According to a United Nations’ (UN) World Meteorological Organization report, issued on November 20, 2012, greenhouse gases hit a record high in the atmosphere in 2011.The report found that carbon dioxide, “the single most important anthropogenic greenhouse gas,” increased at a rate of 2 parts per million per year, consistent with the average during the past decade. The report identifies fossil-fuel combustion and land-use changes—largely tropical deforestation—as primary drivers of the increase.

Methane, which the report states contributes about 18 percent to radiative forcing caused by long-lived greenhouse gases (LLGHGs), increased by about 5 parts per billion (ppb). This is about the same rate as during the past three years, although it follows a period of no almost growth from 1999 to 2006. The report indicates that about 40 percent of methane is emitted by natural sources and 60 percent comes from anthropogenic sources, such as ruminants, rice agriculture, fossil fuel exploitation, landfills, and biomass burning.

Nitrous oxide, the third most important contributor to LLGHGs radiative forcing, increased by about 1 ppb, greater than the 0.78 ppb mean growth rate during the past 10 years. About 60 percent of nitrous oxide results from natural sources and about 40 percent comes from anthropogenic sources, including biomass burning, fertilizer use and various industrial processes. According to the report, “The [National Oceanic and Atmospheric Administration] NOAA Annual Greenhouse Gas Index shows that from 1990 to 2011 radiative forcing by [LLGHGs] increased by 30%, with CO2 accounting for about 80% of this increase.”