One of the most powerful forces driving glacial advance and retreat is a property called albedo — the fraction of incoming solar energy that a surface reflects back into space. Fresh snow and glacial ice have an extremely high albedo, reflecting up to 90% of sunlight, while dark ocean water and bare rock reflect as little as 5–10%. This difference creates a self-reinforcing cycle known as the ice-albedo feedback loop. As global temperatures cool and glaciers grow, more of Earth's surface is covered in highly reflective ice, causing less solar energy to be absorbed — which cools the planet further, encouraging even more glacial growth. During the peak of the last ice age (~25,000 years ago), the Laurentide Ice Sheet blanketed much of North America, dramatically increasing Earth's overall albedo and helping to lock the planet into a cold, glacial state. The reverse is equally powerful: as temperatures warm and ice melts, darker land and ocean surfaces are exposed, absorbing more sunlight, accelerating warming, and driving further melting — a runaway process that helped end the last ice age remarkably quickly in geologic terms.
Glaciers and ice sheets are the world's largest reservoir of fresh water, and their growth or melting has a direct and dramatic effect on global sea level. When glaciers advance, they lock up enormous volumes of water that would otherwise flow into the ocean — at the peak of the Laurentide glaciation, so much water was stored in ice that global sea level was approximately 120 meters (400 feet) lower than today. This exposed the continental shelves as dry land: the area that is now the Atlantic Ocean floor south of Long Island was once a vast coastal plain roamed by mastodons and woolly mammoths. As the climate warmed after ~18,000 BP and the glaciers retreated, all of that meltwater poured back into the world's oceans. Sea level rose rapidly — at times gaining several feet per century — slowly flooding the continental shelf, drowning the outwash plains south of Long Island, and ultimately creating Long Island Sound to the north and reshaping the barrier islands along the south shore. Understanding this relationship between ice volume and sea level is critical today, as melting glaciers in Greenland and Antarctica are once again causing measurable sea level rise.
- (A) As ice expands, it absorbs more solar energy, warming the atmosphere and causing further melting.
- (B) As ice expands, it reflects more solar energy back to space, lowering temperatures and encouraging further ice growth.
- (C) As ice expands, ocean temperatures rise, increasing evaporation and adding more water vapor to the atmosphere.
- (D) As ice expands, volcanic activity beneath the ice increases, releasing gases that cool the surrounding atmosphere.
- (A) Increased volcanic activity on the ocean floor caused ocean water to evaporate at a faster rate.
- (B) The weight of ice sheets compressed the ocean floor, creating deeper ocean basins that held less water near the surface.
- (C) Colder global temperatures caused the volume of ocean water to shrink as it contracted due to lower density.
- (D) Enormous volumes of ocean water were locked up as ice in glaciers and ice sheets, removing water from the ocean.
- (A) Glaciers melt → more dark ocean surface exposed → less sunlight absorbed → temperatures decrease → ice re-grows
- (B) Glaciers melt → more white ice surface exposed → more sunlight reflected → temperatures decrease → ice re-grows
- (C) Glaciers melt → more dark ocean surface exposed → more sunlight absorbed → temperatures increase → more melting
- (D) Glaciers melt → less ocean evaporation occurs → precipitation decreases → deserts expand → sea level falls
- (A) As Greenland and Antarctic ice melts, sea level will rise, threatening to erode and eventually flood Long Island's low-lying barrier islands and south shore communities.
- (B) As Greenland and Antarctic ice melts, increased snowfall will build up Long Island's glacial moraines, raising the island above sea level.
- (C) As Greenland and Antarctic ice melts, global temperatures will decrease due to the ice-albedo feedback loop, protecting Long Island from further erosion.
- (D) As Greenland and Antarctic ice melts, longshore drift on Long Island's south shore will slow down, causing barrier islands to grow wider and taller.
Press ▶ Play to animate the glacier retreating across Long Island — the simulation will automatically pause at 5 key dates. At each pause, scroll down to the Observation Data Sheet and record your answers before pressing ▶ Play to continue. Click any 🔴 red pin on the map to explore a Virtual Field Stop with a photo and Regents question.
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