Global projections of rising sea surface temperatures in humpback whale breeding grounds

Written by Hannah von Hammerstein and Martin van Aswegen

We are pleased to share a new publication in Frontiers in Marine Science, entitled:

High-Resolution Projections of Global Sea Surface Temperatures Reveal Critical Warming in Humpback Whale Breeding Grounds

Authors: Hannah von Hammerstein, Renee O. Setter, Martin van Aswegen, Jens J. Currie and Stephanie H. Stack


Anthropogenic greenhouse gas emissions are triggering changes in global climate and warming the ocean. This will affect many marine organisms, particularly those with high site fidelity and habitat temperature preferences, such as humpback whales on their breeding grounds. To study the impacts of a warming ocean on marine organisms, large-scale projections of climatic variables are crucial. Global models are of 0.25 - 1° (~25-100 km) resolution, and not ideal to predict localized changes. Here, we provide 0.05° resolution (~5 km) sea surface temperature (SST) projections, statistically downscaled using the delta method. We illustrate the shifting isotherms of the critical 21 and 28°C boundaries, which border the climatic envelope that humpback whales prefer for their breeding grounds, over the course of the 21st century on a decadal temporal resolution. Results show by the end of the 21st century, 35% of humpback whale breeding areas will experience SSTs above or within 1°C of current thresholds if present-day social, economic, and technological trends continue (‘middle of the road’ CMIP6 greenhouse gas trajectory SSP2-RCP4.5). This number rises to 67% under the scenario describing rapid economic growth in carbon-intensive industries (‘fossil-fueled development’ CMIP6 greenhouse gas trajectory SSP5-RCP8.5). These projections highlight the importance of reducing global greenhouse gas emissions and minimizing further SST increases to preserve ecological integrity of humpback whale breeding areas. In this context, our results emphasize the need to focus on protection of critical ocean habitat and to provide high-resolution climate data for this purpose.


This paper was led by UH graduate students from the Department of Geography & Environment (Hannah von Hammerstein and Renee Setter) and the Marine Mammal Research Program (Martin van Aswegen) in close collaboration with researchers from the Pacific Whale Foundation (Stephanie Stack and Jens Currie). The study was recently published as part of the ‘Whales and Climate’ research topic in Frontiers of Marine Science aiming to investigate the complex relationship between baleen whales and climate change. projects critical warming in Humpback whale breeding grounds over the 21st century globally.

Figure 1: Humpback mum and calf in shallow clear Maui waters. Image credit: Martin van Aswegen (UH Marine Mammal Research Program: NOAA/NMFS Permit 21476)

Humpback whales are known to spend the wintertime in tropical coastal waters where they give birth to their calves. Historically, these areas lay in regions with sea surface temperatures ranging between 21 and 28°C, and the whales typically return to the same sites each year. With anthropogenic climate change warming the oceans at unprecedented rates, it is likely some of these regions will exceed the known temperature range over the next century. It is currently not known whether humpback whales will continue to migrate to breeding grounds with SSTs above 28°C in the long term.

To determine the magnitude of ocean warming in humpback whale breeding grounds, future sea surface temperature projections are a crucial tool. Global projections of sea surface temperatures are of coarse resolution and not detailed enough to show regional temperature patterns as needed to project changes in humpback whale breeding areas.

This study used statistical downscaling methods to increase the resolution of global sea surface temperature projections and track the critical 21 and 28°C isotherms which border humpback whale breeding grounds over the 21st century. Two climate change scenarios were incorporated in the analysis to showcase these changes under ‘fossil-fueled development’ (a future of high development and carbon emissions) and ‘middle of the road’ (a future with medium challenges to climate mitigation and adaptation) trajectories.

The results show that in a ‘fossil-fueled’ future, 67% of humpback whale breeding grounds would heat up to critical sea surface temperatures (within 1°C or surpassing 28°C). Under the ‘middle of the road’ scenario, this number is reduced to 35%.

Figure 2: Visualization of shifting 21°C (blue line) and 28°C (red line) isotherms on decadal mean monthly SST (in °C) for February (northern hemisphere) and August (southern hemisphere) under the SSP2-4.5 and SSP5-8.5 greenhouse gas concentration scenarios. Grey lines are corresponding to 1°C isotherms. Northern and southern hemispheres (February and August means) separated by a black bar. Polygons correspond to humpback whale breeding areas colored according to the Distinct Population Segments’ (DPS) conservation status. Numerical population codes correspond to DPS as listed in Tables 1 and 2.

The Hawaiian humpback whale breeding grounds would reach the critical 28°C SST isotherm by 2100 under the ‘fossil-fueled development’ scenario. Critically, SSTs would stay well below this threshold if global and national institutions work toward sustainable development goals according to the ‘middle of the road’ scenario.

This new publication provides a striking case study, highlighting the urgency to reduce global greenhouse gas emissions and emphasizing the need to focus on protection of critical ocean habitat on the example of humpback whale breeding grounds.

Full citation details: von Hammerstein H, Setter RO, van Aswegen M, Currie JJ and Stack SH (2022) High-Resolution Projections of Global Sea Surface Temperatures Reveal Critical Warming in Humpback Whale Breeding Grounds. Front. Mar. Sci. 9:837772.

doi: 10.3389/fmars.2022.837772

For more information, please contact corresponding author Hannah von Hammerstein (hfvh@hawaii.edu)

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Twitter: @hannah.hvh @pacificwhale @mvanaswegen1 @StephanieHStack @JensCurrie

Instagram: @MMRP_UH @pacificwhalefoundation


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