Thirteen thousand years ago, much of North America was covered in a giant lake now known as Lake Agassiz. Great lobes of ice that would have looked like 'The Wall' in Game of Thrones acted as dams but, when these gave way, the lake drained suddenly and raised the level of the world's oceans, dramatically changing its climate and lives of everyone on Earth.
In 1879, American geologist Warren Upham was surveying the flat fertile plains of Manitoba and Red River Valley in North America when he came across features that could only be beach ridges and wave-cut terraces. Yet he was hundreds of miles from the sea.
Upham realised that he had found signs of a long lost giant lake larger than all of America's Great Lakes combined (and bigger than France). He named it after Swiss-American Louis Agassiz, a scientist who had pioneered the idea of ice ages. Upham realised that the huge vanished lake was deeply connected with past climate events, including the rise and fall of ancient ice ages.
Upham was not wrong but it has taken many later scientific study to discover the massive impact that the serial draining and filling of Lake Agassiz has had on the world's climate; extinctions of huge numbers of megafauna; the lives of hundreds of thousands of early people; and the shaping of much of North America's landscape. The story also has important lessons and warnings for us today as we attempt to understand and prepare for the impacts of sudden climate change.
So how did Lake Agassiz form in the first place? From about 110000 years ago, most of North America was covered by the Laurentide Ice Sheet, which was up to three kilometres thick. About 16,000 years ago, global temperatures began to warm and the ice sheet started to melt. However, as much of the ice sheet's rim was cooler and higher, the water has nowhere to go and pooled inland. A lake formed that covered 440000 square km – roughly the size of today's Black Sea.
It was a very dynamic body of water. As the ice sheet retreated, different drainage paths opened and closed and the lake's boundaries constantly shifted. It first drained south, creating the vast valley of the Minnesota River, which ultimately drained into the Gulf of Mexico. It later found paths through north-east through the Great Lakes Basin and northwest through the the Mackenzie River system.
But the biggest and most dramatic drainage occurred directly north through the Hudson Bay. It was a sudden event about 13000 years ago, when a huge pulse of freshwater from the lake entered the Arctic and Atlantic Oceans in a matter of months. While there is no definitive proof of causation, this event coincides with a 'climatic whiplash' known as the Younger Dryas when global warming was suddenly reversed for 1200 years and the northern hemisphere's climate in particular plummeted by 3°C. Other theories are that the Younger Dryas was caused by volcanic eruptions of even a meteor strike.
Some scientists have argued the sudden influx freshwater from Lake Agassiz overturned the great Atlantic circulation that typically brings warm water north and cold water south. The subsequent abrupt cooling – the Younger Dryas - is blamed for wiping out many species of megafauna in North America, including mammoths, sabre-toothed cats and giant sloths.
Lake Agassiz persisted until around 8400 years ago when the final ice walls gave way and there was again a sudden outpouring of vast amounts of water into the Arctic Ocean in a matter of months or even weeks. This raised global sea levels by up to 2.8m in a year, flooding vast areas.
Many indigenous cultures in North America, including the Ojibwe and Cree peoples, have oral histories that talk of great floods and displacements in the distant past. Could this be connected? Some students of prehistory have suggested that this flooding event could account for persistent flood myths in cultures from Mesopotamia, China and India. Could the great Biblical flood when Noah built the ark also be attributed to Lake Agassiz?
As Lake Agassiz finally disappeared, it left a vast area of rich, fertile flat land (now the Canadian prairies and the American Midwest) and opened up new areas for exploration and colonisation for the people of the time. But not all of it disappeared: Lake Winnipeg, Lake Winnipegosis, Lake of the Woods and Lake Manitoba are all remnants of the greatest North American lake.
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Lake Agassiz offers many lessons and warnings to modern climate scientists in how quickly and dangerously the Earth's climate can change due to sudden ice loss and movements of huge amounts of water. While there are no lakes on the scale of Agassiz remaining on Earth, there are huge number of glacial lakes where melting ice dams hold back vast quantities of water. The issue was very brutally underlined in August this year when an ice and rock dam collapsed on the Nepal-Chinese border resulting in sudden, catastrophic floods in Nepal and Tibet and the loss of thousands of lives.
Top image: Map of Glacial Lake Agassiz and Lake Ojibway ca 7900 YBP. Designed from Teller and Leverington, 2004 (U.S. Geological Survey) Credit: Chris Light
