Off the coast of Cairns, a once-colourful array of vibrant corals has been bleached into a bare, white graveyard, representing just one symptom of climate change. However, hope for the future of this UNESCO World Heritage treasure lies in pockets of untouched coral, protected by cool deep ocean upwelling.

When coral is exposed to high temperatures it becomes stressed and expels the colour providing symbiotic algae, causing it to become white (bleach). As global temperatures rise, so do sea surface temperatures, increasing the occurrence of these bleaching events and leaving the reef bleak and dead. Recent aerial surveys by the Australian Government suggest that up to 73% of the reef is prevalently bleached.

Yet, two regions continually avoid the worst bleaching, the Swains and Pompeys reef complex in the south and the Ribbon reefs off the coast of the Torres Strait Islands.

New research lead by physical oceanographer at the CSIRO, Chaojiao Sun, suggests that while temperatures across the reef rise, upwelling from the deep ocean provides these unbleached reefs with cool respite.

Both refugia are located on continental shelves with tightly packed coral. When tidal currents are strong, they flood narrow reef channels and drive cold, deep water up to the surface. This upwelling maintains average temperatures at 1-degree Celsius cooler than surrounding waters.

The researchers predict that the “currents of cool water will continue to at least 2080,” even without emissions reduction.

These regions may help coral across the GBR to survive past the turn of the century.  The researchers state that “safeguarding these refuges offers the best chance to preserves some of the reef’s rich array of species and – potentially – to allow corals to adapt to new heat regimes and eventually repopulate degraded reefs.” They suggest that these areas be “designated as high-priority protected areas to maximise success for natural adaptation,” and that they might be the key to allowing “some version of the Great Barrier Reef [to] survive.”

However, they warn against reliance on these refugia, stating that “if warming continues, even the deep waters will heat to temperatures unsuitable for coral.” Modelling from other studies has predicted that “refuges will break down above 3 degrees Celsius warming”.  Dr. Sun and her colleagues reinforce that “robust global climate action is the only solution to limit the frequency and severity of exposure to heat stress.”