11,600 BP

The younger dryas period very suddenly ends. Estimates of how rapidly average temperature rose vary, but a study from The Earth Institute at Columbia University states that the temperature over Greenland rose by 10 degrees in just ten years. In human terms the change probably happened in a single generation. If so, it would seem a miraculous transition, with plants and animals growing much more strongly, creating abundant food. The Columbia University study describes this change as ‘spectacular’. Perhaps the reason it happened was spectacular too.

In physical terms, the Earth’s weather is an energetic system. Energy is needed to create weather and energy drives the weather systems. The energy for this of course comes from sunlight. I remember teaching this using sea breezes. In the morning the sun heats the land faster than the sea, causing the warm air created over the land to rise, which pulls air from over the sea to replace it. The breeze in the morning is onshore on a sunny day. By the middle of the day the sea, with its greater thermal capacity, has finally warmed up, even if not as much as the land, so the breeze dies down. Then at night, the land cools very quickly but the sea much more slowly. This means that the air over the sea becomes warmer than other the land, and as this warmer air rises it pulls air from over the land, creating an off-shore breeze.

This is a small example. It is the sunshine in the tropics that is the great driver of weather. It creates very warm air and evaporates huge quantities of water from the sea. At its extreme it creates hurricanes but every day it is the big temperature difference between the day and night sides of the Earth and between the poles and the tropics that create our global weather systems.

The climatologists do not know what caused the temperature to rise so suddenly at the end of the younger dryas period. We can speculate on this as there are no solid theories yet available. Suppose that the event that started the younger dryas period created dust and water clouds right up in the stratosphere, that enveloped the whole Earth in dark clouds, that reflected a considerable part of the sunshine arriving at Earth back into space. Not only would the Earth cool, but the temperature differences between the night and day sides and between the tropics and the poles would be markedly reduced. There would be far less energy to drive weather systems. Winds would lessen as would rain, as there would be no bright sunshine in the tropics to evaporate the sea to create rain.

Very fine particles that volcanoes manage to shoot up into the stratosphere, above our current weather systems, take many years to return to the surface of the Earth. Even single big eruptions have dimmed sunlight globally for several years after erupting, as was noticed in the years after the Mount Tambora eruption in 1815. Fine particles remain suspended in the atmosphere and are not dragged back to earth by gravity as larger particles are. The Krakatau eruption column in 1883 reached up to 40km high, well into the stratosphere which starts about 18km up.

While we have very energetic weather systems, there is considerable washing out of the atmosphere by clouds. Cumulonimbus cloud can reach up to around 12km high, and cirrus clouds are this high. But if the Earth was blanketed by clouds, the energy to lift clouds this high might not have been available. Weather system clouds might have been restricted to well below the stratosphere, reducing washing-out of fine debris in the stratosphere to a very low level.

Could it be possible that a combination of a comet impact vaporising the Laurentide ice sheet and ejecting huge volumes of water vapour into the stratosphere, combined with worldwide volcanic eruptions triggered by the comet impact, ejecting huge volumes of dust right up into the stratosphere, created a global cloud cover in the stratosphere that persisted for 1200 years? If the energy to drive weather systems lower down in the atmosphere was not available, could there also have been a high and thick cloud layer in the troposphere - from the surface of the Earth all the way up to the start of the stratosphere?

As I said, this is pure speculation, but something caused the younger dryas and something made it end very suddenly. If removal of the energy that drives weather systems started this and triggered a stable cloud cover from very high altitude all the way down to close to the Earth’s surface, could this persist for 1200 years. And could there come a point when the cloud cover thinned sufficiently for the sun to start getting through to the surface? And could this rapidly reactivate weather systems sufficiently powerful to break down the remaining clouds in just 10 years?

If there was a generation of humans that saw their world going from complete cloud cover to the kind of weather we now experience this would surely have got recorded in the stories. It would have been a truly spectacular event.

For 1200 years mankind would have been living, generation after generation, with a story passed down of a great flood and disaster, with the moon and stars in the heavens being taken away. For these to suddenly re-appear, to be replaced in their rightful place in the heavens, would indeed be miraculous.

If there are such stories, perhaps these are pointers to my speculation above maybe being true. Something caused the younger dryas period to suddenly end.

Read the story for more.