by Tim Radford, Climate News Network, February 13, 2015
LONDON − Scientists believe they may have cracked the mystery of the end of the last ice age. The temperatures suddenly soared, and the glaciers went into retreat, because the deep southern ocean released huge quantities of carbon dioxide.
And the convincing answers have been delivered by analysis of the composition of calcium carbonate shells of ancient marine organisms.
The link between human burning of fossil fuels and the steady rise in atmospheric carbon dioxide levels was proposed more than a century ago and firmly established in the last 30 years.
But the ups and downs of planetary temperatures before the emergence of human civilisation are harder to explain. Fossil evidence suggests a link with carbon dioxide levels, but not necessarily a cause.
Bygone climates
Now paleoceanographer Miguel Martínez-Botí, from the University of Southampton, UK, and ocean and climate change researcher Gianluca Marino, from the Australian National University, report in Nature that they found their evidence in sediment cores – in effect, annual records of bygone climates – rich in the shells of tiny foraminifera called Globigerina bulloides.
This is a species that flourishes in conditions of high nutrients, acting as a kind of biological pump, gulping carbon from the atmosphere.
They found that high concentrations of carbon dioxide dissolved in surface waters of the southern Atlantic Ocean and the eastern equatorial Pacific coincided with rises in atmospheric CO2 at the end of the last ice age.
The implication is that these regions were the source of the carbon dioxide to the atmosphere.
At their coldest, during the ice ages, carbon dioxide levels fell to 185 parts per million. During the interglacials, when the world warmed and lions and hyenas roamed the plains of Europe, the carbon dioxide levels rose to 280 ppm.
Right now, thanks to human activity, CO2 levels are rising ominously towards 400 ppm.
The oceans are home to about 60 times more carbon than the atmosphere and can, it seems, surrender it rapidly.
“The magnitude and rapidity of the swings in atmospheric CO2 across the ice age cycles suggest that changes in ocean carbon storage are important drivers of natural atmospheric CO2 variations,” Dr Martínez-Botí says.
“Our findings support the theory that a series of processes operating in the southernmost sector of the Atlantic, Pacific and Indian oceans, a region known as the Southern Ocean, changed the amount of carbon in the deep sea.
Into the abyss
“While a reduction in communication between the deep sea and the atmosphere in this region potentially locks carbon away from the atmosphere into the abyss during ice ages, the opposite occurs during warm interglacial periods.”
To arrive at their conclusion, the scientists had to analyse subtle evidence from the isotopic composition of the carbonate shells, and then use mathematical techniques to reconstruct a story of a great, faraway sigh of carbon dioxide from the ocean to the atmosphere.
The finding, based on calculated probabilities, is incomplete as there may have been other forces also at play.
Gavin Foster, associate professor in isotope geochemistry at the University of Southampton, says: “While our results support a primary role for the Southern Ocean processes in these natural cycles, we don’t yet know the full story. Other processes operating in other parts of the ocean, such as the north Pacific, may have an additional role to play.”
Showing posts with label Southern Ocean. Show all posts
Showing posts with label Southern Ocean. Show all posts
Friday, February 13, 2015
Carbon stored deep in Antarctic waters ended the last ice age
by Miguel Martinez-Boti, University of Southampton and Gianluca Marino, Australian National University, The Conversation, February 12, 2015
It’s well known that carbon in the atmosphere is causing global warming. What is less well known, outside of scientific circles at least, is the role oceans have to play in this. Our seas contain 60 times more carbon than the atmosphere, and they can release it at sufficiently rapid rates to cause dramatic changes in the climate. In fact, as we describe in research published in Nature, CO2 released by the oceans brought about the end of the last ice age.More than 50 million cubic kilometres of ice once covered North America and Scandinavia. It melted away between approximately 19,000 and 10,000 years ago, releasing enough water to raise the sea level by about 130 metres. This came after CO2 concentrations increased by approximately 50%, from 180 to 280 parts per million between the last ice age and the current interglacial period. To explain such a pronounced increase, we have to look at the ocean.
Scientists have thought for a long time that the southern sectors of the Atlantic, Indian and Pacific Oceans, a region known as the Southern Ocean, may be key to explaining the increase in atmospheric CO2.
Large volumes of deep water loaded with carbon come to the surface in this area. However, the low concentration of certain nutrients (for example iron) in surface waters limits the metabolism of planktonic organisms, which cannot fully consume all the carbon brought to the surface ocean, resulting in CO2 being “outgassed” to the atmosphere.
We wanted to assess if the ocean contributed to the atmospheric CO2 increase during the last deglaciation, so it made sense to look at areas that are important today for the ocean-atmosphere exchange of carbon: the Atlantic Sector of the Southern Ocean and the Eastern Equatorial Pacific, another area where deep, cold water rises to the surface.
But how can we then go back in time and check if these areas were a source of CO2 in the atmosphere? The answer is buried a few thousand meters below the surface of the oceans.
Research vessels such as the Joides Resolution are capable of drilling the sea floor to recover long sequences of sediments in which the history of the oceans is recorded. The sediments contain, among other things, fossils of tiny organisms that once lived in the upper ocean, called foraminifera. These creatures build chalky shells, and the waters they live in influence their chemical composition.
After death, the shells sink to the bottom of the oceans, where they accumulate. We analysed the sediment cores and looked for the isotopic composition of the element boron present in shells that lived during particular times of interest. Boron tells us pH levels of the waters, which in turn tells us about carbon levels: a high concentration of CO2 in the waters will make them more acidic (lower pH), and vice versa.
We found a link. When the glaciers of the last ice age were melting, and the atmospheric CO2 was increasing, the surface waters of the Southern Ocean and the Eastern Equatorial Pacific were also more acidic. This signalled an increased concentration of CO2 – much higher than those in the atmosphere.
This is the key finding of our research: the ocean was a source of CO2 to the atmosphere during key intervals of the last deglaciation, which explains the large increase in CO2 concentrations.
Where did this carbon come from?
It’s the next obvious question. Previous research has found that the last ice age saw much less carbon exchanged between ocean and atmosphere than we see today, mostly because the Southern Ocean was intensely stratified at the time and deep waters rarely made it to the surface. Nutrients and CO2 were accumulating in the deep Southern Ocean, due to the decay of the organic matter that was being produced in the surface ocean and transported to the abyss.
During the deglaciation, the effective communication between deep and upper ocean was re-established, and this carbon “reservoir” was leaked to the atmosphere.
Since the beginning of the industrial revolution the oceans have absorbed an estimated 155 billion tonnes of carbon, about 30% of the total human emissions. The present atmospheric CO2 concentrations, approximately 400 parts per million, have not been seen on Earth since the Pliocene, around 3 million years ago, and the rate of increase is unprecedented in the period of on-off glaciers we have had since.
Humanity is performing a large scale experiment with the Earth, and the consequences are already being seen in the form of increased atmospheric and oceanic temperatures, raising sea levels and ocean acidification, to name a few. How the oceanic uptake of CO2 is going to operate in the future remains unknown, but studies like ours advance our understanding of how the ocean works to store and release carbon on timescales of millennia and that therefore are way beyond the reach of the instrumental record.
This article was originally published on The Conversation. Read the original article.
Saturday, September 6, 2014
Robert Schribbler: It’s All About Fresh Water — Rapid Sea Level Rise Points To Massive Glacial Melt in Antarctica
by Robert Schribbler, robertschribbler.wordpress.com, September 3, 2014
It’s all about fresh water. In this case, massive freshwater outflows from the vast glaciers covering Antarctica.
This week, a new scientific report published in the journal Nature found that from 1992 through 2012 freshwater outflow from Antarctica’s massive glaciers exceeded 400 gigatons each year. An immense flood of cold, fresh water. One that helped push sea levels rapidly higher around the Antarctic continent.
But with glacial melt on the rise and with mountains of ice now inexorably sliding seaward, these freshwater flows may just be the start of even more powerful outbursts to come. And such prospective future events have far-ranging implications for sea level rise, global weather, sea ice, human-caused climate change, and world ocean health.
Flood of Fresh Water Drives More Sea Level Rise Than Expected
The researchers discovered the tell-tale signature of this vast freshwater flood through chemical analysis of the seas surrounding Antarctica. The analysis pointed to a broad and expanding fresh water layer over-riding a warmer, saltier current issuing in from the Southern Ocean.
Since fresh water is less dense than salt water, the freshwater layer expands at the ocean surface causing sea levels to rise more rapidly. Meanwhile, the heating of the deep ocean surrounding Antarctica is thought to result in additional thermal expansion of the water column.
The researchers note:
Rate of sea level rise in the seas surrounding Antarctica since 1992. Aggregate sea level rise is indicated in black. Individual seas data is broken out by color. Image source: Nature.
Previously, increased rates of sea level rise surrounding Antarctica were thought to have been set off by increasing winds around the continent. The winds were thought to push more water up against the ice faces forming a kind of perpetual, low-grade storm surge. But the current finding provides strong evidence that the source of the sea level rise is due to less dense fresh water over-topping saltier waters flowing in from the Southern Ocean combined with increasing heat along the Antarctic sea bed. And, notably, this is not the first study to find increasing freshwater flows spilling into the Southern Ocean. Last year, a KNMI expedition uncovered similar results.
More Evidence of Large-Scale Melt
The study comes on the back of other recent findings showing that warm water invasion at Antarctic glacier bases had led to more rapid than expected melt and destabilization. In May, two NASA studies showed that a broad section of West Antarctica had destabilized and was sliding at an ever more rapid pace toward the ocean (see reports here and here). These findings held stark implications for global sea level rise as large ice regions of Greenland and West Antarctica, containing enough water to raise seas at least 15 feet, are likely already in a state of irreversible collapse.
Sea level rise anomaly of the region surrounding Antarctica compared with the rest of the Southern Ocean. Red indicates faster than normal sea level rise. Blue indicates slower than normal sea level rise. Image source: Nature.
This intensifying glacial melt and associated freshwater cap expanding out from the pole has implications — not just for sea level rise, but for sea ice, weather, and world ocean system health.
Impacts For Sea Ice
Large outflows of glacial fresh water may well be involved in the recent observed expansion of sea ice in the zone surrounding Antarctica (see recent related study). Fresh water serves as an insulative cap on the ocean surface preventing warm water from entering the top layer from below. The warm, salty water, in the Antarctic instead pools near the bottom or at the base of the great ice sheets.
Fresh water also freezes at a higher temperature than salt water. So sea ice in an expanding freshwater zone around Antarctica would have naturally higher resiliency even to the rising temperatures now occurring due to human-caused warming. Eventually, however, human heat forcing would overwhelm the ice, but not before a period of related, localized negative feedbacks.
The Iceberg Cooling Effect
The fresh water is a haven for sunlight-reflecting sea ice. It is interspersed with ice bergs from the glacial discharge and the large ice bergs cool the surrounding air. The fresh water layer prevents warm water upwelling from the warm, deep waters surrounding Antarctica. And the leading edge of the fresh water would drive salt-water down-welling along its advancing front. This would push warmer waters toward the ocean bottom, resulting in a kind of heat sink. And this is exactly the kind of dynamic that appears to be ongoing in the Southern Ocean now. These combined impacts are what is known as the ice berg cooling effect associated with large-scale glacial outbursts known as Heinrich Events. And we may well be in the process of setting off one of these geological scale nightmares.
Iceberg cooling effect under a mid-range warming scenario when global climate models were set to include the effects of large freshwater outflows from polar glaciers at a fast enough rate to raise seas by 60 cm through 2060 and 144 cm through 2080 [left frames]. Note the cooler zones in the Southern Ocean and North Atlantic adjacent to Greenland. Right frames include mid range emissions/warming scenarios and IPCC projected rates of sea level rise. It is worth noting that the amplifying effects of potential additional ghg release from the global climate system, particularly from Arctic and world ocean carbon stores, are not included in these simulations. Image source: Hansen and Sato.
For global weather, such events have major implications. Regional cooling in the zone of freshwater outflow would juxtapose regional warming in the southern hemisphere meridional zones. This temperature differential would increase with the strength of the fresh water outflow and the rising intensity of the human-driven warming. The result would be a powerfully intensified storm track. Both the intensified storm track and increased atmospheric moisture loading due to human warming would result in much more powerful weather events than we are currently used to and the potential for catastrophic storms would drastically increase.
Amplifying Feedbacks and a Blow to World Ocean Health
Lastly, the expanding flood of fresh water would result in an increasing stratification of the world ocean system. This stratification would drive warm, salty water toward the ocean bottom and deplete already low oxygen reserves in that region. In addition, the extra heat is more likely to destabilize deep-sea clathrates — releasing methane which will speed in the oxygen depletion of the abyssal waters even as it tips the world ocean system to stop storing carbon and to begin releasing it. A combined feedback that is both an ocean killer and an amplifier to the already extraordinarily powerful human heat forcing mechanism.
Links:
Sunday, May 25, 2014
NewScientist: Antarctic wind vortex is strongest for 1,000 years
The winds ripping around Antarctica. (Image: earth.nullschool.net)
by Michael Slezak, NewScientist, May 11, 2014
Our greenhouse gas emissions are helping to spin up a giant vortex of winds around Antarctica.
Antarctica has been warming relatively slowly compared with the rest of the world. The explanation seems to be that the winds spinning clockwise around the continent have been getting stronger, preventing warm air from entering.
In a way, those winds have done us a favour by keeping warm air away from the South Pole. Otherwise it might be melting. But as this atmospheric maelstrom accelerates, it shrinks, leaving the most vulnerable parts of Antarctica out in the warm and dragging winter rain away from Western Australia.
In 2009, it seemed that the hole in the ozone layer above Antarctica was responsible for boosting the winds. Now Nerilie Abram from the Australian National University in Canberra and her colleagues have shown the ozone hole is only part of the story. Global warming is just as important.
Warming powers winds
The team reconstructed Antarctic temperatures over the past 1,000 years, using an ice core from James Ross Island near the Antarctic Peninsula. The temperatures correlated with how strong and tight the winds are, so they could construct a record of wind strength.
They found that the current strength of the winds is unprecedented over the past millennium. But the surge in strength started in the 1940s, decades before the ozone hole.
So Abram's team simulated the last millennium using 8 climate models, driven by actual greenhouse gas levels previously reconstructed from ice cores. All the models predicted that the winds would pick up by the 1940s, suggesting greenhouse gases were playing a role. That may be because the Northern Hemisphere is warming faster than the south – because it has more continents – creating a strong temperature gradient that boosts the winds.
Such historical data is vital, says Wenju Cai from the CSIRO, Australia's national research agency, in Melbourne. In as-yet-unpublished work, he estimates that ozone depletion has caused two-thirds of the impact on the Antarctic winds, with greenhouse gases responsible for the rest.
Futureshock
If greenhouse gases really are contributing to the winds, it changes our expectations for what will happen to the climate in Australia and Antarctica.
The ozone hole is expected to heal in the coming decades, and if it was the only factor controlling the winds they would weaken and expand. So Australia would get its rain back, while the western parts of Antarctica might get some more protection against warming.
However, Abram says rising global temperatures will counteract this weakening effect on the winds. That means Western Australia will stay dry and the western parts of Antarctica, stranded outside the winds, will keep melting.
Cai estimates that, on our current emissions pathway, the two factors will counteract each other until 2045, so the winds will stay constant. After that, without reducing our emissions, greenhouse gases will boost the winds further.
Journal reference: Nature Climate Change, DOI: 10.1038/NCLIMATE2235
Sunday, May 11, 2014
Australia: Wilder winds, less rain, as Roaring Forties become Furious Fifties
by Peter Hannam, Environment Editor, The Sydney Morning Herald, May 11, 2014
Photo: Fairfax Graphics
The Roaring Forties, the Southern Ocean winds which once bore European sailors to Australia and the East Indies, are becoming more like the Furious Fifties as climate change triggers a shift in key weather patterns poleward, an Australian-led team of scientists has found.
Using data derived from Antarctic ice cores and other sources, the researchers found Southern Ocean winds are now stronger than at any time in the past 1,000 years.
Greenhouse gases are what are causing the winds to intensify now and that’s really moving the system beyond the natural range,” said Nerilie Abram of the Australian National University’s Research School of Earth Sciences and lead author of the research, published on Monday in Nature Climate Change.
In the past half century, the westerly winds have quickened 10-15% and moved 2-5 degrees closer to the South Pole – meaning fewer storms are reaching as far north as Australia.
“That isn’t good news for farmers in the southern parts of Australia who are reliant on the winter winds that come out of the Southern Ocean,” Dr Abram said. Winter rainfall has dropped 20% in southwest Western Australia since the 1960s, with cool-season rain tallies also lower in Australia’s southeast.
The stronger winds also help resolve a climate-change conundrum – why Antarctica is not warming as fast as other continents and the Arctic. “Over a large part of Antarctica we don’t get much warming at all,” Dr Abram said. [This may be changing - temperature anomalies over Antarctica have become very significant this year.]
The reason for the discrepancy is that cool air is being trapped over Antarctica, resulting in increased snowfall for some regions. However, areas exposed to stronger winds and warming seas, such as the Antarctic Peninsula, are heating up faster than anywhere else in the Southern Hemisphere.
“The West Antarctic Ice Sheet [adjacent to the peninsula] is probably the bit of the Antarctic ice mass that we’ve been most concerned about for the longest time,” said Matthew England, from the University of NSW’s Climate Change Research Centre, and a co-author of the paper. If it all melted, that ice sheet could lift global sea levels by 4-5 metres, he said.
Professor England said the changes to atmospheric variability that see the band of westerly winds oscillate north or south – known in the Southern Hemisphere as the Southern Annular Mode – are driven roughly equally by the effect of rising greenhouse gases and the ozone hole.
The relative contribution, though, should alter as internationally agreed constraints on the use of chemicals that destroy the protective ozone layer take effect, potentially slowing the pick-up in wind speeds.
“Going forward, the greenhouse aspect will dominate as the ozone hole starts to repair and, of course, greenhouse gases are going terrifyingly upwards in their concentration,” he said.
Wenju Cai, an atmospheric scientist at the CSIRO who was not part of the research team, said the findings would assist the study of other key processes, such as whether the rate at which the Southern Ocean absorbs heat and carbon dioxide is changing.
The faster winds “may have a lot of influences that we do not know now,” Dr Cai said. “We may even solve some of the big issues that have been puzzling scientists for many, many years.”
Saturday, February 2, 2013
Ozone hole over Antarctica causing circumpolar winds to speed up and move polewards
by Lauren Morello, Climate Central, January 31, 2013
NOAA staff at the United States' South Pole research station prepare to release a balloon that will measure the strength of the ozone layer high above Antarctica.
High above Antarctica, the atmosphere is slowly recovering from the decades-long barrage of manmade chemicals that ate a hole in the protective ozone layer.
But the legacy of that destruction lingers. Scientists have linked the ozone hole that forms each Antarctic spring high above Earth to changes in the fierce band of westerly winds that swirls around Antarctica. Those winds, closer to the continent's surface, have grown stronger and moved poleward over the past several decades.
This NASA animation shows variations in the ozone holes that developed over the South Pole each Antarctic spring from 1979 to 2013. Purple and blue areas have the least ozone, while yellows and reds have the most. Credit: NASA.
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And now a new study suggests that the ozone hole has an even broader reach. It finds evidence those shifting winds are speeding circulation patterns in polar waters. That shift is important because it may already be weakening the Southern Ocean's ability to absorb carbon dioxide from the atmosphere and slow the march of manmade climate change.
Even a small change in the Southern Ocean carbon sink could have a noticeable impact, because the region's waters takes in about 40% of the total carbon absorbed by the world's seas.
"The models were indicating there could be some change in ocean circulation (caused by ozone depletion), but there was a lot of debate about whether what the models were saying was actually happening," said lead author Darryn Waugh, a climate scientist at Johns Hopkins University.
His research, published Thursday in the journal Science, bears those models out. It was published alongside a separate study, from researchers at Pennsylvania State University, that affirms the ozone hole has been the main driver of the changes in Antarctica's winds, dwarfing the role played by climate change.
Waugh and colleagues in the U.S. and Australia found that in the subtropical Southern Hemisphere, water 500 to 1,000 meters deep appear to be growing "younger." That's a sign that north-south circulation in the deep ocean has been speeding up, sending surface water from the ocean surface near the pole to those intermediate depths more quickly, he said.
At the same time, the currents closer to Antarctica's shores appear to be pushing more old, deep water up to the ocean surface.
Scientists worry that the increasing upwelling of that water, hundreds of years old and naturally rich in carbon dioxide, is reducing the amount of manmade carbon absorbed by sub-polar waters.
NOAA staff at the United States' South Pole research station prepare to release a balloon that will measure the strength of the ozone layer high above Antarctica.
Click to enlarge. Credit: NOAA.
"The amount of carbon that goes from the atmosphere into the ocean depends on the balance between the amount of carbon in the atmosphere and the amount of carbon in surface water," Waugh said.
If surface waters are already rich in carbon, "that would mean more of the carbon we're producing would stay in the atmosphere, and that would contribute more to climate change," he said.
Michael Meredith, a physical oceanographer with the British Antarctic Survey who was not involved in either study, said the new research drives home the importance of the Southern Ocean carbon sink. "It's doing us a very big favor, if you like, by taking carbon from the atmosphere and slowing the rate of atmospheric climate change," he said.
Meredith, who called the new study "a strong and important paper," said the question now is what will happen as the ozone layer slowly heals and human activities pump out increasing amounts of greenhouse gases.
With the 1987 Montreal Protocol, which bans the use of ozone-destroying chemicals, now in force, researchers expect the ozone layer to recover by mid-century.
"The future of the circulation in the Southern Ocean, and the impact that it has on global climate change now seems to be very strongly tied to what happens to the westerly winds in the future," Meredith said.
The second study, by Pennsylvania State University researchers, is a small step toward answering that question, said Julie Arblaster, a climate scientist at Australia's Bureau of Meteorology, who called the analysis' use of wind speed and direction observations "sophisticated."
It's the first paper to use such observations — not model simulations — to determine what roles ozone depletion and climate change have played in shifting westerly winds poleward.
That could help researchers identify which climate models will do the best job projecting how the wind pattern will change as the ozone layer strengthens and climate change intensifies.
But first, they'll have to figure out just how both factors alter westerly winds — no small task.
"The next step is to understand the mechanism," Arblaster said. "Because if we can understand the mechanism, we can increase our confidence in projections for the future."
Related Content
Winds Seen As Key Driver Of Antarctica's Growing Sea Ice
Forget the Melting Arctic, Sea Ice in Antarctica Is Growing!
The Ozone Hole Opens Up Again. It's Still Not the Same as Climate Change
Report: Most Antarctic Peninsula Warming Human-Caused
Winds Seen As Key Driver Of Antarctica's Growing Sea Ice
Forget the Melting Arctic, Sea Ice in Antarctica Is Growing!
The Ozone Hole Opens Up Again. It's Still Not the Same as Climate Change
Report: Most Antarctic Peninsula Warming Human-Caused
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Wednesday, August 1, 2012
New discovery of how carbon is stored in the Southern Ocean
New discovery of how carbon is stored in the Southern Ocean
A team of British and Australian scientists has discovered an important method of how carbon is drawn down from the surface of the Southern Ocean to the deep waters beneath. The Southern Ocean is an important carbon sink in the world – around 40% of the annual global CO2 emissions absorbed by the world’s oceans enter through this region.
Reporting this week in the journal Nature Geoscience, scientists from British Antarctic Survey (BAS) and Australia’s national research agency, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), reveal that rather than carbon being absorbed uniformly into the deep ocean in vast areas, it is drawn down and locked away from the atmosphere by plunging currents a thousand kilometres wide.
Winds, currents and massive whirlpools that carry warm and cold water around the ocean — known as eddies — create localised pathways or funnels for carbon to be stored.
Lead author, Dr Jean-Baptiste Sallée from British Antarctic Survey says,
“The Southern Ocean is a large window by which the atmosphere connects to the interior of the ocean below. Until now we didn’t know exactly the physical processes of how carbon ends up being stored deep in the ocean. It’s the combination of winds, currents and eddies that create these carbon-capturing pathways drawing waters down into the deep ocean from the ocean surface.“Now that we have an improved understanding of the mechanisms for carbon draw-down we are better placed to understand the effects of changing climate and future carbon absorption by the ocean.”
CSIRO co-author, Dr Richard Matear says the rate-limiting step in the anthropogenic carbon uptake by the ocean is the physical transport from the surface into the ocean interior.
“Our study identifies these pathways for the first time and this matches well with observationally derived estimates of carbon storage in the ocean interior,” Dr Matear says.
Due to the size and remote location of the Southern Ocean, scientists have only recently been able to explore the workings of the ocean with the help of small robotic probes — known as Argo floats. In 2002, 80 floats were deployed in the Southern Ocean to collect information on the temperature and salinity. This unique set of observations spanning 10 years has enabled scientists to investigate this remote region of the world for the first time. The floats are just over a metre in length and dive to depths of 2 km. Today, there are over 3,000 floats in the oceans worldwide providing detailed information used in oceanic climate models.
The team also analysed temperature, salinity and pressure data collected from ship-based observations since the 1990s. The instrument used for this is called a CTD profiler which is a cluster of sensors taking measurements as it’s lowered deep down into the ocean to depths of more than 7 km.
This study complements a paper published in Nature 487, 313–319 (19 July 2012) — Deep carbon export from a Southern Ocean iron-fertilized diatom bloom — by Victor Smetacek, Christine Klaas, Volker H. Strass et al., which outlines the biological processes involved in how carbon is absorbed in the Southern Ocean.
- Eddy
- A massive whirlpool carrying warm and cold water around the ocean that may be formed when a bend in a surface ocean current lengthens and eventually makes a loop which separates from the main current.
- CTD Profiler
- An oceanographic instrument with a cluster of sensors that measure continuous data of depth, salinity and temperature of the ocean. It is deployed from the deck of a research vessel and lowered down deep into the ocean to depths exceeding 10 km.
- Argo floats
- Small, robotic probes that collect high-quality temperature, pressure and salinity data. There are more than 3,000 underwater robots swimming in the Earth’s oceans at this moment. The probes dive as deep as 2,000 m into the ocean for 10 days at a time, after which they re-surface and transmit the data they have collected via satellites. Each float is designed to make around 150 such cycles. International Argo Project
The Natural Environment Research Council (NERC) is the UK’s main agency for funding and managing world-class research, training and knowledge exchange in the environmental sciences. It coordinates some of the world’s most exciting research projects, tackling major issues such as climate change, environmental influences on human health, the genetic make-up of life on earth, and much more. NERC receives around £320 million a year from the government’s science budget, which it uses to fund independent research and training in universities and its own research centres.
CSIRO is Australia’s national science agency and one of the largest and most diverse research agencies in the world. CSIRO’s marine research – delivered through the Wealth from Oceans National Research Flagship – focuses on understanding our oceans and their biodiversity, resources and relationships with the climate system. The Flagship delivers practical science that enables governments, industries and communities to make informed decisions about the sustainable management of marine and coastal resources. Taking a whole-of-system approach to marine science, the Flagship contributes to national and international challenges where oceans play a central role.
Antarctic Climate & Ecosystem Cooperative Research Centre is a multidisciplinary partnership of 21 national and international organizations based at the University of Tasmania, Hobart. It provides science, knowledge and understanding to help Australia meet the challenges of climate change by understanding the crucial role played by Antarctica and the Southern Ocean in global climate, and the impacts of climate change on Australia and the world. The Centre informs governments, the community and scientists about climate change to guide Australia’s future.
Monday, May 21, 2012
60% reduction in 40 years in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents
Latest Southern Ocean research shows continuing deep ocean change
by John Hartz, Skeptical Science, May 21, 2012
This is a reprint of a press release posted by the Australian Commonwealth Science and Industrial Research Organization (CSIRO) May 4, 2012.
New research by teams of Australian and US scientists has found there has been a massive reduction in the amount of Antarctic Bottom Water found off the coast of Antarctica.
Deploying a mooring carrying a suite of monitoring sensors into the sea ice. Credit: Steve Rintoul
Comparing detailed measurements taken during the Australian Antarctic program's 2012 Southern Ocean marine science voyage to historical data dating back to 1970, scientists estimate there has been as much as a 60% reduction in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents.
In an intensive and arduous 25-day observing program, temperature and salinity samples were collected at 77 sites between Antarctica and Fremantle. Such ship transects provide the only means to detect changes in the deep ocean.
The new measurements, which have not yet been published, suggest the densest waters in the world ocean are gradually disappearing and being replaced by less dense waters.
"The amount of dense Antarctic Bottom Water has contracted each time we've measured it since the 1970s," said Dr Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC. "There is now only about 40% as much dense water present as observed in 1970."
The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.
"It's a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important," Dr Rintoul said.
The research was carried out by more than 50 scientists on the Australian Antarctic Division's research and resupply vessel Aurora Australis, which sailed to Commonwealth Bay, west along the Antarctic coast, and returned into Fremantle.
The Australian Antarctic Division's Chief Scientist, Dr Nick Gales, said the findings of the oceanographic study are profoundly important.
"Not only will this research improve our understanding of ocean currents, but will also feed into our knowledge of how the Southern Ocean and the Antarctic continent drives the world's climate processes," Dr Gales said.
Dr Rintoul was Chief Scientist on the recent voyage and has made a dozen voyages to the Southern Ocean. "When we speak of global warming, we really mean ocean warming: more than 90% of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean.
The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change" Dr Rintoul said. "A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future."
The causes of the observed changes in the Southern Ocean are not yet fully understood. Changes in winds, sea ice, precipitation, or melt of floating glacial ice around the edge of Antarctica may be responsible. Data collected on the latest voyage will help unravel this mystery.
A major challenge is the lack of observations at high latitude, where much of the ocean is covered by sea ice in winter. During the voyage scientists deployed nine drifting profilers, called Argo floats, which will transmit profiles of temperature and salinity every 10 days for the next five years. These ice-capable floats in the seasonal ice zone in the Australian sector of the Southern Ocean are funded through Australia's Integrated Marine Observing System.
"The Argo floats have revolutionised our ability to measure the ocean, particularly in winter when ship observations are very rare," said Dr Rintoul. "On this voyage, we deployed a new kind of float designed to survive encounters with the sea ice. These floats will allow us to see how dense water forms in winter for the first time."
The Aurora Australis visited Commonwealth Bay as part of a celebration of the centenary of Sir Douglas Mawson's Australian Antarctic Expedition. Dr Rintoul's team had the opportunity to repeat oceanographic measurements made by Mawson's team 100 years ago, obtaining one of the few century-long records obtained anywhere in the ocean.
"Our measurements collected in 2012 are quite different to those collected by Mawson in 1912," Dr Rintoul said. "This is an indication of a change in the ocean currents that may be related to a reduction in the amount of dense water formed near Antarctica."
"Mawson's expedition really marked the transition from the "Heroic Age" of Antarctic exploration to a period where science was the primary motivation for Antarctic expeditions. I think he would have gotten a real kick out of the idea that measurements made by his team a century ago are still useful and that Australian scientists are continuing his legacy by studying Antarctica and its connection to the rest of the globe."
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