March 12, 2011 — andyextance

A pioneering study to determine how climate influences where humans live has found that populations least responsible for causing climate change are most vulnerable to its effects. Scientists at McGill University, Montreal, Canada, applied a technique known as niche modelling, used to study how climate change affects plants and animals, in humans for the first time. With that approach they predict strongly negative impacts of climate change in Central America, central South America, the Arabian Peninsula, Southeast Asia and much of Africa.

That climate has any influence on where people can be found itself challenges existing ideas, explains lead author Jason Samson. “Previous studies found that climate was not important,” he told Simple Climate. “They would assume that everyone on the planet had the same relationship with climate.” Niche modelling, by contrast, assumes that climate limits species to a certain area. “In the north, normally it becomes too cold for species to be able to live or reproduce and in the south, there’s going to be competition with other species,” Samson said.

PhD student Samson had been using this approach to study beaver populations in the Québec region he’s based in. Whereas much niche modelling assesses only whether or not species can be found in a given area, he sought to develop models for how beaver numbers found changed from place to place. Then, when discussing his results with his PhD supervisor Murray Humphries, the picture of the Earth from space at night Samson had chosen as wallpaper on his computer provided inspiration. “He said, ‘Wow, wouldn’t it be great to be doing that with humans!’,” Samson recalled.

Too hot, too cold, just right

With this thought in mind Samson began trying to use his models to find links between population distribution and climate. He started with the Gridded Population of the World, a data set hosted by the Earth Institute at Columbia University that shows where people live down to a resolution of 18 km. Working at a lower resolution – using areas of one degree latitude by one degree longitude – the scientist tried to establish the connection using nineteen different models.

“On average those models would explain about 50 percent of the variation in human density,” Samson said. That is a stronger link to climate than often seen in niche modelling. “If you compare that to tree species or bird species then we’re pretty high,” the scientist said. In a paper published online in the scientific journal Global Ecology and Biogeography last month, Samson and colleagues found that in warmer parts of the world larger populations were associated with lower temperatures. Meanwhile, in colder areas of the world larger populations are associated with higher temperatures.

This shows that people in different parts of the world act differently, which, when you consider how people acclimatised to different conditions can respond under the same conditions, is understandable, Samson added. “If you’re travelling on a plane you always see someone ask for a blanket and a pair of socks, and you’re just taking your jacket off because you’re feeling way too hot,” he said. “Of course we have different perspectives about climate.”

Moral hazard

The McGill scientists then put the data from a general circulation climate model forecast for 2050 into their model to predict how populations will change over that time. They then used these numbers to give a climate–demography vulnerability index (CDVI) to show what degree of threat different regions faced from climate change. The locations least at risk by this measure are generally those most responsible for emitting greenhouse gases, the team found. This means that people living there are sheltered from the consequences of their actions, a situation known as “moral hazard”. Meanwhile those emitting least are most at risk.

“Montreal, a city of about 3 million people is pretty low vulnerability,” Samson explains. “In 2050 I predict that there are going to be 5 million people in Montreal. Whereas if you take Nairobi in Kenya, a city about the same size, my model predicts there are going to be about 1 million people in 2050.” As only about half of the population distribution is due to climate, Samson underlines that these figures do not necessarily reflect what the cities’ populations will actually be. Instead, they represent how climate change will affect them. “I used the term vulnerability, because I’m convinced actually that climate change will not suddenly make people massively migrate,” he said. “It just means that in terms of adapting to climate change, it’s going to be harder for people in Kenya than it’s going to be for Montreal.”

The idea of linking population distribution to climate has attracted interest from a number of scientists since the paper was published, Samson says. However, it was difficult to get through the process of peer-review, where other scientists assess the research to judge whether it’s worthy of publication. Nevertheless, even before being accepted for Global Ecology and Biogeography, previous reviewers were intrigued. “I tried the top three journals, and I would get at least one out of the three reviewers at each that were interested and thought that it was worthwhile,” he said.

Now it’s been published, Samson hopes that the findings will be useful to people like those trying to organise water supplies for the future. But he acknowledges that his work will continue to be controversial – a fact that he thinks is partly down to the fact that people are reluctant to accept the role climate has in their lives. “I can easily see why people wouldn’t like these kind of findings, because we don’t think that nature can affect us,” Samson says. “I can have peaches and strawberries, and I’m going to be nice and cosy in my 21°C apartment, and there’s going to be about 30 centimetres of snow that will fall tonight. We tend to forget we’re linked to the environment.”