According to the Intergovernmental Panel on Climate Change (IPCC), atmospheric concentrations of greenhouse gases—primarily carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)—have drastically increased as a result of human activities since 1750 (the Industrial Revolution), and now far exceed pre-industrial values determined by ice core data spanning the past 10,000 years. Moreover, global greenhouse gas emissions have increased 70% between 1970 and 2004.
The IPCC figures to the left show changes in atmospheric CO2 and CH4 concentrations over the past 10,000 years. Notice the large spikes on the right side of both graphs which indicate a rapid increase in these atmospheric gases over the past 200 years. The global atmospheric concentration of CO2 in 1750 was 280 parts per million (ppm), increasing to over 380 ppm in the present. The global atmospheric concentration of methane in 2005 was 1732 parts per billion (ppb), which far exceeds the natural range of the last 650,000 years (320 to 790 ppb) as determined by ice core data.
Although ice core data is used to examine earth’s climate tens to hundreds of thousands of years into the past, researchers have made consistent and direct measurements of atmospheric CO2 levels since the late 1950s in Hawaii. Records from Mauna Loa Observatory, Hawaii show a clear increase in global atmospheric CO2 concentrations. Before the industrial period, global average CO2 atmospheric concentrations averaged about 280 ppm. Now, primarily because of fossil fuel use and land-use practices, global atmospheric concentrations are over 380 ppm. The NOAA graph at the bottom right clearly documents a 0.53 percent or two parts per million per year increase in atmospheric carbon dioxide since 1958. (For more information on CO2 measurements at the Mauna Loa Observatory, see the Scripps Institution of Oceanography’s CO2 Program.) It is important to know whether carbon dioxide levels are increasing or decreasing in the atmosphere because CO2 is the most abundant greenhouse gas, and because its pattern closely matches historical temperature changes. For example, the figure below ("Temperature and CO2 concentration") clearly shows that historic atmospheric CO2 levels and global average air temperature changes are positively correlated (both increase at the same time, and both decrease at the same time). The record presented below extends back over the past 420,000 years, reconstructed from ice cores taken at the Vostock ice station in Antarctica.
During the coldest periods, marked by maximum glacial coverage, CO2 concentrations hovered near 190 parts per million (ppm). When temperatures warmed and glaciers retreated (melted), CO2 concentrations were much higher. Even so, CO2 levels during this time have never exceeded 300 ppm until recently.
Because of its sheer abundance in the atmosphere (compared to other greenhouse gases), carbon dioxide can be considered the most important anthropogenic greenhouse gas. Its primary source is the burning of fossil fuels, though land-use practices like deforestation also contribute to rising concentrations.