Most greenhouse gases have both natural and anthropogenic sources. During the pre-industrial holocene, concentrations of these gases were roughly constant. Since the industrial revolution, concentrations of all the long-lived greenhouse gases have increased due to human actions.[5]
Gas Preindustrial Level Current Level Increase since 1750 Radiative forcing (Wm2)
Carbon dioxide 280 ppm 384ppm 104 ppm 1.46
Methane 700 ppb 1,745 ppb 1,045 ppb 0.48
Nitrous oxide 270 ppb 314 ppb 44 ppb 0.15
CFC-12 0 533 ppt 533 ppt 0.17

Ice cores provide evidence for variation in greenhouse gas concentrations over the past 800,000 years. Both CO2 and CH4 vary between glacial and interglacial phases, and concentrations of these gases correlate strongly with temperature. Before the ice core record, direct measurements do not exist. Various proxies and modelling suggests large variations; 500 Myr ago CO2 levels were likely 10 times higher than now.[6] Indeed higher CO2 concentrations are thought to have prevailed throughout most of the Phanerozoic eon, with concentrations four to six times current concentrations during the Mesozoic era, and ten to fifteen times current concentrations during the early Palaeozoic era until the middle of the Devonian period, about 400 Mya.[7][8][9] The spread of land plants is thought to have reduced CO2 concentrations during the late Devonian, and plant activities as both sources and sinks of CO2 have since been important in providing stabilising feedbacks.[10] Earlier still, a 200-million year period of intermittent, widespread glaciation extending close to the equator (Snowball Earth) appears to have been ended suddenly, about 550 Mya, by a colossal volcanic outgassing which raised the CO2 concentration of the atmosphere abruptly to 12%, about 350 times modern levels, causing extreme greenhouse conditions and carbonate deposition as limestone at the rate of about 1mm per day.[11] This episode marked the close of the Precambrian eon, and was succeeded by the generally warmer conditions of the Phanerozoic, during which multicellular animal and plant life evolved. No volcanic carbon dioxide emission of comparable scale has occurred since. In the modern era, emissions to the atmosphere from volcanoes are only about 1% of emissions from human sources.