Scientists monitoring a global coral bleaching event say climate change and this year's El Nino pose a major threat to these key ecosystems. DW spoke to coral reef scientist Ove Hoegh-Guldberg.
DW: Scientists, yourself included, are warning that a global coral bleaching event is underway. What exactly is happening with the word's coral?
Since the early 1980s, we've been having these disturbances where it gets too warm for corals and the symbiosis between corals and tiny plant-like organisms that live in their tissue breaks down. The effect is that the rate of coral dying goes up.
In 1998 and 2010 we've had truly global events, in which corals have been damaged in the three major ocean basins - in the Atlantic, the Indian and the Pacific Ocean - and we've just seen that start to occur again worldwide.
What's very significant is that the conditions that have built up over the last year look very similar to the precursor to the 1998 global mass bleaching event. So we are very concerned that we are about to go into that cycle again, where in 1998 we lost 16 percent of the world's corals.
To add to the concern, we are now heading for a very significant El Nino condition, which also in 1998 added to the natural vulnerability there and boosted sea temperature to where we saw some really very dramatic changes to reefs in many parts of the world.
These coral reefs are not simply tourist destinations. They are an incredibly important resource for people. It has been estimated that 500 million people are supported from income they get from industries associated with reefs - from fishing, and also from some of the other benefits like the fact coral reefs protect coastlines from wave impacts and storms.
To what extent was coral able to recover after that 1998 event?
There have always been cyclones and hurricanes. They can often wipe out corals in an area across several kilometers - but over 15 or 20 years, they will grow back. What we are seeing though, is that these global events are starting to have a return period which is much shorter than the time it takes coral reefs to recover. From 1998 to 2010 you've got 12 years, 2010 until 2015 is just half that interval.
What's concerning is that we're increasing the frequency of the disturbances, while at the same time, through pollution and not treating corals very well along coastlines, we're slowing their ability to bounce back. So what we've seen in many oceans over the past 30 years is 50 percent of corals disappearing - that includes Australia, Southeast Asia and the Indo-Pacific.
What evidence do we have about the time before 1998?
Prior to the late 1970s or early 80s, you don't have any scientific reports or papers published on this phenomenon. Yet for 30 or 40 years prior to the 1980s, we had lots of divers in the water, we had filmmakers, and no one reported it. When you couple that to the experimental evidence and our understanding of why corals get stressed out and what sea temperatures were doing, it's pretty clear that they didn't really occur with any frequency prior to 1980 - and that it's this rapidly warming ocean, exacerbated by ocean acidification, that's driving corals in a direction where they're bleaching more often. They reached that trigger point probably in the early 1980s.
So is climate change a major trigger here, or is this something that could have happened anyway because of natural cycles?
When you look back at sea temperatures in the past, they didn't get as high as they do today. So it's the extremes that are causing the problem. This correlation, or linkage between sea temperature being too warm and bleaching, is so solid that satellites are able to detect when and where bleaching is going to occur, just based on the temperature signal alone.
So we are very sure that it is a temperature-related phenomenon. We know that we are getting greater and greater extremes due to the rising background sea temperature, and we know that rising sea temperature is due to greenhouse gases in atmosphere. So with a great deal of certainly, this is being driven by warming oceans, and is not due to a natural cycle.
When you go further into the future, the message is fairly worrying. You get to a stage where the point at which bleaching occurs will be reached every year. And if you go even further toward the end of century, you find that even winter temperatures become too warm for coral. So it looks like coral reefs will disappear by around the middle of this century - which of course given the resources they provide to humans, is a great concern.
That is a very bleak outlook. Is there anything we can do to save the coral?
There is, and I think it's really important to realize we're actually at crunch time. On our current track - where we're polluting local water, overfishing coral reefs, and rapidly changing the temperature and acidity of the ocean - we won't have coral reefs. It will be a very long time before they come back - probably well after our exit from the Earth.
We are the first generation to see these types of impact, and we are going to be the last that has the chance to do something. So we're trying to communicate with the [COP21] negotiators that we must get to very low CO2 emission rates as soon as possible, hopefully over the next 20 to 30 years.
Because if we don't, it won't just be coral reefs. It will be a large number of other ecosystems that go - and humanity will be in trouble.
Ove Hoegh-Guldberg is Chief scientist of XL Catlin Seaview Survey and Director of the Global Change Institute at the University of Queensland, Australia.