The last major ice age occurred around 12,000 years ago and led to a period of extreme cold known as the Younger Dryas. This abrupt return to near glacial conditions lasted for about 1,300 years before the climate warmed again. Scientists have proposed several theories for what triggered this cooling period after the last glacial maximum around 20,000 years ago.
Earth’s orbital changes
One dominant theory is that changes in Earth’s orbit and tilt on its axis, known as Milankovitch cycles, led to a decrease in solar radiation in the Northern Hemisphere. This reduction in incoming solar energy meant less heating of the atmosphere, oceans, and land. With less heat to moderate the climate, conditions became colder globally and allowed ice sheets to expand again.
Shutdown of thermohaline circulation
Many scientists believe the Younger Dryas was caused by a sudden shutdown of the Atlantic Meridional Overturning Circulation (AMOC), part of thermohaline circulation that drives the global conveyor belt of ocean currents. This shutdown was likely triggered by a massive discharge of freshwater from a giant glacial lake into the North Atlantic Ocean. The added freshwater made the surface water less dense and less able to sink, effectively stopping the overturning circulation pattern. This prevented warm tropical water from reaching the North Atlantic and caused dramatic regional cooling.
Comet impact theory
A more controversial hypothesis known as the Younger Dryas impact theory proposes that the cooling during this period was caused by the air bursts or impacts of multiple fragments of a disintegrating comet hitting Earth around 12,800 years ago. This cosmic impact set parts of the northern hemisphere ablaze, triggered shockwaves and wildfires that led to the extinction of many large animals, injected dust into the atmosphere that blocked out sunlight, and destabilized the ice sheets. While proponents point to evidence like geochemical signatures and microspherules at suspected impact sites, this theory remains disputed and many questions remain about its likelihood.
Volcanic eruptions
Increased volcanic activity leading up to and during the Younger Dryas is another suggested contributing factor for the cooling. Major eruptions in Europe, North America, and New Zealand occurred around this time and likely emitted sulfate aerosols into the atmosphere that reflected sunlight and caused average global temperatures to drop. However, the timing and intensity of these eruptions do not appear to fully align with the abrupt climate changes during the period in most climate records.
Changes in ocean circulation
Along with a potential slowdown of thermohaline circulation in the Atlantic, other shifts in ocean currents may have promoted cooling during the Younger Dryas. Studies of marine sediment cores have found evidence of reorganizations in deep ocean circulation patterns in the Pacific and Indian Oceans at the time, which would have altered heat transport and distribution. In particular, a weakening of tropical Pacific overturning may have amplified the magnitude of Northern Hemisphere cooling.
Melting ice sheet and sea level rise
| Time Period | Global Mean Sea Level Rise |
|---|---|
| 14,000 – 12,000 years ago | 90 meters |
| Younger Dryas onset (12,900 years ago) | 5 meters in less than 140 years |
The onset of the Younger Dryas coincided with a period of substantial sea level rise, indicating significant melting of the North American and Eurasian ice sheets prior to 12,900 years ago. The rapid sea level rise suggesting collapse of ice sheets corresponds to a peak in summer insolation and warming just before the Younger Dryas. However, model simulations have shown that gradual changes in Earth’s orbit could not have caused such an abrupt climate shift by itself. This melting may have led to freshwater flooding into the North Atlantic and disruption of overturning circulation.
Shutdown of NADW formation
The most widely accepted trigger for the Younger Dryas is a nearly complete shutdown of North Atlantic Deep Water (NADW) formation due to freshwater flooding from melting ice sheets and proglacial lakes. This catastrophic release of freshwater reduced the salinity and density of surface waters in the North Atlantic, causing warm tropical waters to no longer sink and circulate northward. The halt of NADW formation severely impacted the Atlantic meridional overturning circulation, ocean currents, and ventilation of the deep ocean. This in turn caused heat redistribution between the hemispheres and altered the climate globally, forcing a dramatic cooling.
Freshwater forcing from glacial lakes
The prevailing hypothesis is that the cold period was caused by the draining of large proglacial lakes like Lake Agassiz that formed from the retreating Laurentide Ice Sheet. As these giant lakes catastrophically drained, they released enormous freshwater flows into the North Atlantic that reduced sea surface salinity, density, and temperature. Isotopic and paleontological data indicates the first major freshwater pulse occurred about 12,900 years ago, which matches the rapid drop in temperatures marking the start of the Younger Dryas. Though other freshwater sources like the Fennoscandian Ice Sheet contributed, the Laurentide meltwater had the greatest impact due to its volume and proximity to areas of NADW formation.
Role of sea ice expansion
Positive feedback mechanisms may have amplified the initial cooling caused by freshwater forcing during the Younger Dryas. One example is the expansion of sea ice. With colder sea surface temperatures in the North Atlantic, winter sea ice extended further south each year. The increased sea ice reflected more sunlight and insulated the ocean from the warmer atmosphere, resulting in even colder conditions that allowed sea ice to spread further. Models show this seasonal sea ice feedback could sustain and intensify an initial climate perturbation like freshwater input.
Increased albedo from snow and ice cover
In addition to sea ice increase, the growth and advance of snow and ice cover on land during the Younger Dryas led to a higher albedo or surface reflectivity that promoted greater cooling. Ice sheet and alpine glacier expansion in North America and Scandinavia due to colder temperatures increased the area covered by bright, reflective snow and ice. These large icy expanses reflected rather than absorbed incoming solar radiation, causing regional cooling that was then propagated globally by atmospheric and ocean circulation changes.
Weakened AMOC and changed ocean circulation
It is clear that a substantial reduction in North Atlantic overturning circulation was associated with the Younger Dryas climate reversal. Recent analyses of marine sediment cores have inferred that Atlantic meridional overturning circulation (AMOC) during the Younger Dryas was only about 10-15% of modern values. This major ocean current essentially shut down within a span of about 20 years due to freshwater forcing. In addition to the severe weakening of AMOC, other ocean circulation changes like tropical Pacific overturning and Southern Ocean upwelling likely contributed to the widespread cooling.
Widespread wildfires and soot deposits
There is extensive evidence of extensive wildfires occurring at the onset of the Younger Dryas, particularly in the Americas but also Eurasia. These widespread fires deposited substantial amounts of black carbon or soot in glacial ice and terrestrial sediments during this period. The prevalence of wildfire may have resulted from drier conditions as the climate cooled and vegetation changes occurred. The injection of soot into the atmosphere from these fires could have promoted cooling by reducing incoming solar radiation.
Methane release declines
The colder Younger Dryas climate caused wetland areas to shrink, which reduced emissions of methane from natural sources. Since methane is a powerful greenhouse gas, lower atmospheric methane levels during the Younger Dryas would have amplified the cooling trend. Analyses of ice cores show reductions in atmospheric methane concentrations by up to 80% below Late Glacial Maximum levels during the Younger Dryas, which likely contributed to the magnitude of the cooling.
Vegetation changes and feedbacks
Pollen records show the Younger Dryas climate reversal induced major vegetation shifts, particularly in plant communities in Europe. Arctic tundra spread southward, while boreal forest retreated northward with the expansion of polar conditions. These natural biome shifts amplified the cooling – tundra’s snow cover reflected more radiation than forest, while boreal forest releases more heat. In addition, the southward expansion of Arctic vegetation contributed to cold conditions through transpiration, increased cloudiness, and other biophysical feedbacks.
Conclusions
In summary, the predominant theory supported by paleoclimate evidence is that the Younger Dryas cold period around 12,900 to 11,600 years ago was triggered by catastrophic freshwater flooding into the North Atlantic from melting glacial lakes. This influx of freshwater reduced North Atlantic overturning circulation, shifting ocean currents and heat transport that caused considerable cooling at high latitudes. Intensification of this cooling into a cold stadial was likely amplified by positive feedbacks like sea ice expansion, increased snow/ice cover, reduced methane emissions, and vegetation shifts. While other factors like comet impacts and volcanic eruptions may have played a role, freshwater input to the North Atlantic was the principal forcing behind the Younger Dryas event based on current scientific data and climate models.