The GBR is not just one reef. It is 3 000 individual coral reefs and it is huge – covering approximately the same area as Germany, bigger than the Australian state of Victoria, and just a little smaller than California. Each reef is usually a few kilometres across and separated from its neighboring reef by between a few hundred metres to 10 km (see Figure 1). It is by far the biggest reef system in the world. Nothing else is comparable.
Importantly, from the point of view of nature conservation, most of the reefs are far off shore – more than 100 km in the southern zone. For this reason, aside from the few hot–spots where most tourists go, most reefs will rarely see a visitor. The adjacent coast, for the whole of the 2 300 km length of the GBR, has a very low population – perhaps a little more than over half a million people. For the northern section of the GBR, along an almost 1 000 km length of coastline, the total human population is only a few thousand. Compare this with the Caribbean Reefs which have similar extent – there are perhaps 50 million people living nearby, roughly 100 times more than the number living near the GBR. The GBR is one of the most remote, unpopulated, untouched and unspoiled areas on earth. It is hoped that it will remain that way.
The GBR in its present form is just the last in a line of many GBRs that has come and gone roughly every hundred thousand years as the climate has varied in the last few million years. When the climate is much colder than at present, which is more than 90 per cent of the time, the sea level is more than 100 metres lower and the GBR becomes dry land. During these periods, the coastline is up to 100 km seaward of its present position. In the relatively brief periods of warmth such as the last ten thousand years, the sea level rises by 100 metres and the GBR reforms yet again. During periods of cold climate and low sea-level, the GBR was just a long group of a 3 000 thousand flat topped hills on the coastal plain, each about 50 – 100 metres high, and formed of old dead coral.
Rebirth of the present GBR started about 18 000 years ago when the sea–level began to rise at the end of the “ice age” (Last Glacial Maximum). The sea–level reached roughly its present level 10 000 years ago (Larcombe et al., 1995). The Aboriginal people would have witnessed the beginning of the latest sea–level rise 18 000 years ago and watched the coastline erode by up to 50 metres per year. The flat–topped hills made of old coral from the previous version of the GBR flooded with seawater and coral started to grow, yet again, on top of the ancient dead coral. By 10 000 years ago, the sea level had almost finished rising but reached its maximum only about 5 000 years ago after which it fell by about one metre until the present day (Larcombe et al., 1995).
The sea–level is now lower than when the Egyptians were building pyramids and when the Earth’s climate, and the GBR, was a degree or so hotter than today.[i] This slow fall in sea–level caused massive coral loss which can be seen on many reefs in the form of dead reef–flats. These are large areas that are now exposed to the air at low tides but would once have been covered by coral when the sea level was higher (figure 2). The old dead reef flats are often mistaken as being areas where coral has died recently and is attributed to farmers or climate change. But this dead coral is far older.
Figure 2 Lodestone Reef showing the white coral sand made almost entirely of broken coral and other organisms. The “reef flat” is largely dead, being killed by the slow sea–level fall that occurred over the last 5 000 years – a fact often not appreciated by marine biologists who generally have little appreciation of the geological history of the GBR. The reef flat is regularly exposed to the air at low tide so corals can no longer survive there. (Picture Shutterstock)
See section on Mediocre fringing reefs
[i] Roche, R.C., Perry, C.T., Smithers, S.G., Leng, M.J., Grove, C.A., Sloane, H.J. and Unsworth, C.E. (2014). Mid-Holocene sea surface conditions and riverine influence on the inshore Great Barrier Reef. The Holocene, 24(8), pp.885–897.
Great Barrier Reef Science Commentary 