Citation: McKinley, G. A., Fay, A. R., Eddebbar, Y. A., Gloege, L., & Lovenduski, N. S. ( 2020). External forcing explains recent decadal variability of the ocean carbon sink. AGU Advances, 1, e2019AV000149. https://doi.org/10.1029/2019AV000149
The day atmospheric carbon dioxide (CO2) accumulation stalled
I set out to primarily study climate science when I started my postdoc in January 2018. I would have never believed that just over a year later I would be talking about shrinking greenhouse gas levels, especially if you consider their seemingly unabated rise since the industrial revolution.
COVID-19 seems to have changed just about everything and this includes how humans interact with the environment. Overall, fossil fuel usage has decreased drastically, mainly as a result of the decreased demand for travel. About 40 percent of all scheduled flights were cancelled by March 25th, 2020 and many personal cars are remaining idle as people work from home.
A recent article published in Advancing Earth and Space Science describes how one group of scientists propose using this unexpected experiment in falling (CO2) levels. To illustrate their point, they use another unexpected climate event, the eruption of Mt Pinatubo, in 1991 to demonstrate how new models can provide better explanations for how the planet responds to fluctuating greenhouse gas levels. They believe that such events can start to remove climate uncertainties and even bolster credibility needed to influence public policy.
These satellite pictures of Wuhan China in 2019 during the Chinese New Year (top panel) and at the same time in 2020 (bottom panel) show the drastic change in air pollution levels, mainly a result of the halt on fossil fuel based travel. It is suspected that greenhouse gas levels followed a similar pattern.
The role of atmospheric CO2 in the ocean
CO2 accumulates in the atmosphere as a greenhouse gas produced as a byproduct of burning fossil fuels but not all that CO2 stays in the atmosphere. The ocean absorbs about 39% of the excess CO2 produced by humans and this absorption is dangerous to the marine life that lives there. As more CO2 is added to the oceans, they become more acidic. The acidity threatens to dissolve the shells of small marine organisms, often at the bottom of the human food web. Such effects make it important for oceanic warming to be included in policy discussions around climate change.
Unfortunately, many aspects of the ocean in response to increasing CO2 are largely not understood, mostly due to the large size and diversity of the ocean. The scientists in this paper used mathematical models to suggest a more comprehensive explanation of how the oceanic ‘carbon sink’ works in response to fluctuating atmospheric CO2. Without a better understanding of the causes of this sink, it is difficult to explain past behavior and make future predictions.
Modeling the past to inform the future
The research group applied a new mathematical model to explain air-sea CO2 flux in the early 21st century. Flux is defined as the difference between CO2 in the ocean and CO2 in the atmosphere. The new model took into account two external effects they believed responsible for the ocean’s CO2 absorption patterns: The slowed growth rate of atmospheric CO2 and the global sea surface temperature response to the 1991 eruption of Mt Pinatubo.
They used two previous models of CO2 flux for comparison based on observations and direct measurements – not explanations. The first model is based on observationally-based products (blue line) and the second was based on Hindcast models (green line). The models both show that in 1992, after a slow in fossil fuel emissions, the flux (CO2 ocean – CO2 atmosphere) becomes more negative as the carbon sink slows and more CO2 remains in the atmosphere. From 1992 – 2001, a rapid increase in dissolved CO2 resulted in the flux flipping and becoming positive. Finally from 2002 onward, the ocean CO2 grows more slowly than the accelerating atmospheric CO2, leading to a once again negative flux. The problem with these models is that up until now, no one has been able to explain the slow-down of the carbon sink in the 1990s and subsequent recovery.
The new model
According to the previous models, the flux should have been positive as less CO2 was emitted into the atmosphere. This is where the new model, called the Upper Ocean Box Model (red line), comes in. According to all the models, the mean sink in the early 1990s was indeed smaller. However, the pattern with the box model showed a sharp decrease and recovery while the other models it was more gradual (compare the blue and green lines with the red line above). What else could be happening?
Mt Pinatube erupted in 1991, injecting large quantities of sulfate aerosols into the atmosphere. In response, the ocean up took a large amount of carbon and oxygen in the next 2-3 years. It was also noted that a drop in sea surface temperature accompanied these observations. The scientists factored the change in sea surface temperature into the box model and saw that this factor indeed drove the increased CO2 flux into the ocean. Compare the difference patterns in the Upper Ocean Box Model when only atmospheric CO2 is taken into account (red dashed line) with the same model when sea surface temperature is included (red bold line). Without the sea surface temperature change, the decreased atmospheric CO2 alone should have inverted the trend.
The oceanic carbon sink in a changing world
The size and diversity of the ocean makes it incredibly difficult to study and to make certain claims about. For instance, we know that increased CO2 dissolved in the ocean affects marine organisms, but how will that affect us and the food web? Some organisms might adapt while others become extinct. It’s nearly impossible to predict how higher organisms will react.
Better understanding of the ocean as a carbon sink will improve predictions of the future with human-driven climate change. For policy to be effective, scientists need advocates to relate why ups and downs of atmospheric CO2 affect the oceans and why we should care about moving the needle. Models like the one used to diagnose the unexpected CO2 oceanic uptake in the 1990s will certainly help investigators explore the nuances of this new unexpected experiment of 2020.