Annual Greenhouse Gas Index (AGGI)

Introduction

Together, scientific evidence and economic statistics indicate that increases in the abundance of atmospheric greenhouse gases (GHGs) since the Industrial Revolution are the result of human activities, either directly or indirectly. Furthermore, the convergence of extensive scientific evidence indicates that increases in long-lived, well-mixed GHG, particularly CO₂, CH₄, nitrous oxide (N₂O), and halogenated compounds (mainly chlorofluorocarbons or CFCs), are the main cause for increases in global temperature over the industrial period. Other factors influencing Earth’s climate, such as solar radiation, surface reflectivity (Earth’s albedo), volcanic influences, and aerosol particles, have changed little over this same time [NRC, 2020; IPCC 2021; Hansen et al., 2005]. Among these factors, the influence of GHGs on Earth’s energy balance is the largest and most accurately quantified. The change in heat being added by GHGs since the Preindustrial Era, i.e., their radiative forcing, has been long understood [Tyndall, 1861; Arrhenius, 1896] and is rooted in fundamental physical properties of atmospheric GHGs that absorb and re-emit infrared light (i.e., long-wave radiation), thus preventing the loss of that radiation to space, effectively adding heat to the Earth System.

Because projections of climate conditions in the future have large uncertainties owing to the complex nature of Earth’s climate system, we present here a simple observational index based on factors having little uncertainty: the measured atmospheric abundances of long-lived gases over time and the radiative heating they add to Earth’s climate system. The NOAA Annual Greenhouse Gas Index (AGGI) is proportional to the change in heating by these long-lived gases since the onset of the Industrial Revolution. It is calculated as the ratio of total effective radiative forcing due to these gases in a given year relative to the forcing in 1990. 1990 was chosen as the reference year for the AGGI because it is the baseline year for the Kyoto Protocol, an early climate agreement. For 2024, the AGGI was 1.54, which represents a 54% increase in effective radiative forcing from these gases since 1990. Most of this increase (81%) stems from the measured atmospheric increase in CO₂ over this period.

While the AGGI is not a predictor of future climate, it encapsulates the net change in radiative forcing arising from long-lived greenhouse gases that is relevant to Earth’s climate. Future climate projections need to account for many additional factors—such as solar radiation, the reflectivity of Earth’s surface (its albedo), aerosol particles, and feedback mechanisms in biogeochemical and hydrological cycles— that individually and collectively have a strong influence on Earth’s climate system. An understanding of how Earth’s climate system might respond to changes in all of these elements is best provided by climate models that incorporate various physical, chemical, and biological processes, aiming to capture the full complexity of the Earth system. Despite their uncertainty, climate models represent our best tools for understanding how Earth’s climate might change in the future.

Observations

Air samples are collected weekly at remote sites throughout the NOAA Global Greenhouse Gas Reference Network (GGGRN) (see Figure 1).

Figure 1. Sites in the NOAA Global Greenhouse Gas Reference Network (GGGRN; https://gml.noaa.gov/ccgg/about.html) where air samples are collected approximately every week. The samples are returned to NOAA’s Boulder laboratory for analysis on multiple instruments. Measured trace-gas abundances (as dry-air mole fractions) are used to estimate global mean atmospheric abundances of greenhouse gases and annual values for the AGGI. Solid red circles are sites active currently, unfilled red circles are sites where sampling is not currently ongoing. Click on image to view full size figure.

Atmospheric abundances of trace gases in these samples are measured at the NOAA Global Monitoring Laboratory and are used to create smoothed north-south latitude profiles from which global atmospheric abundance averages and trends are calculated for the most abundant, or “major” long-lived greenhouse gases: CO₂, CH₄, and N₂O. Data from a smaller number of remote sites are used to derive global abundance averages and trends for 19 less abundant, or “minor” long-lived greenhouse gases (e.g., chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), SF₆, and others) that contribute much less to warming overall than the three major gases. Highly accurate and precise measurements of these 22 GHGs allow us to understand how atmospheric abundances and consequent radiative forcing from these long-lived gases have changed over time (Figure 2).

NOAA’s measurements show that the global abundance of atmospheric CO₂ in the marine boundary layer reached 422.80 ± 0.10 ppm in 2024 and has increased by an average of 1.94 ppm per year over the past 46 years (1979-2024). The annual increase in atmospheric CO₂ is accelerating: while it averaged about 1.6 ppm per year in the 1980s and 1.5 ppm per year in the 1990s, it averaged 2.6 ppm per year during the last decade (2014-2024). The CO₂ increase from January 1, 2024 to January 1, 2025 measured by NOAA was a record-breaking 3.72 ± 0.09 ppm (see https://gml.noaa.gov/ccgg/trends/gl_gr.html). The increase during 2024 was 27% larger than the previous largest recorded annual increase of 2.94 ± 0.07 ppm in 2015.

Measurements of air in bubbles trapped in ice cores indicate that global average CO₂ abundance was stable at between around 270 - 280 ppm between the end of the last ice age and around 1850 at the start of the Industrial Revolution [IPCC, 2021], after which CO₂ began rising in proportion to emissions resulting from fossil fuel combustion [Friedlingstein et al., 2025]. Remarkably, though, only half of the CO₂ arising from fuel combustion has stayed in the atmosphere, with the other half being absorbed primarily by the global oceans and more recently by land ecosystems [e.g., Ballantyne, et al., 2012]. The long-term rise in CO₂ growth rate is driven by increasing fossil fuel combustion, while the year-to-year variability of the growth rate results from variability in the Earth’s net absorption of carbon (i.e. the balance between uptake and releases), in this case primarily variability in terrestrial ecosystem function often resulting from climate modes such as the El Nino/Southern Oscillation (ENSO) [e.g., Betts et al., 2020].

The atmospheric abundance of methane (CH₄) reached 1929.56 ± 0.61 ppb in 2024 and has increased more rapidly over the past few years than at any other point in the ongoing measurement record, which began in 1983. The recent rapid increase follows a period from 1999 to 2006 when the atmospheric CH₄ abundance was nearly constant. The precise processes directly responsible for the post-2007 increase are not fully understood. However, measurements of methane’s carbon-13:carbon-12 ratio in a subset of our air samples demonstrate that the global methane increase since 2007 is largely the result of biogenic or “microbial” processes such as those coming from wetlands and/or agriculture (e.g., farm animals and rice), and not directly from the increase in global use of fossil fuels, particularly natural gas (which is >90% methane) [Schaefer et al., 2016; Nisbet et al., 2019; Lan et al., 2021; Basu et al., 2022; Michel et al., 2024; https://gml.noaa.gov/ccgg/carbontracker-ch4/]. From 2020 to 2024, the global annual increase in methane averaged 12.3 ± 4.3 ppb yr^-1, which was substantially faster than the average annual increase between 2015 to 2019 of 8.5 ± 1.4 ppb yr^-1 and between 2010 and 2014 of 6.7 ± 3.4 ppb yr^-1 (https://gml.noaa.gov/ccgg/trends_ch4/). The annual methane increase during 2024 was 7.1 ± 0.6 ppb and during 2023 it was 8.7 ± 0.8 ppb.

The atmospheric burden of the long-lived gas nitrous oxide (N₂O) also continues to grow over time and in 2024 reached 337.71 ± 0.02 ppb. Furthermore, its annual increase, which averaged 1.06 ± 0.20 ppb yr^-1 over the past decade, has been accelerating. While the annual increases measured for N₂O during 2020, 2021, and 2022 were among the fastest recorded since measurements began (1.28 ± 0.04 ppb yr^-1), the increase during 2024 was slightly lower at 1.02 ±0.02 ppb yr^-1. Emissions of N₂O arise from natural and anthropogenic processes; anthropogenic N₂O emissions have increased substantially (40% or 1.9 Tg N yr^-1) in the past 4 decades (1980–2020), with the vast majority of that increase coming from agricultural-related activities (Tian et al., 2024).

Among the minor greenhouse gases measured by NOAA GML, the main contributors to radiative forcing are the CFCs. The atmospheric abundances of the three most abundant CFCs were all decreasing by the mid-2000s and have since declined. This decrease has persisted even as CFC-11 emissions temporarily increased from 2013 to 2018 [Montzka et al., 2021]. After 2018, CFC-11 emissions rapidly dropped and have since been slowly decreasing. The atmospheric abundances of the first generation substitutes for CFCs, the HCFC, have recently peaked and started to decline (Western et al., 2024). Atmospheric abundances of the second generation substitutes, saturated HFCs, continue to increase in the global atmosphere (Liang et al., 2022). Overall trends for the CFCs and HCFCs reflect large declines in emissions in response to global controls on production and trade of these gases by the fully adjusted and amended Montreal Protocol on Substances that Deplete the Ozone Layer. The gradual phase-down of 18 saturated HFCs began in 2019.

Figure 2. Global average abundances of the major, well-mixed, long-lived greenhouse gases as measured by the NOAA global air sampling reference network since the beginning of 1979. Five of these gases — carbon dioxide, methane, nitrous oxide, CFC-12, and CFC-11 — accounted for about 96% of the effective radiative forcing by long-lived greenhouse gases in 2024. The remaining 4% is contributed by 17 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

Global atmospheric abundances of the major and minor greenhouse gases measured by NOAA are used to calculate changes in effective radiative forcing beginning in 1979, which is when NOAA's global air sampling network expanded significantly. The change in annual average total effective radiative forcing from these gases since the pre-industrial era is considered in calculating the NOAA AGGI. The AGGI was introduced in 2006 based on measurements through 2004 [Hofmann et al., 2006] and has been updated annually since.

As with the 2023 update, this 2024 AGGI update is derived using equations recommended in the IPCC’s most recent assessment to calculate effective 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 (i.e., <1% for the most abundant).

The calculation of radiative forcing has been refined over time. Radiative forcing primarily captures the direct radiative forcing supplied by changes in the abundance of long-lived greenhouse gases relative to their pre-industrial values. It also includes indirect but rapid effects such as the small immediate temperature adjustment that occurs in the stratosphere. While these factors contain the least uncertainty and have formed the basis for the AGGI as formulated in years before 2024, updated scientific evidence indicates that the change in Earth’s energy budget associated with a perturbation (e.g., a changing greenhouse gas abundance) is more accurately estimated when additional, climate-model-derived adjustments associated with changes in tropospheric and stratospheric temperatures, clouds, and water vapor (but not changes in surface temperature) are taken into account. This effective radiative forcing has 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, the AGGI calculation since 2023 has incorporated effective radiative forcing. Using effective radiative forcing instead of direct radiative forcing results in slightly larger estimates for total radiative forcing (by 0.062 W m^-2) and the AGGI (by 0.012) in 2024.

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 is the radiative forcing arising from spatially heterogeneous, short-lived, climate-forcing agents, such as water vapor, aerosols, clouds, and tropospheric ozone, because they are highly variable and have uncertain global magnitudes, and radiative forcing from changes in stratospheric ozone. Incorporation of the updated IPCC recommended equations (as in the 2022 AGGI) and use of effective radiative forcing as opposed to direct radiative forcing leads to slight changes in the calculated forcing and AGGI for all years. The effective radiative forcing from CH₄ is smaller (by 5%) and the forcing from CO₂ (by 5%), N₂O (by 7%), CFC-11 (by 13%), and CFC-12 (by 12%) is larger compared to their direct radiative forcing. Overall, these adjustments result in slightly higher values in 2024 for radiative forcing (by 0.01 W m^-2) and the AGGI (by 0.01).

2024 Results

NOAA’s measurements of atmospheric composition change indicate that the AGGI reached the value of 1.54 in 2024. This value represents a 54% increase in effective radiative forcing arising from long-lived gases since 1990. In absolute terms, this increase equates to an additional 3.54 W m^-2 of heat being retained by Earth’s climate system relative to preindustrial times.

Radiative forcing from the main long-lived gases (CO₂, CH₄, N₂O) and groupings of minor gases (CFCs, HCFCs, and the HFCs primarily) is shown through 2024 in Figure 3. Carbon dioxide was by far the largest contributor to effective radiative forcing from these gases (2.33 W m^-2, or 66% of the total) and methane was the second largest contributor (0.57 W m^-2, or 16% of the total).

Figure 3. Radiative forcing from the 22 long-lived GHGs included in the NOAA Annual Greenhouse Gas Index. The AGGI, which is indexed to 1 for the year 1990, is shown on the right axis. The “CFC*” grouping in the figure includes forcing from some gases other CFCs (e.g., CCl₄, CH₃CCl₃, and Halons, 8 gases in all), but CFCs account for the majority (95% in 2024) of the forcing from this group. 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, HFC-365mfc, HFC-227ea, and HFC-152a) and SF₆ for completeness; SF₆ accounted for 13% of the radiative forcing from this group in 2024. 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 54% from 1990 to 2024 (by ~1.24 W m^-2), CO₂ has accounted for about 81% of this increase (~1.0 W 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 the combined warming influence from both the first- and second-generation substitutes (HCFCs plus HFCs). Over the past decade, summed radiative forcing from these three chemical classes has changed very little (+0.01 W m^-2), as the declines from CFCs and, in recent years, for HCFCs have been offset by increases from HFCs. Of the ozone-depleting gases and their substitutes, the largest contributors to effective radiative forcing in 2024 were CFC-12, followed by CFC-11, HCFC-22, HFC-134a, and CFC-113. While the radiative forcing from HFCs is currently small relative to all other greenhouse gases (1.5% in 2024), the potential for large future increases led to the adoption of controls on HFC production in the Kigali Amendment to the Montreal Protocol. The abundance 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.

A broader context:

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 (firn) 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 pre-industrial times (Figure 4). This longer-term view shows how increases in greenhouse gas abundances 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. It also shows how the long-term change has been dominated by changes in atmospheric CO₂ abundances.

While the AGGI is presented as an index to provide a straightforward (i.e., unitless) representation of changes in the radiative forcing supplied by long-lived gases since 1990, there is value in considering the absolute heat being added to Earth’s climate system from these greenhouse gases at their current abundances. The extra 3.54 W m^-2 of heat retained in 2024 by the increases measured in long-lived greenhouse gas abundances since pre-industrial time represents an increase of 1.47% in the amount of heat retained in the Earth system from the Sun on an ongoing basis (240 W m^-2) after accounting for the energy being reflected back to space (Bond albedo = 0.294; Williams, 2024).

Figure 4. Atmospheric histories since 1700 for CO₂ abundance (black line), CO₂-equivalent abundance based on ongoing measurements of all greenhouse gases reported here (black dashed 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 represent values 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 approximated by the relationship between CO₂ global mole fraction and radiative forcing from all long-lived greenhouse gases (Ramaswamy et al., 2001). The dashed orange lines highlight the AGGI reference year (1990) at which the index is assigned a value of 1.0. 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, Jack Higgs, Sara Morris, Kate Baugh, Stephen DeVogel, Gaby Petron, Josh Mauss, 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.

Disclaimer: The scientific results and conclusions presented here are those of the author(s) and do not necessarily reflect any statement of position or policy by NOAA or the Department of Commerce.

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

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