A bunch of climate change negotiator bigwigs got stranded together overnight in Cape Town on their way to Antarctica. They chatted about climate change and, by the sound of it, freaked each other out. The conclusion they came to over coffee and wine: if their governments don’t deal with climate change decisively, “what we’re talking about…is extended world war.”

[I]t was Stern, former chief World Bank economist, who on Saturday laid out a case to his stranded companions in sobering PowerPoint detail.

If the world’s nations act responsibly, Stern said, they will achieve “zero-carbon” electricity production and zero-carbon road transport by 2050 — by replacing coal power plants with wind, solar or other energy sources that emit no carbon dioxide, and fossil fuel-burning vehicles with cars running on electric or other “clean” energy.

Then warming could be contained to a 2-degree-Celsius (3.4-degree-Fahrenheit) rise this century, he said.

But if negotiators falter, if emissions reductions are not made soon and deep, the severe climate shifts and sea-level rises projected by scientists would be “disastrous.”

It would “transform where people can live,” Stern said. “People would move on a massive scale. Hundreds of millions, probably billions of people would have to move if you talk about 4-, 5-, 6-degree increases” — 7 to 10 degrees Fahrenheit. And that would mean extended global conflict, “because there’s no way the world can handle that kind of population move in the time period in which it would take place.”

The thing is, this has all happened before. It was called the Migration period:

This was what destroyed/changed/forced into retreat the western Roman Empire. It’s what led to the invasion and settlement of Britain by Angles, Saxons and Jutes (and the subsequent fleeing of Britain by the British to Brittany). Basically, it was the precondition that led to Hild.

Yes, I’m being simplistic. People can argue til the cows come home about politics and war, famine and plague, volcanos and earthquakes, cause and effect, but I see it all as water circling the same plug hole: climate.

Climate–whether influenced by all that Roman metal processing and forest clearing, or part of a natural cycle, or catastrophically altered by volcanic eruption–will work upon sea levels, temperatures, breeding and mutation rates of disease, animal migration, crop production, which will in turn lead to starvation and disease, and mass migration. Migration leads to conflict–to war, small scale or large.

The fifth and sixth centuries in Britain used to be called the Dark Ages for a reason. (We don’t call them that now, because we have learnt to interpret evidence, historiographical and archaeological and linguistic, well enough to shed some light on the era. Now we call it Late Antiquity, or Sub-Roman, or Early Medieval, or the Heroic Age.) For a good chunk of the population, these were grim and uncertain times. Britain lost literacy, central heating, medical care, road maintenance, currency, centralised manufacturing. (It gained other things, such as sophisticated weaving skills, fantastic metalwork, expertise in wood working, new farming methods, and more.) But the change was profound. Frankly I worry about history repeating itself.