Humpback whales may steer clear of Hawaiʻi due to climate change

MANOA — Humpback whales may one day avoid Hawaiian waters due to climate change and rising green house gasses, according the findings of a new paper published in Frontiers in Marine Science by a team of researchers including three University of Hawai‘i at Manoa graduate students.

Hannah von Hammerstein and Renee Setter from the UH Department of Geography and Environment in the College of Social Sciences, and Martin van Aswegen from the Marine Mammal Research Program in the Institute for Marine Biology, are co-authors.

Humpback whales are known to migrate toward tropical coastal waters, such as Hawai‘i’s, where they give birth to their calves. These areas lay in regions with sea-surface temperatures ranging between 21 and 28 degrees Celsius (approximately 70 to 82 degrees Fahrenheit), and the whales typically return to the same sites annually.

According to von Hammerstein, Setter, van Aswegen and co-researchers from the Pacific Whale Foundation, anthropogenic climate change is warming the oceans at unprecedented rates. At the current pace, it is likely that some of these breeding grounds will heat up past the 21 to 28°C temperature range over the next century.

Using a statistical “delta downscaling” method to increase the resolution of global sea-surface temperatures and track the critical 21 to 28°C isotherms (lines drawn on a map or chart joining points with the same temperature) that border humpback whale breeding grounds throughout the 21st century, research suggests two possible climate change scenarios:

w By 2100, in a worst-case scenario with continuing high development and unabated carbon emissions, 67% of humpback whale breeding grounds will surpass the critical sea surface temperature of 28°C;

w In a “middle-of-the-road” scenario with global and international institutions working toward emission mitigation goals, that number would fall to 35% of breeding grounds.

“We expected to see critical warming in some of the breeding grounds, but the number of critically affected areas was a surprise,” said von Hammerstein. “While the results of the study are daunting, they also highlight the differences between the two emission scenarios and what still can be won by implementing emission-mitigation measures.

“We expected that some breeding grounds would probably surpass that threshold, but when we looked at that map and saw the isotherms that we had in red creeping over one after another breeding grounds, we were quite surprised,” she said.

“It’s really crucial that we try to reduce our green house gas emissions and really try to stay on that ‘middle-of-the-road’ greenhouse-gas-emissions scenario at the very least, just so that we can save as many of those breeding grounds as possible from surpassing that critical temperature threshold,” said Setter.

The researchers note that, while it is currently not known whether humpback whales will continue to migrate to breeding grounds above 28°C, they hope their findings may be an incentive for policymakers to work toward reducing emissions, not only in Hawai‘i but also on an international level.

“Our findings provide yet another example of what is to come with anthropogenic climate change, with humpback whales representing merely one impacted species,” van Aswegen said. “Improving our understanding of how ecosystems are going to change is critical for the effective and timely implementation of mitigative measures.”

“We expected that some breeding grounds would probably surpass that threshold but when we looked at that map and saw the isotherms that we had in red creeping over one after another breeding grounds, we were quite surprised.”