Climate Change Could Force Huge Migrations for People and Animals Living Near the Equator

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.
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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, Solomon Hsiang, the study’s lead author,  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.
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Under the above assumptions ... 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.  
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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. 

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