New NOAA Greenhouse Gas Index; Total Impact All GHGs - 485 ppm Carbon
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Increases in the abundance of atmospheric greenhouse gases since the industrial revolution are mainly the result of human activity and are largely responsible for the observed increases in global temperature [IPCC 2013]. However, climate projections have model uncertainties that overwhelm the uncertainties in greenhouse gas measurements. We present here an index that is directly proportional to the direct warming influence (also know as climate forcing) supplied from these gases. Because it is based on the observed amounts of long-lived greenhouse gases in the atmosphere, this index contains relatively little uncertainty.
The Intergovernmental Panel on Climate Change (IPCC) defines climate forcing as “An externally imposed perturbation in the radiative energy budget of the Earth climate system, e.g. through changes in solar radiation, changes in the Earth albedo, or changes in atmospheric gases and aerosol particles.” Thus climate forcing is a “change” in the status quo. IPCC takes the pre-industrial era (chosen as the year 1750) as the baseline. The perturbation to direct climate forcing (also termed “radiative forcing”) that has the largest magnitude and the least scientific uncertainty is the forcing related to changes in long-lived, well mixed greenhouse gases, in particular carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halogenated compounds (mainly CFCs).
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Weekly data are used to create a smoothed north-south latitude profile from which a global average is calculated (Figure 2). The atmospheric abundance of CO2 has increased by an average of 1.76 ppm per year over the past 37 years (1979-2015). The CO2 increase is accelerating: it averaged about 1.5 ppm per year in the 1980s and 1990s, and it was 2.0 ppm per year during the last decade (2006-2015). The annual CO2 increase during the past year was nearly 3 ppm for only the second time since 1979. The increase observed for global atmospheric CO2 has resulted in a 50% increase in its direct warming influence on climate since 1990 (Figure 3).
The growth rate of methane declined from 1983 until 1999, consistent with an approach to steady-state. Superimposed on this decline is significant interannual variability in growth rates [Dlugokencky et al., 1998, 2003]. From 1999 to 2006, the atmospheric CH4 burden was about constant, but since 2007, globally averaged CH4 has been increasing again. Causes for the increase during 2007-2008 included warm temperatures in the Arctic in 2007 and increased precipitation in the tropics in 2007 and 2008 [Dlugokencky et al., 2009]. From 2014 to 2015 global methane increased substantially faster (11.5 ppb/yr) than it had from 2007 to 2013 (5.7 ± 1.2 ppb-1). Similarly, the atmospheric burden of nitrous oxide has increased at faster rates in recent years. Radiative forcing from the sum of observed CFC changes ceased increasing in about 2000 and is now declining [Montzka et al., 2011]. The latter is a response to decreased emissions related to the fully adjusted and amended Montreal Protocol on Substances that Deplete the Ozone Layer.
Figure 2. Global average abundances of the major, well-mixed, long-lived greenhouse gases - carbon dioxide, methane, nitrous oxide, CFC-12 and CFC-11 - from the NOAA global air sampling network are plotted since the beginning of 1979. These five gases account for about 96% of the direct radiative forcing by long-lived greenhouse gases since 1750. The remaining 4% is contributed by an assortment of 15 minor halogenated gases including HCFC-22 and HFC-134a, for which NOAA observations are also shown in the figure (see text). Methane data before 1983 are annual averages from D. Etheridge [Etheridge et al., 1998], adjusted to the NOAA calibration scale [Dlugokencky et al., 2005].
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An Annual Greenhouse Gas Index (AGGI) has been defined as the ratio of the total direct radiative forcing due to long-lived greenhouse gases for any year for which adequate global measurements exist to that which was present in 1990. 1990 was chosen because it is the baseline year for the Kyoto Protocol. This index, shown with the direct radiative forcing values in Table 2 and on the right-hand axis of Figure 4, is a measure of the interannual changes in conditions that affect carbon dioxide emission and uptake, methane and nitrous oxide sources and sinks, the decline in the atmospheric abundance of ozone-depleting chemicals related to the Montreal Protocol, and the increase in their substitutes (HCFCs and HFCs). Most of this increase is related to CO2. For 2015, the AGGI was 1.37 (representing an increase in total direct radiative forcing of 37% since 1990). The increase in CO2 forcing alone since 1990 was about 50% (see Fig. 3). The decline in the CFCs has tempered the increase in net radiative forcing considerably. The AGGI is updated each spring when air samples collected during the previous year from all over the globe have been obtained and analyzed.
Changes in radiative forcing before 1978 are derived from atmospheric measurements of CO2, started by C.D. Keeling [Keeling et al., 1958], and from measurements of CO2 and other greenhouse gases in air trapped in snow and ice in Antarctica and Greenland [Etheridge et al., 1996; Butler et al,, 1999]. These results define atmospheric composition changes going back to 1750 and radiative forcing changes since preindustrial times (Figure 4). This longer-term view shows how increases in greenhouse gas concentrations over the past 60 years (since 1950) have accounted for three-fourths (75%) of the total increase in the AGGI over the past 260 years. Changes in radiative forcing before 1978 are derived from atmospheric measurements of CO2, started by C.D. Keeling [Keeling et al., 1958], and from measurements of CO2 and other greenhouse gases in air trapped in snow and ice in Antarctica and Greenland [Etheridge et al., 1996, 1998; Butler et al,, 1999]. These results define atmospheric composition changes going back to 1750 and radiative forcing changes since preindustrial times (Figure 5). This longer-term view shows how increases in greenhouse gas concentrations over the past 60 years (since 1950) have accounted for three-fourths (75%) of the total increase in the AGGI over the past 260 years.
Figure 5. Pre-1978 changes in the CO2-equivalent concentration and AGGI based on the ongoing measurements of all greenhouse gases reported here, measurements of CO2 going back to the 1950s from C.D. Keeling [Keeling et al., 1958], and atmospheric changes derived from air trapped in ice and snow above glaciers [Machida et al., 1995, Battle et al., 1996, Etheridge, et al., 1996; Butler, et al., 1999]. Equivalent CO2 atmospheric amounts (in ppm) are derived with the relationship (Table 1) between CO2 concentrations and radiative forcing from all long-lived greenhouse gases.
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http://esrl.noaa.gov/gmd/aggi/aggi.html
