Book Review Physical oceanography

Oceans absorb more carbon with weaker ocean circulation

Source: DeVries, Tim, Mark Holzer, and Francois Primeau. “Recent increase in oceanic carbon uptake driven by weaker upper-ocean overturning.” Nature 542.7640 (2017): 215-218.

The full carbon story

Just last year, the concentration of carbon dioxide in the atmosphere crossed 400 parts per million permanently, higher than it has been for millions of years; this is bad news for planet earth because carbon dioxide is a potent greenhouse gas. Nations are working together, in efforts such as the recent international Paris Agreement, to reduce global emissions of carbon dioxide and stabilize carbon dioxide concentrations in the atmosphere. However, there is more to the carbon dioxide story than a simple input/output model, because most of the carbon dioxide emitted does not stay in the atmosphere, but instead is absorbed by the ocean or plants on land.

Increasing atmospheric carbon dioxide concentrations from Mauna Loa Observatory in Hawaii. Credit: Ed Hawkins

In fact, since the industrial revolution, the oceans have absorbed 40% of the carbon emitted from burning fossil fuels, preventing all of it from accumulating in the atmosphere. Scientists have only recently found the tools to measure the ocean uptake of carbon, yet within that short time, we have discovered that the amount of carbon absorbed by the ocean can change dramatically from year to year and also shift over the course of many years. Figuring out what drives changes in ocean carbon consumption is key to scientists’ ability to predict how the ocean carbon uptake will change in the future.

The carbon control panel

There are three main factors that control how much carbon dioxide is absorbed by the ocean. The first is temperature;the temperature of the sea surface affects how much carbon dioxide dissolves into the water. Second, ocean circulation moves carbon around and exposes water with different carbon signatures to the atmosphere, where the water can exchange carbon with the atmosphere. The third factor is biology; living organisms such as  as tiny microscopic plants called phytoplankton absorb carbon dioxide by photosynthesis, just like plants on land. Figuring out how these factors interact to control ocean carbon dioxide absorption is tricky, because measurements of carbon in the ocean show the combined effect of all three. We also know that ocean circulation varies on timescales of years to decades. With all these observations in mind a group of scientists from the United States and Australia got together to test whether the ocean circulation could explain the observed variations in ocean carbon uptake.

The global carbon cycle, showing the how the uptake of carbon in the ocean is influenced by circulation and biological processes (photosynthesis and respiration). Credit: University of Washington

Reconstructing the ocean’s past

Measurements of the global ocean circulation are limited to snapshots from ships and from autonomous instruments, so it is challenging to paint a full picture of ocean circulation. To overcome this challenge, the involved researchers used an ocean model to fill in the gaps. Based on available measurements of ocean properties like temperature, salinity, and gas concentrations, they used basic rules of physics that govern circulation in the ocean to reconstruct a best estimate of global ocean circulation for the past few decades. With this tool, the team was able to recreate big picture ocean circulation for the 1980’s-2000’s and look at how it differed in each decade. They found that on average, circulation sped up in the 1990’s compared to the 1980’s and slowed down in the 2000’s. The next step was to try and connect the dots, and see if circulation can explain the variations in ocean carbon uptake observed over the past few decades.

The scientists broke down the total carbon in the ocean into two parts: the natural carbon, which was around before humans started burning fossil fuels, and anthropogenic carbon, which is excess carbon accumulating due to human activities, namely the burning of fossil fuels. Water in the deep oceans is very rich in natural carbon, because it hasn’t been influenced by the atmosphere for hundreds or even thousands of years. This ‘old’ water returns to the surface in the Southern Ocean surrounding Antarctica, in a process called upwelling, releasing natural carbon to the atmosphere. Water at the ocean surface absorbs anthropogenic carbon from the atmosphere and brings it into the interior of the ocean, but ocean circulation is slow and it takes hundreds of years for the signature of this carbon to make it to the deep ocean. So ocean carbon uptake is a delicate balance between the release of natural carbon, mostly in the Southern Ocean, and the absorption of anthropogenic carbon at the surface.

Accelerator, or brakes?

It turns out that according to the authors’ analysis, the release of natural carbon is much more sensitive to changes in ocean circulation than the anthropogenic absorption. This means that the acceleration in ocean circulation in the 1990’s brought more deep water, enriched in natural carbon, to the surface of the Southern Ocean, speeding up the release of natural carbon to the atmosphere. The stronger circulation also absorbed more anthropogenic carbon dioxide but it didn’t fully compensate for the big release of natural carbon, so the total ocean uptake decreased. In the 2000’s the opposite happened; ocean circulation became more sluggish, slowing down this release of natural carbon, leading to a net increase in ocean carbon uptake. This suggests that the speed up and slow-down of ocean circulation drives increased and decreased ocean carbon uptake.

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Figure 3 from DeVries et al. 2017. A schematic showing the response of natural and anthropogenic carbon dioxide to changes in ocean circulation. Black numbers show the carbon flux (negative is ocean uptake), red numbers show increased release from the ocean and blue numbers show increased uptake into the ocean, in units of 1015 grams of carbon per year.

Looking ahead

This balance between two parts of the ocean carbon uptake is clearly very sensitive to ocean circulation. With their model, the authors were able to conclude that ocean circulation could explain most of the observed variations in carbon uptake, without the effects of ocean surface temperature and biology. This means that monitoring the ocean circulation and what drives its changes is necessary to track how ocean carbon uptake is changing and will continue to change in the future. It is still unclear if the ocean will continue to remove carbon from the atmosphere, moderating the impact of carbon dioxide emissions on our climate. However, one thing is clear from this study: ocean circulation can have a dramatic affect on the carbon cycle, and we should not underestimate its role in the future of our climate.

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