Carbon dioxide (CO2) is a greenhouse gas that is emitted naturally through the carbon cycle, and through activities like the burning of fossil fuels (oil, natural gas and coal), solid waste, trees and wood products, and also as a result of other chemical reactions (e.g., manufacture of cement). Carbon dioxide is also removed from the atmosphere (or "sequestered") when it is absorbed by plants as part of the biological carbon cycle.
Since the Industrial Revolution in the 1700s, human activities (e.g. burning of fossil fuels and deforestation) have dramatically increased CO2 concentrations in the atmosphere. High CO2 levels in the atmosphere contribute directly to climate change. In 2005, global atmospheric concentrations of CO2 were 35 percent higher than they were before the Industrial Revolution. (Source: EPA / Photo: Flickr)