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Researchers Link Ice Age Climate-Change Records to Ocean Salinity


tugmistress

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Posted
  • Location: Scrabster Caithness (the far north of Scotland)
  • Location: Scrabster Caithness (the far north of Scotland)

    Sudden decreases in temperature over Greenland and tropical rainfall patterns during the last Ice Age have been linked for the first time to rapid changes in the salinity of the north Atlantic Ocean, according to research published Oct. 5, 2006, in the journal Nature. The results provide further evidence that ocean circulation and chemistry respond to changes in climate.

    Using chemical traces in fossil shells of microscopic planktonic life forms, called formanifera, in deep-sea sediment cores, scientists reconstructed a 45,000- to 60,000-year-old record of ocean temperature and salinity. They compared their results to the record of abrupt climate change recorded in ice cores from Greenland. They found the Atlantic got saltier during cold periods, and fresher during warm intervals.

    "The freshening likely reflects shifts in rainfall patterns, mostly in the tropics," Howard Spero of the University of California at Davis said. "Suddenly, we're looking at a record that links moisture balance in the tropics to climate change. And the most striking thing is that a measurable transition is happening over decades."

    Spero, who is currently on leave at the National Science Foundation's Marine Geology and Geophysics Program, worked with lead author Matthew Schmidt of the Georgia Institute of Technology and Maryline Vautravers of Cambridge University in the United Kingdom to conduct the research.

    During the Ice Age, much of North America and Europe was covered by a sheet of ice. But the ice records the scientists reconstructed show repeated patterns of sudden warming, called Dansgaard-Oeschger Cycles, when temperatures in Greenland rose by 5 to 10 degrees Celsius over a few decades.

    Close to the tropics, warm, moist air forms a zone of heavy tropical rainfall, called the Intertropical Convergence Zone, which dilutes the salty ocean with fresh water. Today, the tropical rainfall zone reaches into the northern Caribbean, but during the colder periods of the Ice Age it was pushed much further south, towards Brazil. That kept fresh water out of the northern Atlantic, so it became more salty, Spero said.

    The circulation, or gyre, in the North Atlantic moves warm, salty water north, keeping Europe relatively temperate. The deep ocean circulation is very sensitive to the saltiness of north Atlantic surface waters, Spero said. Warming climate, higher rainfall and fresher conditions can alter the circulation. During glacial times, reduced circulation caused climate to cool.

    The new results show that as the climate cooled in Greenland, salinity rapidly increased in the North Atlantic subtropical gyre. The build-up of salt during these cold intervals when the conveyor circulation was reduced would have primed the system to quickly restart on transitions into warm intervals, Schmidt said. However, the actual trigger that caused Atlantic circulation to restart during the Ice Age is still unknown, he said.

    Once warming began, melting ice sheets would have contributed fresh water to the Atlantic, but this would have been partly buffered by the elevated saltiness of the Atlantic.

    Article from here

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    Posted
  • Location: Sunny Southsea
  • Location: Sunny Southsea

    Thank you for posting this, Tugmistress; it is an interesting piece in several ways. Essan did provide a link to it in another thread, but many people would have missed it.

    The usual qualification; without reading the actual paper, we can only discuss what the article/press release says, unfortunatley, but I'll give it a stab.

    I am not sure what the claim 'for the first time' means. The connection between ocean salinity, circulation patterns and climate has been under discussion for several years now. Perhaps it is the first time that hard evidence of a link has been given, I don't know, but it seems a strong claim to make given the number of papers on the subject already, including several by Spero and Schmidt, the authors of this paper.

    The 'discovery' that the Atlantic got saltier when the atmosphere was colder, and fresher when it was warmer, is hardly a revelation. Increased temperature leads to increased precipitation is a fairly straightforward connection to make, and hardly a new idea. Also, the amount of sea-ice has a direct relationship to the salinity of the oceans - but this does not seem to be discussed in the paper.

    That such changes can happen over decades is also no surprise; close examination of the early Holocene period (12,000-8000 BC) has already revealed this. The Rapid changes in temperature - (Dansgaard-Oeshger events) are also fairly well researched.

    The Southerly movement of the ITCZ during an ice age is also hardly a surprise, as the area where the sea temperatures would be warm enough to form the ITCZ would, by definition, be further away from the ice.

    The claim that it was this which caused the Atlantic to become more salty is, at best, disputable; there are other explanations which are equally reasonable, and I would need more convincing before accepting this conclusion.

    The three last paragraphs are the trickiest. 'Deep ocean circulation is very sensitive to the saltiness of North Atlantic surface waters' sounds innocuous enough, but this statement cannot be accepted at face value, as we still don't know whether this is the case. The most recent papers I have read indicate that this statement may, in fact, be false.

    Increasing freshness, whatever the source, is likely to affect the circulation, but this is no new claim; it has been around since at least 1995. 'During glacial times, reduced circulation caused the climate to cool' also says less than it seems to. It doesn't tell us whether the climate was affected by the circulation, or vice-versa. In fact, the paper seems to be suggesting both, which is a logical inconsistency, at the very least. the relationship between the two is not established here.

    The final paragraph, claiming that the increased saltiness would have 'primed the system to restart' also says very little. It suggests that this is a necessary condition for a return to a warming climate, but then goes on to say however... still unknown', which means that it probably was not a sufficient condition in and of itself.

    There is more to say on this subject, but it's getting late, and. as always, my post is far too long. Sorry! :) All I will say is, from what I have read, i'm not overly impressed by the findings of this particular paper.

    :)P

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    Posted
  • Location: Scrabster Caithness (the far north of Scotland)
  • Location: Scrabster Caithness (the far north of Scotland)

    ok, i'll add this one in too, just to give you a headache lol. I don't pretend to understand the whys and wherefors, but i do find it interesting :)

    taken from the same source :)

    .......................................

    While cool August temperatures prevented sea ice in the Arctic from reaching its lowest summer extent on record, 2006 continued a pattern of sharp annual decreases due to rising temperatures probably caused by greenhouse warming, according to University of Colorado at Boulder researchers.

    The latest measurements indicate the Arctic sea ice minimum reached on Sept. 14 was the fourth lowest on record in 29 years of satellite record-keeping, said CU-Boulder Research Professor Mark Serreze of the CU-Boulder's National Snow and Ice Data Center. The ice has been declining at about 8.6 percent per decade, or at about 23 million square miles per year -- an area more than half the size of Ohio, he said.

    The sea ice extent is the total area of all artic regions where ice covers at least 15 percent of the ocean surface, Serreze said.

    he record low, which occurred in 2005, was marked by an extent of sea ice 20 percent lower than the average ice extent from 1978 to 2001, or a 500,000 square-mile decrease equal to an area about twice the size of Texas, the team reported. The 2006 low is about 400,000 square miles less than the average, the research group said.

    "If fairly cool and stormy conditions hadn't appeared in August and slowed the rate of summer ice loss, I feel certain that 2006 would have surpassed last year's record low for September sea ice," said Serreze.

    "At this rate, the Arctic Ocean will have no ice in September by the year 2060," said CU-Boulder researcher Julienne Stroeve of NSIDC, which is part of the Cooperative Institute for Research in Environmental Sciences, a joint institute of CU-Boulder and the National Oceanic and Atmospheric Administration. "The loss of summer sea ice does not bode well for species like the polar bear, which depend on the ice for their livelihood."

    Average air temperatures across most of the Arctic Ocean from January 2006 to August 2006 were about 2 degrees to 7 degrees Fahrenheit warmer than the long-term average across the region over the past 50 years, the team said.

    Ice extent from January to the middle of July 2006 was well below 2005 conditions and was consistent with the unusually warm air temperatures scientists have been tracking in the arctic in recent years, said Serreze. High winter temperatures contributed to limited ice growth, and much of the ice that did form was thinner than normal. "Unusually high temperatures through most of July then fostered rapid melt," he said.

    The arctic "heat wave" broke in August and slowed the melt, and storm conditions led to wind patterns that helped spread existing ice over a larger area, Serreze said. But in September temperatures returned to above-normal patterns, which has meant a slow recovery from last month's minimum and indicates the sea ice extent this month could set a new record for the lowest October minimum.

    One of the most notable features of the 2006 season was the development of a large polynya -- an area of persistent open water surrounded by sea ice -- that is visible north of Alaska, said Walt Meier of CU-Boulder's NSIDC. Calculations show that in early September, the polynya was the size of the state of Indiana, a huge feature never seen in the Arctic before, Meier said.

    How it formed it still not clear, although unusual wind patterns might have forced the ice cover to spread apart, Meier speculated. The team also hypothesized that that thin ice moved into the area over the winter, melting out over the summer and creating the polynya. Warm ocean waters may also have risen to the surface in recent months, helping to melt the ice, Meier said.

    While the polynya is not directly attributable to greenhouse warming, continued weakening and thinning of sea ice with increased warming could make such features more common in the future, said Meier.

    According to CU-Boulder scientist Ted Scambos, sea-ice melting in response to rising temperatures creates a positive feedback loop. "Melting ice means more of the dark ocean is exposed, allowing it to absorb more of the sun's energy, further increasing air temperatures, ocean temperatures, and ice melt," he said. "It seems that this feedback, which is a major reason for the pronounced effects of greenhouse warming in the arctic, is really starting to kick in."

    The research team used satellite data from NASA, NOAA and the U.S. Department of Defense, as well as data from Canadian satellites and weather observatories for the study.

    "I'm not terribly optimistic about the future of the ice," Serreze said. "As greenhouse gases continue to rise, the Arctic will continue to lose its ice. You just can't argue with the physics."

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    Posted
  • Location: SE London
  • Location: SE London

    some very good reading there, if not a bit worrying in some parts. e.g

    "At this rate, the Arctic Ocean will have no ice in September by the year 2060,"

    i think the time is rapidly coming where investment into reducing possible effects of greenhouse gases and ozone depletion is needed more so than other projects governments may be spending on.

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