It’s no secret coral reefs are under threat from global warming. For some, however, salvation may lie in higher sea levels, brought on by the same global warming. A new study of reefs, published in Science Advances, where tides are the major cause of water mixing has found predicted sea level increases may be enough to save some from bleaching.
Although we are finding reefs in much deeper waters, most tropical corals have only a thin layer of water above them at low tide. Those close to land can trap a small amount of water between them and the shore, with big effects when low tide coincides with the middle of a summer day. “You can think of it as applying the same heat to a smaller amount of water,” Professor Ryan Lowe of the University of Western Australia, lead author of the study, told IFLScience.
For corals endangered by heat stress, the consequences can be disastrous. As the world warms, pushing many corals towards conditions they cannot survive, hot days with low water levels are the most dangerous times of all. For some coral reefs, wave action serves as the primary force mixing warm surface water with cooler water below. However, Lowe told IFLScience that for around 30 percent of the world’s reefs, tides are more important. In the study Lowe has explored the way tides influence temperature conditions on six such reefs around the world, including those exposed to the enormous tides off the north-west coast of Australia.
If reefs remain at the same height, while sea levels rise, the volume of water to be warmed will be larger. Even with more heat in the system it may not warm as much. At high tide, or even under average conditions, a small rise in sea level may make little difference. However, when the tide is out the relative change becomes much larger.
While average temperatures may be higher, the variation during the day is likely to fall. Some reefs that currentluy experience temperature swings between day and night of +5°C (9°F) could experience a reduction of more than 2°C (4°F) in this range with a 0.7-meter (2.3 feet) sea level rise, Lowe and his co-authors report. The range could be reduced by 4°C (7°F) for a 1.5-meter (five foot) rise. The authors add, “these amounts are comparable in magnitude to projected warming of the tropical oceans by the end of the century” if we don't get emissions of greenhouse gasses under control.
Consequently, Lowe's reefs may maintain stable peak temperatures, even as averages rise. One goal of the research, Lowe told IFLScience, was to explore the different impacts of chronic stress, such as when weeks of high temperatures cause reef bleaching, and more acute threats, including a single very hot day that coincides with a low tide.
Those coral reefs most threatened by acute conditions could be saved by a consequence of the same conditions that are threatening them.