I’ve had an obsession with maps ever since I first sought my bearings in Winnie the Pooh’s “Hundred Aker Wood”, trying to discover where was “nice for piknicks”, and the locations of the characters’ houses. My childhood was spent studying and drawing treasure maps, charting imaginary lands and plotting routes to faraway places I longed to visit.

Today, my home is plastered with the maps I’ve collected or been given – reminders of places that are special to me. By my desk, I have a large world map, the continents distinguished from the oceans by their mosaic of colours. Each coloured patch is a country, separated from its neighbour by a neat line drawn onto this two-dimensional representation of our world.

The borders are cleanly defined, ink separating nationalities destined for different fates. For me, these lines mark exciting possibilities, with the potential for exploration and adventure, to visit foreign cultures with different foods and languages. For others, they are prison walls that limit all possibilities.

Borders define our fate, our life expectancy, our identity, and so much more. Yet they are an invention just like the maps I used to draw. Our borders don’t exist as immutable facets of the landscape, they are not natural parts of our planet, and were invented relatively recently.

It can be argued, however, that most of these imaginary lines are not fit for the world of the 21st Century with its soaring population, dramatic climate change and resource scarcity. Indeed, the idea of keeping foreign people out using borders is relatively recent. States used to be far more concerned about stopping people from leaving than preventing their arrival. They needed their labour and taxes, and emigration still poses a headache for many states.

There are, however, true human borders set not by politics or hereditary sovereigns, but by the physical properties of our planet. These planetary borders for our mammal species are defined by geography and climate. Humans cannot live in large numbers in Antarctica or in the Sahara Desert, for instance. As global temperatures increase, causing climate change, sea level rise and extreme weather over the coming decades, large parts of the world that are home to some of the biggest populations will become increasingly hard to live in. Coastlines, island states and major cities in the tropics will be among the hardest hit, according to predictions by climate scientists.

Unable to adapt to increasingly extreme conditions, millions – or even billions – of people will need to move.

Borders are often now a barrier to movement, but might that have to change in the future? (Credit: Nicolas Economou/Getty Images)

The most densely populated areas of the planet are clustered around the 25-26th north parallels which has traditionally been the latitude of most comfortable climate and fertile land. An estimated 279 million people are packed into this thin band of land, which cuts through countries including India, Pakistan, Bangladesh, China, the United States and Mexico.

But the conditions here are changing. On average, climate niches – the range of conditions at which species can normally exist – around the world are moving polewards at a pace of 1.15m (3.8ft) per day, although it’s far faster in some places. Adapting to the changing climate will mean chasing our own shifting niche – which for much of human history has been within the temperature range -11C to 15C (12F to 59F) – as it migrates north from the equator. True livability limits are the borders we must worry about as the world warms over this century, bringing unbearable heat, drought, floods, fires, storms, and coastal erosion that make agriculture impossible and displace people.

Already record numbers of people are being forced to flee their homes with each passing year. In 2021, there were 89.3 million people, double the number forcibly displayed a decade ago, and in 2022 that number reached 100 million, with climate disasters displacing many more people than conflicts. Floods displaced 33 million people in Pakistan this year, while millions more in Africa have been affected by drought and the threat of famine, from the Horn of Africa to the continent’s west coast.

UN High Commissioner for Refugees Filippo Grandi appealed to global leaders at the COP27 climate change conference to take bold action to tackle the humanitarian consequences of global warming. That change needs to be “transformational” according to the UNHRC. “We cannot leave millions of displaced people and their hosts to face the consequences of a changing climate alone,” says Grandi.

Without action, hundreds of millions people will have to leave their homes by 2050, according some estimates. One study from 2020 predicts that by 2070, depending on scenarios of population growth and warming, “one to three billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000 years”.

With so many people on the move, will this mean that invented political borders, ostensibly imposed for national security, become increasingly meaningless? The threat posed by climate change and its social reper­cussions dwarf those surrounding national security. Heatwaves already kill more people than those who die as a direct result of violence in wars.

Compounding this, the global population is still growing, particularly in some of the regions worst hit by cli­mate change and poverty. Populations in Africa are set to almost triple by 2100, even as those elsewhere slow in growth. This means there will be a greater number of people in the very areas that are likely to be worst affected by extreme heat, drought and catastrophic storms. A greater number of people will also need food, water, power, housing and resources, just as these become ever ­harder to supply.

Meanwhile, most countries in the Global North are facing a demographic crisis in which people are not having enough babies to support an ageing population. Managed mass migration could thus help with many of the world’s biggest problems, reducing the number of people living in poverty and climate devastation, and helping northern economies build their workforce.

But the main barrier is our system of borders – movement restrictions either imposed by someone’s own state or by the states they wish to enter. Today just over 3% of the global population are international migrants. However, migrants contrib­ute around 10% of global GDP or $6.7tn (£5.9tn) – some $3tn (£2.6tn) more than they would have produced in their origin coun­tries. Some economists, such as Michael Clemens at the Center for Global Development in the US, calculate that enabling free movement could double global GDP. In addition, we would see an increase in cultural diversity, which studies show improves innovation. At a time when we have to solve unprecedented environmental and social chal