Two fifths of the planet's most significant marine environment, a stunning expanse of coral reefs stretching across south-east Asia, has been lost and the rest is set to disappear by the end of the century, a World Wildlife Fund report warns.

Pollution, overfishing and climate change is destroying the area known as the Coral Triangle, which covers an area about half the size of the United States and is home to more than 30% of the world's corals and more than 35% of coral-reef fish – around 3,000 species.

"More marine species exist in the Coral Triangle than are found in all the other tropical oceans put together," the report, The Coral Triangle and Climate Change, says.

Within this biological wonderland – which spans Indonesia, Malaysia, Papua New Guinea, the Philippines, Solomon Islands and Timor-Leste – are 18,500 islands rich in mountain forests and woodlands. The escalation of modern practices, such as deforestation, coastal reclamation, destructive fishing and the pumping of pollution and sewage into sea, over the last 40 years have already destroyed about 40% of coral reefs and mangroves in this unique environment.

If such practices are unchecked, half the species in the Coral triangle will continue to disappear at a rate of 1-2% a year.

"You don't have to be a brain surgeon to see that within 40 years we could lose the rest. This may sound alarming but this is not alarmist. This is probably what we are going to experience if we don't get our act together," said Professor Ove Hoegh-Guldberg, writer of the report and director of the Centre for Marine Studies at the University of Queensland.

Based on a review of more than 300 peer-reviewed scientific studies in fields such as biology, economics and fisheries, the report also predicts a dark future for the six nations within the Coral Triangle – and the surrounding the area.

Of the 150 million people who live across this string of islands, about 100 million depend on the mangroves and seagrass beds for food and building materials. The reefs, which also support industries such as fishing and tourism, are also the nursery for numerous fish species.

The report paints two future scenarios. The worst case would be if the global agreement to cut carbon emissions, signed in Copenhagen in December, fails significantly and carbon emissions continue apace. This would, the report predicts, result in region being struck increasingly by severe drought, typhoons and political unrest.

Food supplies would shrink, being halved by 2050 and cut by 80% before the end of the century, while fresh water supplies would be swamped by rising sea levels. Thousands of people would be forced to migrate from the coast to increasingly impoverished and crowded urban areas inland.

Traditional cultural and family structures would be likely to break down as a result and large numbers of people would flee to nearby neighbours such as Australia and New Zealand.

"Reduced food and water security and the resulting social disruption represents a potent threat to regional security," the report says, predicting that a process of radicalisation is likely among those who stay in the region.

But even bleaker is the stark warning in the report that "the pathway that the world is on today exceeds the worst-case scenario described in this report".