The Earth is now 1°C warmer than it was a few decades ago and it is our oceans that have suffered most from global warming. Since 1970 the world’s oceans have absorbed more than 90% of the excess heat we have pumped into the atmosphere, acting as a buffer to extreme temperature increases. This, however, has come at a serious cost to the planet’s health; extreme weather events, ocean acidification, sea surface temperatures, and sea levels have all been on the rise.

Following the Paris Agreement in 2015, global efforts now only attempt to limit temperature rise ‘well under’ 2°C above pre-industrial levels. In other words, what we do now is about damage control. With a finite amount of resources, conservationists must choose where to invest their effort to be most effective and impactful. In an attempt to do just this, Camille Albouy from the National Institute for Ocean Science in France, along with her colleagues, recently investigated just how vulnerable marine mammals are to global warming.

Albouy and her team ranked 123 species of marine mammals by their sensitivity to increasing sea surface temperatures. They did so by looking at different future greenhouse gas emission scenarios to determine how exposed these mammals will be to increasing temperatures in the future. The end result was a list of species ranked by their vulnerability to future climate change, highlighting areas that are so-called vulnerability ‘hotspots’ (locations where many species will face an uncertain future due to climate change).

It may sound complicated, but conceptually what Albouy and her team did it isn’t too difficult. Where a marine mammal ranked on the list of sensitivity to warming oceans depended on its ability to persist in its habitat under predicted future climate conditions. This was done by using an approach that considered various attributes (or traits) of each species and how these will be affected by future temperatures. For example, let’s consider traits related to feeding. It’s useful to think of the ocean as having different layers, separated by depth. Some marine mammals use upper layers for feeding, whilst others use deeper layers. Because upper layers of the ocean will rise in temperature faster than deeper layers, species that aren’t able to utilise deeper layers are more sensitive to global warming, and may be at greater risk, than those feeding deep down below.

Analysing 15 such traits, Albouy and her colleagues found that of all the mammals they examined, the walrus (Odobenus rosmarus), North Pacific right whale (Eubalaena japonica), and the gray whale (Eschrichtius robustus) were the most sensitive to climate change.  However, for a species to be vulnerable, it needs to be both sensitive and exposed to warming oceans. Some species may be less sensitive but more exposed, and vice versa. Therefore, for this study to be of conservation value, the team had to find a way to rank all 123 species in the same way, and that involved understanding how, and where, future climate conditions will change.

To predict future climate conditions, the International Panel on Climate Change uses four different greenhouse gas emission scenarios. Referred to as Representative Concentration Pathways, or RCPs for short, they represent four different emission reduction scenarios, ranging from a situation where the world significantly reduces its emissions, to one where the status quo remains.

Albouy and her colleagues analysed two RCPs; a low emission scenario with strong mitigation, and a high emission scenario with no significant effort to reduce greenhouse gas emissions. Using these data, they created models predicting the Earth’s future climate. Under the ‘business as usual’ scenario, oceans were expected to be between 5-7 times warmer by 2100. However, sea surface temperatures won’t increase uniformly, meaning some species will be more exposed to warmer waters than others. For example, under the high emissions scenario, some regions are expected to warm up to 4.5°C by 2100 (Bering Sea), whilst others may experience a decrease in sea surface temperatures (Southern Ocean). The exposure of marine mammals to warmer oceans will therefore depend on how much their geographical ranges (areas of the ocean where they naturally occur) is predicted to warm.

Albouy and her team found that some marine mammals will be more severely impacted by climate change than others. For example, the North Pacific right whale and southern right whale are both highly sensitive to rising sea temperatures, but because of differences in their geographical ranges, the already-endangered North Pacific right whale is more vulnerable than the southern right whale.

Regardless of whether we curb our current emissions or not, the research by Albouy and her co-workers paints a bleak future for many marine mammals. They predict that by the year 2100, oceans in the northern hemisphere will be experiencing the greatest increase in average sea surface temperatures and will be vulnerability hotspots.  Importantly, many of the species they identified as highly vulnerable under future climate conditions aren’t currently considered threatened. While reports like this can be disheartening, realising just how vulnerable these complex systems have become, offers some hope for conservationists. Conservationists often find themselves having to respond to crises, where they only become aware of environmental problems when it is usually too late. Through studies like this, they now have the opportunity to get ahead of the problems they seek to solve.