Korallenbleiche muss keine Katastrophe sein

Wenn es warm wird, erbleichen die Korallen. Ein tolles Thema zu dem sich schöner Klimaalarm konstruieren lässt. Zum Glück ist es nicht ganz so schlimm, wie es uns der eine oder andere Apokalyptiker glauben lässt. Ein Beispiel aus dem Stern vom 10. April 2017:

Forscher sehen nach zwei Korallenbleichen am Great Barrier Reef schwarz
Wissenschaftler schlagen angesichts des Ausmaßes der Korallenbleiche am berühmten Great Barrier Reef Alarm: Nach zwei Korallenbleichen im vergangenen und diesem Jahr hätten die betroffenen Gebiete vor Australien so gut wie keine Chance mehr, sich von dem Phänomen wieder zu erholen, warnten sie am Montag nach einer Bestandsaufnahme des Riffs aus der Luft.

Sodom und Gomorrha. Der deutsche Sternleser erschaudert und hat in der Folge Probleme beim Einschlafen. Tja, hätte man doch einmal diejenigen gefragt, die es eigentlich am besten wissen sollten, die Taucher. Die korrigierten den Irrtum: In Wahrheit hatte die Bleiche nur 5% aller Korallen betroffen, nicht die Hälfte wie von interessierten Kreisen zuvor behauptet. Das hört sich doch gleich anders an.

Mittlerweile haben sogar die Umweltorganisationen die Nase kräftig voll, dass die Korallen für alarmistische Zwecke ständig instrumentalisiert werden. Die Wildlife Conservation Society gab am 1. Juni 2015 per Pressemitteilung bekannt, dass die Modelle in Punkto Korallenbleiche wohl kräftig überzogen haben:

New Climate Stress Index Model Challenges Doomsday Forecasts for World’s Coral Reefs

Recent forecasts on the impacts of climate change on the world’s coral reefs—especially ones generated from oceanic surface temperature data gathered by satellites—paint a grim picture for the future of the “rainforests of the sea.” A newer and more complex model incorporating data from both environmental factors and field observations of coral responses to stress provides a better forecasting tool than the more widely used models and a more positive future for coral reefs, according to a new study by the Wildlife Conservation Society and other groups. The study authors point out that, according to the climate stress index model first developed in 2008, coral reefs are responding to more factors than temperature and therefore more resilient to rising temperatures. They conclude that global climate change is the greatest global threat to coral reefs but the future of these ecosystems is more varied than predictions from the more widely used “temperature threshold” models.

The paper titled “Regional coral responses to climate disturbances and warming is predicted by multivariate stress model and not temperature threshold metrics” appears in the online edition of Climatic Change. The authors are: Timothy R. McClanahan of the Wildlife Conservation Society; Joseph Maina of the Wildlife Conservation Society and the Australia Research Council Centre of Excellence for Environmental Decisions; and Mebrahtu Ateweberhan of the Wildlife Conservation Society and the University of Warwick. “Our new multivariate stress model suggests that the future of coral reefs is considerably more nuanced and spatially complex than predictions arising from the threshold models,” said Dr. Tim McClanahan, WCS’s Senior Conservation scientist and a co-author on the study. “According to our findings in the Western Indian Ocean, some places will do well and others will not. The key to accurate predictions is using all available environmental data and complementing it with on-the-ground observations on reef cover, coral communities, and other environmental variables that are key to understanding how corals respond to the interaction between all these variables.”

In the study, the authors compared the abilities of three common thermal threshold indices against a stress model that includes temperature but also light and water quality and movement variables and used the models to predict coral cover and susceptibility to bleaching during a past large stress event: specifically the 1997-98 coral bleaching event in the Western Indian Ocean. The field information used in the test included a compilation of 10 years of coral community data before the bleaching event, two years after the bleaching event, and data during the period of coral recovery between 2001-2005. While the three temperature threshold models (sea surface temperature, cumulative thermal stress, and annual thermal stress) were highly variable with little agreement to field data after the 1998 rise in temperature and coral mortality, the multivariate model based on 11environmental variables combined using a fuzzy logic systems revealed a more accurate fit with the recorded coral cover and susceptibility in the recovery period that followed.

“This latest research suggests a more optimistic future for the world’s coral reefs,” said Dr. Caleb McClennen, Executive Director of WCS’s Marine Program. “The ability of certain coral communities to resist and recover from climatic factors provides hope for the future of the oceans. Our imperative is now to seek out and protect those locations that are refuges from climate change, and reduce other human stresses such as fisheries to ensure the long term survival of coral reefs.” This research was supported by the John D. and Catherine T. MacArthur Foundation, the Western Indian Ocean Marine Science Association, and the World Bank Targeted Research Group on Coral Bleaching.

 To access the article, go to: http://link.springer.com/article/10.1007/s10584-015-1399-x

Einen Monat zuvor hatte bereits das Institut de Recherche pour le Développement (IRD) vor Panikmache gewarnt:

A new future for corals: Persistence and change in coral reef communities

Coral reefs, true reservoirs of biodiversity, are seriously threatened by human activities and climate change. Consequently, their extinction has often been heralded. Now, researchers are painting a less gloomy picture: the planet’s reefs are not doomed to disappear. But they will be very different from the ones we presently know. A new coral fauna will emerge, coming from the species that are most resistant to temperature increases.

Some reefs are recovering

Are coral reefs condemned to disappear? During the first decade of the 21st century, the intensification of cyclones, the phenomenon of coral bleaching due to ocean warming, outbreaks of a coral-eating starfish and coral diseases left us with this fear. But today, scientists are revising their pessimistic forecasts from the previous decade. In fact, recent research works show that, while numerous coral species have indeed been declining for more than 30 years, other are holding firm or even increasing in abundance. Consequently, some reefs have recently managed to recover.

Expanding coral genera

During a vast international study over fifteen years, IRD researchers and their partners observed the ecological development of seven coral reefs throughout the world: two in the Caribbean, in Belize and in the American Virgin Islands, and five throughout the Indo-Pacific Ocean in Kenya, Taiwan, Hawaii, Moorea and the Great Barrier Reef in Australia. Consequently, the scientists have shown the increase of certain genera, like the Porites reef corals, real reef builders that can resist temperature rises well.

They have also put these recent changes into perspective with regard to past events recorded in fossil reefs, showing that the abundance and structure of coral populations have already varied greatly over the course of past millennia.

Towards new underwater landscapes

These new data have enabled them to refine their mathematical models and to revise their forecasts for the coming decades. As ocean temperatures continue to rise, a subset of “winning” species will thrive: those that have the greatest heat tolerance, the best population growth rates or the greatest longevity. These species should progressively populate the planet’s reefs, until they dominate them entirely.

Consequently, the underwater landscapes of the future will be very different to the ones that have been known for millennia. However, much remains to be discovered regarding this new coral fauna and its features. One question in particular remains: will these new eco-systems continue to meet the needs of the populations who depend on them?

Überhaupt muss man wohl viel sorgfältiger bei den Korallen unterscheiden. Einige Arten können sich sehr gut von Bleichen erholen, andere nicht so gut, wie das ARC Centre of Excellence in Coral Reef Studies im Januar 2015 zu bedenken gab:

Predicting coral reef futures under climate change

Researchers examining the impact of climate change on coral reefs have found a way to predict which reefs are likely to recover following bleaching episodes and which won’t.

Coral bleaching is the most immediate threat to reefs from climate change; it’s caused when ocean temperatures become warmer than normal maximum summer temperatures, and can lead to widespread coral death. A key unanswered question has been what dictates whether reefs can bounce back after such events, or if they become permanently degraded. An international team of scientists found that five factors could predict if a reef was likely to recover after a bleaching event. “Water depth, the physical structure of the reef before disturbance, nutrient levels, the amount of grazing by fish and survival of juvenile corals could help predict reef recovery,”says study lead author, Dr Nicholas Graham from the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia. “Remarkably, the two most easily measured variables, water depth and the physical structure of the reef before disturbance, predicted recovery with 98% confidence,” Dr Graham says.

As part of the research, published in the journal Nature, researchers from Australia, the United Kingdom and France examined nearly 20 years of coral reef data gathered from the Seychelles. Data was collected before and after an unprecedented coral bleaching event in 1998, in which 90 per cent of the country’s corals across 21 reefs were lost. Of the reefs affected by the episode, twelve recovered while nine did not. The event had a significant impact on the biodiversity of local fish populations, which changed substantially when reefs did not recover. From their data the researchers identified thresholds for the factors that dictated whether reefs would recover. “Putting numbers on the threshold points at which reefs either recover or degrade helps predict reef futures under climate change,” Dr Graham says.

Study co-author, Dr Shaun Wilson from the Department of Parks and Wildlife, Western Australia adds that the findings are important for predicting reef futures under climate change. “The beauty of this study is that easily acquired measures of reef complexity and depth provide a means of predicting long term consequences of ocean warming events,” Dr Wilson says. “The ability to predict which reefs have the capacity to recover is really important for mapping of winners and losers, and risk analysis” Co-author Dr Aaron MacNeil from the Australian Institute of Marine Science says the insights can be applied to studies and management aimed at improving the outlook of coral reefs around the world. “This gives reef management a major boost in the face of the threats posed by climate change and, encouragingly, suggests people can take tangible steps to improve the outlook for reefs,” Dr MacNeil says. “By carefully managing reefs with conditions that are more likely to recover from climate-induced bleaching, we give them the best possible chance of surviving over the long term, while reduction of local pressures that damage corals and diminish water quality will help to increase the proportion of reefs that can bounce back.”

Paper: Nicholas A. J. Graham, Simon Jennings, M. Aaron MacNeil, David Mouillot, Shaun K. Wilson. Predicting climate-driven regime shifts versus rebound potential in coral reefs. Nature, 2015; DOI: 10.1038/nature14140