The mother of all floods

It was around 18,000 years ago that the climate pendulum swung in favour of warmer times. The Earth continued to orbit the Sun as it had done through the ice age but summers in the northern hemisphere started to see a little more warmth. At first, it was just a glimmer; a little extra warmth here and there. Over the next few thousand years, however, the heat from the Sun continued to rise inexorably over the high latitudes of the northern hemisphere. The effect was striking. Temperatures rose 4° to 7 °C and many of the ice sheets in the north began to melt.

In North America, the Laurentide Ice Sheet persisted longer than most, although by 9,000 years ago it was a shadow of its former self. As the ice shrank back to the Hudson Bay, some of the water ponded up inland over what is now central north Canada, building a vast lake known as Lake Agassiz. It was only a matter of time before the ice barrier gave way.

The Black Sea

Sometime between 8,700 and 8,200 years ago the dam finally broke. It was a cataclysmic event. A torrent of freshwater and icebergs flooded into the North Atlantic. It was the largest single pulse of freshwater over the past 100,000 years. The impact was virtually instantaneous. In the North Atlantic, ocean circulation slowed down, stopping warm tropical waters from getting north. Temperatures plummeted and stayed low for nearly 200 years. The most detailed records come from the Greenland ice cores and suggest that it became 6 °C cooler over the ice sheet. Downwind, Europe also felt the crunch, with the main rain belt shifting south and temperatures dropping by nearly 2 °C. Similar things look like they happened in equatorial regions as well, with the tropical rain belts moving south. At the same time, methane levels in the air dropped 15%, suggesting wetland areas became cooler and drier.

Changes in temperature and rainfall weren’t the only thing that happened. Other effects were felt further afield. With all the extra water dumped in the North Atlantic, the world’s sea level rose by up to 1.4 metres. Many areas on the continental shelf suddenly flooded.

On the boundary between east and west, the Black Sea has historically played a major role in trade between Asia and Europe. But back in the last ice age the Black Sea remained separate from the Mediterranean by an enormous ridge known as the Bosporus Sill. The sea was kept back, allowing a large freshwater lake to flourish. Large parts of the old lake have been mapped with sonar. The results show a world 155 metres below today’s sea surface, with meandering river valleys, deltas and beaches. As the world’s ice sheets started to melt, the Mediterranean converged on the Bosporus Sill. Eventually the dam was breached and a cascade of seawater poured into the lake. It looks like it took over 30 years for the Black Sea to fill, flooding an area equivalent to the size of Ireland in the process.

One way to date when the Black Sea flooded is to look at what was living in the water. At one point, the shells of freshwater molluscs change to those that thrive in seawater. Radiocarbon dating these remains shows that the Black Sea flooded around 8,300 years ago, the same time as Lake Agassiz poured into the North Atlantic and Britain got separated from Europe. It looks like the collapse of the last North American ice was enough to make the Black Sea join the rest of the world. It was the straw that broke the camel’s back.

Importantly, it doesn’t look like the pulse of freshwater into the North Atlantic caused instant cooling. Recent modeling results by Allegra LeGrande and colleagues at the NASA Goddard Institute for Space Studies suggest it took several decades after Lake Agassiz burst before the ocean reached its coldest temperatures; as the freshwater poured into the North Atlantic, deepwater formation gradually slowed until half its production had stopped. It appears that the collapse of Lake Agassiz and the flooding of the Black Sea happened before the coldest temperatures.

In 1997, geologists William Ryan and Walter Pitman of the Lamont-Doherty Earth Observatory of Columbia University in Palisades, New York, provoked a huge storm that is still reverberating around the corridors of academia. It had been known for some time that if you look at the archaeological evidence, farming suddenly seemed to sweep across Europe around 8,000 years ago. Might the flooding of the Black Sea have been the push that got farming into Europe? Could it have also been the origin of the Noah’s Ark story and other flood myths in the region?

The earliest evidence for Western agriculture is in the Near East and dates back around 12,900 years ago. It’s here that many wild varieties of today’s crops were first cultivated; wheat, barley and rye all grow wild in the region. But it’s still not clear why people suddenly started domesticating plants and animals at this time. We know that some groups had been living in the region from at least 23,000 years ago. These people used some wild varieties but it doesn’t look like there was any earlier attempt to farm them. Something changed around 12,900 years ago. Perhaps the population had started to grow? Perhaps cool conditions were an extra stimulus to cultivate wild plants and get better yields? Perhaps it was a combination of these and other things? Regardless, the result was that by 11,500 years ago, groups in the Near East were practising early farming. Slowly the newfangled technique started to get a foothold in other regions. Hunter-gathering became less popular. But once farming got across to Greece, expansion seemed to slow down.

When you take a close look at the ages for the earliest farming in Europe, an intriguing pattern emerges. The spread of agriculture out from the Near East into Turkey, Cyprus and then Greece suddenly stops around 8,300 years ago. It was suddenly not the thing to start trying. But this appears to have been a short-lived blip. By 8,200 years ago there was an explosion of farming across continental Europe; by 7,300 years ago it was widespread. By this time the British Isles was separated from the European mainland and took longer to catch on; farming didn’t become didn’t become popular here for another 1,600 years.

One view is that the expansion in farming was the result of hearing about how great it was. As farming became established, word got out into the neighbouring areas. Hunter-gatherers saw the obvious benefits, it was argued, laid down their spears and joined the party; farming became the choice of most alpha one males. This must have happened to some extent, but why the collapse in farming 8,300 years ago and then sudden explosion across the region? Word of mouth doesn’t explain all the evidence.

Colder conditions can’t have been the cause for the slowdown in farming either. The Greenland ice cores show the lowest temperatures happened 8,200 years ago, yet this was when farming was becoming popular again. An alternative is the sudden flooding of large areas of the European coastline and Black Sea forced early people to move on. Maybe it was early farmers living on the shores of the Black Sea. Alternatively, hunter gatherers might have been the ones who had to relocate fast; if so, they would have caused a headache for farmers living in neighbouring areas. Either way, it would have certainly seemed like the end of the world to those living near the shoreline. As the tides rose, did people move around the coast and inland, taking farming and tales of destruction with them? If true, it shows that it’s not just abrupt shifts in temperature that can have a big effect on people. A change in sea level looks like it might have had a disproportionate impact on our ancestors, driving mass migration and cultural change. It’s a disconcerting omen for the future.

Chris’ popular science book Bones, Rocks and Stars: The Science of When Things Happened is now out in paperback (Published by Palgrave Macmillan; ISBN:0-230-55194-7).

Reference: Turney, C.S.M. and Brown, H. (2007) Catastrophic early Holocene sea level rise, human migration and the Neolithic transition in Europe. Quaternary Science Reviews, 26, 2036-2041.