Carbon dioxide levels in the atmosphere are increasing

A wide variety of evidence supports this assertion. The first major set of measurements were made by Charles David Keeling at Mauna Loa Observatory, in Hawaii, starting in 1958. At that time, the average concentration of CO2 in the atmosphere was 315 parts per million (ppm) by volume — which means that out of every million cubic inches of atmosphere, say, 315 of those cubic inches were CO2. By 2007, the average was up to 383 ppm.[[Earth System Research Laboratory, Global Monitoring Division. “Trends in Atmospheric Carbon Dioxide - Mauna Loa.”]] The so-called “Keeling Curve,” which charts CO2 from 1958 to today, has been rising generally upward for 50 years.

But according to recent research, the increase began long before 1958: between 1000 AD and 1750 AD levels of CO2 ranged between 275 and 285 ppm, well below what Keeling measured. Since then, as economies have industrialized, levels have grown at an accelerating rate.[[For example, see  Etheridge, D.M.L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola and V.I. Morgan. “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. 1998.]] Much of the relevant research has been summarized by the Intergovernmental Panel on Climate Change, the world’s leading authority on climate change science. Today, roughly one in four molecules of CO2 in the atmosphere comes from human activities, mostly from burning of fossil fuels like coal, oil and gas.

Because carbon dioxide levels in the atmosphere help set the Earth’s temperature, this increase is making the planet warmer. In turn, this warming has already contributed to a wide range of ripple effects, which are projected to intensify as CO2 levels and warming increase. For effects both observed and projected on a more local level in the US, see this report from the United States Global Change Research Program.