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 2025, the AGGI was 1.56, which represents a 56% 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) (Figure 1). 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 surface abundance averages and trends for 20 “minor” long-lived greenhouse gases (e.g., chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), SF₆, and others) that due to their lower abundance contribute much less to warming than the three major gases. The highly accurate and precise measurements of these 23 GHGs allow us to understand how atmospheric abundances and consequent radiative forcing from these long-lived gases have changed over time (Figure 2).

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.

NOAA’s measurements show that the mean global abundance of atmospheric CO₂ in the remote marine boundary layer reached 425.62 ± 0.05 ppm in 2025 and has increased by an average of 1.94 ppm per year over the past 47 years (1979-2025). The decadal 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 2.5 ppm per year during the last decade (2016-2025). The CO₂ increase from January 1, 2025 to January 1, 2026 measured by NOAA was 2.06 ± 0.08 ppm.

Measurements of air in bubbles trapped in ice cores indicate that global average CO₂ abundance was stable at 270 - 280 ppm between the end of the last ice age and around 1850 at the start of the Industrial Revolution [IPCC, 2021]. After that time CO₂ began rising in proportion to emissions resulting from fossil fuel combustion [Friedlingstein et al., 2025]. 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), primarily variability in terrestrial ecosystem function often related to climate modes such as the El Niño-Southern Oscillation (ENSO) [e.g., Betts et al., 2020].

The mean global abundance of atmospheric methane (CH₄) in the remote marine boundary layer reached 1936.03 ± 0.48 ppb in 2025, which is more than 2.5 times higher than its preindustrial value in 1750. Methane’s rate of increase in the atmosphere has varied throughout the ongoing measurement record, which began in 1983. While methane’s rate of increase averaged 8.8 ± 4.1 ppb yr^-1 from 1983 through 1998, methane’s mean abundance was nearly constant from 1999 to 2006. Starting in 2007, methane’s abundance increased again and peaked at a mean of 15.2 ± 2.4 ppb yr^-1 during 2020-2022. Since 2022, methane’s increase has been slightly less rapid: from 2023 through 2025, its increase averaged 7.0 ± 2.0 ppb yr^-1, and the rate during 2025 was 4.77 ± 0.43 ppb yr^-1 (https://gml.noaa.gov/webdata/ccgg/trends/ch4/ch4_gr_gl.txt).

The processes responsible for the post-2007 increase in atmospheric methane are not fully understood. However, measurements in a subset of our air samples of methane’s carbon-13:carbon-12 ratio 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 primarily 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; Riddle-Young et al., 2025; https://gml.noaa.gov/ccgg/carbontracker-ch4/].

The mean global abundance of the long-lived gas nitrous oxide (N₂O) in the marine boundary layer also continues to grow over time and in 2025 reached 338.86 ± 0.02 ppb. The annual increase during 2025 (1.23 ± 0.01 ppb yr^-1) was tied with 2022 and 2014 as being the 3rd fastest measured by NOAA since measurements began in 2001; faster increases have only been measured during 2020 (1.32 ± 0.11 ppb yr^-1) and 2021 (1.29 ± 0.02 ppb yr^-1). While the rate of increase in atmospheric N₂O is variable from year to year, it has increased on average throughout the measurements period; the mean increase during the past decade was 1.09 ± 0.20 ppb yr^-1 while during the first decade of measurements it averaged 0.78 ± 0.18 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 global surface atmospheric abundances of the three most abundant CFCs were all decreasing by the mid-2000s and have since continued to decline. 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 HCFCs, which are first generation substitutes for CFCs, have recently peaked and started to decline (Western et al., 2024). Overall atmospheric trends measured 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. Atmospheric abundances of all but one saturated HFCs continue to increase in the global atmosphere (Liang et al., 2022); the abundance of HFC-365mfc has decreased since 2022 perhaps owing to controls on HFC production in the Kigali Amendment to the Montreal Protocol.

Figure 2. Global average surface 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 95% of the effective radiative forcing by long-lived greenhouse gases in 2025. The remaining 5% is contributed by 18 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 forms the basis for NOAA’s AGGI. The AGGI was introduced in 2006 based on measurements through 2004 [Hofmann et al., 2006] and has been updated annually since.

This 2025 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 included in the AGGI [Forster et al., 2021 and Smith et al., 2021] (see Table 1). This approach reflects our best understanding of the absorption of light by long-lived greenhouse gases in Earth’s atmosphere [Etminan 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 concept of radiative forcing has been refined over time. Direct radiative forcing primarily represents the warming influence arising from 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. These factors have the least uncertainty and as such have formed the basis for the AGGI calculation in years before 2023. 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 also taken into account. These adjustments have been estimated by Earth System Models for the major greenhouse gases and are considered as being constant over all years. Considering both direct radiative forcing and these adjustments, known as effective radiative forcings, 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. Adoption of the updated IPCC recommended equations and use of effective radiative forcing leads to slight changes in the calculated forcing and AGGI for all years. While the effective radiative forcing for CH₄ is smaller (by 14%) compared to its direct radiative forcing, it is larger for CO₂ (by 5%), N₂O (by 7%), CFC-11 (by 13%) and CFC-12 (by 12%). 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 for the AGGI (by 0.012) in 2025.

As in earlier AGGI updates, 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 forcing arising from spatially heterogeneous, short-lived, climate-forcing agents that are highly variable and have uncertain global magnitudes, such as aerosols, clouds, and tropospheric ozone, and radiative forcing from changes in stratospheric ozone.

2025 Results

NOAA’s measurements of atmospheric composition change indicate that the AGGI reached the value of 1.56 in 2025. This value represents a 56% increase in effective radiative forcing arising from long-lived gases since 1990. In absolute terms, this increase equates to an additional 3.59 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 2025 in Figure 3. Carbon dioxide was by far the largest contributor to effective radiative forcing from these gases (2.37 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 23 long-lived GHGs included in the NOAA Annual Greenhouse Gas Index (AGGI). The AGGI, which is indexed to 1 for the year 1990, is shown on the right axis. The “CFCs*” grouping in the figure includes forcing from some gases other than CFCs (e.g., CCl₄, CH₃CCl₃, and Halons, 8 gases in all), but CFCs account for the majority (95% in 2025) of the forcing from this group. The “HCFCs” grouping includes the three most abundant of these chemicals (HCFC-22, HCFC-141b, and HCFC-142b). The “HFCs*” grouping includes the most abundant HFCs (HFC-134a, HFC-23, HFC-125, HFC-32, HFC-143a, HFC-365mfc, HFC-227ea, and HFC-152a) and SF₆ for completeness; SF₆ accounted for 11% of the radiative forcing from this group in 2025. 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. While the combined effective radiative forcing of these and all the other long-lived, well-mixed greenhouse gases included in the AGGI rose 56% from 1990 to 2025 (by ~1.29 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 0.096 W m^-2 increase, followed by methane with a 0.083 W m^-2 increase.

While radiative forcing from CFCs and related gases (CFC* in Figure 3) has declined in recent years, the warming influence from this group of chemicals remains larger than the combined warming influence from both the first- and second-generation substitutes (HCFCs plus HFCs). Over the past decade, cumulative 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 overall from HFCs. Of the ozone-depleting gases and their substitutes, the largest contributors to effective radiative forcing in 2025 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 (1.7% in 2025), the potential for large future increases led to the adoption of controls on HFC production in the Kigali Amendment to the Montreal Protocol.

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., nondimensional) 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 3.59 W m^-2 of warming influence supplied in 2025 by the increased abundances of long-lived greenhouse gases relative to pre-industrial time represents approximately 1.5% of the heat intercepted by the Earth from the Sun on an ongoing basis (240 W m^-2) after accounting for the energy reflected back to space (planetary albedo = 0.294; Williams, 2024).

Figure 4. Time series since 1700 for atmospheric CO₂ abundance (or mixing ratio; black dashed line), CO₂-equivalent abundance (mixing ratio) 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 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 abundances (in ppm) are approximated by the relationship between CO₂ global mean atmospheric abundance 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 normalized to 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

At the core of the AGGI lies GML’s high-quality trace-gas measurements, 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 Kate Baugh, Scott Clingan, Tom Conway, Andrew Crotwell, Molly Crotwell, Stephen DeVogel, Ed Dlugokencky, Geoff Dutton, James Elkins, Brad Hall, Jack Higgs, Duane Kitzis, Xin Lan, Patricia Lang, Monica Madronich, Josh Mauss, Tom Mefford, Ben Miller, Eric Moglia, Debbie Mondeel, Steve A. Montzka, Sara Morris, John Mund, Rick Myers, Don Neff, Gaby Petron, Carolina Siso, Pieter Tans, Thayne Thompson, Isaac Vimont, Lee Waterman, Sonja Wolter, and other former and current GML staff. We are particularly grateful for our staff and partners worldwide who on a weekly basis steadfastly and carefully collect and ship samples to Boulder for analysis. SAM appreciates and acknowledges K. Shine for useful conversations.

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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