Submarine pictures of coral reefs that have lost all of their bright colours have become iconic for the consequences of climate change. Just like the shrinking Alpine glaciers. The corals on the Australian Great Barrier Reef, for example, are an ecosystem that reacts very sensitively to marine heatwaves. Marine heatwaves are phases during which the sea-surface temperatures are much higher than usual.
When under prolonged exposure to heat, the corals bleach, i.e., they expel the tiny colourful algae living in their tissues, leaving behind the white skeleton. Without these algae, the corals tend to starve, and many eventually die. "Until now the corals were often able to recover from such bleaching events," explains Thomas Frölicher, Assistant Professor for Ocean Modelling at the University of Bern. "However, if the intervals between these events become shorter, the corals will no longer have time to regenerate and irreversible damage can be expected."
The quantification of the future progression of marine heatwaves is the core of Frölicher’s study that was published in Nature. As he and his colleagues Nicolas Gruber and Erich Fischer from ETH Zurich have quantified, the number of marine heatwave days has doubled between 1982 and 2016. And this is just the start. If the average global temperature rises to a 1.5 °C warming level — the temperature goal agreed to by the Paris Convention — a sixteen-fold increase in days during which marine heatwaves occur can be expected. In case of a temperature rise of 3.5 °C, they would increase by a factor of 41. The largest changes are projected in the western tropical Pacific and in the Arctic Ocean.