We have managed to raise the level of CO₂ in the atmosphere, which had been relatively stable for the past 10,000 years at about 275 parts per million (ppm), to 378 ppm today. If we continue along a business-as-usual path of burning fossil fuel, we could reach 840 ppm CO₂ in the atmosphere by the end of this century.

Global Temperature and CO2 Over The Past 450 Thousand Years Source: UN Intergovernmental Panel on Climate Change (IPCC), Third Assessment Report, Climate Change 2001 Graphic: Woods Hole Research Center Concentrations of CO2 in the atmosphere have not been higher than 300 ppm for the past 450,000 years.