Glaciers & Ice Ages

What is Glaciers & Ice Ages?

Glaciers are persistent ice masses that grow when winter snowfall exceeds summer melt year after year, and shrink when the reverse is true. Over the past 2.6 million years, Earth has cycled between frigid glacial periods — when ice sheets a kilometer thick covered Canada and northern Europe — and warmer interglacials like today. The engine behind these cycles is not random: Milankovitch cycles are three slow, predictable changes in Earth's orbit and axial tilt that alter how much sunlight reaches high northern latitudes in summer. Small decreases in summer insolation let winter snow survive into the following year, triggering ice growth and a cascade of feedbacks — more ice means more reflected sunlight, which means more cooling. The simulation lets you adjust orbital parameters and scrub through 800,000 years of glacial history.

Parameters explained

Temperature Anomaly(°C)

Global mean temperature anomaly relative to the modern baseline, in °C (−8 to +4°C). Negative values represent ice-age cooling; positive values represent interglacial warmth. At −6°C (Last Glacial Maximum, ~21 ka) ice sheets covered much of North America and Scandinavia. A +2°C anomaly corresponds roughly to a rapid warming scenario that drives significant glacier retreat.

Accumulation Rate(m/yr)

Annual snowfall accumulation rate on the glacier in m/yr water equivalent (0.5–6 m/yr). High accumulation rates (>3 m/yr) cause glaciers to advance even under moderate warming; low accumulation (<1.5 m/yr) means even mild warming causes rapid retreat. Real alpine glaciers range 0.5–3 m/yr; polar ice sheets receive far less.

Milankovitch Time(kyr ago)

Position in Milankovitch time, in thousands of years before present (0–800 kyr). Scrubbing this control plays back 800,000 years of glacial cycles. Major glacial peaks occur roughly every 100 kyr (eccentricity cycle). Set to 0 for the present; 21 for the Last Glacial Maximum; 125 for the last interglacial optimum.

Common misconceptions

• MisconceptionIce ages are caused by the climate — the climate gets cold, and then ice forms.

  CorrectThe causal direction is the reverse. Orbital changes (Milankovitch cycles) reduce summer insolation first, allowing ice to accumulate. The growing ice sheet then amplifies the cooling through the ice-albedo feedback and by reducing atmospheric CO₂ outgassing from a cooling ocean. Orbital forcing is the trigger; climate feedbacks are the amplifier.

• MisconceptionThe ice-albedo feedback is a minor detail — the orbital forcing alone explains ice ages.

  CorrectOrbital forcing alone is far too weak to produce full glacial conditions. The ~0.2% change in total insolation from eccentricity must be amplified roughly 5–10× by feedbacks — primarily ice-albedo (ice reflects ~80% of sunlight vs. ~6% for dark ocean) and CO₂ release changes — to match observed glacial temperature drops of 5–8°C.

• MisconceptionWe are currently in an ice age because there is still ice at the poles.

  CorrectTechnically Earth is in an ice era (the Quaternary Glaciation) because permanent ice exists at the poles. But within that era we are in an interglacial period — a warmer phase between glacials. An 'ice age' in common usage typically refers to a glacial maximum like 20,000 years ago, not the current warm interglacial.

• MisconceptionPrecession (the wobble of Earth's axis) changes how much total energy Earth receives from the Sun.

  CorrectPrecession does not change Earth's total annual solar energy. It shifts when in the year each season occurs relative to Earth's closest approach to the Sun (perihelion). This changes seasonal contrast — making northern summers slightly warmer or cooler — which is enough to affect whether high-latitude snow survives the summer melt season.

How teachers use this lab

1. Cycle period identification: set playbackSpeed to 3× and run the full 800 kya timeline. Students count the number of major glacial peaks visible in the temperature curve and calculate the average period. Compare to the expected ~100 kyr eccentricity dominant cycle.
2. Obliquity effect probe: pause timePosition at 400 kya, then sweep obliquity from 22° to 24.5° while holding eccentricity constant. Students record ice-sheet extent at each tilt extreme and explain the result using the concept of seasonal contrast at high latitudes — connecting to HS-ESS2-4.
3. Ice-albedo feedback chain: set timePosition to 20 kya (near glacial maximum) and toggle eccentricity from 0.017 to 0.001 (near-circular orbit, reduced forcing). Ask students to predict whether ice extent changes significantly, then observe. Use the discrepancy to introduce feedback amplification as a crosscutting concept of systems and system models.
4. Data collection — temperature anomaly tracking: advance timePosition in 50 kya increments and record temperature anomaly. Students plot temperature vs. time and identify that glacial periods coincide with the deepest cold anomalies, supporting HS-ESS1-4.
5. Prediction challenge: set timePosition to present (0 kya) and current orbital parameters (eccentricity 0.017, obliquity 23.4°). Ask students to predict — based only on orbital mechanics — whether Earth is heading into or out of a glacial period over the next 20,000 years (answer: slowly toward a new glacial in ~50 kyr, delayed by anthropogenic forcing).