Unlike CO2 and CH4, carbon monoxide (CO) does not strongly absorb terrestrial infrared radiation but affects climate through its chemistry. The chemistry of CO affects OH (which influences the lifetimes of CH4 and HFCs) and tro-pospheric O3 (which is by itself a greenhouse gas), so emissions of CO can be considered equivalent to emissions of CH4. Current emissions of CO may contribute more to radiative forcing over decade time scales than emissions of anthropogenic nitrous oxide.
Because the lifetime of CO is relatively short (a few months), the anomaly quickly disappeared and CO quickly returned to pre-1997 levels. Carbon monoxide levels in 2005 were comparable to those found in the early 2000s. The globally averaged CO mole fraction in 2005 was 83.5 ppb, very near the average of the past five years. Since 1991, little trend in globally averaged CO has been observed.
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