The AGGI is a measure of the climate-warming influence of long-lived trace gases in the atmosphere and how that influence has changed since the onset of the industrial revolution. 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 of 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 mainly the result of human activity and are largely responsible for the observed increases in global temperature [IPCC 2014]. Because climate projections have large model uncertainties that overwhelm the uncertainties in greenhouse gas measurements, we present here an observationally based index that is proportional to the change in the direct warming influence since the onset of the industrial revolution (also known as climate forcing) supplied from these gases. This index is based on the observed amounts of long-lived greenhouse gases in the atmosphere and contains 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, forcing changes in the climate. IPCC takes the pre-industrial era (chosen as the year 1750) as the baseline, although some argue that 1800 is more representative. The perturbation to direct climate forcing (also termed “radiative forcing”) that has the largest magnitude and the smallest scientific uncertainty is the forcing related to changes in the atmospheric global abundance of long-lived, well mixed, greenhouse gases, in particular, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and halogenated compounds (mainly CFCs).
Measured global atmospheric abundances of greenhouse gases are used to calculate changes in radiative forcing 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 is also used to define the NOAA Annual Greenhouse Gas Index (AGGI), which was introduced in 2006 based on measurements through 2004 [Hofmann et al., 2006a] and is updated annually.
Air samples are collected through the NOAA Global Greenhouse Gas Reference Network, which provides samples from up to 80 global background air sites, including some collected at 5 degree latitude intervals from ship routes (see Figure 1).
Weekly data are used from the most remote sites appearing in Figure 1 to create smoothed north-south latitude profiles from which global averages and trends are calculated (Figure 2). For example, the atmospheric abundance of CO2 has increased by an average of 1.85 ppm per year over the past 41 years (1979-2020). This increase in CO2 is accelerating — while it averaged about 1.6 ppm per year in the 1980s and 1.5 ppm per year in the 1990s, the growth rate increased to 2.4 ppm per year during the last decade (2009-2020). The annual CO2 increase from 1 Jan 2020 to 1 Jan 2021 was 2.50 ± 0.08 ppm (see https://gml.noaa.gov/ccgg/trends/global.html), which is slightly higher than the average for the previous decade, and much higher than the two decades before that.
The growth rate of methane declined from 1983 until 1999, consistent with its concentration approaching 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 nearly constant, but, since 2007, CH4 has been increasing again. Causes for the increase are not fully understood, but warm temperatures in the Arctic in 2007 and increased precipitation in the tropics during 2007 and 2008 [Dlugokencky et al., 2009] contributed in the early years. Isotopic measurements argue for continued increasing microbial emissions after 2008 (e.g., likely from wetlands or agriculture) [Schaefer et al., 2016; Nisbet et al., 2019]. Since 2015, the global annual increase in methane has become even larger, averaging 9.7 ± 3.3 ppb yr-1 through 2020 compared to an average annual increase of 6.4 ± 2.9 ppb yr-1 between 2008 and 2014 (https://gml.noaa.gov/ccgg/trends_ch4/). The annual methane increase in 2020 was 15.85 ± 0.47 ppb, which is the largest annual increase recorded since 1983 when NOAA's ongoing measurements began.
The atmospheric burden of nitrous oxide continues to grow over time. Furthermore, the annual increase in nitrous oxide's atmoshpheric burden, averaging 1.0 ppb yr-1 over the past decade, is also increasing. The annual increase in 2020 was the largest recorded since measurements began. Radiative forcing from the sum of observed CFC changes ceased increasing in about 2000 and has continued to decline ever since [Montzka et al., 2021]. This continued decline is a response to global controls placed on CFC production and trade by the adjusted and amended Montreal Protocol on Substances that Deplete the Ozone Layer.
To determine the total radiative forcing of the greenhouse gases for the AGGI, we have used IPCC [Ramaswamy et al., 2001] recommended expressions to convert changes in greenhouse gas global abundance relative to 1750, to instantaneous radiative forcing (see Table 1). (In a separate analysis, we use 1800 as the base year and have added additional gases, but this has little effect on the trend or amount of radiative forcing.) These empirical expressions are derived from atmospheric radiative transfer models and generally have an uncertainty of about 10%. By contrast, uncertainties in the measured global average abundances of the long-lived greenhouse gases are much smaller (<1%).
|Trace Gas||Simplified Expression
Radiative Forcing, ΔF (Wm-2)
|CO2||ΔF = αln(C/Co)||α = 5.35|
|CH4||ΔF = β(M½ - Mo½) - [f(M,No) - f(Mo,No)]||β = 0.036|
|N2O||ΔF = ε(N½ - No½) - [f(Mo,N) - f(Mo,No)]||ε = 0.12|
|Other gases||ΔF = ω(X - Xo)||ω = see notes|
The subscript "o" denotes the unperturbed (1750) global abundance.
f(M,N) = 0.47ln[1 + 2.01x10-5 (MN)0.75 +
C is the CO2 global measured abundance in ppm, M is the same for CH4 in ppb,
Co = 278 ppm, Mo = 722 ppb, No = 270 ppb, Xo = 0
Because we seek an index that is accurate, only direct forcing from these gases 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, are highly variable and have uncertain global magnitudes and also are not included here to maintain accuracy.
Figure 3 shows radiative forcing for CO2, CH4, N2O and groupings of gases that capture changes predominantly in the CFCs, HCFCs, and the HFCs through 2020. As expected, CO2 is by far the largest contributor to total forcing from these gases, while methane and the CFCs are becoming relatively smaller contributors to total forcing over time.
The atmospheric abundance and radiative forcing of the three main long-lived greenhouse gases continue to increase in the atmosphere. While the combined radiative forcing of these and all the other long-lived, well-mixed greenhouse gases included in the AGGI rose 47% from 1990 to 2020 (by ~1.02 watts m-2), CO2 has accounted for about 80% of this increase (~0.82 watts m-2), which makes it by far the biggest contributor to increases in climate forcing since 1990. Had 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 greater in 2010 [Velders et al., 2007], or more than half of the increase in radiative forcing due to CO2 alone since 1990 . While direct radiative forcing from CFCs and related gases (CFC* in Figure 3) has declined in recent years, the current warming influence from this group of chemicals is still larger than that from HCFCs and HFCs. Of the ozone-depleting gases and their substitutes, the largest contributors to direct warming in 2020 were CFC-12, followed by CFC-11, HCFC-22, CFC-113 and HCFC-134a. While the radiative forcing from HFCs has been small relative to other greenhouse gases, the potential for large future increases led to the adoption of controls on HFC production in the Kigali amendment to the Montreal Protocol. The concentration of HCFC-22 in the remote atmosphere surpassed that of CFC-11 by the end of 2015 (Figure 2), but the radiative forcing arising from HCFC-22 is still only 88% of that from CFC-11 because CFC-11 is more efficient at trapping infrared radiation on a per molecule basis.
The Annual Greenhouse Gas Index (AGGI) is calculated as the ratio of total direct radiative forcing due to these gases in a given year to its total in 1990. 1990 was chosen because it is the baseline year for the Kyoto Protocol and the publication year of the first IPCC Scientific Assessment of Climate Change. Most of this increase is related to CO2. For 2020, the AGGI was 1.47 (representing an increase in total direct radiative forcing of 47% since 1990).
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 ~70 years (since 1950) have accounted for three-fourths (72%) of the total increase in the AGGI over the past 260 years.
|Global Radiative Forcing (W m-2)||CO2-eq
|Year||CO2||CH4||N2O||CFCs*||HCFCs||HFCs*||Total||Total||1990 = 1||% change *|
Click here to download this table as comma separated values (csv).
Click here to download measured global annual mean dry-air mole fractions used in deriving the radiative forcing values provided in Table 2 and the AGGI.
The core of the AGGI is GML’s high quality data, to which many scientists and technicians at GML have contributed. Attention to detail, calibration, and quality control are hallmarks of the data that go into deriving the AGGI. Many of GML’s staff over the years have contributed to the data used for this index. These include Ed Dlugokencky, Pieter Tans, Andrew Crotwell, Tom Conway, Lee Waterman, Tom Mefford, Patricia Lang, Duane Kitzis, Eric Moglia, Brad Hall, Ben Miller, Rick Myers, Carolina Siso, Isaac Vimont, Matt Gentry, Debbie Mondeel, James Elkins, Thayne Thompson and other former and current GML staff. We are particularly grateful for our staff and partners worldwide who steadfastly and carefully collect and ship samples on a weekly basis to Boulder for analysis.