Das Spiel ist aus: Niederländische Forscher entzaubern Rahmstorfs Golfstrom-Alarmgeschichte

Im März 2015 verbreitete Stefan Rahmstorf (“stefan”) über das IPCC-nahe Klimablog Realclimate eine windige Alarmstory. Eine Kälteanomalie im Atlantik südöstlich vor Grönland wäre der Beweis dafür, dass der Golfstrom langsam schwächer würde, eine Folge der menschengemachten Klimaerwärmung! “stefan” schrieb seinerzeit auf Realclimate:

What’s going on in the North Atlantic?
The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al. 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years. [...] It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway.

Dazu zeigt Rahmstorf das folgende Bild (“Linear temperature trend from 1900 to 2013″):

 

In seinem Beitrag skizzierte der Postdamer PIK-Forscher ein wahres Klimahorrorszenario und suggerierte, dass alles noch viel schlimmer kommen würde als sowieso schon befürchtet.

In der Folge machten sich internationale Fachkollegen an die Validierung der Rahmstorfschen Schreckensthesen. Gut ein Jahr später liegt nun das Ergebnis der Überprüfung vor. Am 29. Juni 2016 gaben Femke de Jong und Laura de Steur vom niederländischen NIOZ-Institut auf der OSNAP-Projektseite im Rahmen einer Pressemitteilung  bekannt, dass Rahmstorf mit seinem Modell wohl heftig daneben lag. Die insbesondere im Winter 2014/15 beobachtete nordatlantische Kälteanomalie hat nichts mit einem schwächelnden Golfstrom zu tun, sondern ist vielmehr Folge von starker vertikaler Vermischung mit dem kalten Tiefenwasser. Im Folgenden die Pressemitteilung:

A new record in mixing of surface and deep ocean water in the Irminger Sea has important consequences for the Atlantic overturning circulation 

Scientists Femke de Jong and Laura de Steur of the NIOZ Royal Netherlands Institute for Sea Research have shown that the recent temperature changes in the Irminger Sea between Iceland and Greenland can be explained through regional ocean-atmosphere interaction during the cold winter of 2014-2015. This rejects a hypothesis that posed that increased meltwater from Greenland weakened deep water formation and caused the cold blob. The article by de Jong and de Steur is accepted by Geophysical Research letters and has appeared online.

Deep water formation is an important process in the global ocean circulation. When high latitude winters are cold enough, the salty surface water of the North Atlantic cools enough to increase its density and mix with underlying deeper waters. This mixing is called deep convection. It forms the vertical link between the warm northward flow near the surface and the cold southward flow along the bottom, which is generally referred to as the Atlantic Meridional Overturning Circulation. The transport of warm water in the overturning circulation is partly responsible for the mild climate in northwestern Europe compared to similar latitudes in North America.

In climate model simulations the strength of convection south of Greenland is related to the strength of the overturning circulation. Some models predict that convection will weaken due to the input of freshwater released from the melting ice on Greenland. Because of the much lower density of freshwater compared to seawater it forms a barrier that isolates the deep ocean from the cold atmosphere. While the majority of the Earth warms as a result of climate change the region around southern Greenland would cool.

In temperature observations of the earth’s surface in 2015 a similar pattern seems to appear. The Earth warmed while the ocean southeast of Greenland cooled. This led to speculation that convection had already weakened as a result of increased melting of Greenland’s icecap. It would mean that the overturning circulation would be affected faster than expected.

Instruments moored in the Irminger Sea, southeast of Greenland, shows that this is not (yet) the case. This mooring, deployed by the Royal Netherlands Institute for Sea Research (NIOZ) at 3 km depth in 2003, shows that convection strengthened in recent years. Record depths of convection were observed in the Irminger Sea in the winter of 2014-2015. This strong mixing was caused by an extremely cold and long winter. Two NIOZ scientists, Femke de Jong and Laura de Steur, have shown that the temperature evolution in the Irminger Sea (including the strong decrease in 2015) can be explained through regional interaction between the ocean and atmosphere. The manuscript that describes the convection and explains the temperature changes is accepted in Geophysical Research Letters. This coincides with a publication by a German group in Nature Geoscience this week, in which they use a model to show that it will take some time before freshwater from Greenland enters the deep water formation regions in large enough amount to weaken convection.

The measurements in the Irminger Sea were partly funded by the European North Atlantic CLIMate (NACLIM) project and are part of the international OSNAP (Overturning in the Subpolar North Atlantic Program). The goal of OSNAP is to quantify the overturning circulation and its heat transport at high latitudes and to establish a relation to convection and wind forcing. On the 26th of July a research expedition will return to the Irminger Sea to recover the NIOZ and other OSNAP moorings and do a hydrographic survey. This cruise can be followed on the OSNAP blog at www.o-snap.org. A short video documentary of last year’s cruise can be found on youtube (https://www.youtube.com/watch?v=a-lhCIQjE4c).

Im Abstract zum Paper in den Geophysical Research Letters lassen Femke de Jong und Laura de Steur keinen Zweifel daran, dass der Golfstrom weiterhin verlässlich strömt:

Strong winter cooling over the Irminger Sea in winter 2014–2015, exceptional deep convection, and the emergence of anomalously low SST
Deep convection is presumed to be vital for the North Atlantic Meridional Overturning Circulation, even though observational evidence for the link remains inconclusive. Modeling studies have suggested that convection will weaken as a result of enhanced freshwater input. The emergence of anomalously low sea surface temperature in the subpolar North Atlantic has led to speculation that this process is already at work. Here we show that strong atmospheric forcing in the winter of 2014–2015, associated with a high North Atlantic Oscillation (NAO) index, produced record mixed layer depths in the Irminger Sea. Local mixing removed the stratification of the upper 1400 m and ventilated the basin to middepths resembling a state similar to the mid-1990s when a positive NAO also prevailed. We show that the strong local atmospheric forcing is predominantly responsible for the negative sea surface temperature anomalies observed in the subpolar North Atlantic in 2015 and that there is no evidence of permanently weakened deep convection.

Ein weiterer wissenschaftlicher Fehlschlag für Stefan Rahmstorf und seine Potsdamer Klimaalarmschmiede. Es ist erfrischend zu sehen, dass sich die internationale Forschergemeinde mit steigender Tendenz gegen die fragwürdigen PIK-Kreationen wehrt. Es wäre an der Zeit, dass die deutsche Presse im Sinne der Ausgewogenheit auch einmal über diese Ergebnisse berichten würde. Die Washington Post hat am 30. Juni 2016 vorgemacht wie es geht:

The mysterious ‘cold blob’ in the North Atlantic Ocean is starting to give up its secrets
[...] Stefan Rahmstorf, an ocean physicist with the Potsdam Institute for Climate Impact Research and author of the study mentioned above, has writtenthat very cold temperatures in the subpolar North Atlantic Ocean in the winter of 2014-2015 “suggests the decline of the circulation has progressed even further now than we documented in the paper.” But in a new study in Geophysical Research Letters reporting on deep ocean measurements from this region, two researchers present an alternative interpretation. They say that they found “exceptional” levels of deep ocean convection, or mixing of surface waters with deep waters of a sort that helps drive the overturning circulation, during in the winter of 2014-2015 — the height of the cold “blob.” And they attribute that temperature phenomenon to natural climate variability, driven by local weather and winds. “We find that the observed temperature variability is explained without invoking a trend in the lateral heat transport that would be representative of an AMOC slowdown,” Femke de Jong and Laura de Steur of the Royal Netherlands Institute for Sea Research write in the paper. [...]

Ganzen Artikel in der Washington Post lesen.