University of Groningen: Current global warming may affect the Earth’s oceans for many millennia to come

The study was led by the Marine Evolution and Conservation group at University of Groningen’s Institute for Evolutionary Lifesciences, based in Groningen (the Netherlands), in collaboration with an international team of 37 researchers from 28 institutions in 15 countries.

Using genetics
The idea behind the study was to see what happened to marine mammals and their prey during periods of global warming in the past, to better anticipate future changes in the oceans and their biota as our globe heats up again. For that purpose, the study employed genetic data to assess changes in the oceans since the end of the last ice age – some 18 thousand years ago.

The DNA (genes and genomes) in living organisms contain information about the past. “The best analogy is perhaps growth rings in trees, the thickness of which tells us which years in the past were good and bad years from a cross section of the trunk. The past can be unraveled from the genomes in present day individuals and populations in sort of the same way but much further back into the past, thousands to millions of years”, explains Professor Per Palsbøll, the senior author on the study. Using genetics, insights can be expanded beyond the history of one individual to the species, including how many individuals there were and how much they moved among areas.

Baleen whales and their prey
The researchers analyzed genetic data from more than 7,000 samples collected over several decades in the Southern and the North Atlantic oceans. The study is novel in its global, multi-species approach. Baleen whales are top predators that migrate across entire oceans every year and feed on fish and krill in temperate and polar waters during the summer. Accordingly, they are likely highly dependent on large scale oceanographic features as well as the abundance of different types of prey. Therefore, the team felt that there was a lot to be gained by looking at many different baleen whale species and in different oceans, but also at their prey species-little food means few whales

Last ice age
In particular, the study focused on the period after the last ice age – some 18,000 years ago. This period was the last time that the Earth underwent rapid, global increases in temperatures. The warmer temperatures in turn melted the large continental ice caps and polar sea ice. As a result, the sea water levels rose ~120 meters and caused a vast increase in marine habitat and productivity.

Southern Ocean and North Atlantic Ocean
The research uncovered large, exponential simultaneous increases in all baleen whales and krill in the Southern Ocean as Earth warmed, indicative of a tremendous increase in ocean-wide productivity. In contrast, the effects on baleen whales and their prey during the same period of global warming were more subtle in the North Atlantic Ocean and differed among species. Part of the explanation perhaps lies with the so-called “8.2 ky event”, when a large “dam” of glacial melting water released massive amounts of glacial melting water into the western North Atlantic. This event is known to have had tremendous negative effects on marine production, which may explain the noticeable changes detected in the study across most North Atlantic whale and prey species.

Changes persisted
But perhaps the most worrisome observation in the study was what Dr. Andrea Cabrera, the first author of the study, emphasized: “Our study suggests that the large-scale oceanic changes set in motion by global warming persisted for many thousands of years after temperatures stabilized. It is a warning from the past as to what the current global warming may already have started”.

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