Questions & explanations
1. Compare the overkill and climate hypotheses.
The overkill hypothesis says that humans directly hunted megafauna to extinction. The climate hypothesis says that changing environments made it impossible for large animals to survive. Overkill focuses on human actions, while climate focuses on natural changes. Overkill explains why extinctions happened soon after humans arrived in new areas. Climate explains why extinctions also happened in areas with few humans, like Europe. Both may have contributed, but scientists debate which was the main driver. Evidence like kill sites supports overkill, while fossil data on habitat change supports climate.
2. What is Ekman transport?
Ekman transport is the net movement of water (or ice) at a 90-degree angle to the wind direction, caused by the Coriolis effect. The Coriolis effect is a force from Earth's rotation that deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In the Arctic, wind pushes sea ice, but instead of moving straight downwind, the ice drifts about 30-40 degrees to the right of the wind. This happens because the wind stress on the ice is balanced by the Coriolis force. The total transport over the entire ice-ocean layer is at a right angle to the wind.
3. Compare the contributions of ice sheet melt and crustal rebound to relative sea level change.
Relative sea level change is the sum of two effects: the addition of meltwater raises sea level (eustatic), while crustal rebound raises or lowers the land. When ice sheets melt, the added water raises global sea level. But near the former ice sheets, the land rebounds upwards, which can offset the rise. Far from ice sheets, the land may sink due to forebulge collapse, enhancing sea level rise. GIA models separate these contributions. For example, in Scandinavia, rebound is stronger than meltwater rise, so relative sea level falls. In the tropics, the opposite happens.
4. What do thermistor strings tell us about sea ice?
Thermistor strings are chains of temperature sensors placed vertically through the ice and snow. They measure temperature at different depths. By monitoring temperature changes, scientists can see when the ice surface melts or when new ice forms at the bottom. The temperature profile shows the heat flow through the ice. It also reveals snow depth because snow insulates and changes the temperature gradient. During freezing, the release of latent heat keeps the bottom temperature near the freezing point. This data is crucial for understanding ice growth and melt rates.
5. Why is it necessary to calibrate production rates of cosmogenic nuclides?
Production rates depend on altitude, latitude, and geomagnetic field strength. They also vary for different nuclides. Without calibration, the calculated age would be inaccurate. Calibration involves measuring cosmogenic nuclides in surfaces of known age. For example, surfaces from glacial retreat or lava flows that are independently dated. The measured concentration gives the real production rate for that location. The CRONUS-Earth project provided a global calibration of production rates. This allows scientists to use the method anywhere with appropriate scaling.
6. Compare upward-looking sonar with satellite altimetry for measuring ice thickness.
Upward-looking sonar measures ice draft from below, while satellite altimetry measures ice freeboard from above. Sonar gives very detailed local time series but covers a small area. Satellites cover wide regions but have lower resolution and require assumptions about snow depth. Sonar works under ice and is not affected by weather, but satellites can map entire polar oceans. Both methods complement each other; sonar provides ground truth for satellite data. A key difference is that sonar directly measures draft, whereas satellites estimate thickness indirectly.
7. Why does the calibration curve fluctuate over time (wiggles)?
The wiggles in the calibration curve are caused by changes in atmospheric carbon-14 production. When solar activity is low, more cosmic rays reach Earth, creating more carbon-14. Also, changes in Earth's magnetic field affect cosmic ray flux. Volcanic eruptions and ocean circulation can also alter carbon-14 levels. These variations cause the radiocarbon age to sometimes be older or younger than the true age. Tree rings record these wiggles as variations in carbon-14 content. The wiggles actually help improve calibration by providing detailed patterns to match.
8. Why is the momentum balance important for sea ice forecasting?
The momentum balance determines how ice moves and deforms, which affects navigation, oil and gas operations, and climate. By solving the balance, forecast models predict ice drift, convergence, and lead location. Accurate forecasts help ships avoid ice hazards and plan routes. In climate studies, the momentum balance controls ice export from the Arctic, influencing global climate. It also links to ocean circulation through ice-ocean stress. Therefore, correctly representing the momentum balance is essential for both short-term and long-term predictions.
9. How does the choice of mantle viscosity profile affect model results?
Different viscosity profiles change the speed and pattern of rebound. For example, a low viscosity upper mantle causes fast rebound in areas like Hudson Bay. A high viscosity lower mantle slows the deeper flow. The profile also affects the shape and migration of the forebulge—a bulge of crust ahead of the ice sheet. If viscosity is too high, models predict too slow rebound. Too low, and they overcorrect. Scientists test multiple profiles (like VM5a) to find the best fit with observations. The choice thus alters predictions of past and future sea level.
10. What is the difference between the IntCal20 and SHCal20 calibration curves?
IntCal20 is for the Northern Hemisphere, while SHCal20 is for the Southern Hemisphere. Because of different ocean circulation and atmospheric mixing, carbon-14 levels differ between hemispheres. The Southern Hemisphere has slightly older radiocarbon ages for the same calendar year. SHCal20 accounts for this offset, which varies over time. Using the wrong curve can give ages that are off by decades. For samples near the equator, an average or a separate calibration is sometimes used. Scientists must choose the appropriate curve based on sample location.
11. Why do scientists disagree about the cause of megafaunal extinctions?
Scientists disagree because both human hunting and climate change happened at the same time. It is hard to tell which one was more important. In some places, extinctions happened soon after humans arrived, but in others there was a long delay. Also, some large animals survived the ice age in areas with fewer humans. Climate models are also uncertain about exactly how environments changed. Different regions have different extinction patterns, so no single explanation fits all. This leads to ongoing debate between the overkill and climate hypotheses.
12. If the floe size distribution follows a power law, what does that tell us?
A power law distribution means that the number of floes decreases steadily as their size increases. For instance, doubling the floe size might make them four times less common. This pattern is common in nature for things like rock fragments. It suggests that ice breaks in a similar way each time, regardless of the starting size. Scientists can use the power law exponent to predict how much ice is in each size class. This helps compare different regions or seasons. It also shows that the ice pack is random but has a consistent fractal pattern.