The Effects of Sea Level Rise

Recent reports would have us believe that coastlines around the world are in danger of being flooded by extreme sea level rise – primarily from melting glaciers and polar ice caps.

But what is the true history of sea level rise? And how did humans cope with it in the past centuries?

The greatest rises in sea level have come at the end of major ice ages during the last few million or so years. Technically, we are presently in what is known as an interglacial period, this one called the Holocene. Major ice ages over the last several million years appear to occur about every 40 to 100 thousand years. The Holocene is about 10,000 years old.

Most of the “recent” (1,000s of years) rise in sea level occurred as the ice sheets of the Wisconsin Glacial Episode were melting, beginning around 20,000 years BP (before the present). Melting “rapidly” (in terms of geologic time) picked up speed around 14,000 years BHP. Most of the ice was gone, and sea level was close to its maximum position about 7,000 years BP. Since then there have been minor adjustments up and down as climate alternately cooled and warmed.

Geological data show that the total rise in sea level during the last 20,000 years has been about 425 feet (130 metres). During the last 7,000 years it has come up only about 20 feet.

Interestingly, the Greenland ice sheet has been estimated to be melting in recent years at about 67 cubic miles (280 cubic kilometres) per year. That sounds like an enormous quantity but given its current volume of 684,000 cubic miles (2.85 million cubic kilometres), it would take over 10,000 years to completely melt. Looking at the last million years of history, the Earth would be well into another major ice age long before then making it highly unlikely the Greenland Ice Cap could ever be gone. Ditto for the Antarctic ice polar cap.

The amount of water contained within the Greenland Ice Cap, if it entirely melted at current rates, would add about 0.2 mm to sea level.

So, what does melting-glaciers and ice caps have to do with genealogy? Well, most of what we look for in terms of family history records was produced during the Little Ice Age (1300 to 1850 AD). During that cold period glaciers advanced and ice packs extended from the polar regions. At its maximum state, sea level dropped about four feet.

A drop of a few feet was not enough to disrupt habitation on a major scale, but in many regions along the sea coasts, it was enough to expose shallow marine shelves. These areas were covered by fine-grained material, eroded from the continents and carried to the oceans. They were also unconsolidated and subject to easy relatively removal during storms and high winds. The results were buildups of coastal dunes and beaches, and in some cases burial of farms and villages. Port areas were also affected as water levels dropped with wharves then having to be built out further seaward.

Family historians may find that coastal communities where their ancestors lived were altered by wind-blown sand covering their fields. Examples include the Culbin Forest, Morayshire, along the northern coast of Scotland, where sands covered the farmland in the late 17^th century. The Forvie area of Aberdeenshire experienced burial by sand dunes over several thousand years but the greatest impact was during the 15^th and 16^th centuries – the Little Ice Age period.

Major storms and wave action provided the energy and means to erode shorelines. Such has been the case along much of the eastern and southern coasts of England. Dunwich in Suffolk, England, is a prime example of where the coastline has been eroded back over one half-mile since the 14^th century, completely eradicating the ancient town.

Sea level rise does not fall into the category of disasters, but gradual changes over decades would have affected lives and livelihoods, even causing people to move. This happened more than a few times before the Little Ice Age – with seas rising during warm climatic periods and falling during the cold ones. Just changes of a few feet affected many coastal areas of Europe.

Some communities, notably the Dutch, found ways of mitigating against the changes in sea level by the construction of dikes. In addition, because they were hemmed in by other groups and could not expand their agricultural base, they learned to convert the swampy areas to productive farmland by pumping out the excess water.

Humans always adapt to Mother Nature’s whims. We are in no danger of seeing major changes in sea level in the future, though, not even the few feet observed during the rise during the Medieval Warm Period (900 to 1300 AD) or the drop during the Little Ice Age. But changes will continue to occur naturally along all coastlines around the world.