The Quaternary Ice Ages (2.58 mn years ago)

A repeated alteration between cold glacial periods and warm interglacial periods.

The Quaternary Period consists of two epochs: Pleistocene (2.58 million to 11,700 years ago) and Holocene (11,700 years ago to the present). This period was dominated by glacial cycles, with the Holocene being a mild interglacial interval. An ice age does not mean a frozen Earth; it means that permanent continental ice sheets exist and hence, technically, Earth is still inside an ice age.

A glacial period is characterized by lower temperatures, huge continental glaciers, lower sea levels, stronger temperature gradients, and lower CO₂. An interglacial period is characterized by a warmer climate, retreating glaciers, higher sea levels, smaller ice sheets, and higher CO₂, similar to today’s climate.

Ice ages repeat because of the combined effects of Milankovitch cycles and climate feedbacks. Tiny orbital changes produce huge climatic shifts through feedback mechanisms, with cool summers playing a major role by preventing snow from melting.

The Last Glacial Maximum (LGM)

Around 21,000 years ago, during the Last Glacial Maximum (LGM), global temperatures were 4–7°C colder than today, and large continental ice sheets covered Canada, the northern United States, Scandinavia, northern Europe, and Siberia. Sea levels were lower because large amounts of water were locked in glaciers, exposing the Bering Land Bridge (connecting Alaska in North America and Siberia in Asia), the Sunda Shelf (Southeast Asia), and Doggerland (Northern Europe), all of which played a huge role in the migration of humans and animals.

Landforms Shaped by Glaciers

Through erosion, glaciers formed U-shaped valleys, fjords, and cirques, whereas moraines, drumlins, eskers, and till plains were formed by glacial deposition. Evidence of these landforms is preserved in modern glacial landscapes.

Deglaciation

Deglaciation is triggered by warmer summers and the opposite feedback of decreased albedo. As the greenhouse effect strengthened, it resulted in rapid warming and the beginning of an interglacial period.

Although we studied that climate evolves gradually, recent discoveries have fascinated scientists, as they found evidence of dramatic climate change occurring within decades during the last glacial period. Greenland ice cores show rapid warming events. Possible causes include changes in ocean circulation, ice sheet collapse, freshwater pulses, and atmospheric feedbacks.

Dansgaard–Oeschger Events

The events were recorded in Greenland ice cores and were characterized by sudden warming followed by gradual cooling, a pattern that was repeated many times. These events demonstrated that Earth’s climate possesses multiple stable states and that even small disturbances can trigger large climatic transitions.

Heinrich Events

The weakening of ocean circulation due to a huge freshwater input into the North Atlantic was caused by massive armadas of icebergs breaking away from the Laurentide Ice Sheet. The thermohaline circulation of the ocean plays a major role in the redistribution of heat across the planet. Surface warm water moves northward, while cool surface water, due to its higher density, sinks and returns southward, forming a global conveyor belt. Freshwater from melting ice decreases salinity, which reduces sinking and weakens Atlantic circulation, resulting in cooling of the Northern Hemisphere. This ocean circulation mechanism explains abrupt regional climatic shifts.

The Holocene, which began 11,700 years ago, is characterized by a warm climate, climatic stability, and rising sea levels. This stable climate helped agriculture develop and led to the emergence of civilizations.

But are we heading towards the next ice age? According to many scientists, today’s climate is largely driven by anthropogenic activities. It has been observed that today’s greenhouse gas concentrations are already higher than at any time in the last 800,000 years, which can play a major role in deciding Earth’s next climatic world.