Since humans started spewing out CO₂ into the atmosphere on an enormous scale, at the beginning of the industrial revolution, we have been extremely lucky that an awful lot of that CO₂ has not stayed there. Approximately half of that CO₂ has dissolved into the oceans. Here it remains in surface waters, is mixed into deeper waters, or becomes incorporated into algae as they take up CO₂ during photosynthesis – eventually part of this biomass sinks into the deep ocean when these organisms die. The bad news for the future is that the ocean may not continue to perform this favour for us so effectively. Already, the amount of CO₂ absorbed by the ocean is lower than at the start of the industrial revolution (about 25% compared to 50%). One simple reason for this is that, like cramming clothes into a suitcase, the more you add, the harder it becomes to add more.

But the oceans do not just take up excess CO₂, they also absorb a lot of the excess heat from the atmosphere: as heat builds up in the atmosphere, a huge amount (about 90%) ends up heating up the ocean. Once again, there is a very real worry that the oceans may not continue to perform this favour for us so effectively in the future. As the surface of the ocean gets warmer, the global circulation of seawater, cycling from the surface to deep-ocean like a giant conveyor belt (driven by temperature and salinity), will begin to slow down – in fact there are already signs that this is happening.

A slowdown in the ocean’s circulation would be bad news for the transfer of CO₂ and heat to deep in the ocean’s interior (where it becomes safely locked-up for thousands of years). It would also stifle the supply of nutrients to the surface, reducing the amount of phytoplankton growing and reduce the amount of biomass sinking to the deep ocean. All of this would lead to a gradual deterioration of the ocean’s ability to absorb CO₂. As human emissions continue to increase, a larger and larger proportion of CO₂ will likely remain in the atmosphere and the effects of climate change will begin to accelerate even further beyond our control.

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This post is an excerpt from “Do Fish Sleep?: and 38 other ocean mysteries”, by David Aldridge. It is available to buy on kindle here for the reduced  price of $1.46 (99p) until the end of January. To read it, you do not need to own a Kindle device, just the Kindle App which is a free download for smartphones or tablets. It can also be read with“Kindle Cloud Reader” on a PC or Laptop.