How is multiyear sea ice different from first-year sea ice?

The primary distinction between first-year and multiyear sea ice lies in their age, salinity, and physical characteristics.

• Age and Formation: First-year ice is sea ice that is no more than one winter old. Multiyear ice, also known as polar pack, is ice that has survived at least one summer melt season.12
• Salinity: As sea ice ages, it undergoes a desalination process. While first-year Arctic ice typically has a salinity of 4–6 parts per thousand, multiyear ice undergoes further desalination through summer gravity drainage and flushing, reducing its average salinity to 3–4 parts per thousand. By the time ice is one year old, it is generally salt-free enough to be melted for drinking water.32
• Physical Appearance and Strength: Multiyear ice is characterized by a smoother, rounded surface and a pale blue color, whereas younger ice appears more jagged and gray. Because the expulsion of salt increases the ice's hardness and structural strength, multiyear ice is significantly tougher than first-year ice.2
• Thickness: First-year ice typically reaches a thickness of up to 6 feet (2 meters), while multiyear ice is much thicker, averaging about 12 feet (4 meters).2