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Telling the story with pictures

Arctic Sea Ice: Heat Shield

Audio Commentary

Three decades of satellite records show ongoing decline in Arctic sea ice. The images here contrast 1979, the first year of this record, with 2007, the year the ice reached its minimum extent so far. This melting appears to be a symptom of climate change: over the same period, the Arctic warmed more than any other region on the planet as large.

But is there any reason to be concerned about a smaller crust of ice floating on top of the far-away Arctic Ocean? After all, melting sea ice does not directly affect sea level, just as melting ice cubes don’t affect the water level in a glass. (In both cases, the floating ice displaces water – exactly the same amount released by melting.)

However, melting Arctic sea ice does embody a so-called feedback mechanism, which accelerates overall warming. Like a giant planetary heat shield, Arctic ice reflects most of the sunlight that falls on it back into space. When the ice melts to reveal darker ocean, less of the light is reflected, and more of the sun’s energy is absorbed. This heats the ocean, which melts the ice more from underneath, and also eventually releases some of the heat back to the air. This further warms the atmosphere and ultimately leads to even more melting.

Scientists now project that the Arctic Ocean may become completely ice-free for part of the summer, possibly before the middle of this century, triggering changes in weather patterns and ocean currents that could affect other parts of the world.

Both images here show the annual minimum ice extent reached one day in September. Every year, the Arctic Ocean’s ice crust melts partly back in summer, exposing open water, and then refreezes in the dark and frigid winter. The images are based on data from NASA’s Nimbus 7 satellite (1979), and the DoD’s Defense Meteorological Satellite Program (2007).