Landgewinn trotz Meeresspiegelanstieg: Küsten haben sich in den vergangenen 30 Jahren um 13.500 Quadratkilometer ins Meer vorgebaut

Am 25. August 2016 gab es in Nature Climate Change ein kleines Wunder zu bestaunen. Eine Forschergruppe um Gennadii Donchyts vom niederländischen Deltares Research Institute veröffentlichte eine Statistik zu Landgewinnen und -verlusten in den Küstenzonen der Erde. Angesichts des steigenden Meeresspiegels war ein gewisser Landverlust zu erwarten. Umso größer die Überraschung, als nun die offiziellen Zahlen präsentiert wurden: Die Küstenzonen sind nicht etwa geschrumpft, sondern es wurden in den vergangenen 30 Jahren sogar 13,565 km2 neu geschaffen. Eine echte Sensation. Die deutsche Presse zog es allerdings leider vor, dazu zu schweigen. Unbequeme Daten, die man der Bevölkerung lieber nicht zumuten wollte. In der Publikation heißt es:

Earth’s surface gained 115,000 km2 of water and 173,000 km2 of land over the past 30 years, including 20,135 km2 of water and 33,700 km2 of land in coastal areas. Here, we analyse the gains and losses through the Deltares Aqua Monitor — an open tool that detects land and water changes around the globe.

Uns interessieren nur die Zahlen aus den Küstenzonen (die anderen Angaben beziehen sich auf Inlandsseen, die weitgehend unabhängig vom Meeresspiegel sind). Die Rechnung ist denkbar einfach:
33.700 km2 Landgewinn MINUS 20.135 km2 Landverlust = 13.565 km2 Netto-Landgewinn.

Ein wenig Angst hatte man im Deltares-Institut vor den Zahlen dann doch. In der Original-Pressemitteilung, sparte man dieses wichtige Resultat einfach aus:

How the earth has changed over the past 30 years

The world has gained 115,000 km2 of water and 173,000 km2 of land over the past 30 years. The Dutch research institute Deltares developed an open tool that analyses satellite data and visualises land and water changes around the globe. The results were published today in Nature Climate Change.

First global-scale tool that shows water and land conversion

The Deltares Aqua Monitor was developed by Gennadii Donchyts, a remote sensing expert at Deltares. It is the first global-scale tool that shows, with a 30-metre resolution, where water has been transformed into land and vice-versa. The Aqua Monitor uses freely available satellite data and Google Earth Engine, a platform for the planetary-scale scientific analysis of geospatial datasets that is now open to the general public. Gennadii Donchyts: “The Aqua Monitor shows that, around the world between 1985 and 2015, about 173 000 km2, an area about the size of Washington State, has been transformed into land. At the same time, an area of 115 000 km2 has been transformed into water. Both documented and undocumented changes due to man-made interventions, natural variability, and climate change have now been revealed.”

Known versus unknown

While many countries report on dam construction, information about more remote or isolated areas has been lacking. In Myanmar, the Global Reservoir and Dams database shows an increase in the water surface between 1985 and 2010 of about 400 km2. Using the Aqua Monitor, we found 1,180 km2 of new surface water during the same period. The damming of the Rimjin River in North Korea close to the border with South Korea resulted in a storage surface of 12.4 km2 that was actually due to the Hwanggang Dam, which was thought to be located 35 km to the east. These unknown reservoirs may have had a severe impact on the displacement of people and on the ecology. These issues still have to be investigated.

Created by nature or humans

The results of the Aqua Monitor show only the compound impact of natural and human change or variability. It is often hard to tell what has caused a change without determining the details of the local water and sediment budget. While changes in meanders in the Brahmaputra delta are clearly natural, the Mondrian-like shapes near Taiji Nai’er lakes in China are clearly man-made.

Big data at everyone’s fingertips

Universally-available analytics for big satellite data may have major implications for monitoring capacity and associated actions. At the very local scale, members of the general public can now make assessments without expert assistance if their houses are threatened by coastal erosion. At the regional scale, a downstream riparian state can conduct year-to-year monitoring to see whether upstream neighbours are establishing new impoundments. Finally, at the planetary scale, global agencies such as the United Nations International Strategy for Disaster Reduction can monitor the appearance of new reservoir storage capacity that may reduce flood hazards.

Jaap Kwadijk, the Deltares scientific director: “This has never been done before. So it is difficult to imagine all the new applications that will be made using this tool. But the tool can be used by everybody and so I am sure multiple applications will emerge in the next few years”.

Die BBC fuhr hier einen transparenteren Kurs und berichtete auch explizit über den Landgewinn an den Küsten:

Coastal areas were also analysed, and to the scientists surprise, coastlines had gained more land – 33,700 sq km (13,000 sq miles) – than they had been lost to water (20,100 sq km or 7,800 sq miles). “We expected that the coast would start to retreat due to sea level rise, but the most surprising thing is that the coasts are growing all over the world,” said Dr Baart. “We’re were able to create more land than sea level rise was taking.”

Wie funktionierts? Jeder Geologe lernt in den ersten Semestern, dass sich Deltas in das Meer vorbauen, wenn die Rate des Sedimenteintrags den Meeresspiegelanstieg übersteigt. Dies scheint also derzeit der Fall zu sein. Der Meeresspiegelanstieg ist zu gering, als dass es hier zu einer weiten Überflutung der Küstenebenen kommen könnte. Das System befindet sich im Zustand der Regression, wobei sich viele Sandküsten derzeit leicht in Richtung Meer vorschieben. Nach Ende der letzten Eiszeit vor 10.000 Jahren sah dies noch anders aus. Damals besaß der Meeresspiegelanstieg ein Vielfaches der heutigen Rate und die Küsten wurden vom Meer überrannt. Man nennt dies im geologischen Fachjargon “Transgression”. Vielleicht sollte man politischen Entscheidungsträgern einen geologischen Grundkurs als Pflicht auferlegen, damit sie sich in der Klimadiskussion besser zurechtfinden.

Zum Online Aqua-Monitor geht es hier. Siehe auch Bericht zur Studie auf watson.ch.