16 February 2017 - Knowing the rate at which the oceans absorb carbon pollution is a key to understanding how fast climate change will occur.
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.