5.1: Measuring Past and Current CO2 Concentrations

 

How do Scientists Measure Current Atmospheric CO₂ Concentrations?

We track long-term changes in atmospheric CO₂ using the Keeling Curve, which represents CO₂ concentrations measured in Mauna Loa in Hawaii. The Keeling Curve below, named after Charles David Keeling, highlights temporal patterns, or patterns across time, in CO₂ concentrations at both long-term (across years) and short-term (within years) scales.

Carbon dioxide concentrations at Mauna Loa Observatory by NOAA is licensed under the Public Domain. Original Source.
Note: Due to the eruption of the Mauna Loa Volcano, measurements from Mauna Loa Observatory were suspended as of Nov. 29, 2022. Observations from December 2022 to July 4, 2023 are from a site at the Mauna Kea Observatories, approximately 21 miles north of the Mauna Loa Observatory. Mauna Loa observations resumed in July 2023.

 

Behind the Scientist \(\PageIndex{1}\)

It was 1953, and Charles David Keeling was a post-doctoral researcher in Pasadena, California. While his research focused on nuclear power, he became interested in carbonate chemistry and decided to go on a side quest: he started taking CO₂ measurements under varying conditions – along industrialization gradients, at the tops of mountains and in temperate rainforests, during the night and during the day, and in different seasons. Dr. Keeling never stopped taking these measurements and later that decade he received funding to install continuous CO₂ monitors at Mauna Loa, Hawaii. The resulting dataset has become the longest-running time series of CO₂ data in the world and has been integral to our understanding of the natural and anthropogenic drivers of CO₂. The Keeling Curve showcases the impact that one person can have on science and the importance of collecting long-term datasets.

 

The video below shows the process by which scientists measure atmospheric CO₂ over time.

 

How do Scientists Know Ancient-Earth CO₂ Concentrations?

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before instrumental sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice sheets. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO₂ concentrations were about 260–280 ppm immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years. The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years. During this time, the atmospheric carbon dioxide concentration has varied between 180 and 210 ppm during ice ages, increasing to 280–300 ppm during warmer interglacial periods. This core is known as the "Vostock Ice Core" and was drilled at Russia's Vostock Station in East Antarctica in collaboration with the United States and France.

Modified From: Over 400,000 years of ice core data: Graph of CO₂ (green), reconstructed temperature (blue) and dust (red) from the Vostok ice core. Available with a CC-BY-SA 3.0 License.

 

Natural vs. Current Variation in CO₂

Natural increases in carbon dioxide concentrations have periodically warmed Earth’s temperature during ice age cycles over the past million years or more. The warm episodes (interglacials) began with a small increase in incoming sunlight in the Northern Hemisphere summer due to variations in Earth’s orbit around the Sun and its axis of rotation. 

That little bit of extra sunlight caused a little bit of warming. As the oceans warmed, they outgassed carbon dioxide—like a can of soda going flat in the heat of a summer day. The extra carbon dioxide in the atmosphere greatly amplified the initial, solar-driven warming.

Based on air bubbles trapped in mile-thick ice cores and other paleoclimate evidence, we know that during the ice age cycles of the past million years or so, atmospheric carbon dioxide didn't get any higher than 300 ppm. Before the Industrial Revolution started in the mid-1700s, atmospheric carbon dioxide was 280 ppm or less.

 

Atmospheric carbon dioxide (CO₂₎ in parts per million (ppm) for the past 800,000 years based on ice-core data (light purple line) compared to 2024 concentrations (bright magenta dot). The peaks and valleys in the line show ice ages (low CO₂) and warmer interglacials (higher CO₂). Throughout that time, CO₂ was never higher than 300 ppm (light purple dot, between 300,000 and 400,000 years ago). The increase over the last 60 years is 100 times faster than previous natural increases. In fact, on the geologic time scale, the increase from the end of the last ice age to the present value of 422.8 ppm (dashed magenta line) looks virtually instantaneous. Graph by NOAA Climate.gov based on data from Lüthi, et al., 2008, via NOAA NCEI Paleoclimatology Program.

 

Predicting Future Concentrations of CO₂

If global energy demand continues to grow rapidly and we meet it mostly with fossil fuels, human emissions of carbon dioxide could reach 75 billion tons per year or more by the end of the century. Atmospheric carbon dioxide amounts could be 800 ppm or higher—conditions not seen on Earth for close to 50 million years.

Possible future pathways for yearly global carbon dioxide emissions (left) and the resulting atmospheric carbon dioxide concentrations (right) through the end of the century. These possible futures are based on different shared socioeconomic pathways ("SSPs" for short). Each pathway is an internally consistent set of assumptions about future population growth, global and regional economic activity, and technological advances. Climate models use the pathways to project a range of possible future atmospheric carbon dioxide amounts; for simplicity, the image shows the only the mean value predicted by the models for each pathway, not the full range of uncertainty. NOAA Climate.gov graphic adapted from figure TS.4 in the IPCC Sixth Assessment Report Technical Summary.