see BBC News 24. Nov. 2005 :
CO2 highest for 650,000 years

BBC News Oct 2009 :
CO2 levels over the last 20 million years

Since the beginning of the industrial revolution, atmospheric con-centrations of carbon dioxide have increased nearly 30%, methane concentrations have more than doubled, and nitrous oxide concen-trations have risen by about 15%. These increases have enhanced the heat-trapping capability of the earth's atmosphere.

Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increased concentration of carbon dioxide. Plant respiration and the decomposition of organic matter release more than 10 times the CO2 released by human activities; but these releases have generally been in balance during the centuries leading up to the industrial revolution with carbon dioxide absorbed by terrestrial vegetation and the oceans.

What has changed in the last few hundred years is the additional release of carbon dioxide by human activities. Fossil fuels burned to run cars and trucks, heat homes and businesses, and power factories are responsible for about 98% of U.S. carbon dioxide emissions, 24% of methane emissions, and 18% of nitrous oxide emissions.

Increased agriculture, deforestation, landfills, industrial production, and mining also contribute a significant share of emissions. In 1997, the United States emitted about one-fifth of total global greenhouse gases.

The 30% increase in atmospheric CO2 observed since pre-industrial times cannot be explained by natural causes. CO2 concentrations have varied naturally throughout Earth's history.
However, CO2 concentrations are now higher than any seen in at least 650 000 years.

Sources: Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-1998 (April 2000) and
Carbon Dioxide Information Analysis Center.