Questions: Ice-Sheet Dynamics and Climate Feedbacks

Question 1

Milankovitch orbital cycles redistribute solar heating across Earth's surface by roughly 10 W/m² but do not significantly change the total solar energy received by Earth. Yet they are associated with glacial-interglacial temperature swings of 5–6°C globally. What primarily accounts for this amplification?

• A. The direct warming from orbital changes is sufficient to explain 5–6°C swings when integrated over geological timescales
• B. The ice-albedo feedback amplifies the initial orbital forcing: warming shrinks ice cover, exposing dark land and ocean that absorb more heat, driving further warming
• C. Increased volcanic activity during interglacials injects greenhouse gases that amplify the orbital signal
• D. Ocean heat capacity delays the temperature response so that multiple orbital cycles stack before temperatures rise

Answer: (B)

Orbital forcing alone is too weak to drive 5–6°C global temperature swings. The amplification comes from positive feedbacks — primarily the ice-albedo feedback. When orbital changes cause slight warming, ice margins retreat; the newly exposed dark ocean and land surface absorb far more solar radiation than the high-albedo ice they replaced, causing further warming and further ice retreat. This positive feedback roughly doubles the temperature response to orbital forcing. Without feedbacks, Milankovitch cycles would produce only modest climate oscillations. The fact that ice cores and marine sediments show the same ~100,000-year periodicity as orbital eccentricity cycles is strong evidence that the ice-albedo feedback is transmitting and amplifying the orbital signal.

Question 2

During a Heinrich event — a massive discharge of icebergs into the North Atlantic — what is the most direct climatic consequence in that region?

• A. Warming of the North Atlantic due to the latent heat released as icebergs melt
• B. Weakening or shutdown of the thermohaline circulation as freshwater reduces surface water density, causing cooling in the North Atlantic
• C. Rapid sea level rise that floods coastal regions and changes atmospheric circulation
• D. Enhanced ice-albedo feedback as sea ice expands, causing further cooling of the Southern Hemisphere

Answer: (B)

Freshwater from melting icebergs is less dense than saline ocean water. In the North Atlantic, the thermohaline circulation depends on surface waters cooling, becoming dense, and sinking — this drives the Atlantic Meridional Overturning Circulation (AMOC) that transports heat northward. A pulse of freshwater 'caps' the surface, preventing sinking and weakening or shutting down this overturning. The result is reduced northward heat transport and dramatic cooling in the North Atlantic region, despite the global warming trend during deglaciation. This produces the 'bipolar seesaw': the Southern Hemisphere actually warms during these events as heat accumulates there instead of being transported north.

Question 3

Ice sheets grow slowly over tens of thousands of years but can collapse over just a few thousand years because warming feedbacks become mutually reinforcing once triggered.

• T. True
• F. False

Answer: True

Growth and decay of ice sheets are highly asymmetric in rate. Ice sheet growth requires sustained cold: snow must accumulate faster than it melts over many millennia. This is a slow process constrained by orbital forcing and accumulation rates. But once warming begins, multiple feedbacks engage simultaneously and reinforce each other: ice-albedo feedback exposes more dark surface; meltwater disrupts ocean circulation which redistributes heat; isostatic rebound as ice thins changes the geometry of remaining ice. These positive feedbacks can drive rapid collapse once a threshold is crossed. The paleoclimate record shows this asymmetry: the Laurentide ice sheet took ~80,000 years to grow during a glacial but collapsed in roughly 6,000–8,000 years during deglaciation.

Question 4

Post-glacial isostatic rebound — the slow rise of land as ice sheets melt — has no practical relevance today because deglaciation was completed thousands of years ago.

• T. True
• F. False

Answer: False

Isostatic rebound is still ongoing today, thousands of years after the major ice sheets retreated. Scandinavia is still rising at up to ~8 mm/year, and Canada is also rebounding. The Earth's mantle is viscous and responds slowly — the timescale for full rebound is tens of thousands of years. This has practical consequences: in Scandinavia, relative sea level is actually falling in some areas despite global sea level rise, because the land is rising faster. It also matters for future ice-sheet stability: regions where ice is currently thinning will see their bedrock slowly rise in coming millennia, potentially exposing more land above sea level and altering the stability of remaining ice.

Question 5

Explain why the ice-albedo feedback is called a 'positive feedback' and describe its role in amplifying Milankovitch orbital forcing into full glacial-interglacial cycles.

Model answer: A positive feedback is one where an initial change triggers a response that reinforces and amplifies the original change. The ice-albedo feedback works as follows: if warming causes ice to retreat, the newly exposed land and ocean have much lower albedo (they absorb rather than reflect solar radiation), which causes additional warming, which causes further ice retreat, and so on. This self-reinforcing loop amplifies the original warming signal. Milankovitch orbital forcing provides only modest direct warming (~10 W/m² redistribution), insufficient on its own to produce the observed 5–6°C swings. The ice-albedo feedback, along with CO₂ and water vapor feedbacks, amplifies this signal into full glacial-interglacial cycles. The same feedback runs in reverse during cooling: a little cooling causes ice to expand, raising albedo, causing more cooling and more ice growth.

The distinction between 'positive' and 'amplifying' is worth noting: 'positive' in feedback terminology means reinforcing, not beneficial. Ice-albedo feedback is 'positive' because it reinforces the initial direction of change, whether warming or cooling. This is in contrast to 'negative feedback' (which opposes change and stabilizes the system). Understanding the sign and strength of climate feedbacks is the central challenge in estimating climate sensitivity to greenhouse gas forcing.