Coral reefs are fragile ecosystems and become increasingly dominated by lettuce-like and encrusting morphologies at mesophotic depths. Credit: Peter Mumby
Key points:
- Researchers found that deep water can insulate coral reefs against the global warming effects experienced during surface heatwaves.
- But the protection resulting from deep water will be lost and reefs will be pushed past coral mortality if global warming exceeds 3°C above pre-industrial levels.
- The study gives hope that some reefs are resilient to the current level of climate change, but also provides a warning about the impact of human activity on this resilience.
Climate change projections for coral reefs are based on sea surface temperatures, but these projections do not account for the fact that deeper water does not experience the same warning as the surface. A new study, published in Proceedings of the National Academy of Sciences, reveals how changing temperatures will impact mesophotic corals at a depth of 30-50 meters.
Researchers calculated mesophotic reef warming projections using factors such as wind, tidal mixing of water, and local complexities. They found that separation between warm buoyant surface water and cooler deeper water can insulate reefs from surface heatwaves. However, this protection will be lost if global warming exceeds 3°C above pre-industrial levels and pushes mesophotic temperatures in the Great Barrier Reef past the threshold for coral mortality.
As a result of their analyses, the research team estimated that by 2050-60, bottom temperatures on the Great Barrier Reef will increase by 0.5-1°C under lower projected greenhouse gas emissions and 1.2-1.7°C under higher emissions.
“Coral reefs are the canary in the coalmine, warning us of the many species and ecosystems affected by climate change,” said Jennifer McWhorter of the universities of Exeter and Queensland. “Coral bleaching is a dramatic sign of the impact humans are having on the planet.”
Similar changes to the deeper water insulation may occur on other reefs worldwide, but the local conditions will dictate how the water moves and mixes, meaning the degree to which deeper water coral refuges exist will vary.
The current study gives hope that some reefs, particularly in deeper waters, are resilient to the current level of climate change. At the same time, it provides a warning about the impacts of climate change and human activity on this resilience.
“Reefs face multiple threats—not just climate change,” explained Paul Halloran, professor at Exeter University. “By targeting management of these threats on reefs that have the best chance of escaping the worst impacts of climate change, hopefully some healthy reefs can be maintained.”