Carbon cycles naturally through the environment. Plants and other photosynthetic organisms extract carbon dioxide from the air and, in the presence of light, make food in the form of the carbohydrate glucose, which provides energy and is used to build and repair structures. Both animals and plants return CO₂ to the air when they release the energy stored in food molecules through the process of respiration. Other carbon-based molecules cycle more slowly through the environment, remaining stored underground or at the bottom of the ocean for long periods of time.

When we extract coal and oil from Earth's crust and then burn these fossil fuels to provide energy for transportation, heating, cooking, electricity, and manufacturing, we add carbon to the atmosphere more rapidly than it is naturally removed through photosynthesis and sedimentation. The result is that atmospheric carbon dioxide concentrations are higher today than they have been for at least 400,000 years.

This human-caused increase is a concern to climate experts because carbon dioxide, water vapor, methane, nitrous oxide, and a few other compounds are responsible for keeping Earth warm in the first place. Scientists' concern is that an increase in greenhouse gases is likely to enhance the greenhouse effect.

Greenhouse gas molecules, each composed of three or more component atoms, readily absorb infrared radiation coming from the surface of Earth. When they do, they vibrate and ultimately re-emit the radiation they have absorbed. Often, they simply pass their energy to nearby greenhouse gas molecules. This absorption-emission-absorption cycle in the lower atmosphere keeps heat near the surface of Earth, effectively insulating it from the cold of space.

In the proper balance, greenhouse gases make life on Earth possible. However, any increase in their concentration poses the risk of altering the natural balance and changing global temperatures dramatically. Indeed, temperature data from a variety of sources show that average global temperatures have risen slightly less than one degree Celsius in the past century. What may sound like a small change actually suggests an alarming trend to many climate experts. After all, ice ages and the warm interglacial periods between them are marked by changes of just four or five degrees Celsius. A warming trend on top of the current interglacial period could have devastating consequences for human societies.

To learn more about the role CO₂ plays in Earth's temperature, check out Global Warming: The Physics of the Greenhouse Effect.

To learn more about evidence suggesting a link between human activities and global warming, check out CO2 Concentrations at Mauna Loa Observatory, Hawaiʻi.

To learn more about evidence of regular, extreme climate change throughout Earth's distant past, check out Climate Change and Greenland Ice Sheet Project 2: A Record of Climate Change.

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