Great Barrier Reef Coral Bleaching

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

Coral bleaching on the Great Barrier Reef in Australia was worse than expected last year, and a further decline is expected in 2017 according to the Great Barrier Reef Marine Park Authority. In a document issued this week they noted that, along with reefs across the world, the Great Barrier Reef has had widespread coral decline and habitat loss over the last two years.

We’ve written about coral bleaching before, as it’s a real barometer of climate change. To put the importance of the Great Barrier Reef into context:

  • It’s 2300 km long and covers an area of around 70 million football pitches;
  • Consists of 3000 coral reefs, which are made up from 650 different types of hard and soft coral; and
  • Is home to over 1500 types of fish and more than 100 varieties of sharks and rays.

Coral bleaching occurs when water stress causes coral to expel the photosynthetic algae, which give coral their colours, exposing the skeleton and turning them white. The stress is mostly due to higher seawater temperatures; although cold water stresses, run-off, pollution and high solar irradiance can also cause bleaching. Whilst bleaching does not kill coral immediately, it does put them at a greater risk of mortality from storms, poor water quality, disease and the crown-of-thorns starfish.

Last year the Great Barrier Reef suffered its worst bleaching on record, aerial and in-water surveys identified that 29% of shallow water coral reefs died in 2016; up from the original estimation of 22%. The most severe mortality was in an area to the north of Port Douglas where 70% of the shallow water corals died. This is hugely sad news to Sam and I, as we explored this area of the Great Barrier Reef ourselves about fifteen years ago.

Whilst hugely concerning, there is also a little hope! There was a strong recovery of coral in the south of the Great Barrier Reef, as bleaching and other impacts were less.

Images from the Copernicus Sentinel-2A satellite captured on 8 June 2016 and 23 February 2017 show coral turning bright white for Adelaide Reef, Central Great Barrier Reef. Data courtesy of Copernicus/ESA, and contains modified Copernicus Sentinel data (2016–17), processed by J. Hedley; conceptual model by C. Roelfsema

The coral bleaching event this year has also been captured by Sentinel-2. Scientists from ESA’s Sen2Coral project have used change detection techniques to determine bleaching. Images between January and April showed areas of coral turning bright white and then darkening, although it was unclear whether the darkening was due to coral recovery or dead coral being overgrown with algae. In-water surveys were undertaken, which confirmed the majority of the darkened areas were algal overgrowth.

This work has proved that coral bleaching can be seen from space, although it needs to be supported by in-situ work. ESA intends to develop a coral reef tool, which will be part of the open-source Sentinel Application Platform (SNAP) toolkit. This will enable anyone to monitor the health of coral reefs worldwide and hopefully, help protect these natural wonders.

Is This The Worst Global Coral Bleaching Event Ever?

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

It was announced last week that 93% of the Great Barrier Reef has been hit by coral bleaching due to rising sea temperatures from El Niño and climate change. We first wrote about the third worldwide coral bleaching in October 2015, noting this year’s event could be bad. Those fears would appear to be coming true with the results of Australia’s National Coral Bleaching Task Force aerial survey of 911 coral reefs which found 93% had suffered from bleaching; of which 55% had suffered severe bleaching.

Coral bleaching occurs when water stresses cause coral to expel the photosynthetic algae, which give coral their colours, exposing the skeleton and turning them white. The stress is mostly due to higher seawater temperatures; although cold water stresses, run-off, pollution and high solar irradiance can also cause bleaching.

Bleaching does not kill coral immediately, but puts them at a greater risk of mortality. Recovery is also possible if the water stress reduces and normal conditions return, which is what is hoped for in the Northern Sector of the reef above Port Douglas, where around 81% of corals had suffered severe bleaching – the water quality in this area is good, which should also aid recovery. The reefs fared better further south. Within the Central Sector, between Port Douglas and Mackay, 75 of the 226 reefs suffered from severe bleaching. Whilst in the Southern Sector below MacKay only 2 reefs suffered severe bleaching and 25% had no bleaching.

The news is not all bad. A survey of the coral reefs of the Andaman and Nicobar Islands, a territory of India that marks the dividing line between the Bay of Bengal & Andaman Sea, also published this week shows no evidence of coral bleaching. This survey is interesting for remote sensors as it was undertaken by a remotely operated vehicle, PROVe, developed by India’s National Institute of Ocean Technology. As well as mapping the coral reefs, PROVe has a radiometer attached and is measuring the spectral signatures of the coral in the area, which could be used to support the monitoring of corals from satellites.

Monitoring coral bleaching from space has been done before. For example, Envisat’s MERIS sensor was determined to be able to detect coral bleaching down to a depth of ten metres, or the Coral Bleaching Index (Ziskin et al, 2011) which uses the red, green and blue bands to measure increases in spectral reflectance in bleached corals. Given the size, geographical area and oceanic nature of corals, satellite remote sensing should be able to offer valuable support to the monitoring of their health.

Following the second global bleaching event, in 1997/98, research confirmed that 16 percent of the world’s coral died. Who knows what the outcome of the current event will be?

Landscape Features Visible From Space

Eye of the Sahara from Landsat 8 on 7th July 2015. Data courtesy of NASA/USGS.

Eye of the Sahara from Landsat 8 on 7th July 2015.
Data courtesy of NASA/USGS.

One of my favourite facts growing up was that the Great Wall of China was the only manmade feature visible from space. Of course, I now know that everything about that statement was wrong, and it is not a fact at all!

Firstly, it is actually very difficult to see the Great Wall of China from space due to the narrowness of the wall and the pixel size of satellites. For example, Landsat has a pixel size of 30 m meaning that it is impossible to distinguish anything smaller than 30 m and features need to be significantly larger to be visible. The astronauts astronauts Chris Hadfield from Canada and China’s Yang Liwei, both said they could not see the Great Wall with the naked eye when orbiting the Earth. US astronaut Leroy Chiao took a picture of the Great Wall from the window of the International Space Station in 2004; however, even this needed to be magnified to be able to see the Great Wall. Sadly, the other side of my childhood fact is also untrue; there are a number of manmade features that can be seen from space, including the Great Pyramids at Giza, the greenhouses of Almeria in Spain and Palm Tree Island in Dubai.

There are, of course, also many natural features visible from space. From the obvious Great Barrier Reef, Uluru (also known as Ayres Rock) and the Grand Canyon; to the more unusual and less well known features such as the Eye of the Sahara, which is the image at the top of the blog. I become aware of the Eye of Sahara, also known as the Richat, through a recent New Scientist article. It’s a 40 kilometre wide series of concentric rings of rocks of different ages, located in the Sahara Desert near Ouadene in Mauritania.

It’s not known precisely how this feature was created, nor why it is so circular; however, it is an interesting anomaly visible from space. The concept of exploring unusual Earth features seen from space is the basis of a new television series due to begin on Discovery UK at the end of this month. The series, Into The Unknown, will see presenter Ed Stafford travel to unusual and unexplained landscape features that have been spotted from satellites.

Who says there is nothing left to discover on Earth? Start scouring your satellite pictures; you never know what you might discover!