The Frozen Sponge on Ice Sheets: How Firn Holds Back Meltwater

Takeaway: Firn is the old snow that works like a frozen sponge, and as it fills with ice, more meltwater can escape to the sea and weaken ice shelves.Ice sheets do not just melt from the top and send water straight to the ocean. Much of that water first meets firn: older snow packed with tiny air spaces, like a cold sponge spread across Greenland and Antarctica. This episode matters because that sponge helps decide how fast ice sheets add to sea level, how vulnerable Antarctic ice shelves are to cracking apart, and how scientists read climate history from air bubbles trapped in ice.We unpack a major review of what researchers know about firn: how snow turns into ice, how meltwater percolates, refreezes, forms ice layers or hidden aquifers, and why warming can make this natural buffer less reliable. Greenland already retains about half of its surface meltwater in firn, but warming has reduced average firn pore space there by about 5% since 1980. In Antarctica, firn on ice shelves can keep meltwater from ponding in cracks; when that storage space disappears, water can help force fractures open.Along the way, we talk through the field science behind the findings: snow pits, firn cores, radar, satellites, weather stations, and models that try to follow water through a changing, layered material. The story is practical, uncertain in important places, and deeply human: to understand future sea level, scientists have to understand what happens inside old snow.Citation: The Firn Symposium team. “Firn on ice sheets.” Nature Reviews Earth & Environment 5, 79–99 (2024). https://doi.org/10.1038/s43017-023-00507-9Disclosure: This Waterlines episode uses AI-generated voices for the host conversation.