Am 23. April 2016 berichtete Volker Mrasek, den wir auch aus dem Deutschlandfunk kennen, im SRF über das ARGO-Messnetz in den Weltozeanen, mit dem wichtige Informationen zum ozeanischen Wärmeinselinhalt (Ocean Heat Conent) gewonnen werden. In der Ankündigung der Sendung heißt es:
Argo Tauchroboter Messnetz
Wir wissen mehr über den Mond, als über die Ozeane, so hört man manchmal. Doch unbemerkt von der Öffentlichkeit füllt eine riesige Flotte von Tausenden von Mini-U-Booten diese Datenlücke. Argo heisst das Projekt, und es feiert gerade den 15-Geburtstag.
Ein interessanter Beitrag, der in der Sendung bei Minute 13:35 beginnt und hier online nachzuhören ist. Einziger Kritikpunkt: Im Beitrag wird so ganz lässig nebenbei die ziemlich gewagte These aufgestellt, das ARGO System hätte eindeutig bewiesen, dass die fehlende Wärme aus dem Temperatur-Hiatus gänzlich im Meer verschwunden wäre, das Rätsel somit endgültig gelöst wäre. Das ist natürlich murks und ein Schlag ins Gesicht all jener Klimaforscher, die noch immer auf der Suche nach einer guten Erklärung für den Hiatus sind. Siehe z.B. „Michael Mann, Ben Santer, Gerald Meehl: Erwärmungshiatus existiert und ist erklärungsbedürftig. Klimamodelle müssen auf den Prüfstand„.
Aber wir nehmen den SRF-Beitrag gerne zum Anlass, uns in Punkto Ozeanwärme auf den neuesten Stand zu bringen. Das haben wir letztmalig im August 2014 getan, als wir in der Tiefe des Ozeans eine gänzlich andere Geschichte entdeckten (siehe „Suche nach der angeblich im Ozean versunkenen Wärme endet mit Fehlschlag: Tiefe Meeresschichten kühlten sich in den letzten 20 Jahren ab„). Der Beitrag wurde auch auf The Hockeystick in englischer Sprache veröffentlicht.
Zunächst schauen wir bei Ole Humlum auf Climate4You vorbei, der die Temperaturentwicklung der Weltozeane für die obersten 700m geplottet hat:
Wie wir bereits in unserer Übersicht 2014 festgestellt hatten, gibt es in den oberen Wasserschichten in der Tat keinen Erwärmungshiatus. Die obersten 700m der Weltzozeane erwärmen sich seit Mitte der 1980er Jahre, nachdem sie sich zuvor leicht abgekühlt hatten. Am 19. Januar 2016 meldete die Webplattform Wissenschaft Aktuell:
Klimawandel: Ozeane heizen sich immer schneller auf
Etwa die Hälfte der zusätzlichen Wärme nahmen die Weltmeere in den vergangenen 20 Jahren auf […] Doch heizten sich die Weltmeere besonders in den vergangenen beiden Jahrzehnten besonders schnell auf. Über dieses Ergebnis, das für zukünftige Klimamodelle von großer Bedeutung ist, berichten nun amerikanische Wissenschaftler in der Fachzeitschrift „Nature Climate Change“. „In jüngster Zeit haben sich die Ozeane substanziell erwärmt und das Wärmesignal reicht in immer tiefere Wasserschichten“, sagt Peter Gleckler vom Lawrance Livermore National Laboratory.Ganzen Artikel auf Wissenschaft Aktuell lesen.
Bereits im Juli 2015 hatte die Süddeutsche Zeitung getitelt:
Meere heizen sich zu schnell auf
Aber weiß man das so genau, wenn das ARGO-Messnetz erst 15 Jahre alt ist? Und wie sieht es unterhalb von 700m aus? Kann die anhaltende Erwärmung der oberen 700m den Hiatus erklären? Wir begeben uns auf Spurensuche.
Im August 2014 erschien im Journal of Physical Oceanography ein Paper von Carl Wunsch und Patrick Heimbach, das den ozeanischen Wärmeinhalt der letzten zwei Jahrzehnte beschrieb.
Bidecadal Thermal Changes in the Abyssal Ocean
A dynamically consistent state estimate is used for the period 1992–2011 to describe the changes in oceanic temperatures and heat content, with an emphasis on determining the noise background in the abyssal (below 2000 m) depths. Interpretation requires close attention to the long memory of the deep ocean, implying that meteorological forcing of decades to thousands of years ago should still be producing trendlike changes in abyssal heat content. Much of the deep-ocean volume remained unobserved. At the present time, warming is seen in the deep western Atlantic and Southern Oceans, roughly consistent with those regions of the ocean expected to display the earliest responses to surface disturbances. Parts of the deeper ocean, below 3600 m, show cooling. Most of the variation in the abyssal Pacific Ocean is comparatively featureless, consistent with the slow, diffusive approach to a steady state expected there. In the global average, changes in heat content below 2000 m are roughly 10% of those inferred for the upper ocean over the 20-yr period. A useful global observing strategy for detecting future change has to be designed to account for the different time and spatial scales manifested in the observed changes. If the precision estimates of heat content change are independent of systematic errors, determining oceanic heat uptake values equivalent to 0.1 W m−2 is possibly attainable over future bidecadal periods.
Im Paper erteilen die beiden Spezialisten der Deutschlandfunk-Hypothese eine klare Absage. Es gibt noch viel zu wenig Daten, insbesondere aus den tiefen Ozeanschichten, um die fehlende atmosphärischen Erwärmung durch versunkene Ozeanwärme zu erklären:
Direct determination of changes in oceanic heat content over the last 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalize e.g., the apparent “pause” in warming.
Im Oktober 2014 dann zwei weitere Paukenschläge. Llovel et al. und Durack et al. veröffentlichen in Nature Climate Change zwei Papers, die ebenfalls den Schluss zulassen, dass die Erwärmungspause an der Erdoberfläche nicht mit der Ozeanwärme gelöst werden kann. Während sich die oberen Wasserschichten schneller erwärmt haben als gedacht, kühlten sich die tiefen Wasserschichten unterhalb von 2 km überraschenderweise eher ab. In den Tiefen des Ozeans kann sich daher die vermisste Wärme nicht verstecken. Reporting Climate Science (via Judith Curry) hat die Papers zusammenfassend kommentiert:
The implication of this is that a build up of heat in the deep oceans is not the solution to the so called missing energy mystery that has puzzled climate scientists trying to match the observed heat build up on the planet with what the theory of global warming suggests should be happening. A number of climate scientists had previously suggested that heat is accumulating in the deep oceans and that this accounts for the missing energy.
An analysis of ocean data together with satellite measurements suggests that the warming rate for the top 700m of ocean in the southern hemisphere has been underestimated – at least from 1970 until the early 2000s when an array of measurement buoys, known as Argo, began to collect data.
Separately, an analysis of satellite measurements and ocean temperature data has revealed that that the deeper half of the ocean (below 2 km depth) has, on average, not warmed from 2005 to 2013 and may have cooled – in contrast to the prevailing view, based on sparse ship-based measurements, that had suggested deep ocean warming between the 1990s and 2005.
Papers relating to both pieces of research have been published in Nature Climate Change. Both papers are important because they shed light on the debate around the so called missing energy mystery. Essentially, they imply that heat has accumulated faster than had been thought in the upper ocean but not, as many have suggested, in the deeper ocean below 2km.
Die NASA gab am 6. Oktober 2014 die folgende Pressemitteilung zur Llovel et al.-Studie heraus:
NASA Study Finds Earth’s Ocean Abyss Has Not Warmed
The cold waters of Earth’s deep ocean have not warmed measurably since 2005, according to a new NASA study, leaving unsolved the mystery of why global warming appears to have slowed in recent years.
Scientists at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, analyzed satellite and direct ocean temperature data from 2005 to 2013 and found the ocean abyss below 1.24 miles (1,995 meters) has not warmed measurably. Study coauthor Josh Willis of JPL said these findings do not throw suspicion on climate change itself.
„The sea level is still rising,“ Willis noted. „We’re just trying to understand the nitty-gritty details.“
In the 21st century, greenhouse gases have continued to accumulate in the atmosphere, just as they did in the 20th century, but global average surface air temperatures have stopped rising in tandem with the gases. The temperature of the top half of the world’s oceans — above the 1.24-mile mark — is still climbing, but not fast enough to account for the stalled air temperatures.
Many processes on land, air and sea have been invoked to explain what is happening to the „missing“ heat. One of the most prominent ideas is that the bottom half of the ocean is taking up the slack, but supporting evidence is slim. This latest study is the first to test the idea using satellite observations, as well as direct temperature measurements of the upper ocean. Scientists have been taking the temperature of the top half of the ocean directly since 2005, using a network of 3,000 floating temperature probes called the Argo array.
„The deep parts of the ocean are harder to measure,“ said JPL’s William Llovel, lead author of the study published Sunday in the journal Nature Climate Change. „The combination of satellite and direct temperature data gives us a glimpse of how much sea level rise is due to deep warming. The answer is — not much.“
The study took advantage of the fact that water expands as it gets warmer. The sea level is rising because of this expansion and the water added by glacier and ice sheet melt.
To arrive at their conclusion, the JPL scientists did a straightforward subtraction calculation, using data for 2005-2013 from the Argo buoys, NASA’s Jason-1 and Jason-2 satellites, and the agency’s Gravity Recovery and Climate Experiment (GRACE) satellites. From the total amount of sea level rise, they subtracted the amount of rise from the expansion in the upper ocean, and the amount of rise that came from added meltwater. The remainder represented the amount of sea level rise caused by warming in the deep ocean. The remainder was essentially zero. Deep ocean warming contributed virtually nothing to sea level rise during this period.
Coauthor Felix Landerer of JPL noted that during the same period warming in the top half of the ocean continued unabated, an unequivocal sign that our planet is heating up. Some recent studies reporting deep-ocean warming were, in fact, referring to the warming in the upper half of the ocean but below the topmost layer, which ends about 0.4 mile (700 meters) down.
Landerer also is a coauthor of another paper in the same journal issue on 1970-2005 ocean warming in the Southern Hemisphere. Before Argo floats were deployed, temperature measurements in the Southern Ocean were spotty, at best. Using satellite measurements and climate simulations of sea level changes around the world, the new study found the global ocean absorbed far more heat in those 35 years than previously thought — a whopping 24 to 58 percent more than early estimates.
Both papers result from the work of the newly formed NASA Sea Level Change Team, an interdisciplinary group tasked with using NASA satellite data to improve the accuracy and scale of current and future estimates of sea level change. The Southern Hemisphere paper was led by three scientists at Lawrence Livermore National Laboratory in Livermore, California.
NASA monitors Earth’s vital signs from land, air and space with a fleet of satellites and ambitious airborne and ground-based observation campaigns. NASA develops new ways to observe and study Earth’s interconnected natural systems with long-term data records and computer analysis tools to better see how our planet is changing. The agency shares this unique knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.
Im Mai 2015 konnte das Forscherquartett Liang, Wunsch, Heimbach und Forget dann sogar im Journal of Climate zeigen, dass sich die tiefen Ozeanschichten derzeit abkühlen und Wärme an obere Wasserstockwerke abgeben. Die Wissenschaftler weisen daraufhin, dass sich die Ozeane zu Beginn des anthropogenen Klimawandels keineswegs in einem Gleichgewichtszustand befanden und dies berücksichtigt werden muss. Hier der Abstract:
Vertical Redistribution of Oceanic Heat Content
Estimated values of recent oceanic heat uptake are on the order of a few tenths of a W m−2, and are a very small residual of air–sea exchanges, with annual average regional magnitudes of hundreds of W m−2. Using a dynamically consistent state estimate, the redistribution of heat within the ocean is calculated over a 20-yr period. The 20-yr mean vertical heat flux shows strong variations in both the lateral and vertical directions, consistent with the ocean being a dynamically active and spatially complex heat exchanger. Between mixing and advection, the two processes determining the vertical heat transport in the deep ocean, advection plays a more important role in setting the spatial patterns of vertical heat exchange and its temporal variations. The global integral of vertical heat flux shows an upward heat transport in the deep ocean, suggesting a cooling trend in the deep ocean. These results support an inference that the near-surface thermal properties of the ocean are a consequence, at least in part, of internal redistributions of heat, some of which must reflect water that has undergone long trajectories since last exposure to the atmosphere. The small residual heat exchange with the atmosphere today is unlikely to represent the interaction with an ocean that was in thermal equilibrium at the start of global warming. An analogy is drawn with carbon-14 “reservoir ages,” which range from over hundreds to a thousand years.
