How do current greenhouse gas concentration compare to previous levels?
Atmospheric greenhouse gas concentrations have increased since the Industrial Revolution, primarily as the result of human activities. • Atmospheric carbon dioxide has ranged from 275 to 285 parts per million (ppm) in preindustrial times (AD 1000 – 1750), but has risen 36 percent to 379 parts per million in 2005. This increase is primarily due to the burning of fossil fuels associated with transportation, construction and manufacturing. Deforestation also releases CO2 and reduces its uptake by plants. • Similarly, atmospheric methane has varied between 400 and 700 parts per billion (ppb) over the last 650,000 years. In 2005, the global average abundance of CH4 was roughly 1,774 parts per billion. For the most part, atmospheric methane has increased as a result of human activities related to agriculture, natural gas distribution and landfills. • Since 1750, atmospheric nitrous oxide is estimated to have increased from 270 parts per billion to 319 parts per billion in 2005, primarily as
