Atmospheric Concentrations of Greenhouse Gases - Global atmospheric concentrations of carbon dioxide, methane, nitrous oxide, and certain manufactured greenhouse gases have all risen over the last few hundred years. Before the industrial era began in the late 1700s, carbon dioxide concentrations measured approximately 280 ppm. Concentrations have risen steadily since then, reaching an annual average of 394 ppm in 2012—a 41 percent increase. Almost all of this increase is due to human activities.
The concentration of methane in the atmosphere has more than doubled since preindustrial times, reaching about 1,826 ppb in 2012. It is very likely that this increase is predominantly due to agriculture and fossil fuel use.
Climate Forcing - In 2011, the Annual Greenhouse Gas Index was 1.30, an increase in radiative forcing [Radiative forcing is any change in the Earth’s energy flow as a result of changing of any physical factor, in this case, increase in greenhouse gas concentrations.] of 30 percent since 1990. Of the greenhouse gases, carbon dioxide accounts for by far the largest amount of radiative forcing, and its contribution continues to grow at a steady rate. By 2011, radiative forcing due to carbon dioxide was 40 percent higher than in 1990. Carbon dioxide accounts for approximately 80 percent of the overall increase in radiative forcing since 1990. [For more on climate forcing, see http://ossfoundation.us/projects/environment/global-warming/radiative-climate-forcing .]