INTRODUCTION

The AGGI is a measure of the warming influence supplied by long-lived trace gases and how that influence is changing each year. The index was designed to enhance the connection between scientists and society by providing a normalized standard that can be easily understood and followed. The warming influence supplied by long-lived greenhouse gases is well understood by scientists and has been reported by NOAA through a range of national and international assessments. Nevertheless, the language of scientists often eludes policy makers, educators, and the general public. This index is designed to help bridge that gap. The AGGI provides a way for this warming influence to be presented as a simple index.

Increases in the abundance of atmospheric greenhouse gases since the industrial revolution are largely the result of human activity and are largely responsible for the observed increases in global temperature [IPCC 2007]. 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 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 (arbitrarily 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 and well mixed greenhouse gases, in particular carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and halogenated compounds (mainly CFCs).

Atmospheric global greenhouse gas abundances are used to calculate changes in radiative forcing for the period beginning in 1979 when NOAA's global air sampling network expanded significantly. The change in annual average total radiative forcing by all the long-lived greenhouse gases since the pre-industrial era (1750) is used to define the NOAA Annual Greenhouse Gas Index (AGGI), which was introduced in 2004 [Hofmann et al., 2006a] and has been updated annually since.

OBSERVATIONS

Figure 1. The NOAA Earth System Research Laboratory global cooperative air sampling network used to determine the AGGI.

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Global monitoring of atmospheric greenhouse gases, in particular carbon dioxide (CO₂), has been a goal of the U.S. government for over 40 years. The NOAA monitoring program provides high-precision measurements of the global abundance and distribution of long-lived greenhouse gases that are used to calculate changes in radiative climate forcing.

Air samples are collected through the NOAA/ESRL global air sampling network, including a cooperative program for the carbon gases which provides samples from about ~80 global clean air sites, including measurements at 5 degree latitude intervals from ship routes (see Figure 1).

Weekly data are used to create a smoothed north-south latitude profile from which a global average is calculated (Figure 2). The growth rate of CO₂ has averaged about 1.68 ppm per year over the past 31 years (1979-2011). The CO₂ growth rate has increased over this period, averaging about 1.4 ppm per year before 1995 and 1.9 ppm per year thereafter. 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]. The approach to steady state may have been accelerated by the economic collapse of the former Soviet Union and decreased emissions from the fossil fuel sector. From 1999 to 2006, the CH₄ burden was about constant, but since 2007, globally averaged CH₄ has begun increasing again. Causes for the recent increases are warm temperatures in the Arctic in 2007 and increased precipitation in the tropics in 2007 and 2008 [Dlugokencky et al., 2009]. Nitrous oxide continues to increase at a relatively uniform growth rate, while 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 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 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 (see text). Methane data before 1983 are annual averages from Etheridge et al. (1998), adjusted to the NOAA calibration scale [Dlugokencky et al., 2005].

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RADIATIVE FORCING CALCULATIONS

To determine the total radiative forcing of the greenhouse gases, we have used IPCC [IPCC 2001] recommended expressions to convert greenhouse gas changes, relative to 1750, to instantaneous radiative forcing (see Table 1). These empirical expressions are derived from atmospheric radiative transfer models and generally have an uncertainty of about 10%. The uncertainties in the global average abundances of the long-lived greenhouse gases are much smaller (<1%).

Because we seek an index that is accurate, only the direct forcing has been included. Model-dependent feedbacks, for example, due to water vapor and ozone depletion, are not included. Other spatially heterogeneous, short-lived, climate forcing agents, such as aerosols and tropospheric ozone, have uncertain global magnitudes and also are not included here to maintain accuracy. Figure 3 shows the results for carbon dioxide global monthly averages for the period 1979-2011. An index based on the total of these contributions to radiative forcing would be similar to the Consumer Price Index, for example. It would include all the important components but not all the components of climate forcing. In contrast to climate model calculations, the results reported here are based mainly on measurements and have small uncertainties.

Figure 3. Radiative forcing, relative to 1750, due to carbon dioxide alone since 1979. The percent change from January 1, 1990 is shown on the right axis.

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2011 RESULTS

Figure 4 shows radiative forcing for the major gases and a set of 15 minor long-lived halogenated gases (CFC-113, CCl₄, CH₃CCl₃, HCFCs 22, 141b and 142b, HFCs 134a, 152a, 23, 143a, and 125, SF₆, and halons 1211, 1301 and 2402). Except for the HFCs and SF₆, which do not contain chlorine or bromine, these gases are also ozone-depleting gases and are regulated by the Montreal Protocol. As expected, CO₂ dominates the total forcing with methane and the CFCs becoming relatively smaller contributors to the total forcing over time. The five major greenhouse gases account for about 96% of the direct radiative forcing by long-lived greenhouse gas increases since 1750. The remaining 4% is contributed by the 15 minor halogenated gases.

Figure 4. Radiative forcing, relative to 1750, of all the long-lived greenhouse gases. The NOAA Annual Greenhouse Gas Index (AGGI), which is indexed to 1 for the year 1990, is shown on the right axis.

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Of the five long-lived greenhouse gases that contribute 96% to radiative climate forcing, CO₂ and N₂O are the only ones that continue to increase at a regular rate. Radiative forcing from CH₄ increased from 2007 to 2011 after remaining nearly constant from 1999 to 2006. While the radiative forcing of the long-lived, well-mixed greenhouse gases increased 30% from 1990 to 2011 (~0.66 watts m^-2), CO₂ has accounted for nearly 80% of this increase (~0.52 watts m^-2). Had the ozone-depleting gases not been regulated by the Montreal Protocol and its amendments, it is estimated that climate forcing would have been as much as 0.3 watt m^-2 higher in 2010 [Velders et al., 2007], or more than half of the increase in radiative forcing due to CO₂ alone since 1990.

An Annual Greenhouse Gas Index (AGGI) has been defined as the ratio of the total 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 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, and the decline in the atmospheric abundance of ozone-depleting chemicals related to the Montreal Protocol. Most of this increase is related to CO₂. For 2011, the AGGI was 1.30 (representing an increase in total radiative forcing of 30% since 1990). The increase in CO₂ forcing alone since 1990 was about 39% (see Fig. 3). The decline in the CFCs has tempered the increase in net radiative forcing considerably. The AGGI will be updated each year when air samples from all over the globe for the previous year have been obtained and analyzed.

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