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19. August 2014: Has the puzzle of rapid climate change in the last ice age been solved? New report published in Nature shows that small variations in the climate system can result in dramatic temperature changes

Bremerhaven, Germany, August 19th, 2014.  Over the past one hundred thousand years cold temperatures largely prevailed over the planet in what is known as the last ice age. However, the cold period was repeatedly interrupted by much warmer climate conditions. Scientists have long attempted to find out why these drastic temperature jumps of up to ten degrees took place in the far northern latitudes within just a few decades. Now, for the first time, a group of researchers at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), have been able to reconstruct these climate changes during the last ice age using a series of model simulations. The surprising finding is that minor variations in the ice sheet size can be sufficient to trigger abrupt climate changes. The new study was published online in the scientific journal Nature last week and will be appearing in the 21 August print issue.

 

During the last ice age a large part of North America was covered with a massive ice sheet up to 3km thick. The water stored in this ice sheet is part of the reason why the sea level was then about 120 meters lower than today. Young Chinese scientist Xu Zhang, lead author of the study who undertook his PhD at the Alfred Wegener Institute, explains. “The rapid climate changes known in the scientific world as Dansgaard-Oeschger events  were limited to a period of time from 110,000 to 23,000 years before present. The abrupt climate changes did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today - they occurred during periods of intermediate ice volume and intermediate sea levels.” The results presented by the AWI researchers can explain the history of climate changes during glacial periods, comparing simulated model data with that retrieved from ice cores and marine sediments.

 

How rapid temperature changes might have occurred during times when the Northern Hemisphere ice sheets were at intermediate sizes (see graphics below)

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, even as far as the French coast.  Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions.

During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again.

“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” explains Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute, Germany. In doing so he illustrates the new study’s significance with regards to contemporary climate change. “At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes.”

At present, the extent of  Arctic sea ice is far less than during the last glacial period.  The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions. 

“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” summarises Gerrit Lohmann. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”

 

Notes for Editors:

The original paper was published in Nature under the following title :

Xu Zhang, Gerrit Lohmann, Gregor Knorr, Conor Purcell:Abrupt glacial climate shifts controlled by ice sheet changes. Nature, DOI: 10.1038/nature13592

 

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The Alfred Wegener Institute conducts research in the Arctic, Antarctic and oceans of the high and mid-latitudes. It coordinates polar research in Germany and provides major infrastructure to the international scientific community, such as the research icebreaker Polarstern and stations in the Arctic and Antarctica. The Alfred Wegener Institute is one of the 18 research centres of the Helmholtz Association, the largest scientific organisation in Germany.

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Abbildung C: Schematische Darstellung der heutigen Klimabedingungen auf der Nordhalbkugel.

In der Gegenwart ist die Ausdehnung des arktischen Meereises sehr viel geringer als in der letzten Eiszeit. Gleichzeitig verschwand der Laurentidische Eisschild als wichtigster Taktgeber für die eiszeitliche Ozeanzirkulation. Das heutige Klima verhält sich in Modellsimulationen deshalb sehr viel robuster gegen graduelle Veränderungen als in den Phasen mittlerer Vereisung und mittlerer Meersspiegelhöhen, in denen es während der letzten Eiszeit die rasanten Temperatursprünge auf der Nordhalbkugel gab. Abbildung: Alfred-Wegener-Institut

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Abbildung B: Die Nordhalbkugel im „Interstadial“ (kurze, warme Zwischenphasen während der Eiszeit)

In den lang anhaltenden Kaltphasen wuchsen die Eisschilde weiter an. Bei höheren Eisschilden über Nordamerika, wie sie dann in Zeiten mittlerer Meeresspiegelhöhen typisch waren, trennten sich die vorherrschenden Nordwestwinde in zwei Arme auf. Die Hauptwindströmung verlief nördlich des so genannten Laurentidischen Eisschildes und sorgte dafür, dass auch die Meereisgrenze vor der europäischen Küste sich gen Norden verlagerte. Im eisfreien Meer kann ein Wärmeaustausch zwischen Atmosphäre und Ozean stattfinden. Gleichzeitig trieb der Südarm der nordwestlichen Winde wärmeres Wasser in die eisfreien Bereiche des Nordost-Atlantiks und verstärkte dadurch den Wärmetransport in den Norden zusätzlich. Die veränderten Bedingungen kurbelten die Ozeanzirkulation an. Im Ergebnis führte ein hoher Laurentidischer Eisschild über Nordamerika also zu einer verstärkten Ozeanzirkulation und damit zu einem größeren Wärmetransport Richtung Norden. Das Klima auf der Nordhalbkugel wurde innerhalb weniger Jahrzehnte wärmer, bevor es aufgrund des dann einsetzenden Gletscherrückgangs über Nordamerika und den erneuten Änderungen der Windverhältnisse wieder abzukühlen begann.

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Abbildung A: Die Nordhalbkugel im „Stadial“ (Kaltphasen)

In der eiszeitlichen Kaltphase bedeckten riesige Eisschilde den Norden Amerikas und Europas. Starke Nordwestwinde trieben das arktische Meereis bis zur französischen Küste hinunter. Weil der ausgedehnte Eisdeckel auf dem Nordatlantik den Wärmeaustausch zwischen Luft und Meer unterband, fehlte den Meeresströmungen die starke Antriebskraft wie unter heutigen Bedingungen. Die Ozeanzirkulation als mächtiges Förderband der Weltmeere war deshalb sehr viel schwächer ausgeprägt als in der Gegenwart und transportierte nur wenig Wärme in den Norden.

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Figure C: Schematic depiction of current climate conditions in the Northern Hemisphere

At present, the extent of the Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Model simulations demonstrate that today’s climate is much more robust in resisting the changes which existed during phases of intermediate ice thickness and intermediate sea levels. It was then, during the last ice age, that the most rapid temperature swings in the Northern Hemisphere took place. Map: Alfred-Wegener-Institut

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Figure B: The Northern Hemisphere in a warm phase (a brief, warm interstadial phase during glacial climates)

During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing northwest winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again. Map: Alfred-Wegener-Institut

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Figure A: The Northern Hemisphere in a cold (stadial) phase

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. Map: Alfred-Wegener-Institut

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