carbon dioxide

Take a deep breath and exhale slowly. Your body just released carbon dioxide, a colorless gas that's essential to life on Earth. Carbon dioxide is one of the greenhouse gases that absorbs and holds heat in Earth's atmosphere, keeping our planet warm enough for life to exist. But humans are increasing carbon dioxide levels in the atmosphere above natural levels. As carbon dioxide levels rise, so do global temperatures.

Definition: a colorless, heat-absorbing gas that
is naturally present in the atmosphere
Formula: CO2 (1 carbon atom + 2 oxygen atoms)
Unit of Measure: PPM (parts per million);
number of molecules in 1 million molecules of air
Sources: burning fossil fuels, plant and animal respiration, decay of organisms, weathering
of rocks
Cool Fact: Without greenhouse gases like carbon dioxide, Earth's surface would be frozen!

Image credits: main image, courtesy of AMNH/Stephanie Fotiadis.