NOAA index tracks how greenhouse gas pollution amplified global warming in 2020

Extra heat trapped in the atmosphere by human-caused greenhouse gas pollution continued to exacerbate global warming in 2020, driven by historically high emission levels that were largely unaffected by the economic slowdown stemming from the pandemic, NOAA scientists reported.

NOAA’s Annual Greenhouse Gas Index, known as the AGGI, tracks increases in the warming influence of most heat-trapping gases being added to the atmosphere principally from human activity, including carbon dioxide, methane, nitrous oxide, chlorofluorocarbons, and other chemicals. The top five greenhouse gases account for about 96 percent of the increased heat trapped in the atmosphere due to human activity since 1750, the start of the Industrial Revolution. The AGGI also tracks 16 secondary greenhouse gases responsible for the remaining 4 percent of human-caused warming.

These graphs depict the 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 plotted since the beginning of 1979. These five gases account for about 96% of the direct radiative forcing by long-lived greenhouse gases since 1750. Credit: NOAA Global Monitoring Laboratory

The AGGI is based on thousands of air samples collected from sites around the world each year from NOAA’s Global Greenhouse Gas Reference Network. The concentrations of these greenhouse gases and other chemicals are determined through the analysis of those samples at NOAA’s Global Monitoring Laboratory in Boulder, Colorado. Scientists then calculate the amount of extra heat being trapped in the Earth system by these gases as a result of human activity in the past year, and how much that has changed over time. 

“This analysis of samples collected by NOAA’s Global Greenhouse Gas Reference Network provides a measure of the excess heat trapped in the atmosphere by greenhouse gas pollution,” said James Butler, the Global Monitoring Laboratory’s director. “The contributions of different greenhouse gases have evolved over time, but the conclusion that humans are nearly 100 percent responsible for their increase is inescapable.”  

One number tracks human impact on climate

NOAA scientists released the first AGGI in 2006 as a way to help policymakers, educators, and the public understand the cumulative impact of greenhouse gases on climate over time. The AGGI converts the complex scientific computations of how much extra heat is trapped in the atmosphere by human-caused emissions each year into a single number that can easily be compared to previous years. The AGGI is updated each spring after nearly all the air samples collected during the previous year have been obtained and analyzed. 

This figure describes the heat-trapping influence ( or radiative forcing), of all the long-lived greenhouse gases relative to 1750. The NOAA Annual Greenhouse Gas Index (AGGI), which is indexed to 1 for the year 1990, is shown on the right axis. Credit: NOAA Global Monitoring Laboratory

Scientists benchmarked the AGGI to the year 1750, the onset of the Industrial Revolution, assigning it a value of zero. An AGGI value of 1.0 was assigned to 1990 - the year of the Kyoto Protocol, an international treaty that called on the global community to reduce greenhouse gas pollution. The AGGI is useful in tracking  the relative change in heat being trapped in the atmosphere by human-caused emissions of greenhouse gases since the Kyoto Protocol.

In 2020 - the AGGI reached a value of 1.47, and the change from 2019 to 2020 was similar to previous years despite an economic slowdown caused by the pandemic. That means 47 percent more heat attributable primarily to human activity was captured by the Earth’s climate system in 2020 than in 1990. 

Analysis of samples collected in 2020 showed the global average burden of methane, the second-most abundant anthropogenic greenhouse gas, reached 1,879 parts per billion (ppb), continuing a rapid rise that began in 2007 following a 10-year plateau. The year-to-year jump was almost 16 ppb, the largest increase detected in the observational record dating to 1984, when the global average was 1,645 ppb.

While a given amount of methane is 28 times more efficient at trapping heat over a 100-year time frame than carbon dioxide (CO₂), the global warming influence of methane is 4 times less than CO₂ because of its much lower concentrations in the atmosphere.

CO₂ is by far the most abundant human-emitted greenhouse gas. Roughly 40 billion metric tons of CO2 are generated each year by transportation, electrical generation, cement manufacturing, deforestation, agriculture, and many other practices. 

This chart displays pre-1978 changes in the CO2-equivalent abundance and AGGI based on the ongoing measurements of all greenhouse gases reported here, measurements of CO2 going back to the 1950s from C.D. Keeling and atmospheric changes derived from air trapped in ice and snow above glaciers Equivalent CO2 atmospheric amounts (in ppm) are derived with the relationship between CO2 concentrations and radiative forcing from all long-lived greenhouse gases. Credit: NOAA Global Monitoring Laboratory

The NOAA measurements showed the global average level of CO₂ in 2020 was 412.5 parts per million (ppm), a 2.6 ppm increase over 2019. It was the sixth-largest annual increase since 1980, when NOAA’s first global average calculation showed the average atmospheric abundance of CO2 was 338.9 ppm. The 2020 increase might have been the largest on record were it not for the pandemic-induced economic slowdown, NOAA scientists said. 

Since 1990, the amount of extra heat trapped in the atmosphere by increases in CO₂ alone represents 66 percent of the total from all the major long-lived greenhouse gases.

The climate influence of one powerful set of greenhouse gases - ozone-depleting gases such as chlorofluorocarbons (CFCs) once commonly used as refrigerants and foam-blowing agents - continued to decline in 2020, largely due to controls adopted by the Montreal Protocol. 

For more information, contact Theo Stein, NOAA Communications, at theo.stein@noaa.gov.