Lab 11.1 – Clues in the Curve: Tracking CO₂ Through Time

Fundamental concept: Atmospheric CO₂ is increasing over time due to human activity.
Estimated time to complete: 25 minutes
Data skills preparation: TBD
Materials needed: None

In this lab activity you will learn about atmospheric carbon dioxide (CO₂) and its link to the ocean. 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.

The Keeling curve showing the concentration of carbon dioxide increasing over time from 1958 to 2024

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.

Before answering the questions below, take a look at the curve and watch this video about how scientists measure atmospheric CO₂.

Orientation Questions

  1. What are the independent and dependent variables on the Keeling curve?
  2. What range of CO₂ concentrations do you see in the Keeling Curve?
  3. What were the CO₂ levels when Dr. Keeling started taking his measurements of CO₂ in the 1950s?
  4. What graph type is this (hint: Look back at Lab 2)? Choose from below:
    1. Time Series
    2. Station Profile
    3. Bubble Chart
    4. Bathymetric Chart

Interpretation Questions

  1. What temporal pattern is evident in this curve at long timescales (across years)? What do you think is driving this pattern?

Biological Impacts on CO₂ in the Atmosphere

CO₂ is essential for photosynthesis—a biological process through which plants and other photosynthetic organisms use sunlight as energy to combine carbon dioxide and water to produce sugars (glucose) and oxygen – substances that all animals need to survive! Animals (and plants, too!) then break down this sugar through cellular respiration, a process that releases energy for biological functions and returns CO₂ to the atmosphere. 

Now let’s get a bit more comfortable with the basic photosynthesis equation. Given what we’ve discussed so far, do your best to place the components of this equation in the correct spot.

Plants and other photosynthetic organisms (algae) consume CO₂ in the surface ocean, where sunlight is readily available, which mitigates increased CO₂ concentrations in the atmosphere. This process also produces oxygen, helping to keep the planet hospitable for life. The growth of plants and algae varies throughout the year due to seasonal changes in sunlight and nutrients.

Human Population’s Influence on CO₂ concentrations in the atmosphere

A landscape-oriented infographic titled "Anthropogenic Causes of Rising CO₂ Levels." In the center is an illustration of Earth with "CO₂" above it and an upward orange arrow indicating increased CO₂ emissions. Two black arrows curve inward toward Earth from both sides. On the left, the label "Industrialization" sits above an image of factory smokestacks and coal. On the right, the label "Deforestation" appears above a tree stump, a small tree, and a chainsaw. Below this, the text reads: "Plants remove CO₂."

While CO₂ is a naturally occurring and biologically important gas, increased burning of fossil fuels (such as coal and oil) due to industrialization since the last century has contributed to an atmospheric level of CO₂ that Earth has not experienced since roughly 3 million years ago (Lindsey 2025). Deforestation, the cutting down of trees to make way for development or for lumber, also contributes to rising CO₂ levels as we are removing plants which are a natural sink of CO₂. Every year, human activities are adding more CO₂ to the atmosphere than is being removed, and so the total amount of CO₂ in the atmosphere goes up. 

You may have learned that CO₂ is a greenhouse gas and leads to global warming by trapping more heat near the earth’s surface. In the next activities in this lab, you will learn that CO₂ from the atmosphere is taken up by the ocean and is also leading to ocean acidification.

Watch this video showing temporal and spatial patterns in CO₂ throughout the year on earth. While you watch this video, identify any patterns you see, and try to think about what might be causing them. Answer the questions that follow the video.

Interpretation Questions

  1. Based on the video, how do concentrations of CO₂ vary spatially? Are they higher in the Northern Hemisphere or in the Southern Hemisphere?
  2. Now focus on the Northern Hemisphere in the video – how do concentrations of CO₂ vary seasonally? 

Application Questions:

  1. Looking back at the Keeling Curve, describe the  temporal pattern that is evident in this curve at short timescales (within a year)? What do you think is driving these patterns?

From Fossil Fuels to Ocean CO₂

The ocean and atmosphere are coupled systems that are constantly exchanging dissolved gasses that are very important to living organisms, particularly oxygen (O₂), carbon dioxide (CO₂), and nitrogen (N₂). The ocean absorbs about 30% of the CO₂ that is released in the atmosphere. As levels of atmospheric CO₂ increase from human activity such as burning fossil fuels (e.g., car emissions) and changing land use (e.g., deforestation), the amount of  CO₂ absorbed by the ocean also increases. 

Reflection Question

  1. If you were to create a Keeling curve for the ocean, how do you think it would compare to the curve shown for atmospheric CO₂? 
  1. The CO₂ concentration in the ocean would not change at all over time
  2. The CO₂ concentration in the ocean would change faster than the atmospheric CO₂ concentration 
  3. The CO₂ concentration in the ocean would change slower than the atmospheric CO₂ concentration 
  4. The CO₂ concentration in the ocean would change at a similar rate to the atmospheric CO₂ concentration 

Behind the Scientist

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.

Refernces:

Lindsey, R. (2025). Climate change: Atmospheric carbon dioxide. Retrieved from https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide

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