Chapter 1, Key Finding 1

Atmospheric carbon dioxide (CO₂) has increased from a preindustrial abundance of 280 parts per mil- lion (ppm) of dry air to over 400 ppm in recent years—an increase of over 40%. As of July 2017, global average CO₂ was 406 ppm. Methane (CH₄ has increased from a preindustrial abundance of about 700 parts per billion (ppb) of dry air to more than 1,850 ppb as of 2017—an increase of over 160%. The current understanding of the sources and sinks of atmospheric carbon supports the dominant role of human activities, especially fossil fuel combustion, in the rapid rise of atmospheric carbon (very high confidence).