Atmospheric Mixing Ratio of Greenhouse Gases and Radiative Forcing

Abstract

AbstractThe state of the earth’s climate is determined by the level of minor constituents with chemical reactivity and/or radiative forcing. Historically, emissions and removals on the earth’s surface or photo-chemical production and loss in atmosphere led to changes in atmospheric burden of the atmospheric species. Concentrations of greenhouse gases (GHGs) have increased significantly due to emissions driven by human activities. Globally, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have increased by 44%, 131%, and 22%, respectively, from 1850 to 2019. Other well-mixed GHGs of purely anthropogenic origin, such as chlorofluorocarbons (CFCs), have reached peak concentrations in the 1990s, and the CFC-replacements are growing fast since the mid-1990s. The balance of net incoming solar radiation and earth’s outgoing radiation had reached a quasi-steady state millions of years ago, when concentration of CO2, CH4, and N2O varied between 180–285 ppm, 340–727 ppb, and 209–293 ppb, as observed during 800ky-0 AD. However, the recent increase of well-mixed GHGs have changed this balance to a net positive radiative forcing by absorbing a greater fraction of the outgoing radiation and the energy imbalance leading to global warming. Overall, the increase in GHGs alone produced an effective radiative forcing of about 3 W m−2 during 1850–2019 with 65% contribution from CO2 alone. Metrics like global warming potential (GWP, GWP*) and global temperature change potential (GTP) are discussed to help understanding the impacts of GHGs on the global surface air temperature increase. It is discussed that the regional GHGs emissions should be managed wisely to achieve the ambitious climate goals.