Neues vom ungeliebten Hiatus

Die letzten anderthalb Jahrzehnte geriet die globale Erwärmung ins Stocken. Erst der kürzliche El Nino konnte den Hiatus beenden. Noch immer rätselt die Fachwelt, wie es zum Hiatus kommen konnte. Im Monatstakt erscheinen neue Papers zur Ursachenforschung. Im Januar 2017 publizierten Garfinkel und Kollegen eine Studie zur Erwärmungspause in Europa, deren Ursache sie in der Stratosphäre sehen, nämlich in einem geschwächten polaren Wirbel. Kurios: Ursache für die Abschwächung des Wirbels soll der Anstieg der Treibhausgase sein:

Stratospheric variability contributed to and sustained the recent hiatus in Eurasian winter warming
The recent hiatus in global-mean surface temperature warming was characterized by a Eurasian winter cooling trend, and the cause(s) for this cooling is unclear. Here we show that the observed hiatus in Eurasian warming was associated with a recent trend toward weakened stratospheric polar vortices. Specifically, by calculating the change in Eurasian surface air temperature associated with a given vortex weakening, we demonstrate that the recent trend toward weakened polar vortices reduced the anticipated Eurasian warming due to increasing greenhouse gas concentrations. Those model integrations whose stratospheric vortex evolution most closely matches that in reanalysis data also simulate a hiatus. While it is unclear whether the recent weakening of the midwinter stratospheric polar vortex was forced, a properly configured model can simulate substantial deviations of the polar vortex on decadal timescales and hence such hiatus events, implying that similar hiatus events may recur even as greenhouse gas concentrations rise.

Im April 2017 gab es dann ein Paper von Hedemann und Kollegen in Nature Climate Change. Zu den Autoren gehört übrigens auch Jochem Marotzke. Zunächst einmal wird die Existenz des Hiatus eingeräumt, ebenso die Tatsache, dass Klimamodelle ihn nicht haben kommen sehen. Als Auslöser wird dann eine ominöse Gleichgewichtsstörung am oberen Rand der Atmosphäre genannt. Schließlich wird noch erklärt, dass man das Phänomen wohl niemals aufklären werden wird. Das ist doch recht ernüchternd. Abstract:

The subtle origins of surface-warming hiatuses
During the first decade of the twenty-first century, the Earth’s surface warmed more slowly than climate models simulated1. This surface-warming hiatus is attributed by some studies to model errors in external forcing2,3,4, while others point to heat rearrangements in the ocean5,6,7,8,9,10 caused by internal variability, the timing of which cannot be predicted by the models1. However, observational analyses disagree about which ocean region is responsible11,12,13,14,15,16. Here we show that the hiatus could also have been caused by internal variability in the top-of-atmosphere energy imbalance. Energy budgeting for the ocean surface layer over a 100-member historical ensemble reveals that hiatuses are caused by energy-flux deviations as small as 0.08 W m−2, which can originate at the top of the atmosphere, in the ocean, or both. Budgeting with existing observations cannot constrain the origin of the recent hiatus, because the uncertainty in observations dwarfs the small flux deviations that could cause a hiatus. The sensitivity of these flux deviations to the observational dataset and to energy budget choices helps explain why previous studies conflict, and suggests that the origin of the recent hiatus may never be identified.

Etwas verständlicher sind da vielleicht zwei neue Papers (2017) eines Teams um Nicola Scafetta. Im ersten Teil beschreiben die Autoren die Rolle der Ozeanzyklen auf die Temperaturentwicklung. Hier der Abstract von Scafetta et al. 2017a (pdf hier):

Natural climate variability, part 1: Observations versus the modeled predictions
During the whole history of the planet, astronomical factors (orbital and solar variability) have determined the energy balance of the Earth and generated natural climate oscillations affecting the life of plants, animals and human beings at all time scales. During the last decades, severe concerns have been raised about whether human activities could have been so influential as to deeply modify the natural variability of the global climate and, in particular, could have caused a significant warming since the beginning of the 20th century. To properly address the latter issue, it is required to understand the phenomenology of the natural climate fluctuations. These are well emphasized by several climate indexes such as the Atlantic Multidecadal Oscillation, the Pacific Decadal Oscillation, the El Niño–Southern Oscillation and others. This complex natural dynamic is still not reproduced by the general circulation models (GCMs) supporting the Anthropogenic Global Warming Theory (AGWT), which is mainly advocated by the Intergovernmental Panel on Climate Change (IPCC). In this “part 1” of our work we briefly introduce the general topic and statistically compare observed and GCM modeled global surface warming trends from 1860 to 2016. We find that the models have significantly overestimated the observed warming during the historical record. In addition, we compare observed and modeled temperature trends of three significant periods: from Jan/1922 to Dec/1941, from Jan/1980 to Dec/1999 and from Jan/2000 to Dec/2016. We find that only during the 1980-1999 period the observed and synthetic records show compatible warming trends within the 95% confidence level. The severe discrepancy between observations and modeled predictions found during the 1922-1941 and 2000-2016 periods further confirms, according to the criteria proposed by the AGWT advocates themselves, that the current climate models have significantly exaggerated the anthropogenic greenhouse warming effect.

Im zweiten Teil wird der Hiatus diskutiert. Der kürzliche El Nino wird als Sonderereignis abgezogen. Er kann nicht als Beleg dafür herangezogen werden, dass sich die Erwärmung nach dem Hiatus nun fortsetzen würde. Hier der Abstract von Scafetta et al. 2017b (pdf hier):

Natural climate variability, part 2: Interpretation of the post 2000 temperature standstill
The period from 2000 to 2016 shows a modest warming trend that the advocates of the anthropogenic global warming theory have labeled as the “pause” or “hiatus.” These labels were chosen to indicate that the observed temperature standstill period results from an unforced internal fluctuation of the climate (e.g. by heat uptake of the deep ocean) that the computer climate models are claimed to occasionally reproduce without contradicting the anthropogenic global warming theory (AGWT) paradigm. In part 1 of this work, it was shown that the statistical analysis rejects such labels with a 95% confidence because the standstill period has lasted more than the 15 year period limit provided by the AGWT advocates themselves. Anyhow, the strong warming peak observed in 2015-2016, the “hottest year on record,” gave the impression that the temperature standstill stopped in 2014. Herein, the authors show that such a temperature peak is unrelated to anthropogenic forcing: it simply emerged from the natural fast fluctuations of the climate associated to the El Niño–Southern Oscillation (ENSO) phenomenon. By removing the ENSO signature, the authors show that the temperature trend from 2000 to 2016 clearly diverges from the general circulation model (GCM) simulations. Thus, the GCMs models used to support the AGWT are very likely flawed. By contrast, the semi-empirical climate models proposed in 2011 and 2013 by Scafetta, which are based on a specific set of natural climatic oscillations believed to be astronomically induced plus a significantly reduced anthropogenic contribution, agree far better with the latest observations.