Coral growth in Australia's Great Barrier Reef has slowed to its most
sluggish rate in the past 400 years.

The decline endangers the species the reef supports, say researchers
from the Australian Institute of Marine Science.

They studied massive porites corals, which are several hundred years
old, and found that calcification has declined by 13.3% since 1990.

Global warming and the increasing acidity of seawater are to blame,
they write in Science journal.

Coral reefs are central to the formation and function of ecosystems
and food webs for tens of thousands of other marine organisms.

The Great Barrier Reef is the largest in the world, composed of over
2,900 individual reefs and 900 islands.

Dr Glenn De'ath and colleagues investigated 328 colonies of massive
Porites corals, from 69 locations.

The largest corals are centuries old - growing at a rate of just 1.5cm per year.

By looking at the coral skeletons, they determined that calcification
- or the deposit of calcium carbonate - has declined by 13.3%
throughout the Great Barrier Reef since 1990.

Such a decline is unprecedented in at least the past 400 years, they write.

The researchers warn that changes in biodiversity are imminent, both
at the Great Barrier Reef and at other reef systems throughout the
world's oceans.