chemical Formula


Global Warming Potential (GWP*)

Water Vapor


Evaporation from oceans,  the water cycle

Carbon Dioxide


Industrial burning of fossil fuels, cement production, volcanoes, weathering of rocks




Organic decomposition, rice production, livestock, decay from landfills, and fossil fuel production.


Nitrous Oxide


The oxidation of nitric oxide, chemical production, fertilizers, land conversion to agriculture.


*GWP = Measure of the impact of a molecule relative to carbon dioxide. Source: Grubb et al. (1999)

While fluctuation in temperatures is nothing new on this planet, the rate at which global temperatures are climbing exceeds anything in human record. Seventeen of the past twenty years have ranked warmer than any year on record since 1870. June 2010 was the warmest month ever in recorded history (as of the writing of this lesson) followed by June 2005 (NASA, 2010).  Climate scientists estimate that global temperatures have increased from 0.56°C to 0.92°C between 1906 and 2005, with most of the increase taking place in the second half of that period.

Climate scientists have resolved that an increase of over 2­­­°C would result in serious and irreversible consequences.

Two degrees Celsius doesn’t seem like a lot. What’s the big deal?

A two-degree difference in temperature might not feel terribly different to you, but remember that this increase is a global average increase in temperature: it is not evenly distributed across the earth’s surface. Some areas will experience much higher increases, and few regions will experience little to no increase. Firstly, since water stores and disperses heat energy better than land, temperature is projected to increase less over oceans than over land. Antarctica and the Arctic will warm more rapidly than “lower latitudes” due to positive feedback effects that result from melting ice.
Surface Temperature Change
Figure 3: Increasing temperatures are not uniformly distributed across the globe. Higher lattitudes and land cover will experience larger increases than oceans.

The highly reflective surface of polar ice caps reflect solar energy back into outer space, reducing the amount of heat trapped in the atmosphere. As the planet heats up, ice begins to melt, reducing reflectivity or albedo. When ice melts, the darker surface beneath the ices absorbs, rather than reflects solar radiation. This increases surface temperature, which causes ice to melt.  In other words, the melting of glacial ice results in a vicious cycle the result of which is ever-increasing temperatures.  Scientists estimate that 2­­­°C is threshold at which this and several other positive feedback cycles will begin. For example, the permafrost (ground that is permanently frozen) at northern latitudes in places like Siberia effectively stops methane gas—a greenhouse gas twenty-one times stronger than CO2—from entering the atmosphere. In some places, permafrost is already beginning to melt. Once methane escapes into the atmosphere, it will exacerbate the greenhouse effect, resulting in more warming and what authors have referred to as “runaway” climate effects (Orr, 2009).
Positive Feedback Loops 

Figure 4:

Therefore, while 2­­­°C might not seem drastic in any one place and time, the highly uneven distribution of heat and resulting positive feedback effects make the seemingly modest temperature increase a point that human civilization would be best advised to avoid.

Where do anthropogenic greenhouse gases from?

As explained above, anthropogenic greenhouse gases come from the burning of fossil fuels. Carbon dioxide is the most common anthropogenic GHG, and the largest current contributor to the greenhouse effect. In the United States, about 44 percent of anthropogenic CO2 comes from the burning of petroleum, followed by coal (36 percent) and natural gas (20 percent).
Billion Metric Tons CO2e

How does this break down by activity?  About 39 percent of CO2 emissions come from the production of electricity used in homes, offices, and industries. When you turn on a light switch, you might not be emitting CO2 on-site, but there’s a good chance that a power plant somewhere in your region is burning fossil fuels twenty-four hours a day to make sure that that electricity is ready to flow. A similar proportion—about 34 percent—of CO2 emissions come from the tailpipes of automobiles, trucks, buses, and airplanes.  The remaining CO2 emissions come from the direct burning of fossil fuels in our homes and places of work. When you light a gas stove, turn up the heat in your home, or work in a factory that generates its own energy through on-site burning of fossil fuels, you emit GHGs into the atmosphere.
Million Metric Tons CO2e

How do we know climate change is happening? Who is studying climate change?
Although several well-funded groups in the United States continue to deny the human contribution to climate change—alleging that recent rises in temperature are the result of “natural” cycles or denying altogether that change is occurring—the consensus amongst climate scientists around the world is remarkably strong. This consensus is articulated in a series of reports issued by the Intergovernmental Panel on Climate Change (IPCC), the most recent of which was released in 2007. The fifth assessment report is due to be released in 2014.

What about this winter’s record snowfall? It sure doesn’t seem like “global warming” is really happening. Sometimes it even seems like global “cooling.”

Be careful! More snow doesn’t necessarily more “cold.” The “warming” associated with the increased greenhouse effect is not the sole repercussion of increased GHG levels.  Many climate policy advocates have ceased using the term “global warming” and instead opted to use the terms “climate change” or “climate destabilization” because the uneven distribution of heat is likely to influence weather patterns such as precipitation and storm strength in different place. Some regions are forecast to experience more frequent and heavier rains, while other regions are forecast to experience punishing drought. Increased snowfall is likely the result of increasing levels of moisture in the atmosphere. Higher temperatures result in more rapid evaporation from oceans, which is one reason for record snowfall—despite warmer temperatures.
Other potential effects of climate change

  • Rising Sea Levels will result from two principal phenomena: thermal expansion and ice melt. Firstly, as the temperature of oceans increase, water expands, forcing water onto areas that are currently dry land. Secondly, as glaciers and ice caps melt, water transitions from solid to liquid form, increasing the volume of water in the ocean. A satellite image of the world at night reveals the overwhelming number of cities on or near the ocean. Rising sea levels are especially troubling considering that about 40 percent of the world’s population lives within 100 kilometers of the coast. Where will current coastal inhabitants move when their homes are underwater? This is an important question that governments world-wide will have to address in coming decades.

  • Stronger hurricanes. While the frequency of hurricanes may remain unchanged, the strength of hurricanes is predicted to increase as these storms thrive upon warmer water. Already, climatologists have been surprised by the appearance of hurricanes in higher latitudes and regions of the world in which hurricanes have never been recorded. Hurricane Katrina, which decimated areas of New Orleans and resulted in over 1,000,000 displaced persons in 2005, increased from a category three storm to a category five storm as it crossed a warm area in the Gulf of Mexico.


  • Heat Waves have grown much more common and more intense in recent years.

Who is talking about it?

How do we know climate change is happening? Who is studying climate change?

Although several well-funded groups in the United States continue to deny the human contribution to climate change—alleging that recent rises in temperature are the result of “natural” cycles or denying altogether that change is occurring—the consensus amongst climate scientists around the world is remarkably strong. This consensus is articulated in a series of reports issued by the Intergovernmental Panel on Climate Change (IPCC), the most recent of which was released in 2007. The fifth assessment report is due to be released in 2014.


How does this relate to planning?

Where we live, how we live there, and how we travel around are all  closely connected to the emission of GHGs and ensuing climate change. If you recall, GHGs come from the burning of fossil fuels in our power plants, automobiles, and buildings—three sectors over which urban planning has a direct influence. Planning can approach climate change with both mitigation and adaptation strategies; both are crucial if we hope to avoid the most cataclysmic effects of climate change and many strategies are both mitigating and adaptive. Mitigation involves reducing climate change by eliminating carbon sources and increasing carbon sinks. For example, by creating cities and neighborhoods in which residents can walk or cycle to work rather than drive, planning can help reduce total GHG emissions. You will be introduced to additional mitigation strategies in later modules. Adaptation involves planning for a warmer and rapidly changing future. We are just beginning to experience the effects of GHG emitted in the early 1980s—there is a thirty-year time lag between when GHGs are emitted and when we experience their effects.  Therefore, even if we reduce our GHG emissions TODAY, we are guaranteed to experience higher temperatures, rising sea levels, stronger and less predictable weather, droughts, floods,  food shortages, and all the societal stress that comes with it. We can reduce this potential stress by creating more resilient places, and foster government services that are able to flexibly and quickly respond to emergency. This strategies will also appear in later modules.


NASA (National Aeronautics and Space Associaiton), 2010.  Global Climate Change: NASA’s Eyes on the Earth. Retrieved 10 September 2010 at M C and Sen A K. 1993. The Quality of Life. Oxford University Press
EIA (United State Energy Information Administration). 2011. Annual Energy Outlook 2011. <>>
An animation that demonstrates warming patterns on Earth:
A fascinating animation that demonstrations fluctuations of atmospheric carbon from prehistory to present-day: