CO2 levels in the atmosphere help set the planet’s temperature
Scientists first postulated carbon dioxide’s role as a heat-trapping gas in the 1800s,[[Arrhenius, Svante. “On the Influence of Carbonic Acid in the Air Upon the Temperature of the Ground.” Philosophical Magazine 41. 1896. ]] and that idea is now universally accepted.[[A thorough history of our understanding (and misunderstanding) of how atmospheric CO2 affects climate can be found at this site by the American Institute of Physics.]] Visible light from the Sun passes through the cloud-free regions of Earth’s atmosphere without much interference. After striking the Earth’s surface, some of this energy is re-emitted in the form of infrared radiation, or infrared light. Carbon dioxide and other so-called greenhouse gases, such as methane and water vapor, absorb some of this infrared radiation, which heats up the air. That in turn leads to warming of the land and water below (Earthguide shows an animated diagram of how it works).
Measurements made by drilling deeply into ancient ice in greenland and Antarctica show that temperature and atmospheric CO2 levels have varied in lock-step going back as far as 800,000 years.[[Jouzel, Jean; Masson-Delmotte, Valerie; Cattani, Olivier; Dreyfus, Gabrielle; Falourd, Sonia; et. al. “Orbital and millennial Antarctic climate variability over the past 800,000 years.” (Abstract) Science, 317(5839), 2007. 793-797]] [[Petit, Jean-Robert; Jouzel, Jean; Raynaud, Dominique; Barkov, N I; Barnola, Jean-Marc, et. al.: “Climate and atmospheric history of the past 420,000 years from the Vostok ice core,” (Abstract) Antarctica. Nature, 399(6735), 1999. 429-436]]
Without any CO2 in the atmosphere to trap heat, the oceans would freeze solid.[[See this Tutorial on the Greenhouse Effect for an example]] But levels of CO2 have been increasing over the past 250 years or so.[[D.M. Etheridge, L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola and V.I. Morgan. 1998. “Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores.” (Abstract) In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.]] An overwhelming body of evidence, much of it summarized by the IPCC establishes the link between the modern increase in carbon dioxide in the atmosphere with the observed increase in temperatures around the globe. This temperature increase has in turn contributed to a wide variety of effects, effects projected to intensify in the future.