As humans burn fossil fuels and release greenhouse gases, those gases enter the atmosphere where they cause increases in global temperatures and climate consequences such as more frequent and severe heat waves, droughts, changes to rainfall patterns, and rising seas. But for many years scientists have known that not all of the carbon dioxide we emit ends up in the atmosphere. About 40% actually gets absorbed in the ocean waters.
I like to use an analogy from everyday experience: the ocean is a little like a soda. When we shake soda, it fizzes. That fizz is the carbon dioxide coming out of the liquid (that is why sodas are called “carbonated beverages”). We’re doing the reverse process in the climate. Our carbon dioxide is actually going into the oceans.
The process of absorption is not simple – the amount of carbon dioxide that the ocean can hold depends on the ocean temperatures. Colder waters can absorb more carbon; warmer waters can absorb less. So, a prevailing scientific view is that as the oceans warm, they will become less and less capable of taking up carbon dioxide. As a result, more of our carbon pollution will stay in the atmosphere, exacerbating global warming. But it’s clear that at least for now, the oceans are doing us a tremendous favor by absorbing large amounts of carbon pollution.
How much carbon dioxide is being absorbed by the oceans is an active area of research. In particular, scientists are closely watching the oceans to see if their ability to absorb is changing over time. Such a study is the topic of a very recent paper published in the journal Nature. The authors studied recent ocean carbon dioxide uptake and in particular the mystery of why it appears the oceans are actually becoming more absorbing.
The authors describe a slowdown in a major ocean current called the overturning circulation. That circulation brings dense salty water from the surface to the depths of the ocean while simultaneously bringing colder but less salty and dense water upwards. Why is this important current slowing down? It’ possible that global warming is a culprit.
In fact, a slowdown of the current is a prediction of global warming. As the Earth warms, ice melt - especially near the Arctic - flows into the oceans. That meltwater has less salt and therefore is less dense than the surrounding waters. In a certain sense, the freshwater can block the overturning circulation, making it difficult for water near the surface to sink to the ocean depths. But it is also possible that the circulation just changes naturally.
To conduct the study, the scientists used what are called ‘tracer data.’ Tracers are chemicals in the ocean that are used to track ocean currents. Sort of like a message in a bottle. If you release a bottle off the eastern coast of North America and someone finds the bottle in Northern Europe, you can be sure the ocean currents pass from North America to Europe. Their tracer data spanned decades, back to the 1980s.
From the tracer results, the scientists reconstructed ocean current patterns. Based on the changes in circulation, they predicted a decrease in carbon absorption in the 1990s followed by an increase in the 2000s. The authors explained that ocean circulation currents tend to bring deep carbon-rich fluids to the surface where it can then escape to the atmosphere. When the ocean circulation gets weaker, less of this carbon rich water is at the surface so the transfer of carbon to the atmosphere is less intense.
The findings are a bit counter-intuitive because many scientists expect the deeper, colder waters to be able to hold more carbon dioxide. The authors of this study remind us that the transfer of carbon dioxide between the atmosphere and the oceans is a two-way street - carbon can go either way. Changing the emission of carbon dioxide from the ocean or the ability of the ocean to absorb are two ways to alter the overall carbon uptake. These processes are two sides of the same coin but they operate under different time frames. The first process (changes to carbon emissions) are more rapidly responding while the second process is slower.
So, we can be grateful that the oceans are doing us a service by reducing the amount of carbon dioxide in the atmosphere. It buys us more time to reduce our dependency on fossil fuels. On the other hand, it isn’t a free gift. The increased carbon uptake by the ocean means that the ocean waters will become acidic more rapidly than they otherwise would. This acidification threatens many base-level components of the food chain.
I asked the lead author why this was important and he responded:
Human CO2 emissions are the major contributor to global warming and climate change, so it’s really important to know where these CO2 emissions end up. Oceanographers have known for decades that the ocean acts as a sponge for CO2, absorbing it from the atmosphere. But the actual amount of CO2 that is absorbed by the oceans is pretty sensitive to a range of processes, including ocean currents, temperatures, and biological activity.
The tracers we studied showed us that the amount of CO2 absorbed by the ocean can trend up and down due to changes in ocean currents that move water between the surface and deep ocean. This is important because climate models predicts big changes in these ocean currents as the climate warms. Knowing how the ocean responds to these changes can help us adapt to climate change.