Um Antwort wird gebeten: Warum verschweigt das Geesthachter Institut für Küstenforschung die Mittelalterliche Wärmeperiode in seiner Pressemitteilung zum Zweiten Ostee-Klimazustandsbericht?

An: Frau Jessica Klepgen, Pressestelle des Helmholtz-Zentrums Geesthacht, Zentrum für Material- und Küstenforschung
Von: Sebastian Sebastian Lüning

Gesendet: 6.6.2015
Antwort: 10.6.2015 und 15.6.2015 (siehe unten)

Sehr geehrte Frau Klepgen,

Mit Interesse sah ich Ihre Pressemitteilung zum neuen Ostsee-Klimabericht.

In Ihrer Pressemitteilung thematisieren Sie in korrekter Weise die starke Erwärmung des Ostseeraumes während der vergangenen 150 Jahre seit Ende der Kleinen Eiszeit und verwenden hierzu sogar eine plakative Zwischenüberschrift “Erwärmung schreitet voran”.

Unerwähnt bleibt in Ihrer Pressemitteilung jedoch, dass der Klimabericht auch zwei höchst interessante Kapitel zum Paläoklima der vergangenen 12.000 Jahre bzw. 1.000 Jahre enthält. Weshalb war Ihnen dies keine Erwähnung wert? Dankenswerterweise ist der Bericht Open Source so dass ich mir die beiden Kapitel im Detail anschauen konnte. Besonders beeindruckt hat mich die Passage zum Klima der Zeitspanne 8000-4500 Jahre vor heute. Damals war es laut Bericht um 1,0-3,5°C wärmer als heute:

…a warm and stable climate with air temperature 1.0–3.5 °C above modern levels (8000–4500 cal year BP)…”.

Interessant ist auch das Kapitel zur Mittelalterlichen Wärmeperiode. Hierzu heißt es:

“Recent investigations of Fennoscandia by Ljungqvist (2010) showed that the MWP [Medieval Warm Period] occurred between 800 and 1300. At that time, warm-season (May-September) temperatures exceeded the contemporary warming of the end of twentieth century by about +0.5°C.”

Laut Bericht war also die Mittelalterliche Wärmeperiode (MWP) im Ostseeraum wärmer als heute bzw. zumindest so warm wie heute.

Meine Frage an Sie: Weshalb lassen Sie diese wichtigen Kontextinformationen in Ihrer Pressemitteilung aus? Weshalb war es zur Zeit der MWP bereits schon einmal so warm wie heute, obwohl der atmosphärische CO2-Gehalt damals außerordentlich niedrig war? Mir geht es nicht darum, dem CO2 seine Klimawirkung abzusprechen, die es sicherlich hat. Jedoch sollten wir die aktuelle Klimaentwicklung stets im Kontext der vorindustriellen Klimadynamik sehen. Ich habe den Eindruck, dass Sie sich aus mir unbekannten Gründen vor diesem Vergleich scheuen.

Ich würde ich über eine Antwort Ihrerseits freuen. Im Sinne der Transparenz möchte ich Ihre Reaktion gerne auf meiner Webseite bekanntgeben.

Mit freundlichen Grüßen

Dr. habil. Sebastian Lüning


Frau Klepgen antwortete dankenswerterweise bereits wenige Tage später und bat darum, die Frage im Blog des Ostsee-Forschungsprogramms („BACC Blog“) in englischer Sprache zu posten. Im Blog waren einige Beiträge zur Veröffentlichung des Ostseeberichts zu finden, allerdings bisher nahezu ohne Benutzerkommentare. Lüning wählte einen geeigneten Beitrag aus und stellte dort am 10. Juni 2015 seine Frage ein. Im Gegensatz zu einigen anderen Instituten stellte sich das Helmholtz-Zentrum Geesthacht der Herausforderung. Vielen Dank dafür. Am 15. Juni 2015 gab es im BACC-Blog gleich zwei Antworten:


Von: Marcus Reckermann (BACC-Projektkoordinator)
An: Sebastian Lüning

Datum: 15.6.2015

Thank you for your comment which touches an important point. In fact, the first two chapters on the Holocene conditions in the Baltic Sea region were added to this new book to put the recent warming into perspective, to treat this important subject. The results are clearly spelled out in the chapters and also in the Executive Summary in Chapter 1 so there is surely no hiding or downplaying of findings. Of course you can criticize that the postglacial warming was not explicitly mentioned in the press release, but a press release cannot repeat the complete content of the book but needs to be short and concise, and we need to choose which points wrap up the book in a few sentences as good as possible. In fact other aspects which other interest groups may find of outstanding importance are also not mentioned in the press release.

The BACC project is defined as purely scientific project, not being influenced by vested interests, be it by Oil Companies or Greenpeace. It is our intention to let the scientific evidence speak (in the form of published knowledge) and we try to describe the scientific consent but also we aim to state issues where there is not enough or contradicting evidence.

May I add that this book contains three chapters (Part 6) which attempt to draw together the state of knowledge concening the question if there are other factors also responsible for the regional contempory warming in the Baltic Sea region next to atmospheric CO2 (aerosols, land cover change). Please keep in mind that the findings in the book all refer to the Baltic Sea region and there is no claim made for the global scale.


Von: Hans von Storch (zusammen mit zwei Kollegen Leiter des Instituts für Küstenforschung)
An: Sebastian Lüning

Datum: 15.6.2015

Mr. Lüning you wrote:

“I was surprised to see that the press release does not mention key content from the two palaeoclimate chapters. Of particular interest I found this statement: “…a warm and stable climate with air temperature 1.0–3.5 °C above modern levels (8000–4500 cal year BP)…”.
In the chapter on the Medieval Warm Phase (MWP )I discovered this paragraph which in my opinion is crucial for understanding the context of the 20th century climatic evolution: “Recent investigations of Fennoscandia by Ljungqvist (2010) showed that the MWP [Medieval Warm Period] occurred between 800 and 1300. At that time, warm-season (May-September) temperatures exceeded the contemporary warming of the end of twentieth century by about +0.5°C.””

I have to ask for your understanding that it took a little before you get this answer, but I needed some time to consider your inquiry.

Indeed, I asked Eduardo Zorita, who is one of the authors of the chapter on the Holocene Chapter 2, and his response was “The external climate forcings in the Mid-Holocene were different as today. For instance, not only the concentrations of greenhouse gases was different, but also the orbital configuration relative to the sun was different and as a consequence the solar insolation was also different. Especially at seasonal and regional scales, the solar insolation at high latitudes in the Northern Hemisphere was considerably stronger than today, and therefore, it is not surprising that the temperature in the Baltic Sea area during this period is believed to have been higher than today, specially so during the summer season.” Additionally one has to keep in mind the then very different geographic configuration of the Baltic Sea region.

The issue of the MWP is considered in Chapter 3.4, “The Medieval Warm Period”. Eduardo Zorita’s assessment is: “The case of the MWP is not so clear-cut, and its causes are still a matter of research. The energy output of the sun during this time could have been higher, although the uncertainty in its reconstruction are still broad. The recent PAGES2K reconstructions of continental temperatures indicate warmer temperatures than the mid-20th century over some continents, but not everywhere. Thus, it is not yet clear if the MWP was a global or regional phenomenon.” Here I would add that the sentence “Recent investigations … showed that the MWP [Medieval Warm Period] occurred between 800 and 1300. At that time, warm-season (May-September) temperatures exceeded the contemporary warming of the end of twentieth century by about +0.5°C.” is indeed potentially misleading, as it asserts an elevated temperature of about 0.5°C without making the uncertainty explicit. From the rest of the chapter it is clear (see: Abstract) that the warmer periods were not uniform across time. The sentence, as contained in the report, is based on one recent paper, making use of tree-ring width and wood density; that these data reproduce only part of the past variability and suffer from substantial uncertainty is long known, and we are good advised to show restraint in putting too much into such inversions in terms of temperature and precipitation. From the context the uncertainty becomes clear, but it may have been better if the formulation would have been less definite.


Von: Sebastian Lüning
An: Marcus Reckermann

Datum: 16.6.2015

Dear Dr Reckermann,

I am thankful that you took the time to reply to my comments. As you rightly say, my criticism is not about the report – which represents a solid piece of work. I am more concerned that the press release actually does not follow your principle of open and unbiased reporting. I am fully aware that a press release has to be selective due to space constraints. But a conscious decision was taken by your team and the institute to highlight the temperature evolution in the press release. In my opinion it would have been your ethical duty to provide the full context of the 20th century warming. In your press release you write:

„The current study takes into consideration observed climate changes for approximately the last two hundred years“.

This is incorrect. The truth is that the report also looked at the climate evolution of the past 10,000 years. Why are you hiding this in the press release? Maybe because inconvenient questions could have been asked like „how was the climate during that time“?

Let’s not discuss the Mid Holocene Climate Optimum which indeed may be related to Milankovic cyclicity. Still it is an eye-opening fact that temperatures were much higher at the time than today.

I am more concerned with the Medieval Warm Phase, 1000 years ago. In your press release you have a subheading „Warming continues“. It should have been your duty to provide the context of this „warming“ to the public, that prior to the warming there was cooling in the Baltic area, of roughly the same amount. The whole meaning of the statement „Warming continues“ changes 180°. A better subheading would have been „Significant Warming during 20th century, now matching temperatures of the Medieval Warm Phase 1000 years ago“. I simply cannot accept that there was „not enough space“ in the press release, this fact is just too important to leave it out. Omission in this case has a very strong political flavour to me.

I am fully aware that your report has a regional and not global character. We need more of these comprehensive regional reports for different regions of the world in order to build a solid palaeoclimatological and climatological basis for our understanding. It will be hard to unravel the overall drivers only from one region alone.

As you may be aware, I am of the opinion that we currently seriously underestimate the role of the fluctuating sun in the climate equation. A good example is Leal-Silva & Velasco Herrera (2012) who found that sea ice distribution in the Baltic Sea was largely controlled by solar activity changes:

I have run a full text search through your open access book (thanks for making it freely available!), but could not find this important paper. Why?

I want to urge you to keep your principle of unpolitical and unbiased work. In this case, however, it has failed at the stage of the press release.

Best regards

Sebastian Lüning


Von: Sebastian Lüning
An: Hans von Storch

Datum: 16.6.2015

Dear Mr von Storch,

Thanks a lot also to you for providing these additional information. Independent of the press release that I criticize, I find the question of the Medieval Warm Period of great importance. There are good papers out there that show a very clear MWP, such as Ljungqvist et al. 2012.

I will have to study the PAGES2K report in more detail before I comment. It is clear that there is never a 100% worldwide snychronicity to be expected. Even today, Antarctica has been cooling for the last few decades. Does it question the overall warming of the 20th century? No, it does not. Let’s put it like this: At the moment we cannot rule out that the MWP is of a similar extent as the present Modern Warm Phase. None of the climate models can reproduce a „global“ MWP. This worries me.

But it is true, more work has to be done on the palaeoclimatology, unaffected by political interests. We will present a couple of regional reviews on the MWP on the website in the coming months. I am curious myself what will be the outcome.

Best regards

Sebastian Lüning