Introduction

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 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 so 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.. The perturbation to 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).

In their recent assessments, the IPCC has concluded that the change in Earth’s energy budget associated with a perturbation (e.g., a changing greenhouse concentration) is more accurately estimated when adjustments that are associated with changes in tropospheric and stratospheric temperatures, clouds, and water vapor (but not changes in surface temperature) are taken into account. These “effective radiative forcings” have been estimated for the major greenhouse gases and, despite the use of general circulation models to derive them, their use has “led to a much-improved understanding and increased confidence in the quantification of radiative forcing” [Forster et al., 2021]). As a result, these adjustments have been incorporated into the calculation of the 2023 AGGI.

Measured global atmospheric abundances of greenhouse gases are used to calculate changes in effective radiative forcing beginning in 1979 when NOAA's global air sampling network expanded significantly. The change in annual average total effective 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.

Observations

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).

Figure 1. The NOAA Global Greenhouse Gas Reference Network (GGGRN), where greenhouse gases are measured in ambient air. Results from subsets of these sites are used to determine global background greenhouse gas concentrations and the AGGI. Click on image to view full size figure.

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 1.90 ppm per year over the past 45 years (1979-2023). 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 2.5 ppm per year during the last decade (2014-2023). The annual CO₂ increase from 1 Jan 2023 to 1 Jan 2024 was 2.83 ± 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 (i.e., from wetlands or agriculture) [Schaefer et al., 2016; Nisbet et al., 2019; Lan et al., 2021; Basu et el., 2022; see also: https://gml.noaa.gov/ccgg/carbontracker-ch4/]. From 2019-2023, the global annual increase in methane has averaged 13.2 ± 3.5 ppb yr^-1 compared to an average annual increase of 9.1 ± 2.4 ppb yr^-1 over the preceding 5 years (between 2014 and 2018) and 5.1 ± 0.4 ppb yr^-1 during the previous 5-year period (2009-2013; https://gml.noaa.gov/ccgg/trends_ch4/). The annual methane increase during 2023 was 9.9 ± 0.6 ppb and during 2022 it was 13.2 ± 0.8 ppb.

The atmospheric burden of nitrous oxide continues to grow over time. Furthermore, its annual increase, which averaged 1.1 ppb yr^-1 ± 0.2 ppb yr^-1 over the past decade, is also increasing. While the annual increases measured for N₂O during 2020, 2021 and 2022 are among the fastest recorded since measurements began (1.3 ± 0.05 ppb yr^-1), the increase in 2023 was 1.03 ± ppb yr^-1. Considering other gases, effective 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]. While radiative forcing from HCFCs has recently peaked and started to decline [Western et al., 2024], the increases in radiative forcing from 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 reflect global controls placed production and trade of CFCs, HCFCs, and HFCs by 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 since the beginning of 1979. These five gases account for about 96% of the effective radiative forcing by long-lived greenhouse gases since 1750. The remaining 4% is contributed by 15 other halogenated gases including HCFC-22 and HFC-134a, for which NOAA observations are also shown here. Methane data before 1983 are annual averages from D. Etheridge [Etheridge et al., 1998], adjusted to the NOAA calibration scale [Dlugokencky et al., 2005]. Click on image to view full size figure.

Radiative Forcing Calculations

In this 2023 AGGI update we have used updated equations recommended in the IPCC’s most recent assessment to calculate radiative forcing for all years from the greenhouse gases defining the AGGI [Forster et al., 2021; 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%).

In addition, the 2023 AGGI calculation is now based on effective radiative forcing for the main greenhouse gases, given the 2021 IPCC-assessed advances in quantification of the adjustments associated with a trace gas concentration change [Forster et al., 2021; Smith et al., 2021]. As in earlier AGGI updates, however, the more complex and less certain model-dependent responses to changes in surface temperature, collectively known as climate “feedbacks”, are not included. Also not included are the radiative forcings arising from spatially heterogeneous, short-lived, climate forcing agents, such as aerosols, clouds and tropospheric ozone, because theay are highly variable and have uncertain global magnitudes and also are not included here.

Incorporation of the updated IPCC recommended equations (as in the 2022 AGGI) and use of effective radiative forcing in the 2023 AGGI leads to decreased forcing for CH₄ (by 5%), and increases in forcing for CO₂ (by 5%), N₂O (by 7%), CFC-11 (by 13%), and CFC-12 (by 12%). Overall, these adjustments result in slightly higher values for radiative forcing (by 0.01 W m^-2 in 2023) and a slight increase in the AGGI (by 0.01 in 2023).

2023 Results

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 2023. Carbon dioxide was by far the largest contributor to effective radiative forcing from these gases(2.29 W m^-2, or 66% of the total) and methane was the second largest contributor 0.56 W m^-2, or 16% of the total).

Figure 3. Radiative forcing, relative to 1750, of virtually all 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. The “CFC*” grouping includes some other long-lived gases that are not CFCs (e.g., CCl₄, CH₃CCl₃, and Halons), but the CFCs account for the majority (95% in 2023) of this radiative forcing. The “HCFC” grouping includes the three most abundant of these chemicals (HCFC-22, HCFC-141b, and HCFC-142b). The “HFC*” grouping includes the most abundant HFCs (HFC-134a, HFC-23, HFC-125, HFC-143a, and HFC-152a) and SF₆ for completeness, although SF₆ only accounted for a small fraction of the radiative forcing from this group in 2023 (13%).
Click on image to view full size figure.

The atmospheric abundance and effective radiative forcing of the three main long-lived greenhouse gases continue to increase in the atmosphere. While the combined effective radiative forcing of these and all the other long-lived, well-mixed greenhouse gases included in the AGGI rose 51% from 1990 to 2023 (by ~1.18 watts m^-2), CO₂ has accounted for about 80% of this increase (~0.95 watts m^-2), which makes it by far the largest contributor to increases in effective radiative forcing from long-lived gases since 1990. The second largest contributor to the increase since 1990 was N₂O with a 0.09 W m^-2 increase, followed by methane with a 0.08 W m^-2 increase.

While 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 effective radiative forcing in 2023 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 (1.3% in 2023), 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 effective radiative forcing arising from HCFC-22 is still only 74% 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 effective 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 2023, the AGGI was 1.51, which represents a 51% increase in effective 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 of the total increase in the AGGI over the past 270 years.

Figure 4. Atmospheric histories since 1750 for CO₂ abundance (black dashed line), CO₂-equivalent abundance based on ongoing measurements of all greenhouse gases reported here (black line), and the AGGI (red line, right-hand scale).  The measurements of CO₂ between the 1950s and 1978 are from C.D. Keeling [Keeling et al., 1958].  Prior to 1978, atmospheric abundances are 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. The dashed orange lines highlight the reference year for the AGGI being assigned a value of 1.0 in 1990.
Click on image to view full size figure.

Download this table as comma separated values (csv).

Data Sources

• Table of global annual mean dry-air mole fractions used in deriving the AGGI.
• Listing of web links to the raw measurement data and the appropriate doi citations for the data used in the AGGI.

Acknowledgements

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.

For more information, contact: Stephen.A.Montzka@noaa.gov.

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