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The concentrations of carbon dioxide, methane, and other greenhouse gases are currently increasing over time. The carbon dioxide concentration, measured as the mole fraction in dry air, on Mauna Loa, Hawaii, constitutes the longest record of direct measurements of CO₂ in the atmosphere. The average Mauna Loa CO₂ level for 2017 was 407 ppmvd (based on the monthly averages) compared to 316 ppmvd in 1959. The measurements were started by C. David Keeling of the Scripps Institution of Oceanography in March of 1958 at a facility of the National Oceanic and Atmospheric Administration (Keeling et al. 1976). NOAA started its own CO₂ measurements in May 1974, and they have run in parallel with those made by Scripps since then (Thoning et al. 1989).

Table 1.1 Global warming potential (GWP) – relative to CO₂ – as well as atmospheric concentration and lifetime of selected greenhouse gases.

Source: Data are based on CDIAC (2010).

Gas	Chemical formula	Recent tropospheric concentration	GWP (100 yr time horizon)	Atmospheric lifetime (yr)
Carbon dioxide	CO2	≈386 ppm	1	100
Methane	CH4	≈1800 ppb	25	12
Nitrous oxide	N2O	≈320 ppb	298	114
Ozone	O3	≈34 ppb	N/A	h/d
CFC-11	CCl3F	≈240 ppt	4750	45
CFC-12	CCl2F2	≈536 ppt	10 900	100
CFC-113	CCl2FFClF2	≈76 ppt	6 130	85
HCFC-22	CHClF2	≈200 ppt	1 810	12
HCFC-141b	CH3CCl2F	≈20 ppt	725	9
HCFC-142b	CH3CClF2	≈20 ppt	2 310	18
Halon 1211	CBrCIF2	≈4 ppt	1 890	16
Halon 1301	CBrCIF3	≈3 ppt	7 140	65
HFC-134a	CH2FCF3	≈52 ppt	1 430	14
Carbon tetrachloride	CCL4	≈88 ppt	1 400	26
Methyl chloroform	CH3CCl3	≈10 ppt	146	5
Sulfur hexafluoride	SF6	≈7 ppt	22 800	3200

For a much longer time frame, data from the Vostok ice core provide an insight into the variations in CO₂ levels for the past four glacial–interglacial cycles. In January 1998, the collaborative ice-drilling project between Russia, the United States, and France at the Russian Vostok station in East Antarctica yielded the deepest ice core ever recovered, reaching a depth of 3623 m (Petit et al. 1999). Data from this project are displayed in Figure 1.1. One conclusion from the authors was that present-day atmospheric burdens of CO₂ seem to have been unprecedented during the past 420 000 years. Note that the mean resolution for the CO₂ measurements was about 1500 years.