A sudden thaw
Deep in the sediment layers of Bølling Sø (Bølling Lake), a quiet body of water in central Jutland, records one of Earth's most abrupt climate shifts. Around 14,700 years ago, the planet, which had been locked in a deep freeze for millennia, suddenly warmed. Ice core records from Greenland show that temperatures there may have increased by as much as 7°C or more in just a few decades. This rapid warming is the beginning of the Bølling-Allerød interstadial, a warm period near the end of the last glacial period that lasted until about 12,890 years ago.
The evidence for this change was first pieced together in Denmark through the science of palynology, the study of pollen grains and other microscopic plant structures. In 1901, geologists Nikolaj Hartz and Vilhelm Milthers began studying deposits from a clay pit near the town of Allerød and later from the drained peat bogs of Bølling Sø. They discovered distinct layers containing abundant birch tree pollen (Betula) sandwiched between layers dominated by pollen from cold-adapted tundra plants, such as mountain avens (Dryas octopetala). This shift in vegetation from open, treeless landscapes to birch-dominant forests showed a rapid and substantial warming event. Birch trees showed that temperatures had become warm enough to support forest growth where once only hardy, low-lying shrubs could survive.
An ocean-atmosphere reshuffle
The Bølling-Allerød warming is linked to major changes in ocean circulation. A sudden strengthening of the Atlantic Meridional Overturning Circulation (AMOC), a large system of ocean currents that transports warm water from the tropics northward into the North Atlantic. Prior to the warming, during a period known as Heinrich stadial 1, a thick layer of cold freshwater from melting glaciers sat atop a deeper, warmer, and saltier layer of ocean water.
Research suggests that this unusual stratification stored immense amounts of heat at depth. Eventually, this stored energy was released abruptly, reinvigorating the AMOC and transferring a massive amount of heat to the Northern Hemisphere. This event triggered the rapid warming seen in the climate records and led to the collapse of ice sheets, causing a global sea-level rise of about 16 meters, an event known as Meltwater Pulse 1A. The warming was not global; while the Northern Hemisphere heated up, the Southern Hemisphere experienced an equivalent cooling phase, a phenomenon described as the "polar seesaw". The abrupt warming period eventually ended with a return to near-glacial conditions during a cold snap called the Younger Dryas, which started around 12,900 years ago.