A New Tool for Understanding Where Carbon Dioxide Goes

Source: Quay, P.; Sonnerup, R.; Munro, D.; Sweeney, C., Anthropogenic CO₂ accumulation and uptake rates in the Pacific Ocean based on changes in the ¹³C/¹²C of dissolved inorganic carbon. Global Biogeochem. Cycles (2017), 31 (1), 59-80, doi: 10.1002/2016GB005460.

The ocean’s role in climate change

We generally focus on the atmosphere in discussions of carbon dioxide (CO₂) emissions caused by human activity and their role as a greenhouse gas. The fact that roughly one-third of those emissions ends up in the ocean is typically not highlighted.

The ocean is currently taking up a significant amount of CO₂ from the atmosphere, helping lessen the rise in global temperatures. The atmosphere and ocean equilibrate by exchanging gases at the sea surface; if there’s more CO₂ in the air than in the water at a given location, the water will take up CO₂ to lessen the disparity. But this process relies on a number of factors, including on the ocean containing less CO₂ than the atmosphere, and it is not certain that the ocean will draw down CO₂ indefinitely. Understanding exactly how much human-emitted CO₂ it’s currently absorbing is essential to predicting the ocean’s future ability to take up CO₂.

Thanks to iconic graphs such as the Keeling Curve (Figure 1), it is easy to see the result of human activity on atmospheric CO₂ levels. But there’s no such poster-child of climate change for the ocean, largely because it is so challenging to tease out which ocean properties are a result of human activity. Scientists are continuously looking for new ways to accomplish this. In this study, researchers show that tracking the ratio of heavy to light carbon atoms (also known as carbon isotopes) in the water over time can be used to infer how much human-emitted CO₂ is ending up in the Pacific Ocean.

Carbon isotopes: trail of crumbs

Carbon atoms exist in nature with different weights. Most carbon atoms have a mass of 12 atomic mass units (amu), and these atoms are referred to as carbon-12 (¹²C). Though in much lower abundance, heavier carbon atoms weighing 13 amu also exist, and are referred to as carbon-13 (¹³C). These carbon atoms with different weights comprise carbon’s natural isotopes.

When plants photosynthesize, they selectively use CO₂ containing ¹²C as the carbon atom over CO₂ containing ¹³C. And since fossil fuels are made from buried plant matter, the carbons in fossil fuels have a lower ratio of ¹³C to ¹²C than the ratio across all carbon isotopes in nature. So when humans burn fossil fuels, the CO₂ we release into the atmosphere has a lower ¹³C/¹²C ratio.

As a result of fossil fuel burning, the atmosphere has been accumulating the lighter carbon isotope. The ocean has been absorbing this CO₂ from the atmosphere, and thus the proportion of ¹³C to ¹²C in the ocean has been decreasing. These researchers leveraged this fact and tracked the change in the ¹³C/¹²C ratio (denoted δ¹³C) in the ocean, knowing that a decrease in this ratio could be attributed to CO₂ from fossil fuels.

Pacific getting lighter

Quay et al. used a combination of cruise data and model output of δ¹³C data in the Pacific Ocean from the 1990s and 2000s, as well as measurements of δ¹³C from the Hawaii Ocean time series station (HOT; 23˚N, 158˚W) and Drake Passage Time Series (55˚S-63˚S and 68˚W-58˚W). By comparing data for the two 10-year periods, Quay et al. found a significant decrease in δ¹³C across the entire Pacific Ocean in that time interval (Figure 2). From this decrease, they calculated that the Pacific Ocean has been taking up 4.401 ± 5.501 billion tons of CO₂ every year. This means that from this estimate, the Pacific Ocean alone is taking up about 13% of the annual CO₂ emissions from fossil fuels and industry every year. This value is in line with estimates from other methods and ocean models, validating the δ¹³C method as a way of measuring ocean accumulation of CO₂ from fossil fuel burning. The results also confirmed that the Pacific Ocean is a significant sink for human-emitted CO₂, affirming the importance of the ocean in moderating atmospheric CO₂ levels.

The future by way of the present

We have known that the ocean is taking up CO₂ from the atmosphere, but it’s difficult to measure how much. This paper shows that comparing the ratio of heavy to light carbon isotopes in the ocean over time is another valid way to find how much CO₂ the Pacific Ocean is absorbing. While the results show that the ocean is a significant sink for human-emitted CO­₂, it’s possible that its ability to draw down CO₂ could weaken should the ocean approach a limit for CO₂ uptake. Losing this sink would mean faster CO₂ accumulation in the atmosphere, with potentially catastrophic consequences for global temperatures. Tracking and measuring the oceanic absorption of CO­₂ from fossil fuel burning is essential for predicting the future state of the climate.

Julia Dohner

Julia is a PhD student at Scripps Institution of Oceanography in La Jolla, California. Her focus is on biogeochemistry, which, as the name suggests, centers on the combined effects of biological, geological and chemical processes on the earth system. She is advised by Dr. Ralph Keeling and is modeling the global carbon cycle to better understand how much carbon dioxide ends up in the atmosphere. When not at her computer writing code, Julia can usually be found reading and/or thinking about food.