Climate Change: Annual greenhouse gas index

August 14, 2020

Ever since people began grappling with the realization that human activities are changing the climate, scientists and decision-makers have struggled to come up with simple ways to talk about the topic with each other and the public. To help with that challenge, NOAA climate experts created a simple index that answers one of the most frequently asked questions: how much are human-produced greenhouse gases influencing the climate today compared to the past?

NOAA’s Annual Greenhouse Gas Index (AGGI) is a yearly report on the combined influence of long-lived greenhouse gases (atmospheric gases that absorb and radiate heat) on Earth’s surface temperature. The index compares the combined warming influence of these gases each year to their influence in 1990, the year that countries who signed the U.N. Kyoto Protocol agreed to use as a benchmark for their efforts to reduce emissions. By the end of 2019, the warming influence of human-produced greenhouse gases had risen 45 percent above the 1990 baseline.

Explore this interactive graph: Click and drag to display different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag. This graph (source data) shows the combined warming influence of long-lived greenhouse gases as a fraction of their 1990 influence.

Amplifying the greenhouse effect

Like other gases in the atmosphere, including oxygen and nitrogen, greenhouse gases are largely transparent to incoming sunlight. Unlike those more abundant gases though, greenhouse gases are not transparent to heat (longwave infrared radiation). The sun-warmed surface of Earth radiates heat day and night. Some heat escapes freely to space, but some is absorbed by greenhouse gas molecules. These gas molecules radiate warmth back into their surroundings; thus, they are also known as heat-trapping gases. 

The natural warming influence of greenhouse gases—the greenhouse effect—keeps Earth’s temperature friendly to life; without greenhouse gases, the planet’s average temperature would be below freezing. Since the onset of the Industrial Revolution, however, human activities, especially burning coal and oil for fuel, have increased the abundance of heat-trapping gases in the atmosphere and amplified the greenhouse effect. Earth’s average global temperature is rising as a result.

To calculate how much warming we can expect, scientists need to know the combined influence of all greenhouse gases. The answer is complex because each type of greenhouse gas absorbs and releases different amounts of heat energy. Additionally, the amount of each gas in the air is different, and the concentration changes over time and from place to place.

Combined influence of GHGs

This graph shows the heating imbalance in watts per square meter relative to the year 1750 caused by all major human-produced greenhouse gases: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons 11 and 12, and a group of 15 other minor contributors. Today's atmosphere absorbs more than 3 watts of incoming solar energy over each square meter of Earth's surface. According to NOAA's 2019 Annual Greenhouse Gas Index (right axis), the combined heating influence of all major greenhouse gases has increased by 45% relative to 1990. Graph by NOAA based on data from NOAA ESRL.

According to the 2019 AGGI report, the combined heating influence of the long-lived, human-produced greenhouse gases is 3.14 Watts for every square meter of Earth's surface. Just over 80 percent of that is due to carbon dioxide (66%) and methane (16%).  Nitrous oxide, which comes from burning fossil fuels as well as from different agricultural and industrial activities, including wastewater treatment, accounted for just over 6% of the total heating imbalance. Most of the remaining influence comes from ozone-depleting CFCs (chlorofluorocarbons) that were used in cooling systems and as aerosol propellants in the mid-1900s. These substances and their replacements are now regulated under the Montreal Protocol, but they are extremely long-lived in the atmosphere, so they continue to play a small role in Earth's heating imbalance.  

Making the AGGI

Researchers in NOAA’s Global Monitoring Division calculate the AGGI using air samples collected every week at about 100 clean-air sites around the world. Technicians use state-of-the-art instruments to measure the abundance of greenhouse gases. From these observations, the researchers generate a smoothed global average. At the end of the year, the weekly data are combined into an annual average, which is then compared (indexed) to 1990. Indexing each year against 1990 makes it easier to compare one year to another.

The year 1990 was chosen as the baseline year because it marked the first time that countries around the world seriously considered how they might work together to reduce emissions of heat-trapping gases. Under the U.N. Kyoto Protocol, many industrialized nations around the world agreed to reduce their collective greenhouse gas emissions by about 5 percent of their 1990 amounts by 2012. Although the goals of this agreement have not been met, the year 1990 remains an important point of reference for efforts to limit greenhouse gas emissions.

NOAA researchers have also calculated AGGI values back to the year 1750—the onset of the industrial revolution. To accomplish this, they used data on greenhouse gases concentrations estimated from air bubbles trapped in ice cores collected at a network of sites around the globe. Using these historic data provide a more complete picture of humanity’s long-term influence on climate change. In the 2019 AGGI report, experts from NOAA's Earth System Research Laboratory state, "This longer-term view shows how increases in greenhouse gas concentrations over [just] the past ~70 years (since 1950) have accounted for three-fourths (75%) of the total increase in the AGGI over the past 260 years."

Butler, J. and Montzka, S. (2019). The NOAA Annual Greenhouse Gas Index (AGGI). Earth System Research Laboratory, Global Monitoring Division Website. Accessed August 14, 2020. 

U.N. Framework Convention on Climate Change. (n.d.) The Kyoto Protocol. Accessed July 20, 2017.

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