Kein Grund zur Panik: Der oszillierende Golfstrom

Der Golfstrom liefert „Fernwärme“ für Westeuropa. Einige Klimaaktivisten malen Horrorszenarien an die Wand, dass sich der Golfstrom aufgrund des Klimawandels verlangsamt oder gar stoppt – mit fatalen Folgen für Europa. Andere Wissenschaftler sehen hierfür in den harten Daten jedoch keinen Hinweis. Den bisherigen Verlauf der Diskussion finden Sie in unserem dkS-Archiv. Heute im Blogpost: Was gibt es Neues zum Golfstrom?

Im Februar 2019 berichteten Lozier et al. in Science, dass die Modelle nicht einmal die wichtigsten Antriebe des Golfstroms korrekt quantifizieren. Der dominante Faktor sei nicht die Labradorsee östlich Kanadas, sondern das Nordmeer östlich von Grönland. Siehe auch Bericht in The Daily Caller.

Der Golfstrom ist offenbar gar nicht so anfällig gegen den Klimawandel wie gedacht. ScienceNews am 31. Januar 2019:

Climate change might not slow ocean circulation as much as thought

New findings from an international ocean observing network are calling into question the long-standing idea that global warming might slow down a big chunk of the ocean’s “conveyor belt.” The first 21 months of data from sensors moored across much of the North Atlantic are giving new insight into what controls the strength of the Atlantic Meridional Overturning Circulation, a system of currents that redistributes heat around much of the Western Hemisphere.

Researchers had thought the strength of that circulation, known by the acronym AMOC, was largely influenced by the sinking of cold freshwater in the Labrador Sea, between Greenland and Canada. And climate simulations suggest that the sea’s deepwater formation might slow as the world continues to warm — which also could slow down the entire Atlantic current system and possibly make temperatures on land in the northeastern United States and the United Kingdom plunge. That concept inspired the (otherwise unrealistic) 2004 climate apocalypse film The Day After Tomorrow.

Weiterlesen auf ScienceNews

Über fundamentale Verständnisprobleme handelt auch dieser MDR-Beitrag aus dem Juli 2018:

Bringt uns ein schwacher Golfstrom Wärme?
Eine neue Studie sorgt für Aufregung und viel Kritik. Darin geht es um den Golfstrom. Bisher hieß es: Wenn er schwächer wird und womöglich ganz ausbleibt, dann wird es kalt. Ein Forscherteam aus China und den USA behauptet nun das Gegenteil. Sie sagen, ein schwächer werdender Golfstrom heizt die globale Temperatur nochmal so richtig an.

Weiterlesen beim MDR.

Siehe auch Artikel zur Studie in der Welt. Anhänger der harten IPCC-Linie waren entsetzt. Klimareporter findet die Studie gar nicht gut und nennt sie „provokant“. Auch die dem Klimaalarm zugeneigte SZ war unzufrieden. Am besten ist es, die Ergebnisse ungefiltert direkt von den Autoren erklärt zu bekommen. Hier die dazugehörige Pressemitteilung der University of Washington vom 18. Juli 2018:

Atlantic Ocean circulation is not collapsing – but as it shifts gears, global warming will reaccelerate

A huge circulation pattern in the Atlantic Ocean took a starring role in the 2004 movie “The Day After Tomorrow.” In that fictional tale the global oceanic current suddenly stops and New York City freezes over. While many aspects of the movie are unrealistic, oceanographers are concerned about the long-term stability of the Atlantic Ocean circulation, and previous studies show that it has slowed dramatically in the past decade. New research from the University of Washington and the Ocean University of China finds the slowdown is not caused by global warming but is part of regular, decades-long cycle that will affect temperatures in coming decades. The paper [Chen & Tung 2018] was published July 18 [2018] in Nature.

“Climate scientists have expected the Atlantic overturning circulation to decline long-term under global warming, but we only have direct measurements of its strength since April 2004. And the decline measured since then is 10 times larger than expected,” said corresponding author Ka-Kit Tung, a UW professor of applied mathematics with an adjunct appointment in atmospheric sciences.

“Many have focused on the fact that it’s declining very rapidly, and that if the trend continues it will go past a tipping point, bringing a catastrophe such as an ice age. It turns out that none of that is going to happen in the near future. The fast response may instead be part of a natural cycle and there are signs that the decline is already ending.”

The results have implications for surface warming. The current’s speed determines how much surface heat gets transferred to the deeper ocean, and a quicker circulation would send more heat to the deep Atlantic. If the current slows down, then it will store less heat, and Earth will be likely to see air temperatures rise more quickly than the rate since 2000.

“The global climate models can project what’s going to happen long-term if carbon dioxide increases by a certain amount, but they currently lack the capability to predict surface warming in the next few decades, which requires a knowledge of how much the excess heat trapped by greenhouse gases is being absorbed by the oceans,” Tung said.

The Atlantic Meridional Overturning Circulation, or AMOC, is a conveyor belt that brings surface water northward in the Atlantic; from there, the heavier salty water sinks and returns at depth from the Labrador and Nordic seas, near the North Pole, all the way south to the Southern Ocean. Most people are interested in what happens at the surface — the Gulf Stream and associated Atlantic currents carry warmer water north, bringing mild temperatures to Western Europe.

But the new paper argues that the most important step, from a climate perspective, is what happens next. In the North Atlantic, the saltier water from the tropics sinks almost a mile (1,500 meters). As it does, it carries heat down with it away from the surface.

Changes in the strength of the AMOC affect how much heat leaves our atmosphere. The new study uses a combination of data from Argo floats, ship-based temperature measurements, tidal records, satellite images of sea-surface height that can show bulges of warm water, and recent high-tech tracking of the AMOC itself to suggest that its strength fluctuates as part of a roughly 60- to 70-year, self-reinforcing cycle.

When the current is faster, more of the warm, salty tropical water travels to the North Atlantic. Over years this causes more glaciers to melt, and eventually the freshwater makes the surface water lighter and less likely to sink, slowing the current.

When the AMOC is in a slow phase, the North Atlantic becomes cooler, ice melt slows, and eventually the freshwater melt source dries up and the heavier saltier water can plunge down again, which speeds up the whole circulation.

The new study argues that this current is not collapsing, but is just transitioning from its fast phase to its slower phase – and that this has implications for heating at the surface.

From 1975 to 1998, the AMOC was in a slow phase. As greenhouse gases were accumulating in the atmosphere, Earth experienced distinct warming at the surface. From about 2000 until now, the AMOC has been in its faster phase, and the increased heat plunging in the North Atlantic has been removing excess heat from the Earth’s surface and storing it deep in the ocean. “We have about one cycle of observations at depth, so we do not know if it’s periodic, but based on the surface phenomena we think it’s very likely that it’s periodic,” Tung said.

The new paper supports the authors’ previous research showing that since 2000, during which observations show a slowdown in surface warming, heat has accumulated deep in the Atlantic Ocean. The new study shows this is the same period when Atlantic overturning circulation was in its fast phase.

Recent measurements of density in the Labrador Sea suggest the cycle is beginning to shift, Tung said. That means that in coming years the AMOC will no longer be sending more of the excess heat trapped by greenhouse gases deep into the North Atlantic. “The good news is the indicators show that this slowdown of the Atlantic overturning circulation is ending, and so we shouldn’t be alarmed that this current will collapse any time soon,” Tung said. “The bad news is that surface temperatures are likely to start rising more quickly in the coming decades.”

The first author is Xianyao Chen at the Ocean University of China and Qingdao National Laboratory of Marine Science and Technology. The study was funded by the U.S. National Science Foundation, the Natural Science Foundation of China, the National Key Basic Research Program of China and a Frederic and Julia Wan Endowed Professorship.



Figure: The top panel shows global average surface temperature changes since 1950, with two periods of slower change and a period of rapid warming from 1975 to 2000. The lower panels show the strength of the Atlantic overturning circulation. The blue (and, on the right, purple) curve is the salinity north of 45N, an indirect measure, or proxy, for the AMOC strength. The green curve is an established proxy of AMOC.Ka-Kit Tung/University of Washington


Die Welt am 21. Juli 2019:

„Bereits seit den 40er-Jahren werden Tiefentemperaturen im Nordatlantik gemessen“, berichtet Professorin Monika Rhein von der Universität Bremen. Diese Temperaturen sind ein indirekter Hinweis auf die Stärke des Golfstroms. „Diese Messdaten zeigen starke Schwankungen, aber keinen Trend in irgendeine Richtung“, bilanziert die Ozeanografin die langen Messreihen. Eine Abnahme des Golfstroms ist also bislang nicht festzustellen.

Das Deutsche Klimakonsortium gab im Juli 2018 eine Broschüre zum Golfstrom heraus.

Thibodeau et al. 2018 analysierten Sauerstoffisotope von Foraminiferen eines Nordwestatlantischen Sedimentkerns und fanden, dass die Abschwächung des Golfstroms (AMOC) im 20. Jahrhundert in den 1970er Jahren zu einem Golfstrom-Minimum führte. Besonders schwach war der Golfstrom aber auch während der Kleinen Eiszeit, einer natürlichen Kaltphase. Siehe auch Bericht in der Daily Mail.

McCarthy et al. 2018 fanden, dass ein Teil der Variabilität des Golfstroms mit dem NAO-Ozeanzyklus zusammenhängt.

Yan et al. 2018 bemängeln, dass Klimamodelle die starke Variabilität des Golfstroms nur in sehr abgeschwächter Form abbilden, ein bedeutender Mangel der Simulationen.

Todd et al. 2018 dokumentieren kurzfristige Änderungen des Golfstroms nach starken Hurrikanen.

Good et al. 2018:

The observed AMOC overturning has decreased from 2004–2014, but it is unclear at this stage whether this is forced or is internal variability.

Lique & Thomas 2018 warnen, dass sich die Lage des Golfstroms im Laufe der Zeit ändern könnte. Bericht auf Scinexx dazu:

Nordatlantik: Klimawandel verschiebt Umwälzpumpe

Absinkzonen der atlantischen Umwälzströmung könnten sich stark verlagern

Verschobener Strömungsmotor: Der Klimawandel könnte die atlantische Umwälzströmung nicht nur schwächen, sondern auch verschieben, wie eine Simulation nun nahelegt. Die großen Absinkzonen warmen Wassers lägen dann nicht mehr vor Grönland wie heute, sondern im Nordpolarmeer und im subtropischen Atlantik. Das jedoch könnte die Strömung stark beeinflussen – und auch den Wärmeaustausch und die Pufferwirkung des Ozeans verändern, wie die Forscher im Fachmagazin „Nature Climate Change“ berichten.

Weiterlesen auf Scinexx