NOAA Global Monitoring Laboratory, R/GML, 325 Broadway, Boulder, CO 80305-3328
The AGGI is a measure of the direct 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 direct 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 2021]. Because climate feedbacks and future 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 (see https://gml.noaa.gov/ccgg/ghgpower/). 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 direct radiative forcing beginning in 1979 when NOAA's global air sampling network expanded significantly. The change in annual average total direct 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 1.88 ppm per year over the past 43 years (1979-2022). 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 averaged 2.4 ppm per year during the last decade (2012-2022). The annual CO2 increase from 1 Jan 2022 to 1 Jan 2023 was 2.19 ± 0.08 ppm (see https://gml.noaa.gov/ccgg/trends/global.html).
The atmospheric burden of methane has increased more rapidly over the past few years than at any other point in the on-going measurement record, which began in 1983. The recent rapid increase follows a period from 1999 to 2006 when the atmospheric CH4 burden was nearly constant. Causes for the recent 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, Lan et al., 2021, Basu et el., 2023]. Since 2017, the global annual increase in methane has averaged 12.0 ± 4.2 ppb yr-1 compared to an average annual increase of 7.6 ± 3.2 ppb yr-1 over the preceding 5 years (between 2011 and 2016; https://gml.noaa.gov/ccgg/trends_ch4/). The annual methane increase during 2022 was 14.07 ± 0.58 ppb and during 2021 it was 17.7 ± 0.47 ppb.
The atmospheric burden of nitrous oxide continues to grow over time. Furthermore, its annual increase, which averaged 1.1 ppb yr-1 over the past decade, is also increasing. The annual increases measured for N2O during 2020, 2021 and 2022 are among the fastest recorded since measurements began. Considering other gases, direct radiative forcing from the sum of observed CFC changes ceased increasing in about 2000 and has continued to decline ever since, despite a temporary increase in CFC-11 emissions from 2013 to 2018 [Montzka et al., 2021]. Increases in HCFCs and HFCs have offset the decline from CFCs so that radiative forcing from the sum of these three chemical classes has changed very little over the past decade (+0.01 W m-2). These trends are in response to global controls placed on CFC and HCFC production and trade by the fully adjusted and amended Montreal Protocol on Substances that Deplete the Ozone Layer.
In this 2022 AGGI update we have used updated equations recommended in the IPCC’s most recent report to recalculate direct radiative forcing for all years from the greenhouse gases defining the AGGI [Forster et al., 2021 and Smith et al., 2021] (see Table 1). This revision reflects an improved understanding of the absorption of light by long-lived greenhouse gases in Earth’s atmosphere, with the largest changes being noted for methane [Etminian et al., 2016; Meinshausen et al., 2020; Forster et al., 2021; Smith et al., 2021]. These empirical expressions are derived from atmospheric radiative transfer models. As such, they generally have an uncertainty of about 10%, which is substantially smaller than uncertainties associated with climate projections. Uncertainties in the measured global average abundances of the long-lived greenhouse gases are even smaller (<1%).
Incorporation of the updated IPCC recommended equations results in slightly higher values for radiative forcing and CO2-equivalent mole fraction in all years but a slightly smaller relative increase in radiative forcing since 1990. As a result, the revised AGGI values in recent years are slightly smaller (by ~0.02) than when derived using the previous calculation methods.
|Trace Gas||Simplified Expression
Radiative Forcing, ΔF (Wm-2)
|CO2||ΔF = (α' + c1*√N) · ln(C/Co)
where α' = d1 + a1(C - Co)2 + b1(C - Co)
|CH4||ΔF = (a2√M + b2√N + d2) * (√M - √Mo)|
|N2O||ΔF = (a3√C + b3√N + c3√M + d3) * (√N - √No)|
|Other gases||ΔF = ω(X - Xo)|
* These updated relationships and values for ω are given in Smith et al. (2021) and Meinshausen et al. (2010). Subscripted numbers on coefficients a, b, c, and d refer to the different chemicals (e.g., c1 refers to the constant -2.15 x E10-3 associated with CO2; d3 refers to the constant 0.12173 associated with nitrous oxide). The subscript “o” denotes the unperturbed (1750) global abundance.
Co = 277.15 ppm, Mo = 731.41 ppb, No = 273.87 ppb, and Xo = 0 ppt
C is the CO2 global measured abundance in ppm, M is the same for CH4 in ppb,
Because we seek an index that is accurate, only direct forcing from these gases has been included. Model-dependent feedbacks and adjustments, for example, due to induced changes in water vapor, atmospheric circulation, and stratospheric ozone depletion, are not included. Other spatially heterogeneous, short-lived, climate forcing agents, such as aerosols, clouds 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 2022. Carbon dioxide is by far the largest contributor to total forcing from these gases and methane is the second largest contributor.
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 49% from 1990 to 2022 (by ~1.11 watts m-2), CO2 has accounted for about 78% of this increase (~0.88 watts m-2), which makes it by far the largest contributor to increases in climate forcing since 1990. Methane and N2O contributed similarly to the increase in radiative forcing since 1990 (0.086 to 0.81 watts m-2 or approximately 7.5% for each). 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 combined. Of the ozone-depleting gases and their substitutes, the largest contributors to direct radiative forcing in 2022 were CFC-12, followed by CFC-11, HCFC-22, HCFC-134a and CFC-113. While the radiative forcing from HFCs has been small relative to all other greenhouse gases (1.4% in 2022), 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 83% 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 2022, the AGGI was 1.49, which represents a 49% increase in total direct radiative forcing from human-derived emissions of these gases 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 nearly three-fourths (71%) of the total increase in the AGGI over the past 270 years.
|Global Radiative Forcing (W m-2)||CO2-eq
|Year||CO2||CH4||N2O||CFCs*||HCFCs||HFCs*||Total||Total||1990 = 1||% change *|
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, Xin Lan, Andrew Crotwell, Tom Conway, Lee Waterman, Tom Mefford, Patricia Lang, Monica Madronich, John Mund, Don Neff, Sonja Wolter, Duane Kitzis, Eric Moglia, Brad Hall, Geoff Dutton, Isaac Vimont, Ben Miller, Molly Crotwell, Rick Myers, Carolina Siso, Debbie Mondeel, Scott Clingan, James Elkins, Thayne Thompson, Steve Montzka 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.