Acidification of the deep Atlantic Ocean is accelerated by ocean circulation

Much has been written about the impacts of sea level rise under climate change.

However, an equally drastic transformation creeps from the ocean depth: ocean

acidification.

Ocean acidification starts with the mixing of atmospheric carbon dioxide (CO2) in the

surface ocean. Carbon dioxide reacts with water to produce carbonic acid. Carbonic acid,

in turn, separates into bicarbonate ions as well as stray hydrogen ions. The prevalence of

these stray hydrogen ions determines the acidity of the seawater; excessive hydrogen

ions make the surface seawater more acidic.

Compared to the surface ocean, the deep ocean is even more acidic. CO2 that dissolves in

the surface ocean descends into the ocean’s depth with dead biotic materials, and

accumulates there. Thus, ocean acidification is gradually happening from the bottom of

the ocean upwards.

Ocean acidification is accelerating as a result of climate change. In the surface ocean,

30% more hydrogen ions are present compared to preindustrial times. In the deep

ocean, the zone where seawater acidity becomes inhabitable is creeping upwards. In a

recent article published in Nature, researcher Fiz Perez from Spain’s Instituto

Investigaciones Marinas and colleagues warn that ocean acidification might be

happening even faster than we think.

Compared to other oceans in the world, the North Atlantic Ocean in particular possesses

a natural balancing mechanism that has helped it to slow acidification in the past.

There, oceanic circulation causes the less acidic surface water to constantly sink and mix

with the more acidic deep water. This process has kept the deep Atlantic Ocean in far

less acidic conditions than the north Pacific Ocean, where surface and deep waters are

segregated.

The naturally less acidic North Atlantic Ocean has become a safe haven calcifying

marine organisms. These marine organisms, such as shellfish and deep cold water

corals, detest acidic conditions. The acidic water corrodes their shells and skeletons.

Past a certain depth, the seawater is too acidic for these organisms to survive. In the

North Atlantic, cold water corals thrive in massive colonies as deep as 2000 meters

under the surface. By comparison, the same corals struggle to survive under 500 meters

in the Pacific.

Today, the surface water that descends into the ocean depth is becoming more and more

acidic. With more CO2 mixing, the Atlantic’s natural balancing mechanism, on which its

deep biotic communities rely on, could be weakened. Through computer modeling of

ocean monitoring data, the study by Perez and colleagues demonstrates that the very

ocean circulation which made the North Atlantic Ocean so benign for calcifying

organisms today also makes it more susceptible to changes in the surface ocean. In fact,

the rate at which the deep Atlantic Ocean is changing has exceeded previous

expectations. There, the lower limit of hospitable seawater zone is rising towards the

ocean surface at a rate of 10 to 15 meters per year.

The Pacific Ocean today – with its patchy, scarce deep coral coverage – offers us a

glimpse of the future of the North Atlantic. This research paper extends our

understanding of the urgency of climate change. It reminds us of the breadth of impacts

climate change has on every facet of the Earth’s biotic communities – even those that

live 2000 meters below the ocean’s surface.