Last Glacial Maximum

The Last Glacial Maximum (LGM) was the most recent period of peak ice coverage during the Last Glacial Period. This era occurred between 26,000 and 20,000 years ago. During this time, massive ice sheets stretched across much of Northern North America, Northern Europe, and Asia.

This period is also known as the Last Glacial Coldest Period. It profoundly reshaped the Earth's climate, geography, and biological landscapes. The LGM was characterized by much colder temperatures and significantly drier conditions across most of the globe.

To form these massive ice sheets, the Earth required two specific conditions: prolonged cold and heavy precipitation in the form of snow. As snow accumulated over long periods, it compressed into ice, growing into vast sheets. This process locked up enormous quantities of water that would otherwise be in the oceans. Consequently, the sea level dropped significantly, reaching about 125 meters (410 feet) lower than it is today.

This drop exposed continental shelves and created new coastal plains. The ice also altered atmospheric circulation. Large clouds formed over the cooling northern Pacific and Atlantic oceans. These clouds reflected sunlight back into space, which amplified the global cooling effect.

Different regions experienced the LGM in distinct ways. In the northern hemisphere, massive continental ice sheets dominated. However, East Asia remained largely unglaciated despite having cold temperatures. This happened because the ice sheets in Europe created extensive anticyclones, or large-scale weather systems, above them. These systems generated very dry air masses that reached Siberia and Manchuria. Because the air was so dry, there was not enough precipitation to form glaciers in most of Asia. Other factors, such as the relative warmth of the Pacific Ocean and large mountain ranges, also prevented continental glaciation there.

Researchers use various methods to determine when the LGM began and ended. They study terrestrial cosmogenic nuclides and radiocarbon dating to track ice sheet margins. Because different scientists use different data sets and criteria, there is no single agreed-upon timeline. For example, archaeologist Jennifer French dates the onset to 27,500 years ago. She suggests ice sheets reached their maximum by 26,000 years ago, with deglaciation starting between 20,000 and 19,000 years ago. In Britain, this period is called the Dimlington Stadial and is dated between 31,000 and 16,000 years ago.

The scale of change during the LGM was immense. The average global temperature about 21,000 years ago was approximately 6 °C colder than present-day levels. According to the USGS, permanent summer ice covered about 8% of the Earth's surface and 25% of the land area.

In contrast, today only about 3.1% of Earth's surface is covered in year-round ice. The atmosphere was also much dustier. Dust levels were 20 to 25 times higher than they are now. This was caused by reduced vegetation, stronger global winds, and less rain to clear the air.

Vegetation patterns shifted dramatically across the planet. Most deserts expanded, and many subtropical regions lost their forests to open woodlands. In Southern Australia, shifting sand dunes covered half of the continent.

The Amazon rainforest was split into two large blocks by extensive savanna. However, some areas experienced unexpected changes. In the western United States, changes in the jet stream brought heavy rain to areas that are now deserts. This created large pluvial lakes, such as Lake Bonneville in Utah. Similarly, a major lake formed in the Dasht-e Kavir in Afghanistan and Iran.

One of the most striking effects of the LGM was the connection of landmasses. Because the sea level was so low, many islands became part of the mainland. In Southeast Asia, the Indonesian islands like Borneo and Bali joined the continent to form a landmass called Sundaland.

In the southern hemisphere, the Australian mainland, New Guinea, and Tasmania joined to form Sahul. These connections allowed for different patterns of movement and life across the globe. Understanding the LGM helps scientists study how the hydrological cycle and global temperatures interact to change the face of our world.