Oceans all over the world have been getting warmer, but none as much as the Arctic Ocean, which is the world’s smallest and shallowest ocean. Concerned scientists have been monitoring changes in temperature in this ocean for the past 20 years, using satellites, and have identified that warmer waters flowing into the Arctic Ocean from the North Atlantic may be exacerbating the phenomenon.
However, a new study conducted by an international team of scientists, has quantified the flow of Atlantic waters into the Arctic Ocean, a process known as Atlantification, from pre-industrial and early industrial times. They have found that the warming of the Arctic waters had begun by early in the 20th Century, decades earlier than warming was first recorded by modern instruments. Their study is published today in the journal Science Advances.
“The rate of warming in the Arctic is more than double the global average, due to feedback mechanisms,” said co-lead author Dr Francesco Muschitiello from the University of Cambridge. “Based on satellite measurements, we know that the Arctic Ocean has been steadily warming, in particular over the past 20 years, but we wanted to place the recent warming into a longer context.”
The researchers analyzed the marine sediments in the eastern Fram Straight, a gateway through which the Atlantic and Arctic Oceans exchange waters. Fram Straight lies between Greenland and Svalbard. They used biochemical markers found in the shells of tiny marine organisms in the sediments to reconstruct the changes in water column properties over the past 800 years.
The sediments were dated precisely using a combination of methods and the researchers looked for diagnostic signs of Atlantification, such as changes in temperature and salinity. Atlantic Ocean waters are both warmer and more saline than the waters of the Arctic.
“When we looked at the whole 800-year timescale, our temperature and salinity records look pretty constant,” said co-lead author Dr Tesi Tommaso from the Institute of Polar Sciences of the National Research Council in Bologna. “But all of a sudden at the start of the 20th century, you get this marked change in temperature and salinity – it really sticks out.”
The results indicate that the flow of warmer Atlantic waters into the Arctic began to increase in the early 1900s. This evidence provides the first historical perspective on Atlantification of the Arctic Ocean and reveals that the Arctic’s connection with the North Atlantic Ocean is much stronger than previously thought. The connection is capable of shaping Arctic climate variability, which could have important implications for sea-ice retreat and global sea level rise as the polar ice sheets continue to melt.
Scientists expect that, as the Arctic Ocean gets warmer, the ice in the polar region will melt which will, in turn lead to global rises in sea level. As the ice melts, it exposes more of the ocean’s surface to the sun, releasing heat and raising air temperatures. As the Arctic continues to warm, it will melt the permafrost, which stores huge amounts of methane, a far more damaging greenhouse gas than carbon dioxide.
Interestingly, the findings suggest that increasing Arctic Atlantification at the start of the industrial era was probably driven by a change in the North Atlantic ocean’s circulation pattern that brought warmer, sub-tropical waters further northwards than before, noted the study authors. This warming was not, at the time, influenced by the activities of humans.
“The reason for this rapid Atlantification at the gate of the Arctic Ocean is intriguing,” said Muschitiello. “We compared our results with the ocean circulation at lower latitudes and found there is a strong correlation with the slowdown of dense water formation in the Labrador Sea. In a future warming scenario, the deep circulation in this subpolar region is expected to further decrease because of the thawing of the Greenland ice sheet. Our results imply that we might expect further Arctic Atlantification in the future because of climate change.”
The researchers say their results also expose a possible flaw in climate models, because they do not reproduce this early Atlantification at the beginning of the last century.
“Climate simulations generally do not reproduce this kind of warming in the Arctic Ocean, meaning there’s an incomplete understanding of the mechanisms driving Atlantification,” said Tommaso. “We rely on these simulations to project future climate change, but the lack of any signs of an early warming in the Arctic Ocean is a missing piece of the puzzle.”