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Climbing levels of CO2 in the atmosphere are causing global warming

For more than a century, carbon dioxide has been known to help set the planet’s temperature.1 But levels of CO2 have been increasing over the past 250 years or so. This suggests that the observed rise in temperature over the past century is due to the CO2 increase.

How do we know that some other factor is not the primary cause of warming? One way scientists test this is with computer models of the climate system. They run these models as though carbon dioxide levels had been steady rather than rising — and those simulations show that without the extra CO2, other, natural factors would not explain the current warming.2 This is true for each continent as well as for the earth as a whole. 

Another test is to look at the pattern of warming. If CO2 is really trapping heat in the lower levels of the atmosphere, close to the Earth’s surface, you would expect the upper atmosphere actually to get cooler while the lower atmosphere warms. In fact, that is just what temperature measurements are showing.3

Finally, since most of the Earth’s heat comes from the Sun, it would be reasonable to wonder whether the Sun has increased in brightness recently. Although our Sun is quite steady in the amount of energy it emits, it does vary slightly. The most precise way to measure the Sun’s energy is by looking from space, so you don’t have to peer up through the murky atmosphere. Satellites have been doing that since the 1970s, and their observations make it clear that an increase in Solar brightness does not account for the warming.4 5

The increase in global average temperature has led in turn to a wide variety of effects, from sea level rise to more frequent extreme weather.  Effects are generally projected to intensify in the future as CO2 levels and warming increase.  

References
  1. Arrhenius, Svante (1896). "On the Influence of Carbonic Acid in the Air Upon the Temperature of the Ground." (PDF) Philosophical Magazine 41: 237-276
  2. Hegerl, G.C., F. W. Zwiers, P. Braconnot, N.P. Gillett, Y. Luo, J.A. Marengo Orsini, N. Nicholls, J.E. Penner and P.A. Stott: Understanding and Attributing Climate Change. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  3. V. Ramaswamy, M.-L. Chanin, J. Angell, J. Barnett, D. Gaffen, M. Gelman, P. Keckhut, Y. Koshelkov, K. Labitzke, J.-J. R. Lin, A. O’Neill, J. Nash, W. Randel,R. Rood, K. Shine,8M. Shiotani, and R. Swinbank, "Stratospheric Temperature Trends: Observations and Model Simulations" (PDF) Reviews of Geophysics, 39, 1.  February 2001 pp. 71–122
  4. Benestad, R. E., and G. A. Schmidt (2009), "Solar trends and global warming." (Abstract) J. Geophys. Res., 114
  5. For a more informal discussion, see Duffy, Phil B., B.D. Santer, and T.M.L. Wgley, “Solar variability does not explain late-20th century warming.” (PDF) Physics Today, January, 2009. pp. 48-49.