What the hell is the DC Metro’s “climate change will increase shark bites” ad talking about? An investigation

Ever since I moved to Washington, DC last summer, I’ve been fascinated by an ad campaign for the DC Metro. The premise of the campaign is simple: taking public transit reduces your carbon footprint compared with driving yourself. It highlights various negative consequences of climate change, and points out how riding the Metro can help fight them.

Many of these ads highlight well-known consequences of climate change:

Photos by David Shiffman

Others highlight less well-known consequences of climate change, but are still on solid scientific ground:

But one ad in particular has been perplexing me for months:

(In case you can’t read it, it says “more CO2 could lead to increased shark bites.
Keep the sharks at bay. Take Metro”

The idea that more humans will be bitten by sharks as a consequence of climate change is…not one I’ve heard stated so directly by any reputable sources. So I decided to ask an international team of experts in the topics of shark behavior, negative shark-human interactions, and how climate change will affect wildlife in general a simple question: in your expert opinion, what the hell is this ad talking about?

Before we get into possible explanations for what this ad could be talking about, let’s review the well-documented impacts of climate change on marine wildlife, including but not limited to sharks.

Climate change results in range shifts, moving animals into geographic areas and habitats where they didn’t used to be.

Basically, when an animal’s current habitat becomes unsuitable, they move. When current habitat becomes too warm, you move where it’s colder (north in the northern hemisphere, south in the southern hemisphere). This has been documented in lots of marine animals, including many that are shark prey. It has also been documented in sharks, by a paper led by SFS’s own Chuck Bangley (I am a coauthor on this paper).

Climate change (and ocean acidification) affects the sensory abilities of marine animals, making it harder for them to find and consume their normal prey.

With sharks specifically, changes in ocean pH appear to affect the sense of smell, which may result in taking longer to find food, eating less food, and growing more slowly than a well-fed animal might. This paper noted that since shark’s size affects what they eat, ocean acidification may result in sharks eating different prey that they didn’t used to it. (Note: a new paper this month found the opposite of this).

Climate change and ocean acidification make some sharks more active

This is believed to be due to changes in blood chemistry as a result of changes in ocean pH. In a presentation I saw about this research, the increased activity was described as more fidgety than athletic in nature.

Climate change and ocean acidification make fishes bolder, less afraid of things that would normally keep them away

This change in behavior is probably also related to disruptions to a fish’s sense of smell- a sense they use to detect other living things in their environment and determine if they are friend or foe. (Note: a new paper this month found the opposite of this).

Climate change can disrupt the timings of many animal behaviors

“Many species change their behaviour and ranges in response to changes in the climate,” said Dr. Dani Rabaiotti, a researcher at the Zoological Society of London. “This can be changes in the time of activity, or where that activity happens, which can bring wildlife into more (or less) conflict with people.” (There’s a great review of this for marine animals here).

So with this background information in mind, let’s consider several theories of how climate change could result in an increase in shark bites, arranged in order of decreasing absurdity. These attempts to explain what the hell the metro ad could possibly be talking about come from conversations with interviewed experts, as well as suggestions from my twitter followers.

(I should note that not one of the experts that I spoke to thought that this particular ad was solidly based on a reasonable, straightforward interpretation of science and facts, with one reporting that the question made him choke on his morning coffee and another calling it “stupid nonsense.”)

Most absurd/least likely: climate change will result in sea level rise which will result in sharks invading our habitat and biting us in our homes. (This was suggested by several twitter followers, some obviously in jest, some…not).

Y’all…Y’ALL. No. That’s not how this works. While unchecked sea level rise is likely to flood some coastal cities, we won’t have humans living and going about their days in neck deep water, vulnerable to sharks as we walk around. Cities will be uninhabitable before they’re under several feet of water, and people will, like, have to leave. (And in the meantime, no, those “sharks in flooded city streets” photos continue to be fake).

Still extremely absurd and unlikely: sharks will bite us because they can no longer tell the difference between us and their normal prey, or because they can no longer catch their normal prey, or because their normal prey migrated away so they eat us because we’re there. (Various versions of this were suggested on twitter).

Dr. Charlie Huveneers, the leader of the Southern Shark Ecology Group at Flinders University, calls this a “very imaginative interpretation” of the available data. We’re just not on the menu, folks, even if regular prey becomes harder to find or obtain. Sharks don’t eat humans and changing environmental conditions aren’t going to change that. (Might cases of bites due to mistaken identity become more likely when one of the senses used to confirm identity work a little less well? Maybe, but probably not much).

Dr. Jessica Meeuwig from the University of Western Australia also pointed out that more CO2 in the water also has another relevant, less commonly discussed effect. “More CO2 means less O2 in the water which means oxygen stress,” she told me. “I would think O2 stress would make animals more sluggish, not more aggressive.” (Indeed, the increased boldness and increased activity levels described above weren’t related to hunting and feeding, just general swimming).

Kinda-sorta unlikely: range shifts bring dangerous shark species into contact with humans, and more dangerous shark species around humans means an increased chance of shark bites.

This phenomenon was the subject of a 2016 article in TIME Magazine entitled “how climate change is fueling a rise in shark attacks.” (This is an extremely bad article, which notes that there were slightly more shark bites in 2016 vs. 2015, and also noted that climate change was shifting the ranges of some fish including sharks. It did not note that the shark species identified as shifting their ranges were responsible for zero of the bites that year.)

As noted above, climate change induced range shifts are happening already, and the species we documented this phenomenon in, bull sharks, is a species that has been known to bite humans. Indeed, there were some bull shark bites in the area included in our study. However, we were very careful to never say that “climate change is going to increase shark bites off North Carolina,” because it’s a lot more complicated than that. Lead author Dr. Chuck Bangley explains: “Relating the chance of being bitten by a shark directly with climate change is a bit of a stretch,” he told me. ” Our findings in North Carolina indicate that changing ocean temperatures are likely already causing sharks and other migratory marine species to occur more often in areas where they were previously rare. Some of these areas might be places where people aren’t used to seeing certain species like bull sharks. However, just because sharks are in an area more often or in greater numbers doesn’t mean they’re any more likely to bite someone. Bites on humans are exceedingly rare regardless of whether the sharks used to be there or not. To me, range expansion among bull sharks isn’t scary because of the sharks, it’s scary because it’s a sign that climate change is already moving entire marine ecosystems.”

Additionally, Dr. Dani Rabaiotti had thoughts on this possible explanation for the metro ad, based on our paper. “There is some evidence that the range of certain shark species will change as temperatures rise, which has the potential to increase the number of shark bites in some areas, but would be just as likely to decrease shark bites in others,” she told me.

More likely, but boring and a weird thing to blame on sharks: when it’s hotter, more people go swimming.

Unlikely events are more likely to happen when they have more chances to happen. More people going in the ocean means more shark bites, even when any given shark bite is a very unlikely event.

“The number one variable in human-shark interactions is the number of warm sunny days – and the fact that this results in more people in the water,” said Dr. Christopher Pepin-Neff of the University of Sydney. “So if we believe that longer, hotter summers will be a result of climate change then I would say, yes, there is likely to be an increase in shark bites because the sheer numbers of people using the ocean is likely to be higher.”

Dr. Andrew Chin of James Cook University, the author of a review of climate change’s impacts on sharks and their relatives, agrees that this is the most likely explanation. “If you want me to dream up some widely speculative scenarios, they all boil down to ‘it’s getting hot in here’ – i.e. more heat, more beachgoers, higher encounter risk,” he told me, noting that he had never heard of any scientific evidence that climate change would make shark bites more likely. Dr. Chin also noted that climate change also increases the frequency and intensity of storms, which results in fewer sharks in coastal areas where most humans go in the water.

The most likely, least satisfying explanation: the people who made this ad are wrong.

Climate science is complicated stuff, and, as noted by Southern Fried Science founder Dr. Andrew Thaler on twitter, “metro ads are not peer reviewed.” Maybe they just got it wrong, and I’m the weird one for spending months thinking about it and bothering a dozen global experts on this topic.