The average cost of carbon dioxide gas is $ per gram. CO2 is the most important greenhouse gas produced by human activities, mainly through the combustion of fossil fuels. At 380 parts per million, atmospheric mixing ratios for carbon dioxide are now higher than ever before. The current rate of increases is about 2 ppm per year. Meanwhile, the US has been working out various mechanisms to limit CO2 emissions. In fact, it has pledged to reduce the 2005 levels of CO2 emissions by 17% by 2020. The US is already moving from coal-based electricity production to a system based on neutral gas.