Climate change at the poles
Understand how warming affects polar regions. September Arctic sea ice extent has declined over the satellite record since 1979, although individual years vary. Greenland and Antarctica are losing land ice, contributing to sea-level rise. Thawing permafrost can release carbon dioxide and methane. Sea-ice loss and greenhouse-gas release can each reinforce warming through different feedback loops.
What a learner can do afterwards
- Describe the long-term decline in September Arctic sea ice extent since satellite tracking began in 1979, while recognizing that individual years vary
- Explain the ice-albedo feedback: warming melts sea ice, exposed dark ocean absorbs more sunlight as heat, and the added warmth helps melt more ice
- Explain how land-ice loss raises sea level and how permafrost thaw can release greenhouse gases that add to warming
1 · Read
Sea ice floats on the ocean. It grows in winter and shrinks in summer. Satellites have tracked Arctic sea ice since 1979. Sea ice extent is the area of ocean with at least 15% ice cover. September extent has declined over many decades, although individual years vary. Comparing the same month across many years helps separate the long-term trend from seasonal changes.
White sea ice reflects more sunlight than dark ocean water. When sea ice melts away, it exposes dark water. The water absorbs more sunlight as heat. Added warmth can melt more nearby ice. This repeating chain is the ice-albedo feedback. Albedo means how much sunlight a surface reflects. A positive feedback strengthens the starting change; positive does not mean helpful.
Greenland and Antarctica have ice sheets on land. Both are losing ice overall. When ice is lost from land into the ocean, it adds water and raises sea level. Melting floating sea ice has little direct effect on sea level because it already pushes aside seawater.
Permafrost is ground that has stayed frozen for at least two years. When it thaws, microbes can break down old plant matter. This can release carbon dioxide and, in wet conditions, methane. Both are greenhouse gases: they absorb some heat energy Earth gives off and slow its escape to space.
Gases released from thawed ground can add to warming. Added warmth can thaw more ground, allowing more gas release. This is a permafrost feedback. It differs from the ice-albedo feedback: one involves greenhouse gases, while the other involves a change in sunlight reflection.
A made-up practice model starts with 90 ice units and removes 10 twice. Subtract once for each loss: 90 - 10 = 80, then 80 - 10 = 70. These are practice numbers, not satellite observations or a prediction.
September Arctic sea ice has declined over decades. Sea-ice loss can add warming. Land-ice loss raises sea level. Permafrost thaw can release gases that add warming and cause more thaw.
This lesson, practice and flashcards were written for this dataset. Ice-unit calculations use made-up practice values.
The readings were written for this dataset. The linked videos credit Victor J. Donnay and Lee Hotz and are recorded as TED-Ed resources. Their licensing terms have not been verified.
More to read
Comparing sea ice across years
Arctic sea ice changes during the year. More ocean freezes in winter, and more ice melts in summer. Comparing winter with summer shows this seasonal pattern.
To study change over decades, scientists compare the same time of year. Satellites have tracked Arctic sea ice since 1979. September sea ice extent has declined over that record, although individual years vary.
Extent measures the area of ocean with at least 15% ice cover. It does not mean every part of that area is solid ice. Extent also does not tell us the ice's thickness.
A useful chart names the measurement, dates, units and source. Made-up arithmetic values can help with practice, but they must not be labelled as satellite observations.
Land ice and floating ice
The location of ice matters when explaining sea-level rise. Glaciers and ice sheets store water on land. When that water enters the ocean, it adds to the sea.
Floating sea ice already pushes aside seawater. Its melting has little direct effect on sea level. Losing sea ice can still affect climate because dark ocean absorbs more sunlight than bright ice.
Warming seawater also raises sea level by expanding. It takes up more space even without added meltwater. Ocean expansion and loss of land ice are two different causes of rising seas.
An explanation should name the process involved. Saying that all melting has the same effect hides the difference between water added from land and ice already floating in the sea.
Changes in thawing ground
Permafrost is ground that has stayed frozen for at least two years. It may contain soil, rock and ice. Some also contains old plant matter.
When frozen ground thaws, microbes can break down that plant matter. They can release carbon dioxide. In wet conditions, they can also release methane. Both gases can add to greenhouse warming.
Thawing can change the ground itself. Where ice within the ground melts, the surface can settle or become less stable. Along some coasts, thawed ground can be more easily worn away by waves.
These effects are different. Gas release changes the atmosphere. Erosion removes material from a shore. Neither should be described as floating sea ice directly adding water to the ocean.
Why distant coasts are connected
Ice sheets are far from many towns, but their water is part of Earth's water system. Ice lost from land into the ocean contributes to sea-level rise beyond the polar regions.
Higher sea levels let high tides and storm water reach farther onto low land. Roads, homes and freshwater supplies near a coast can be affected.
The change is not identical at every coast. Land can rise or sink, and ocean conditions differ between places. Scientists study local records as well as the global average.
A global sea-level estimate is useful for understanding the whole ocean. It cannot by itself give an exact future water level for every town. Local evidence is needed too.
Two different warming feedbacks
A feedback happens when a change causes effects that influence that change again. A positive feedback strengthens the starting change. The word positive does not mean that the result is helpful.
In the ice-albedo feedback, warming melts sea ice. Dark ocean is exposed and absorbs more sunlight as heat. Added warmth can melt more ice. This chain changes how much sunlight is reflected.
In a permafrost feedback, warming thaws frozen ground. Microbes can release greenhouse gases as they break down old plant matter. Those gases can add warming and cause more thaw. This chain changes how heat escapes to space.
Both can reinforce warming, but they do so through different processes. Naming each step helps explain the difference.
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2 · Watch
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Where it sits
Optional connections
Where this leads
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.