* Astronomy

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RE: Thermohaline Circulation

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Fears that a shutdown of ocean currents is about to plunge Europe into a mini ice age receded last week. New measurements have failed to show clear evidence that the current is weakening, and models of the North Atlantic show that a shutdown would not occur in the way oceanographers had expected.

Currents in the North Atlantic, dominated by the Gulf Stream, carry warm water north from the tropics towards Europe. During the winter, this water warms the westerly winds travelling from America, keeping the climate in western Europe milder than it would otherwise be.
The circulation is driven by density differences in the water arising from variations in temperature and salinity. Global warming reduces temperature differences, because higher latitudes warm more than the tropics, and salinity differences could be affected by increased mel****er from the Greenland ice sheet flowing into the sea.

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Ice Age Climate-change

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Continued warming of the Arctic Ocean

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New Weather Centre

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RE: Thermohaline Circulation

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New research shows that a rapid rise in global temperature 55 million years ago caused major disruption to ocean currents, and that the disruption took 140,000 years to reverse.

Writing in the journal Nature, the scientists say the phenomenon may be important for understanding the impact of present day climate warming.
Recent research suggests north Atlantic currents which bring heat to northern Europe may be weakening.
The new study, by Flavia Nunes and Richard Norris from the Scripps Institution of Oceanography in La Jolla, California, looked at tiny fossil animals called foraminifera in marine sediments from 14 ocean-floor locations around the world.
Analysing the ratios of two isotopes of carbon in the shells of these foraminifera allowed them to determine ocean current patterns at the time the creatures died.

The time in question was an extraordinary epoch in Earth history - the Palaeocene-Eocene Thermal Maximum (PETM), when the global average temperature rose by anything between four and seven Celsius in a few thousand years.
It has been cited as the reason for the spread of mammals around the world, and for the evolution of bats.
Computer models of modern climate suggest that temperature changes could affect ocean currents, and recent research has found indications that it is happening now in the north Atlantic.

But the disruption 55 million years ago took in more than a single ocean; the entire global system appears to have altered course.
Before the PETM, Nunes and Norris found, surface waters sank principally in the southern hemisphere, with deep currents then flowing north.
As temperatures rose, this pattern abruptly reversed. The new north-to-south system endured for 40,000 years, and currents took a further 100,000 to return to their previous polarity.
The reason why temperatures shot up during the PETM are unclear; but carbon dioxide concentrations in the atmosphere appear to have been extremely high, about a thousand times higher than currently.

The suspicion is that some kind of feedback mechanism may have been involved.

One theory is that an initial warming changed the distribution of heat in the oceans so that deposits of gas hydrates on the sea floor were released, with carbon dioxide and methane rising to the surface and entering the atmosphere, causing further greenhouse warming.
The new research provides some support for this theory, as well as demonstrating that abrupt temperature changes can have a long-term impact on ocean currents which are, as the Gulf Stream demonstrates, intimately tied to weather systems.
Some researchers have raised concern that release of gas hydrates could contribute to present-day global warming.

Evolutionary trends in coiling of tropical Paleogene planktic foraminifera
Richard D. Norris1 and Hiroshi Nishi
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The close link between temperatures in the North Atlantic and the strength of ocean circulation is underlined by a new analysis of sea-floor sediments.

The sediments were drilled from Blake Outer Ridge, from 30° to 24°N between the northern Bahamas and 71°W, off the US east coast.

They contain traces of naturally occurring radioactive atoms in ratios that are a giveaway for the speed of ocean waters going back 60,000 years.
The work by a team from the Woods Hole Oceanographic Institution is critical to our understanding of climate change.
If human activity alters circulation speeds, as many scientists suspect may happen, it could have a dramatic impact on temperatures.
This is of particular concern in northern Europe, which benefits from the strong flow of warm waters that sweep past it from the tropics and keep winters mild.

"We have to get more observational data and improve our models" - Andreas Schmittner, Oregon State University.

These waters eventually sink, or overturn, at high latitudes and return to the tropics as a deep, cold flow. If this great circulation, or conveyor, slows - and there is some evidence this is happening - then European winters should become harsher.

"Warm periods in the past are generally associated with strong ocean circulation, or overturning; and cold periods are generally associated with a weak overturning circulation. We have documented these changes in the past and associated them with abrupt climate change" - Dr Candace Major, from Woods Hole.

Dr Major was speaking at the American Geophysical Union (AGU) Fall Meeting.

She was presenting research by her team that builds on work reported last year by another group. The 2004 results linked past temperatures, worked out from Greenland ice cores, to the strength of ocean circulation for the past 20,000 years.

The new data greatly extends that record and reinforces the significance of the relationship.
It is done by using what scientists call a geochemical proxy - an indirect record of past activity.
In the case of ocean sediments, this is a ratio of two atoms - protactinium and thorium - that appear in the water as naturally occurring uranium undergoes radioactive decay.
It happens that as sediments fall through the water, thorium tends to "stick" to them and is buried; while the not-so-sticky protactinium has a tendency to be washed out of the North Atlantic basin.

"So, the stronger the circulation, the more the protactinium will be exported out of the North Atlantic and the less of it will be buried in the sediment. In the case of a sluggishly moving ocean, the more will be buried in the sediments" - Dr Candace Major.

Dr Major displayed graphs here tracking temperature and ocean circulation from about 25,000 to 60,000 years ago that were beautifully aligned - they were virtually one for one.

The big question for science now is, which way will the graphs go in the future?

Modelling indicates global warming will slow North Atlantic circulation.
What happens is that as Arctic ice melts and Arctic rivers flow faster - trends which have both been documented - the northern ocean fills with fresh water and becomes less salty.
Less salinity means a lower density; the waters then cannot overturn, so the circulation weakens.
The previous modelling work had predicted that if this "great conveyor" turned off completely, Europe would cool by perhaps four to six degrees Celsius.

And Andreas Schmittner, of Oregon State University, came to the AGU to present the results of the very latest work in this field - a broad analysis that combined nine different computer models of future climate change. These suggested there would not be a shutdown in the next 100 years, but rather a slowdown of about 25%.
This produced a fair amount of muttering among journalists in the hall who just this past fortnight had reported on actual measurements collected in the North Atlantic which showed there had already been a 30% weakening in strength in the past 50 years.

"This is an apparent contradiction between the models and the observations. We have to reconcile these differences, obviously. We have to get more observational data and improve our models" - Andreas Schmittner.

"It could be that the models are pointing - if you like - to average trends in ocean circulation, but that there could be significant inter-annual variability which is what the observational data has picked up; we just don't know" - Dr Candace Major.

From BBC article

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Scientists have found that there is a 70 percent chance of shutting down the thermohaline circulation in the North Atlantic Ocean over the next 200 years, with a 45 percent probability of this occurring in this century. The likelihood decreases with mitigation, but even the most rigorous immediate climate policy would still leave a 25 percent chance of a thermohaline collapse.

“This is a dangerous, human-induced climate change. The shutdown of the thermohaline circulation has been characterized as a high-consequence, low-probability event. Our analysis, including the uncertainties in the problem, indicates it is a high-consequence, high-probability event.” - Michael Schlesinger, professor of atmospheric sciences at the University of Illinois at Urbana-Champaign.

Schlesinger will present a talk “Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation” on Dec. 8 at the United Nations Climate Control Conference in Montreal. He will discuss recent work he and his colleagues performed on simulating and understanding the thermohaline circulation in the North Atlantic Ocean.
The thermohaline circulation is driven by differences in seawater density, caused by temperature and salinity. Like a great conveyor belt, the circulation pattern moves warm surface water from the southern hemisphere toward the North Pole. Between Greenland and Norway, the water cools, sinks into the deep ocean, and begins flowing back to the south.

“This movement carries a tremendous amount of heat northward, and plays a vital role in maintaining the current climate. If the thermohaline circulation shut down, the southern hemisphere would become warmer and the northern hemisphere would become colder. The heavily populated regions of eastern North America and western Europe would experience a significant shift in climate.” - Michael Schlesinger.

Higher temperatures caused by global warming could add fresh water to the northern North Atlantic by increasing the precipitation and by melting nearby sea ice, mountain glaciers and the Greenland ice sheet. This influx of fresh water could reduce the surface salinity and density, leading to a shutdown of the thermohaline circulation.

“We already have evidence dating back to 1965 that shows a drop in salinity around the North Atlantic. The change is small, compared to what our model needs to shut down the thermohaline, but we could be standing at the brink of an abrupt and irreversible climate change.” -Michael Schlesinger.

To analyse the problem, Schlesinger and his colleagues first used an uncoupled ocean general circulation model and a coupled atmosphere-ocean general circulation model to simulate the present-day thermohaline circulation and explore how it would behave in response to the addition of fresh water.
They then used an extended, but simplified, model to represent the wide range of behaviour of the thermohaline circulation. By combining the simple model with an economic model, they could estimate the likelihood of a shutdown between now and 2205, both with and without the policy intervention of a carbon tax on fossil fuels. The carbon tax started out at $10 per ton of carbon (about five cents per gallon of gasoline) and gradually increased.

“We found that there is a 70 percent likelihood of a thermohaline collapse, absent any climate policy. Although this likelihood can be reduced by the policy intervention, it still exceeds 25 percent even with maximal policy intervention.” - Michael Schlesinger.

Because the risk of a thermohaline collapse is unacceptably large, “measures over and above the policy intervention of a carbon tax – such as carbon capture and sequestration – should be given serious consideration.”

Collaborators on this research are U. of I. research programmer Bin Li, Princeton University researchers Sergey Malyshev and Jianjun Yin, University of Michigan research scientist Natasha Andronova, and Wesleyan University economics professor Gary Yohe.

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