The world – all with the exception of the US federal government, that is – has woken up to the threat of climate change. It is no longer an abstract scientific concept; it is the visceral experience of homes being burned down in wildfires, coastal communities being swamped, and major metropolitan centers being submerged by storm surges.

There are now also verified climate refugees: those that have fled from rising sea levels, increasingly potent cyclones, extreme heat waves, a lack of water and food, and climate-triggered conflict zones. Today, they number in the thousands, but according to a new Land Use Policy study, there will be a staggering 2 billion of them by the end of the century.

By then, there will be around 11 billion people on the planet, which means that over one-fifth of the entire population of the planet will be fleeing from climate change nightmares.

Today, climate refugees are mostly coming from sub-Saharan Africa, the Middle East, and small Pacific islands sinking beneath the waves. Within half a century, millions will be emerging from the shrinking coastlines of plenty of countries, including some of the most populous like Indonesia, Japan, India, China and, of course, the world’s sole superpower.

Without a doubt, people will be fleeing from Miami and the San Francisco Bay Area as the tides encroach ever closer.

A stunning animation of sea level anomalies over time. CSIRO/Emma Reed

By 2100, the mass migration of humans across the planet, from the shores to the more hilly cities and towns, will become a common occurrence. This will place a major strain on the economies of the metropolises seeing the greatest influxes, while those of the abandoned communities will decline rapidly.

It’s not even a simple matter of just moving people to new cities, either. “Even in the absence of land degradation, permafrost conversion, the trade-offs accompanying cities, roads, and dump sites, there are geopolitical barriers-to-entry spanning large areas,” the team from Cornell University note in their study.

Apart from the fact that this would empower a small handful of landowners to control who should be permitted to move into their territory, the team also point out that “ongoing wars and regional conflicts are a much-underestimated barrier to entry.”

The 2 billion figure is a worst-case scenario estimate. It assumes that climate change mitigation measures, like the Paris agreement, fail and fossil fuels do not get pushed out by low-carbon energy sources like wind, solar and nuclear power.

Although the authors aren’t entirely pessimistic about the future, they suggest that proactive measures need to be taken, as coastal climate refugees are all but guaranteed at this stage.

“Whatever the correct estimates turn out to be, intelligent and judicious future-proofing of the planet will require unprecedented transboundary effort, commitment, and collaboration,” they conclude.