From Eurekalert: Earth’s climate change 20,000 years ago reversed the circulation of the Atlantic Ocean
Global warming today could have similar effects on ocean currents and could accelerate climate change
The Atlantic Ocean circulation (termed meridional overturning circulation, MOC) is an important component of the climate system. Warm currents, such as the Gulf Stream, transport energy from the tropics to the subpolar North Atlantic and influence regional weather and climate patterns. Once they arrive in the North the currents cool, their waters sink and with them they transfer carbon from the atmosphere to the abyss. These processes are important for climate but the way the Atlantic MOC responds to climate change is not well known yet.
An international team of investigators under the leadership of two researchers from the UAB now demonstrates the response of the Atlantic MOC to climate change in the past. The new research results will be published on 4 November 2010 in the international front-line journal NATURE. The research project was led by Rainer Zahn (ICREA researcher) and Pere Masque, both of the UAB at the Institut de Ciència i Tecnologia Ambientals (ICTA) and Department of Physics. With collaborators at the universities of Seville, Oxford and Cardiff (UK) they investigated the distribution of isotopes in the Atlantic Ocean that are generated from the natural decay of uranium in seawater and are distributed with the flow of deep waters across the Atlantic basin. The young investigator Cesar Negre studied the natural abundance of these isotopes in the seafloor sediments 2.5 km deep in the South Atlantic and achieved a PhD degree in the Environmental Science and Technology doctoral programme at ICTA.
The study shows that the ocean circulation was very different in the past and that there was a period when the flow of deep waters in the Atlantic was reversed. This happened when the climate of the North Atlantic region was substantially colder and deep convection was weakened. At that time the balance of seawater density between the North and South Atlantic was shifted in such a way that deep water convection was stronger in the South Polar Ocean. Recent computer models simulate a reversal of the deep Atlantic circulation under such conditions while it is only now with the new data generated by UAB scientists and their colleagues from Seville and the UK that the details of the circulation reversal become apparent. This situation occurred during the ice age 20,000 years ago.
Although this was far back in time the results are relevant for our climate today and in the near future. The new study shows that the Atlantic MOC in the past was very sensitive to changes in the salt balance of Atlantic Ocean currents. Similar changes in seawater salt concentration are expected to occur in the North Atlantic in the course of climate warming over the next 100 years. Therefore the data to be published in Nature offer the climate modelling community the opportunity to calibrate their models and improve their capacity to predict reliably future ocean and climate changes.
The research has been funded by the Spanish Ministry for Science and Innovation (MICINN).
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This is just the usual press release with no link to the paper and no real scientific details to go with it, allowing compliant media minds to reprint it without questioning it. I’ll be interested to find out how they determined this reversal. If anybody locates a copy of the paper, please leave a note in comments.
I also think this headline could be true:
Atlantic Ocean ocean current reversal 20,000 years ago changed the Earth’s climate.
– Anthony
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Peter Taylor says:November 5, 2010 at 8:23 am
There are two factors that ‘warming’ might induce: warmer surface waters in the North Atlantic zone of downwelling; and freshening of the North Atlantic due to either glacial melt or increased river flows in the Arctic Basin.
It requires more precipitation to have more streamflow. More precipitation requires more evaporation/condensation from the ocean, which increases the salt content. More clouds take heat to the tropopause and reflect sunlight.
Warmer surface waters increase evaporation/precipitation increasing net deposition of snow/ice reducing net glacial ice loss.
Doesn’t add up.
Tim Clark:
I’d be the first to agree it doesn’t add up! But if data are accurate, then there was a freshening in the North Atlantic (recently ceased) and the sea surface area south of Greenland has got warmer and still is. Both of these could theoretically slow the THC.
As to the wider picture – there is data on increased glacial melt and run-off from Siberian rivers; a 14% increase in cloudiness in the Arctic Basin; and during the positive phase of the PDO increased precipitation (and warmth) in Alaska; but during this same period of about 25 years, global cloud cover decreased by about 4%.
So warmer seas globally seem to have produced less cloud globally (or vice versa – more likely!).
Personally, I think the tracking of cyclones and anticyclones influenced by the jetstream is key to understanding northern hemisphere dynamics – when solar UV flux was high from 1980-2003, the jetstream moved northward and the polar vortex was tight, when the Sun’s reduced its flux and extended that reduction for a year or more, the jetstream shifted south and the polar vortex expanded.
Charles Perry has published work suggesting that oceanic warm water pools create feedback via warm rising air that affects the jetstream – for example, a warm North Pacific during positive PDO, and perhaps therefore also during a warm North Atlantic during a positive AMO. I am not convinced of the potential of this feedback.
If you look at the jetstream right now, it is massively kinked and the kinks do not appear to correlate with SSTs – I still do not know what causes these kinks.
Take a look at:
http://www.squall.sfsu.edu/crws/jetstream.html
I would suggest that the contortions of the jetstream initiate cyclonic vortices and somehow also direct the anticyclone centres too. A more northerly jet will dump more rainfall in Fennoscandinavia and Siberia from the North Atlantic, and more in Alaska from the North Pacific. Greenland ice melt may be due to warmer seas.
It will be interesting to see what happens this winter, with the solar flux still relatively low – and a negative AMO with high pressure systems in the sub-Arctic, the jet should flatten and run due west-east into Iberia with a repeat of the devastating floods of last winter (especially in the Sierra Nevada). Sediment studies from southern Spain show this happened last during the Little Ice Age.
The problem is that globalised average data don’t provide much of a guide as to what happens in the two key regions of the northern Pacific and northern Atlantic – which I am convinced actually drive the long term cycles such as MWP/LIA and the shorter 60-70 year cycles such as the 1945-1975 cool period (now known NOT to be due to industrial aerosols – see my chapter on ‘natural cycles’ in ‘Chill’ for references).
“Global warming today could have similar effects on ocean currents and could accelerate climate change”
And 20,000 years ago what caused the temperatures to rise similar to today?
pkatt, 20,000 years ago, the continents were in the same position they are now. One of the potentially major differences would have been in Antarctica. The southern tip of South America nearly bridges with Antarctica (the sea floor there has a mountainous ridge). The massively thick floating sea ice may have touched the bottom of that narrow channel thus diverting the circumpolar Antarctic current into the Atlantic Ocean. That diversion would have significantly cooled the Atlantic. It would be an easy guess that this will happen again. I think the cyclical Milankovitch Antarctic ice buildup (and thus the diversion of that cold current into the Atlantic basin) is what cools the Atlantic enough to allow a growing ice cap in the Arctic. Hence the onset of an ice age. Just my musings.
Bill Yarber
Nov 4, 9:05 am
I have a hunch baased on Arctic sea temperatures that the strength of the MOC (and of an impotant component of the MOC – the gulf stream) oscillates with the AMO.
Thus we should see MOC strength coming back down soon.