How climate change is impacting the ocean’s ability to fix carbon dioxide

Researchers from the Danish Centre for Ocean Life at DTU have been the first to document that the important contribution made by zooplankton to the carbon cycle is changing, and that zooplankton in the temperate regions of the North Atlantic are today removing significantly less carbon dioxide from the atmosphere than they did 55 years ago.

However, in the colder regions of the North Atlantic from Iceland and down to the northern USA, the situation is the opposite: Here, the zooplankton are transporting more carbon from the ocean surface towards the seabed. The findings were published on 11 February 2019 in an article in the prestigious scientific journal Nature Ecology & Evolution.

The sea plays a major role in the Earth’s carbon cycle, because atmospheric carbon dioxide is fixed as organic carbon in the surface and subsequently transported by zooplankton to the ocean interior. Thereby carbon dioxide is removed from the atmosphere for many years, where it would otherwise contribute to climate change.

Storage in the sea is carried out by different processes, collectively known as the biological pump, and which is estimated to remove as much carbon dioxide from the atmosphere as that emitted by human activities.

Plankton affected by climate change

The article in Nature Ecology & Evolution deals with marine copepods, mm-sized crustaceans that are the dominant group of zooplankton and which are found everywhere.

“The changes we see in the North Atlantic are largely due to the fact that climate change is affecting marine copepods. In the central and eastern part of the North Atlantic, there are areas with fewer or smaller copepods than previously, and these changes in the biomass are significant for how much carbon can be transported from the atmosphere and down into the ocean,” explains Professor Thomas Kiørboe from the Centre for Ocean Life, who has lead the study.

In the north-western parts of the North Atlantic near Iceland and the USA on the other hand, the study shows that the copepod population has increased since 1960, so more carbon can be transported here from the atmosphere and down into the ocean. However, these are complex biological, chemical, and physical processes that are not yet sufficiently well understood to allow us to predict what the changes will mean for the climate in future.

“Our study is the first attempt to describe the global significance of the contribution made by copepods to the carbon cycle. We know that the changes in copepod populations affect the global carbon cycle directly through the changes in the transport of carbon from the ocean surface to the seabed, but as yet we are unable to quantify the consequences,” says Thomas Kiørboe.

The study is based on a unique data set which extends from 1960 to 2014, and which contains 200,000 observations of the occurrence of different copepod species. The researchers have combined this data with information on copepod body size, together with their food, migration behaviour, etc., and processed the data in complex models.

(Karin Stubgaard)