The movie The Day After Tomorrow saw the planet globally affected by the cessation of the ocean conveyor belt, or, more precisely known as the thermohaline circulation (THC). The northern hemisphere suffered massive drops in temperature, rises in sea level and a variety of other climate conditions.
Putting aside the fantastical nature of the speed with which this happened, the base science is sound; that an increase in freshwater could slow or shutdown the thermohaline circulation, causing an unexpected and unhelpful ice age.
Now to be fair, the freshwater increase would necessitate the entire Greenland ice sheet to dissipate for this to happen, but given a timeframe of decades to centuries, this is not entirely out of the question. In addition, we know for a fact that a sudden increase in freshwater will have significant effects in the northern hemisphere, because Earth has already seen it happen.
The Younger Dryas was a brief period of cold climate approximately 12,800 to 11,500 years Before Present. Prior to the Younger Dryas, a period of massive deglaciation took place across North America, leaving behind freshwater lakes fed by glacial runoff. One of these freshwater lakes was Lake Agassiz, a lake that measured in at approximately the size of Iraq, covering much of Manitoba, western Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan (see image below).
Climatologists believe that Lake Agassiz drained through the Great Lakes and Saint Lawrence River into the Atlantic Ocean, at approximately 11000 BC, causing the strangling of the THC.
Scientists note that if such an event were to occur, there is nothing we could do to stop it; we will have gone too far already. However Ed Hawkins and Rowan Sutton of the University of Reading, UK, have used a climate model developed by the Met Office’s Hadley Centre to give us at least a warning.
The study “is the first to demonstrate that such rapid changes are potentially predictable,” Hawkins says. “It is a first step in designing a possible ‘early warning system’.”
Their climate simulation ran for 1100 virtual years, based on pre-industrial C02 data, and showed several natural fluctuations big enough to make the THC slow or speed up by about 15% within a decade. In the data though, a few locations within the North Atlantic Ocean showed signs of the oncoming change, giving a warming of as much as 10 years.
These indicators were a drop in the sea-surface temperature and salinity of water in the Nordic seas, as well as a drop in flow through the Denmark Strait.
“Ten years’ warning would not be enough to do anything about the change itself, but could aid adaptation planning on regional scales,” Hawkins says. As depicted in the movie, when the cessation of the THC happened, the planet was unable to cope, and millions died, while millions more streamed across their southern borders looking for warmer weather.
This simulation is not necessarily helpful in terms of a world in the grips of climate change, as it only looked at natural variations. “The markers for a slowdown due to global warming might be very different,” Hawkins says. And accortding to the latest IPCC report, if global warming continues as it has been, it is “very likely” that the current will slow this century.
Australia, South America, Africa, be prepared for an influx of residents sometime in the next few decades!
Source: New Scientist