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How Do We Know Mauna Loa Carbon Dioxide Measurements Don’t Include Volcanic Gases?

Aerial view of recent lava flows near Mauna Loa’s summit in November 2022. The National Oceanic and Atmospheric Administration (NOAA) monitors greenhouse gases at Mauna Loa Observatory, which is 4 miles (6 kilometers) away from the summit crater. Despite the proximity, volcanic gases aren’t common at the observatory, and when they do happen, these temporary spikes are not included in the long-term data. The decades-long trend of increasing CO₂ at Mauna Loa matches that from sampling sites all around the world.

USGS/J. Schmith

The amount of CO₂ in the atmosphere is measured at Mauna Loa Observatory, Hawaii, and all around the world. NASA also measures CO₂ from space. Data from around the planet all shows the same upward trend.

The longest record of direct measurements of carbon dioxide (CO₂) in Earth’s atmosphere comes from Mauna Loa Observatory in Hawaii, operated by the National Oceanic and Atmospheric Administration. These measurements began in 1957 and continue today, painting a clear picture of how CO₂ has been changing over the seasons, years, and decades.

Atmospheric carbon dioxide (CO2) has been measured since 1957 at the Mauna Loa Observatory in Hawaii, which is operated by the National Oceanic and Atmospheric Administration (NOAA). By December 2021, the amount of CO₂ in the atmosphere had increased by 50% compared to before the Industrial Revolution. Since then, atmospheric CO2 has continued to increase. Credit: NASA

Mauna Loa is an active volcano, though, so how can scientists be sure that the volcano itself is not the cause of rising CO₂ measured there?

There are several answers to this question.

Mauna Loa Observatory sits in an isolated location so it can capture clean air flowing over the Pacific Ocean. There is little direct influence from pollution due to the vastness of the ocean, but there are some local, natural processes that can change the amount of CO₂ over short amounts of time (such as a few hours). For instance, nearby plants and trees can temporarily draw CO₂ out of the air through photosynthesis.

Gases from Mauna Loa’s crater, which is 4 miles (6 kilometers) away from the observatory, can temporarily raise the amount of CO₂ in the air. Prevailing winds generally prevent volcanic gases from reaching the observatory, but when the wind is light, it can happen. However, the influx of volcanic CO₂ is immediately noticeable because the readings jump upward sharply, and by amounts far larger than normal seasonal fluctuations. These brief spikes are not included in the final data because they’re caused by short-term, local variations, and they don’t reflect the long-term average that the observatory strives to measure.

Another way to answer the question is to look at CO₂ data from other parts of the world. If Mauna Loa data were contaminated by the volcano, then those measurements would look different from air samples collected in Alaska or at the South Pole. Air samples are regularly collected from observatories, tall towers, aircraft, and weather balloons at 86 locations worldwide. These data from around the planet all follow the same trend.

A line graph with four different lines. The vertical axis is labeled "Atmospheric CO2, in parts per million (ppm)," and the horizontal label is labeled with years from 1972 to 2020. The four lines show CO2 measured at different stations. A light blue line is for Barrow, Alaska; a navy blue line is for Mauna Loa, Hawaii; a green line is for American Samoa (South Pacific); and a pink line is for the South Pole. All the lines go up and down in a wave pattern over time, with a long-term trend in CO2 rising from around 330 ppm to above 410 ppm.

Long-term records from Alaska, Hawaii, Samoa, and the South Pole all capture the rising trend in CO₂. The yearly “sawtooth” pattern is caused by plant growth. During the growing season, plants use CO₂ for photosynthesis. Over the winter, plants decompose and return some CO₂ to the atmosphere. This seasonal cycle is largest in the Northern Hemisphere, because it has more land area than the Southern Hemisphere. But regardless of the seasonal swings, the long-term increase in CO₂ is similar around the globe.

NASA

Lastly, NASA’s Orbiting Carbon Observatories, OCO-2 and OCO-3, were designed to measure CO₂ from space. Instead of collecting samples from individual locations, these instruments use their lofty vantage points to make around 100,000 measurements around the world each day. Continuous global observations can see short-term changes in CO₂ concentrations, and can be used to estimate CO₂ emissions from cities and individual power plants. These missions also help scientists learn more about how carbon dioxide is stored and released by natural processes, which is important because the natural environment soaks up more than half of the CO₂ that is emitted by human activities.

Accurate measurements of greenhouse gases are essential to our understanding of climate change, and the amount of CO₂ in the atmosphere is measured by many different methods and all around the world. By using more than one approach, scientists can be sure they’re measuring a global trend, as well as local variations.

NASA Earth Observatory: How do scientists know that Mauna Loa’s volcanic emissions don’t affect the carbon dioxide data collected there?

FAQ: What do volcanoes have to do with climate change?

Atmospheric Infrared Sounder (AIRS)

Orbiting Carbon Observatory (OCO-2)

Orbiting Carbon Observatory (OCO-3)