̽��ֱ�� of Cambridge - Francesco Muschitiello

Arctic Ocean started getting warmer decades earlier than we thought

sc604 — Wed, 24 Nov 2021 18:56:45 +0000

̽��ֱ��Arctic Ocean has been getting warmer since the beginning of the 20th century – decades earlier than records suggest – due to warmer water flowing into the delicate polar ecosystem from the Atlantic Ocean.

Changes in ocean 'conveyor belt' predicted abrupt climate changes

sc604 — Wed, 20 Mar 2019 00:00:01 +0000

In the Atlantic Ocean, a giant ‘conveyor belt’ carries warm waters from the tropics into the North Atlantic, where they cool and sink and then return southwards in the deep ocean. This circulation pattern is known as the Atlantic Meridional Overturning Circulation (AMOC) and it’s an important player in the global climate, regulating weather patterns in the Arctic, Europe, and around the world.

Evidence increasingly suggests that this oceanic current system is slowing down, and some scientists fear it could have major effects, such as causing temperatures to dive in Europe and warming the waters off the eastern coast of the USA, potentially harming fisheries and exacerbating hurricanes.

A new study published in Nature Communications provides insight into how quickly these changes could take effect if the ocean current system continues weakening.

An international team of scientists studied one of the key sections of the AMOC –where North Atlantic water sinks from the surface to the bottom of the ocean. They confirmed that changes in the ocean conveyor belt preceded abrupt and major climatic changes during the transition out of the last ice age, referred to as the last deglaciation. ̽��ֱ��study is the first to determine the time lags between past changes to the AMOC and major climate changes.

“Our reconstructions indicate that there are clear climate precursors provided by the ocean state — like warning signs, so to speak,” said lead author Francesco Muschitiello from the ̽��ֱ�� of Cambridge’s Department of Geography, who completed the work while a postdoc at Columbia ̽��ֱ��.

Until now, it has been difficult to resolve whether past changes in the ocean conveyor belt occurred before or after the abrupt climate shifts that punctuated the last deglaciation in the Northern Hemisphere. To overcome the usual challenges, the team pieced together data from a sediment core drilled from the bottom of the Nordic Seas, a lake sediment core from southern Scandinavia, and ice cores from Greenland.

Scientists typically rely on radioactive carbon (carbon 14) dating to determine the ages of sediments. This relationship is tricky in ocean sediments, though, because carbon 14 is created in the atmosphere, and it takes time for the carbon to make its way through the ocean. By the time it reaches the organisms at the bottom of the water column, the carbon 14 could already be hundreds or thousands of years old. So the team needed a different way to date the sediment layers in the marine core.

̽��ֱ��researchers solved this puzzle by measuring carbon 14 levels from a nearby lake sediment core and matching it to the marine core layers. Next, they compared the real age of the marine sediments to the deep ocean carbon 14 measurement, giving them a record of ocean circulation patterns in this region over time. ̽��ֱ��final piece of the puzzle was to analyse ice cores from Greenland, to study changes in temperature and climate over the same time period.

Comparing the data from the three cores revealed that the AMOC weakened in the time leading up to the planet’s last major cold snap around 13,000 years ago. ̽��ֱ��ocean circulation began slowing down about 400 years before the cold snap, but once the climate started changing, temperatures over Greenland plunged quickly by about 6 degrees.

A similar pattern emerged near the end of that cold snap, transitioning out of the ice age; the current started strengthening roughly 400 years before the atmosphere began to heat up dramatically, when Greenland warmed up rapidly — its average temperature climbed by about 8 degrees over just a few decades, causing glaciers to melt and sea ice to drop off considerably in the North Atlantic.

For now it’s not fully clear why there was such a long delay between the AMOC changes and climatic changes over the North Atlantic.

It’s also difficult to pinpoint what these patterns from the past could signify for Earth’s future. Recent evidence suggests that the AMOC began weakening again 150 years ago. However, current conditions are quite different from the last time around, says Muschitiello: the global thermostat was much lower back then, winter sea ice stretched farther south than New York Harbour, and the ocean structure would have been much different. In addition, the past weakening of the AMOC was much more dramatic than today’s trend so far.

“It is clear that there are some precursors in the ocean, so we should be watching the ocean. ̽��ֱ��mere fact that AMOC has been slowing down, that should be a concern based on what we have found,” said Muschitiello.

̽��ֱ��study should also help to improve the physics behind climate models, which generally assume the climate alters abruptly at the same time as AMOC intensity changes. ̽��ֱ��model refinements, in turn, could make climate predictions more accurate. As Svensson puts it: “As long as we do not understand the climate of the past, it is very difficult to constrain the climate models needed to make realistic future scenarios.”

Reference:
Francesco Muschitiello et al. 'Deep-water circulation changes lead North Atlantic climate during deglaciation.' Nature Communications (2019). DOI: 10.1038/s41467-019-09237-3

Adapted from Columbia ̽��ֱ�� story.

A bold response to the world’s greatest challenge
̽��ֱ�� ̽��ֱ�� of Cambridge is building on its existing research and launching an ambitious new environment and climate change initiative. Cambridge Zero is not just about developing greener technologies. It will harness the full power of the ̽��ֱ��’s research and policy expertise, developing solutions that work for our lives, our society and our biosphere.

A new study is the first to measure the time lags between changing ocean currents and major climate shifts.

There are clear climate precursors provided by the ocean state — like warning signs, so to speak

Francesco Muschitiello

Muschitiello et al

A simplified diagram of the Atlantic Meridional Overturning Circulation

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Sea ice acts as ‘pacemaker’ for abrupt climate change

sc604 — Wed, 06 Mar 2019 19:00:00 +0000

An international study involving researchers from the UK, Norway, Germany Australia, South Korea and the US has confirmed that changes in sea ice cover in the Norwegian Sea played a key role in driving abrupt climate change events between 32,000 and 40,000 years ago, where global temperatures shifted as much as 15 degrees Celsius.

̽��ֱ��results, reported in the journal Science Advances, indicate that initial sea ice reduction started before the abrupt warming over Greenland, and that sea ice expansion started before the end of the warm periods in Greenland.

̽��ֱ��Arctic sea ice is a key element of the global climate system and the strong ongoing warming of the Arctic Ocean can have major impacts on the stability of the Greenland Ice Sheet, first and foremost accelerated sea level rise.

̽��ֱ��Nordic Sea system and its water column structure during the last glacial cycle is the closest analogue to the present-day Arctic Ocean, which makes it a perfect natural laboratory to understand the role of rapid disappearance of regional sea ice on abrupt warming on the Greenland Ice Sheet.

̽��ֱ��last glacial period, 10,000–100,000 years ago, was marked by repeated abrupt climate changes with global implications. Within a matter of decades, temperature shifts of as much as 15 degrees Celsius occurred around Greenland, but the mechanisms driving these changes –known as Dansgaard-Oeschger events – are not fully understood.

“One of the most challenging aspects of palaeoclimatology is to precisely resolve and reconstruct the exact timing of the events that took place across the major climate transitions of the last glacial cycle,” said co-author Dr Francesco Muschitiello, from the ̽��ֱ�� of Cambridge’s Department of Geography. “As a result, a detailed account of the temporal relationship between changes in sea-ice extent in the Nordic Seas, and North Atlantic ocean circulation and climate across Dansgaard-Oeschger events has been so far elusive.”

̽��ֱ��researchers investigated specific organic molecules in a sediment core from the southern Norwegian Sea, one of which was produced by algae that live in sea ice and others that were produced by organisms living in open, ice-free waters thousands of years ago.

̽��ֱ��new sea ice reconstruction based on organic molecules in marine sediments was also evaluated by means of results from a model simulation of past sea ice conditions.

“Our data suggest that there were substantial changes in the sea ice cover in the southern Norwegian Sea between 32,000 and 40,000 years ago,” said Henrik Sadatzki from the ̽��ֱ�� of Bergen and the paper’s first author. “Most extensive sea ice conditions occurred at the onsets and early parts of cold periods over Greenland and the most pronounced open-ocean conditions occurred at the onsets of the abrupt changes to warm periods over Greenland.”

̽��ֱ��results support that an enhanced sea ice cover contributed to insulation of the cold, high-latitude atmosphere from relatively warmer waters that were present in the Norwegian Sea beneath the sea ice lid.

In turn, sea ice reduction allowed for heat release from the exposed Norwegian Sea waters to the atmosphere, which was a prime ingredient in shaping the abrupt warming of the Dansgaard-Oeschger climate events in Greenland.

̽��ֱ��Dansgaard–Oeschger climate events have stirred interest in documenting that the climate system contains mechanisms that may lead to abrupt and surprising climate changes.

These findings clarify the series of events taking place in the high-latitude North Atlantic across the abrupt Dansgaard–Oeschger cycles of the last glacial period. However, further work is needed to ultimately identify the physical mechanisms linking the current sudden demise of Arctic sea ice to abrupt Greenland Ice Sheet changes.

̽��ֱ��research was funded in part by the European Research Council.

Reference:
Henrik Sadatzki et al. ‘Sea ice variability in the southern Norwegian Sea during glacial Dansgaard-Oeschger climate cycles.’ Science Advances (2019). DOI: 10.1126/sciadv.aau6174

Adapted from a ̽��ֱ�� of Bergen press release.

Substantial variations in past sea ice cover in the Norwegian Sea were instrumental for several abrupt climate changes in large parts of the world, researchers have found.

United Nations Photo

Icebergs in Ilulissat Icefjord, Greenland

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