Lecture 13 – Orbital Variations and Ice Sheets (p.1)

Q: What are the three orbital parameters that control solar radiation reaching Earth?

Obliquity (axial tilt), eccentricity, and precession.

Q: What is the current axial tilt of the Earth, and what is its range?

Currently 23.5°, varying between a minimum of 22.2° and a maximum of 23.5°.

Q: What is the periodicity of the axial tilt (obliquity) cycle?

Approximately 41,000 years between two maxima or two minima.

Q: How does increased axial tilt affect polar regions?

It brings 10–15% more solar radiation to polar regions in summer and less in winter, increasing seasonality.

Q: What is orbital eccentricity?

A measure of how elliptical Earth's orbit is; e = 0 is circular, e > 0 is elliptical.

Q: What are Earth's current, minimum, and maximum eccentricity values?

Current: 0.0167; minimum: 0.0050; maximum: 0.0607.

Q: What are the two major eccentricity cycles?

~100,000 years (varies 95,000–131,000) and ~413,000 years (larger amplitude variations).

Q: What is precession?

The wobbling of Earth's rotational axis, causing the position of solstices and equinoxes to shift in ~23,000-year cycles.

Q: Why does precession matter for climate?

Because Earth's orbit is elliptical, precession changes when each hemisphere is closest/farthest from the Sun, altering seasonal solar radiation by up to 12%.

Q: When is Earth currently closest to the Sun (perihelion) and farthest (aphelion)?

Perihelion on January 3rd; aphelion on July 4th.

Q: How does the current orbital configuration affect N hemisphere seasons?

N hemisphere winters receive slightly more solar radiation (closer to Sun) and summers receive slightly less (farther from Sun), compared to a circular orbit.

Q: How was the orbital configuration different 11,500 years ago?

Perihelion was in June (N hemisphere summer got more radiation) and aphelion in December (N hemisphere winter got less), the reverse of today.

Q: What is Earth's average, minimum, and maximum distance to the Sun?

Average: 155.5 million km; minimum: 153 million km (perihelion); maximum: 158 million km (aphelion).

Q: How does the tilt cycle affect seasonality compared to the eccentricity and precession cycles?

Tilt has the strongest seasonal effect (up to 15% in polar regions); eccentricity has a small effect (~0.2%); precession affects seasonal radiation by up to 12%, especially at low latitudes.

Q: What enhanced anthropogenic greenhouse effect was noted in the Lecture 12 recap?

Anthropogenic emissions of CO₂, CH₄, and N₂O have added 2.7 W/m² of radiative forcing, causing >1°C global warming in the last 100 years — unprecedented for the Holocene.

Q: What are the solar radiation effects of each orbital parameter (tilt, eccentricity, precession)?

Tilt: up to 15% seasonal variation, mainly polar regions. Eccentricity: 0.2% variation, affects all areas equally. Precession: up to 12% variation, mainly low latitudes.

Q: What is insolation?

The amount of solar energy received by the Earth.

Q: What did Milankovitch propose about glacial and interglacial cycles?

Major glaciations are initiated by variations in Earth's orbital parameters (eccentricity, obliquity, precession), which alter the amount and distribution of solar radiation by latitude.

Q: What is the typical accumulation rate for continental ice sheets?

Less than 0.5 m of ice per year.

Q: Why does extreme cold limit ice sheet growth?

If it's too cold, not enough moisture can be transferred to glaciers for rapid accumulation.

Q: At what temperature does ablation begin, and how does it change?

Ablation begins at mean annual temps of −10°C and increases exponentially at higher temps, exceeding 3 m ice/year.

Q: Describe ice sheet mass balance at different temperature ranges.

Below −20°C: slow positive growth. −15 to −10°C: fast positive growth. At the equilibrium line: no net change. Above −10°C: ablation exceeds accumulation, ice melts.

Q: What is the equilibrium line (climate point) of an ice sheet?

The point where accumulation equals ablation — the ice sheet grows in winter and shrinks in summer with no net change.

Q: Does winter or summer insolation control ice sheet growth, and why?

Summer insolation — no matter how much snow falls, it can always be melted in the following summer if warm enough. Colder winters may actually reduce accumulation.

Q: What is Milankovitch's key hypothesis about ice sheet growth?

Low summer insolation at high northern latitudes (~65°N) is necessary for growing continental ice sheets.

Q: What orbital conditions favor N hemisphere ice sheet growth (low summer insolation)?

Small axial tilt (less seasonality), summer solstice at aphelion (farthest from Sun), and high eccentricity.

Q: What orbital conditions favor N hemisphere ice sheet melting (high summer insolation)?

High axial tilt, summer solstice at perihelion (closest to Sun), and low eccentricity.

Q: How much do summer insolation variations amount to at 65°N?

Up to 12% around the mean value, equivalent to approximately ±15 W/m².

Q: What were the major N hemisphere ice sheets during the LGM (~20,000 years ago)?

The Laurentide and Cordilleran ice sheets, which extended much farther south than today.

Q: How does elevation affect ice sheet mass balance?

Ice sheet growth is enhanced at higher elevations because temperature decreases with altitude, favoring accumulation.

Q: How do orbital-scale changes in summer insolation affect the climate point?

They move the climate point (equilibrium line) northward or southward — low summer insolation pushes it south, expanding ice coverage. The movement is proportional to the change in insolation.