Medieval Warm Period

following a path that had been walked by their parents and grandparents and great-grandparents before them.

And then the pass was abandoned, the climate cooled, the medieval warm period gave way to the Little Ice Age, the ice advanced, the route became impassable, and the path that had been walked for generations disappeared under a growing blanket of frozen precipitation.

What had been a road became a barrier, and the ice preserved not a single dramatic event.

Not a battle, or a catastrophe, or a moment of historical significance, but a cross-section of human traffic.

The pass sits at an elevation of roughly 2,756 metres, too high to cross comfortably in all but the warmest conditions, and completely impassable when the ice covers it.

The artefacts acted as a kind of proxy record for temperature, because people only used the pass when it was warm enough for the ice to retreat, and the dates of the artefacts clustered into distinct groups corresponding to known warm periods in the climate record.

The Roman Warm Period The Medieval Warm Period and further back, warm, intervals during the Bronze Age and the Neolithic that are known from other climate proxies, tree rings, pollen records, lake sediments, but are here confirmed by the simple, eloquent evidence of human objects lying on ground that is only exposed when the temperature is high enough.

The pass was a thermometer made of shoes and pins and fragments of birch bark and it had been taking readings for 5,000 years but not all of what the ice returns is ancient.

From England to Ireland to the Shetland Islands and Faroes, Iceland was simply the most distant land that they colonized.

In this, the Norse were helped by something known as the Medieval Warm Period, a four-century summer holiday in which temperatures spiked around Europe, making even an island on the edge of the Arctic Circle easily habitable.

All told, about 20,000 people would settle Iceland, and that was a major problem for Erik and his pa.

By the time they showed up in the 1960s, all the good land was already gone.

Or sorry, what these data indicate are that temperature, the sea surface temperature, in this case, taken from the Makassar Strait between the islands of Kalimantan or Borneo and Sulawesi in Indonesia.

These, the sea surface temperature has fluctuated over the past two thousand years, where we have warm periods and cold periods also similar to the temperature, to the world temperature, where we have warm periods in the Iron Age and then cold period in the Dark Age, warm period again in the High Middle Ages during the Medieval Warm Period, and then the cold period again in a little ice age similar to the temperature.

And then we have our current warm period where we see a warming of the sea surface temperature, which again, is a recovery from the little ice age.

And the fact that we had warm and cold periods happening while the CO2 concentration remains constant at about three hundred and eighty parts per million prior to the Industrial Revolution also is an indication that CO2 is not the only factor that affects sea surface temperature.

... that's, that's a Western Hemisphere-centric way of thinking about an anomaly in climate, because actually in other parts of the world it was colder at that time.

So we now call it the Medieval Climate Anomaly.

Okay.

And then that was the period when the Vikings were able to colonize Greenland, for example.

Just climate shifts.

So we're moving out of the Medieval Warm Period into the Little Ice Age here-

Mm

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