_:genid1 "Free to use with credit to the original figure source."^^ . "heattrapping-gas-levels" . "33.4"^^ . "Heat-Trapping Gas Levels"^^ . "Present-day atmospheric levels of carbon dioxide, methane, and nitrous oxide are notably higher than their pre-industrial averages of 280, 0.7, and 0.27 parts per million (ppm) by volume, respectively (left). Air sampling data from 1958 to 2013 show long-term increases due to human activities as well as short-term variations due to natural biogeochemical processes and seasonal vegetation growth (right). (Figure sources: (left) Forster et al. 2007; (right) Scripps Institution of Oceanography and NOAA Earth Systems Research Laboratory)."^^ . _:genid1 . . . . . . . . . .