Image in header: Average temperatures on Greenland over the last 17 thousand years, reconstructed from the examination of deep drill cores in the ice. Unfortunately, the temperature scale is in Fahrenheit. -20°F is about -29°C (top y-axis) and -60°F is about -51°C (bottom y-axis). Source image; Researchgate (‘credits: Azimuth Project 75’).
Wild climate changes after the last great ice age
Our planet has moved into a colder climate over the last 3 million years. Within this period, fluctuations can be seen between longer ice ages or glacials and shorter warmer periods or interglacials. These repeating cycles are named after researcher Milancovic. They are caused by repeating variations in ’tilt’ of the Earth (obliquity and precession) and the shape of Earth’s orbit around the Sun (eccentricity). The Weichselian was the last of these ice ages and began about 116 thousand years ago. At the end of this Weichselian, the warmer period would begin (in which we are still living today), but this intergalactic started with very remarkable and violent climate fluctuations, to say the least, most pronounced in the northern hemisphere. Briefly, around 14 thousand years ago, it very suddenly got 10 degrees warmer on average (end of the glacial). But immediately afterwards – around 12 700 years ago – the average temperature dipped sharply again just as suddenly. This new mini-ice age is called the Young Dryas. It lasted for about 1,200 years, before turning back to warmer climate. Around 11,560 years ago, the warmer interglacial began until today.
During the Young Dryas, North America and Europe had tundra and cold deserts south of the newly formed ice sheets. One of the plants that grew a lot in that tundra was Dryas octopetala or silverweed (now found in the Alps). Hence the name of this sudden mini-ice age.
According to some estimates, the Young Dryas started with an average cooling of 10°C in just 10 years! These determinations are based on ice drills in Greenland. Possibly the cooling was less severe in southern areas, but even then we are talking about a few decades at most. This is extremely fast. How can this be explained?
Help ! Blocked conveyor belt
There are several hypotheses. Some relate it to a meteorite impact that threw up enough dust to partially shield the Earth from solar energy for long enough. But the most popular explanation is the cessation of the ‘conveyor belt’, the global sea currents that transport tropical heat to the poles. The Gulf Stream between Mexico and northern Europe is said to have shut down due to the influx of ernome amounts of freshwater from North America. This went as follows:
Around 14,700 years ago, global warming started, and you got a rapid melting of huge ice sheets in Europe and America. In North America, this allowed the Agassiz Sea to form. A giant inland sea of cold meltwater in the region north of ‘The Great Lakes’ lie. Northeast of the Agassiz Sea, the giant meltwater lake was blocked by large ice dams. As these ice dams were also melting, the Agassiz water began to escape towards the Atlantic Ocean. Some of the meltwater also flowed eastward and southward towards the US interior (there are worldwide legends and stories of deluges from that period), but the northeastward flow towards the Atlantic Ocean gradually became much larger. As a result, millions of cubic metres entered the ocean, mixing with the Gulf Stream saltwater flows. The mixing with freshwater makes the seawater lighter. As a result, the Gulf Stream water could no longer sink near northern Europe. The entire ‘conveyor belt’ of heat eventually stopped on this. The effect was greatest and fastest in the continents immediately bordering the North Atlantic (Europe, Greenland, North America), but was also felt elsewhere in the world.
This well-documented event nicely illustrates the importance of heat transport via ocean currents. A similar scenario is feared by current climate changes due to fossil carbon (fuel) reintroductions. If the ice sheets in Greenland and other Arctic regions melt completely, this could again block the Gulf Stream.