Forschergruppe um Michael Mann: Es könnte noch bis zu zwanzig Jahre dauern, bis die Erwärmung wieder kräftig anzieht

In Diskussionen offenbaren Klimaaktivisten immer wieder erschreckende Wissenslücken. Unglaublich aber wahr: Es gibt immer noch Klimakämpfer, die nicht mitbekommen haben, dass die legendäre Hockeystick-Kurve nicht mehr existiert, von Michael Mann einige Jahre später selber aus dem Verkehr gezogen und durch eine realistischere Temperaturkurve für die vergangenen 1000 Jahre ersetzt worden ist. Mindestens genauso schlimm ist das Beharren darauf, es gäbe in den letzten anderthalb Jahrzehnten keinen Erwärmungshiatus. Dieser Irrglaube hat sich in den Hirnen offenbar tief festgesetzt. Hiatus bedeutet dabei auch nicht unbedingt komplette Erwärmungspause, sondern eine starke Verlangsamung, auf englisch “Slowdown”. Prognostiziert wurde eine Erwärmung von 0,2°C pro Jahrzehnt, eingetroffen ist sehr viel weniger.

Die Wissenschaft hat den Hiatus bzw. Slowdown bereits lange akzeptiert, da wirkt der einsame Kampf einiger Klimaaktivisten schon ziemlich lächerlich. Wer es immer noch nicht glauben wollte, konnte es am 11. Mai 2016 in der Mitgliederzeitschrift Eos der American Geophysical Union (AGU) schwarz auf weiß nachlesen:

Was the Recent Slowdown in Surface Warming Predictable?
The temporary deceleration in warming across the Northern Hemisphere earlier this century could not have been foreseen by statistical forecasting methods, a new study concludes.

From the early 2000s to the early 2010s, there was a temporary slowdown in the large-scale warming of Earth’s surface. Recent studies have ascribed this slowing to both internal sources of climatic variability—such as cool La Niña conditions and stronger trade winds in the Pacific—and external influences, including the cooling effects of volcanic and human-made particulates in the atmosphere.

Several studies have suggested that climate models could have predicted this slowdown and the subsequent recovery several years ahead of time—implying that the models can accurately account for mechanisms that regulate decadal and interdecadal variability in the planet’s temperature. To test this hypothesis, Mann et al. combined estimates of the Northern Hemisphere’s internal climate variability with hindcasting, a statistical method that uses data from past events to compare modeling projections with the already observed outcomes.

Weiterlesen in Eos.

Hockeystick-Autor Michael Mann räumt den Hiatus/Slowdown offen ein. Im Abstract der Arbeit, die im April 2016 in den Geophysical Research Letters erschien, werden die pazifischen und atlantischen Ozesanzyklen (PDO, AMO) als Ursache der fehlenden Erwärmung genannt:

Predictability of the recent slowdown and subsequent recovery of large-scale surface warming using statistical methods
The temporary slowdown in large-scale surface warming during the early 2000s has been attributed to both external and internal sources of climate variability. Using semiempirical estimates of the internal low-frequency variability component in Northern Hemisphere, Atlantic, and Pacific surface temperatures in concert with statistical hindcast experiments, we investigate whether the slowdown and its recent recovery were predictable. We conclude that the internal variability of the North Pacific, which played a critical role in the slowdown, does not appear to have been predictable using statistical forecast methods. An additional minor contribution from the North Atlantic, by contrast, appears to exhibit some predictability. While our analyses focus on combining semiempirical estimates of internal climatic variability with statistical hindcast experiments, possible implications for initialized model predictions are also discussed.

Bei aller Freude wundert man sich aber schon über die Behauptung, die pazifische Ozeanzyklen wären unverhersehbar. Das ist natürlich Quatsch, denn die PDO folgt allgemein einem klaren 60-Jahreszyklus, der die Temperaturnen systematisch beeinflusst. Die bestehenden Vorhersagemethoden berücksichtigen dies noch viel zu wenig. Das gleiche Forscherteam hatte übrigens bereits ein Jahr zuvor (Februar 2015) in Science den Hiatus/Slowdown durch Ozeanzyklen erklärt. Der ORF berichtete damals:

Kühler Pazifik bremst globale Erwärmung
Trotz Klimawandels sind die globalen Temperaturen in den vergangenen 15 Jahren kaum gestiegen. Die Ursache dafür ist in den Ozeanen zu finden, schreiben US-Forscher in einer Studie. Ein neuerlicher Anstieg der Temperaturen sei zu erwarten – spätestens in zwanzig Jahren.

Eine echte Offenbarung. Es könnte gemäß Ozeanzyklenverlauf noch zwanzig Jahre dauern, bis die Erwärmung wieder kräftig anzieht. Ob die Politik diese Aussage mitbekommen hat? Hier die entsprechende Pressemitteilung der Penn State University:

Interaction of ocean oscillations caused ‘false pause’ in global warming

The recent slowdown in climate warming is due, at least in part, to natural oscillations in the climate, according to a team of climate scientists, who add that these oscillations represent variability internal to the climate system. They do not signal any slowdown in human-caused global warming.

“We know that it is important to distinguish between human-caused and natural climate variability so we can assess the impact of human-caused climate change on a variety of phenomena including drought and weather extremes,” said Michael Mann, Distinguished Professor of Meteorology, Penn State. “The North Atlantic and North Pacific oceans appear to be drivers of substantial natural, internal climate variability on timescales of decades.”

Mann, Byron A. Steinman, assistant professor of geological sciences, University of Minnesota-Duluth and a former Penn State National Science Foundation postdoctoral fellow and Penn State researcher Sonya K. Miller looked at a combination of real-world observational data and state-of-the-art climate model simulations used in the most recent report of the Intergovernmental Panel on Climate Change to understand the competing contributions to climate variability in the Northern Hemisphere during the historic era. They report their results today (Feb 26) in Science.

The Atlantic Multidecadal Oscillation (AMO) describes how North Atlantic sea-surface temperatures tend to oscillate with a periodicity of about 50 to 70 years. The Pacific Decadal Oscillation (PDO) varies over a broader range of timescales. The researchers looked only at the portion of the PDO that was multidecadal — what they term the Pacific multidecadal oscillation (PMO).

Using a wide variety of climate simulations, the researchers found that the AMO and PMO are not significantly correlated; they are not part of the global “stadium wave” oscillation, as some researchers had claimed. What they found was that the Northern Hemisphere was warming more slowly, not because of the AMO — which has been relatively flat — but because of a sharply down-trending PMO.

The researchers conclude that the down-trending PMO and the unusual slowing of warming over the past decade are tied to heat burial beneath the tropical Pacific and a tendency for sustained La Niña type conditions. While there is paleoclimate data suggesting that this type of response could come from subtle features of climate change itself that climate models do not currently capture, the researchers note that the most likely explanation is the random excursions of the AMO.

“Our findings have strong implications for the attribution of recent climate changes,” said Mann. “Internal multidecadal variability in Northern Hemisphere temperatures likely offset anthropogenic warming over the past decade.”

The researchers conclude that given past historical patterns of variation in the AMO and PMO, this situation will likely reverse and add to human induced warming in the future.

Dabei ist es jedoch wohl erher Wunschdenken, dass AMO und PDO erratisch und ungekoppelt schwingen. Judith Curry hatte bereits überzeugend darlegen können, dass die Ozeanzyklen global zusammenhängen (siehe “La Ola im Ozean: Die klimazyklische Stadionwelle“).

Die von Mann und Kollegen behauptete Unvorhersagbarkeit des Hiatus/Slowdown wird von Kollegen angezweifelt. Eine Gruppe um Yoshimitsu Chikamoto veröffentlichte im April 2015 in Nature Communications eine Arbeit, die auf Basis der Entwicklung im Pazifik eine mehrjährige Prognose vorstellt. Hier die entsprechende Pressemitteilung der University of Hawaii – SOEST:

Extending climate predictability beyond El Niño

Tropical Pacific climate variations and their global weather impacts may be predicted much further in advance than previously thought, according to research by an international team of climate scientists from the USA, Australia, and Japan. The source of this predictability lies in the tight interactions between the ocean and the atmosphere and among the Atlantic, the Pacific and the Indian Oceans. Such long-term tropical climate forecasts are useful to the public and policy makers.

At present computer simulations can predict the occurrence of an El Niño event at best three seasons in advance. Climate modeling centers worldwide generate and disseminate these forecasts on an operational basis. Scientists have assumed that the skill and reliability of such tropical climate forecasts drop rapidly for lead times longer than one year.

The new findings of predictable climate variations up to three years in advance are based on a series of hindcast computer modeling experiments, which included observed ocean temperature and salinity data. The results are presented in the April 21, 2015, online issue of Nature Communications.

“We found that, even three to four years after starting the prediction, the model was still tracking the observations well,” says Yoshimitsu Chikamoto at the University of Hawaii at Manoa International Pacific Research Center and lead author of the study. “This implies that central Pacific climate conditions can be predicted over several years ahead.”

“The mechanism is simple,” states co-author Shang-Ping Xie from the University of California San Diego. “Warmer water in the Atlantic heats up the atmosphere. Rising air and increased precipitation drive a large atmospheric circulation cell, which then sinks over the Central Pacific. The relatively dry air feeds surface winds back into the Atlantic and the Indian Ocean. These winds cool the Central Pacific leading to conditions, which are similar to a La Niña Modoki event. The central Pacific cooling then strengthens the global atmospheric circulation anomalies.”

“Our results present a paradigm shift,” explains co-author Axel Timmermann, climate scientist and professor at the University of Hawaii. “Whereas the Pacific was previously considered the main driver of tropical climate variability and the Atlantic and Indian Ocean its slaves, our results document a much more active role for the Atlantic Ocean in determining conditions in the other two ocean basins. The coupling between the oceans is established by a massive reorganization of the atmospheric circulation.”

The impacts of the findings are wide-ranging. “Central Pacific temperature changes have a remote effect on rainfall in California and Australia. Seeing the Atlantic as an important contributor to these rainfall shifts, which happen as far away as Australia, came to us as a great surprise. It highlights the fact that on multi-year timescales we have to view climate variability in a global perspective, rather than through a basin-wide lens,” says Jing-Jia Luo, co-author of the study and climate scientist at the Bureau of Meteorology in Australia.

“Our study fills the gap between the well-established seasonal predictions and internationally ongoing decadal forecasting efforts. We anticipate that the main results will soon be corroborated by other climate computer models,” concludes co-author Masahide Kimoto from the University of Tokyo, Japan.