Rising concentrations of a large number of potent anthropogenic greenhouse gases other than carbon dioxide have collectively contributed an amount of radiative forcing comparable to that of CO2 since preindustrial times. Many of these non-CO2 gases (e.g., CH4, N2O, CF2O2, SF6) are emitted at the Earth's surface and contribute directly to this forcing (Prinn, Chapter 9, this volume). They are characterized by atmospheric lifetimes of decades to millennia (lifetime as used here is the amount of the gas in the global atmosphere divided by its global rate of removal). Other non-CO2 gases (e.g., isoprene, terpenes, nitric oxide [NO], carbon monoxide [CO], sulphur dioxide [SO2], dimethyl sulphide [C^^S), most of which are also emitted at the Earth's surface, contribute indirectly to this forcing through production of either tropospheric ozone (which is a powerful greenhouse gas) or tropospheric aerosols (which directly reflect sunlight back to space, absorb it, or indirectly change the reflection properties of clouds). This second group of climatically important non-CO2 gases is characterized by much shorter lifetimes (hours to months).
The role of non-CO2 carbon gases is not typically included in global carbon budgets, because the sources and sinks for these gases are not well understood. Figure 2.2 summarizes, albeit very simply, the basic cycles and fluxes of the major non-CO2 trace gases relevant to climate. Note that the emissions of the carbon-containing gases alone contribute about 2.3 PgC y-1 to the carbon cycle (Prinn, Chapter 9, this volume). To aid the handling of the non-CO2 gases in the policy processes under the United Nations Framework Convention on Climate Change (UNFCCC), scientists have calculated so-called global warming potentials (GWPs). These dimensionless GWPs, which range from 20 to 20,000 for the major non-CO2 gases, are intended to relate the time-integrated radiative forcing of climate by an emitted unit mass of a non-CO2 trace gas to the forcing caused by emission of a unit mass of CO2. The GWP concept has difficulties, because the removal mechanisms for many
gases (including CO2 itself) involve complex chemical and/or biological processes and because the time period (e.g., decade, century) over which one integrates the instantaneous radiative forcing of a gas to compute its GWP is somewhat arbitrary (Manne and Richels, Chapter 25, this volume). Nevertheless, by multiplying the emissions of each major non-CO2 greenhouse gas by its GWP, we obtain equivalent amounts of CO2 emissions, which are comparable to the total actual emissions of CO2 (e.g., current total emissions of CH4 and N2O are equivalent to 3.8 and 2.1 PgC y-1, respectively). For this reason, a "CO2 emissions only" approach for global warming pol icy may lead to significant biases in the estimation of global warming abatement costs (Manne and Richels, Chapter 25, this volume). Thus, a multi-gas approach for studying the carbon cycle and how it relates to climate change needs to be implemented.
Was this article helpful?