Herds of wildebeest after crossing a river in Kenya on Sept. 4, 2015. (Carl de Souza/AFP/Getty Images)

It’s no surprise that populations living in the Arctic — whether human, animal or otherwise — have become the poster children for global warming’s terrible consequences...

The Arctic is warming faster than anywhere else on the planet, and its rapid ice loss has not only become a major contributor to global sea-level rise, but is also destroying habitats for polar bears and people alike.  

But a new study, published Thursday in the the journal Scientific Reports, argues that, while the stakes at the poles are high, we may want to be paying a little more attention to what’s going on near the equator, as well. The research suggests that even a moderate amount of warming could force populations in the tropics to undergo huge migrations — longer journeys than they’d have to take if they lived anywhere else on the planet — to get to cooler ground.

“The point we’re trying to make in the paper is people are not really talking about the tropics,” said Solomon Hsiang, the chancellor’s associate professor of public policy at the University of California Berkeley and the new study’s lead author. “The tropics, because of very specific patterns that occur [there], are a place where we might actually be really concerned.”  

The study centers on the idea that many types of organisms, including humans, insects and even plants, have adapted for life in a certain type of climate — and as temperatures warm, they may relocate to continue living in those same types of conditions. It’s becoming a well-documented phenomenon. Numerous studies on specific types of organisms, from butterflies to fish, have found that animals already seem to be moving around in response to climate change. And at least one large meta-analysis in the past few years has found that terrestrial animals are shifting their ranges even faster than expected, although these migration rates differ heavily from one species to the next.

The problem presented in the study is that temperatures throughout the tropics — which stretch from about 23 degrees north of the equator to 23 degrees south — tend to be fairly uniform. It’s only once you move into the subtropics that temperatures begin to cool significantly as you travel north or south toward the poles. And as climate change causes temperatures to rise, the tropics also warm more or less to the same extent throughout.

Even a little bit of warming at the equator can be devastating to the organisms living there, Hsiang pointed out, simply because it’s so hot already. “Once it’s 95 degrees, one more degree can actually be very damaging to crops, very damaging to human health,” he said.

But because of the uniformity of temperatures near the equator, if populations in the tropics want to move to cooler areas as the climate warms they’ll likely need to cross through the entirety of the tropics to do so. And that has the potential to be a pretty long trip. The situation presents a kind of catch-22, in which moving from the tropics to cooler areas can involve a lengthy and difficult migration, but staying put can lead to serious consequences, as well.

To demonstrate just how much of a problem this could be as the planet continues to warm, Hsiang teamed up with atmospheric scientist Adam Sobel of Columbia University to create a model showing how populations in the tropics might be forced to migrate under future warming scenarios.  

The researchers adopted a scenario in which the average annual temperature on Earth rises by 2 degrees Celsius, a pretty modest warming scenario. (In fact, keeping global temperatures within 2 degrees of their pre-industrial levels is one of the major goals of the recently adopted Paris Agreement.) They then assumed that populations all over the planet — which were nonspecific in this study, and could represent any type of animal or other organism — would migrate to new areas that warmed to the same temperatures as their old habitats used to be. They also assumed that populations would move poleward to get to cooler ground, rather than simply changing their elevation (by climbing farther up a mountain, for instance, or swimming deeper into the sea).

These assumptions are not exactly intended to be realistic. After all, there are a host of factors that play into an organism’s migration patterns, and not every population will necessarily move in the ways assumed in this model — or even at all, for that matter. Rather, the study is intended to highlight certain properties about the tropics and the ways that they could, theoretically, exert pressure on the inhabitants there in the future.

Under the above assumptions, though, the researchers found that up to a quarter of the continental land encompassing the tropics would require inhabitants to travel at least 1,000 kilometers (that’s more than 600 miles) to get to the right conditions again. And since the ocean tends to warm in different patterns, up to 75 percent of oceanic locations in the tropics would require inhabitants to travel that far.

Assuming a uniform distribution of organisms throughout the world, the researchers found that the result is a massive pile-up at the edges of the tropics. While populations outside the tropics are able to migrate at more or less the same rate to get to cooler ground, populations close to the equator are forced to make a beeline for the subtropics to reach their ideal conditions. This means essentially the entirety of migrating populations living in the tropics end up halting and concentrating in more or less the same places.  

“It’s kind of analogous to when you’re driving on the highway and suddenly the cars in front of you all slow down,” Hsiang said. “It creates a traffic jam.” 

This kind of response could cause massive problems for regions at the edges of the tropics, he noted. The study suggests that populations in these regions — again, not referring to any specific type of animal at this point — could grow by 300 percent after the migrations simulated by the model. “That would be really hard for any system to handle, whether it’s an ecosystem or a human system,” Hsiang said.

Competition for resources, increased spread of infectious disease and even civil unrest or war in the case of human populations are just some of the consequences that could occur under such a scenario, the authors note in the paper. 

As an added experiment, the researchers applied their model to the current distribution of human populations on Earth. They found that, under all the same assumptions, 12.5 percent of the global population would be forced to migrate at least 1,000 kilometers, and up to a third of the population would have to move more than 500 kilometers. 

And even the movements of plants and animals in real life depend on a diverse array of factors, the authors have acknowledged. Some organisms may opt to shift their elevations in search of better climates, for instance, rather than moving poleward (although, as the researchers have noted, space tends to become limited the higher you climb, which can cause populations that adopt this method to shrink). And, of course, the availability of food and water, the type of landscape and the composition of ecosystems in other places all play into a species’ migration patterns as well.  

In this way, the new paper isn’t a wholly realistic depiction of what might happen in and around the tropics in the future. Rather, Hsiang said, it’s intended to present a theoretical framework that can help raise awareness about the unique properties of the tropics — and the challenges they could cause to their inhabitants.  

So the paper, unrealistic though some of its assumptions may be, gives one more reason to worry about the future effects of climate change. The 2-degree warming scenario presented in the study is considered a target under the Paris Agreement, but some experts remain dubious that it’s possible to keep global temperatures within those constraints without significantly greater commitments to carbon-cutting from nations around the world. And, of course, the effects described in Hsiang and Sobel’s paper would only be amplified under higher warming scenarios.  

“There’s got to be a lot more research into it — how powerful the dynamics are that sort of govern what we’re talking about,” Hsiang said. “We sort of are trying to raise it as an issue that people should look more into.”

By Chelsea Harvey June 9

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