With Global Heating, Marine Mammals Are Headed for Dangerous Waters

Daniel Smuskowitz - Science & Technology

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The journey of life that began around 3.7 billion years ago is a story of awe, wonder and imagination. It is the process by which microscopic organisms, through evolution, transformed to display the array of life that we now see before us. It is truly baffling to think, after witnessing animals as gigantic as the blue whale and as unique as a dugong, of the distant ancestors we all hold. This spectacle of life was perhaps the very first art form, but just like a vase needs only a second to shatter. Indeed, recent science suggests the Climate Crisis may very well be the tipping point that pushes our already depleted marine mammals to a point of no return.

While greenhouse gas emissions have shot up on land, our oceans have carried the vast majority of the resulting burden. Since the 1970’s the world’s oceans have absorbed more than 90% of the excess heat in our atmosphere from Global Heating. As a result, ocean acidification, sea surface temperatures, and sea levels have and will continue to rise.

Following the Paris Agreement in 2015, global efforts are now directed at limiting temperature rises to ‘well under’ 2°C above pre-industrial levels. Human activity has already resulted in global warming of around 1°C, which has increased the occurrence of extreme weather events and ecological disruption. With a finite amount of resources, conservationists must choose where to invest their effort for the most impact. A recently published study in the journal Nature aimed to do just that.

The mostly French team behind the investigation looked into the vulnerability of marine mammals to global heating. In short, they ranked 123 species of marine mammals by their sensitivity to increasing sea surface temperatures, and then looked at different emission scenarios to see how exposed the species would be.

Their vulnerability rank depended on two factors: how sensitive to increases in sea surface temperatures they were, and, how exposed they would be to those increases under different future greenhouse gas emission scenarios. The end result was a list of species ranked by their vulnerability to climate change, highlighting areas of vulnerability hotspots, vulnerable species, and implications for aquatic ecosystems.

One of the traits used was related to feeding, specifically; which level of the ocean the species use to feed. The ocean is vast expanse and should be thought of as separate layers, some deeper than others. The higher layers of the ocean, such as the upper layer, are more exposed to warming than the deeper layers.

The process continued similarly for another 14 traits. This lengthy task allowed the team to create a list of the 123 different species all ranked by how sensitive to global warming they were.

Figure 2- North Pacific right whale. Photo by B. Rone.
https://www.mmc.gov/priority-topics/species-of-concern/north-pacific-right-whale/

The findings showed that the walrus (Odobenus rosmarus) and North Pacific right whale (Eubalaena japonica) (figure 2), followed by the gray whale (Eschrichtius robustus), were the most sensitive species out of the group.

But 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 others may be more sensitive but less exposed. For this study to mean something, the team had to find a way to rank all 123 species in the same way.

Projected future sea surface temperatures won’t all increase the same amount over the entire ocean. This means that some species will be more exposed to warmer waters than others. Once the team was able to estimate how each species would be exposed based on its location, they could use this, and the sensitivity score together to assign ranks for vulnerability to global warming.

This very useful tool allows us to predict (and map) the changes in sea surface temperatures for the world’s oceans into the future. How exposed a species was depended how much its geographical range (area of the ocean) was predicted to warm. From this mapping, what we see is that in the future, changes in sea surface temperatures will be different around the world. For example, using the International Panel on Climate Change’s (IPCC) ‘high emissions scenario’, also known as the ‘business-as-usual’ projection, some oceans are expected to warm up to 4.5 °C by 2100 (Bering Sea), whilst other regions may even experience a decrease in sea surface temperatures (Southern Ocean).

So what does this mean? A species that is less sensitive than another but has a geographical habitat range in a region expected to have a greater effect of warming, may still be more vulnerable. One example of this is the North Pacific right whale and southern right whale (Eubalaena australis). The results showed that both whales were highly sensitive to climate change, but because the species have different geographical ranges, it means they are not equally exposed to warming oceans. Instead, the already endangered North Pacific right whale, for both scenarios, was more vulnerable than the southern right whale.

Figure 3 - Vulnerable species concentrations under high emissions scenario for the year 2100. Red indicates high concentrations of vulnerable species. From Albouy et al. (2020)

Regardless of the scenario, the study found that by the year 2100, oceans in the northern hemisphere are expected to have the greatest increase in average sea surface temperatures, and, will be hotspots for species vulnerability to global warming (as seen in figure 3). For both high and low scenarios, the North Pacific right whale and the gray whale were the most vulnerable species to climate change on the list.

One of the highlights from the study was the creation of two lists for the 20 most vulnerable species to global warming for both high and low emissions scenarios, the latter referring to emissions consistent with 2 degrees of warming. An important message from this study, is that some species that are seen on the top 20 vulnerability list, are currently not even listed as threatened. For example, the gray whale currently shows strong population numbers and is not considered a threatened species. However, in this study the gray whale is listed as the second most vulnerable species on both the high and low emissions lists. This is also the case for other species on the top 20 list, like the pacific white-sided dolphin (Lagenorhynchus obliquidens) (figure 1). The team hopes that through the findings in this study conservationists will be able to identify species that will need our protection into the future.

Some of the species listed in the top 20 lists play a key role in how their ecosystem functions. The best way to understand this is through an example, we’ll use a species we have already discussed. Gray whales feed by raking up sediment on the sea floor (figure 4). In the process, animals (amphipods), the size of a grain of rice are pushed up to the surface and become food for seabirds. If we were to now imagine that the gray whale no longer existed, the seabirds would definitely notice something was different, what is termed ‘functional significance’.

Figure 4 – A gray whale filter feeding. Photo by F. Nicklin. https://www.nationalgeographic.org/article/gray-whale-past-present-and-future/

The top 20 list for the high emissions scenario contained far more species that were functionally significant. The dugong (Dugong dugon) and the walrus are two examples of species that made the high emissions top 20 list, but not the low emissions top 20 list. They are both functionally important for their ecosystems. This goes to shows that our actions today have enormous importance not just for these species, but also the health of the vast ecosystems they are integral to.

Reading through the findings in this study imply a stark future, but the truth is that this is just the science, the future will be based on the actions we take today to mitigate the effects of climate change. One avenue of hope is that this report didn’t take into consideration the species’ possible ability to move, adapt or evolve in response to warming oceans. What we can take away however, is that the choices we make today matter, not only for ourselves, but also for the vast variety of life that exists on this remarkable blue planet of ours.

Written by Daniel Smuskowitz

Having graduated recently from Commerce at UNSW, Daniel has made a turn to conservation work in Zimbabwe and Australia and is currently undertaking a Masters of Conservation Biology at Macquarie University.

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