🌍 Earth-Sun Dynamics

Unit 4: Climate Change β€” Earth and Space Science

How do we know that natural cycles aren't causing climate change today?

ENGAGE

How has the amount of radiation reaching Earth varied in the past?

The Big Questions

  • How do we know that climate change isn't happening because of natural cycles?
  • How has the amount of radiation (sunlight) reaching Earth varied in the past?
  • Does the amount of radiation the Sun produces vary over time?
  • Did natural factors contribute to temperature changes during Earth's past?
  • Do they contribute to global warming today?

How Do Scientists Know About the Past?

Scientists reconstruct Earth's climate history using proxy data:

  • Ice cores β€” Trapped air bubbles reveal atmospheric composition; oxygen isotopes indicate temperature
  • Tree rings β€” Width and density reflect growing conditions
  • Ocean sediments β€” Fossils and chemistry record ocean temperatures

These records extend 800,000+ years into the past.

Scientists also use mathematical models to calculate how much solar energy (insolation) reached Earth at different times and latitudes.

Why 65Β°N Latitude?

  • Ice sheets form and grow at high latitudes
  • Scientists discovered a strong correlation between summer insolation at 65Β°N and the timing of glacial-interglacial cycles
  • Summer matters because it determines whether ice melts faster than it accumulates

πŸ”‘ Key Idea: If summer insolation at 65Β°N is high enough, more ice melts in summer than forms in winter β†’ ice sheets shrink. If summer insolation is low, ice sheets grow.

Glacial and Interglacial Periods

Glacial periods (Ice Ages)

  • Cooler global temperatures
  • Large ice sheets cover high latitudes
  • Lower sea levels
  • Last ~90,000 years each

Interglacial periods

  • Warmer global temperatures
  • Reduced ice sheet coverage
  • Higher sea levels
  • Last ~10,000–15,000 years each

The cycle repeats approximately every ~100,000 years.

We are currently in an interglacial period (the Holocene).

Insolation and Temperature Over 350,000 Years

Refer to the graph in your packet.

What to look for:

  • Temperature line β€” cycles between glacial and interglacial periods
  • July insolation at 65Β°N β€” rises and falls in a cyclical pattern
  • The two patterns appear to rise and fall together

Correlation vs. Causation

Correlation β€” Two variables appear to be related; when one changes, the other tends to change in a predictable way.

Causation β€” One variable directly causes the other to change. There is a mechanism that explains why and how.

We observe a correlation between insolation at 65Β°N and glacial-interglacial cycles.

The question we need to answer:

Is there a causal mechanism that explains this relationship? What evidence do we need?

EXPLORE

How does Earth's position affect the amount of radiation reaching Earth's surface?

Three Orbital Factors

As Earth orbits the Sun, three things change over tens to hundreds of thousands of years:

Factor What Changes Cycle Length
Precession The direction Earth's axis points ~26,000 years
Obliquity The degree of Earth's axial tilt ~41,000 years
Eccentricity The shape of Earth's orbit ~100,000 years

These are called Milankovitch Cycles, named after Serbian scientist Milutin Milanković who calculated their effects on Earth's climate.

Orbital Factor 1: Precession

The direction of Earth's tilt with respect to the Sun

Cycle length: ~26,000 years

  • Earth's axis wobbles like a spinning top
  • This changes which hemisphere faces the Sun when Earth is closest (perihelion) vs. farthest (aphelion)

Configuration A:
NH tilted toward Sun at perihelion (closer)
β†’ More intense NH summers

Configuration B:
NH tilted toward Sun at aphelion (farther)
β†’ Less intense NH summers

Precession: Key Takeaways

Does precession change the total radiation reaching Earth?

No. The total annual energy Earth receives stays roughly the same.

Does it change radiation at 65Β°N?

Yes! Precession redistributes energy between hemispheres and seasons.

  • When NH is tilted toward the Sun at perihelion β†’ more intense summer radiation at 65Β°N
  • When NH is tilted toward the Sun at aphelion β†’ less intense summer radiation at 65Β°N

Precession changes when each hemisphere gets the most intense sunlight β€” not how much total energy Earth receives.

Orbital Factor 2: Obliquity

The degree of Earth's axial tilt

Cycle length: ~41,000 years

Earth's tilt varies between 22.1Β° and 24.5Β°
(Currently ~23.4Β° and decreasing)

Less tilt (22.1Β°)

  • Milder seasons
  • Less direct sunlight at high latitudes
  • Less extreme difference between summer and winter

More tilt (24.5Β°)

  • More extreme seasons
  • More direct sunlight at high latitudes in summer
  • Greater difference between summer and winter

Obliquity: Key Takeaways

Does obliquity change the total radiation reaching Earth?

No. The total annual energy stays roughly the same.

Does it change radiation at 65Β°N?

Yes! Greater tilt means sunlight hits 65Β°N at a more direct angle during summer.

  • More tilt β†’ more direct summer radiation at 65Β°N β†’ warmer high-latitude summers
  • Less tilt β†’ less direct summer radiation at 65Β°N β†’ cooler high-latitude summers

Obliquity changes the angle at which sunlight strikes high latitudes, which changes how concentrated the energy is.

Orbital Factor 3: Eccentricity

The shape of Earth's orbit

Cycle length: ~100,000 years

Earth's orbit changes from nearly circular to more elliptical and back.

Low eccentricity (nearly circular)

  • Small difference between perihelion and aphelion distances
  • Seasons less affected by orbital distance

High eccentricity (more elliptical)

  • Larger difference between closest and farthest approach
  • Orbital distance has a bigger effect on seasonal intensity

Current eccentricity is relatively low (~0.017).

Eccentricity: Key Takeaways

Does eccentricity change the total radiation reaching Earth?

Yes, slightly. Higher eccentricity increases total annual radiation by a small amount.

Does it change radiation at 65Β°N?

Yes. Higher eccentricity means a bigger difference between perihelion and aphelion seasons.

Eccentricity amplifies the effects of precession. When eccentricity is high, the difference between being close to or far from the Sun matters more β€” so precession has a bigger impact on seasonal intensity.

Summary: Three Factors Working Together

Factor Changes... Timescale Effect on 65Β°N
Precession Direction of tilt ~26 kyr Redistributes seasonal energy between hemispheres
Obliquity Degree of tilt ~41 kyr Changes angle/concentration of sunlight at high latitudes
Eccentricity Orbit shape ~100 kyr Amplifies seasonal differences; slightly changes total energy

No single factor alone explains glacial-interglacial cycles. It is the combined effect of all three that drives the pattern.

EXPLAIN β€” Part 1

Did changes in Earth's position cause glacial-interglacial cycles?

Applying Our Models to Real Data

We will now use what we learned about the three orbital factors to analyze specific time periods in the glacial-interglacial record:

Time Period Climate Transition
144 kya β†’ 130 kya Glacial β†’ Interglacial
130 kya β†’ 113 kya Interglacial β†’ Glacial
25 kya β†’ 9 kya Glacial β†’ Interglacial

For each period, we will examine how each orbital factor changed, predict the effect on 65Β°N summer insolation, and compare to the actual data.

Features of a Good Explanatory Model

Models are used to represent a system under study. Explanatory models should:

  1. Represent real-world objects or ideas
  2. Illustrate and/or predict relationships between components of a system
  3. Use scientific vocabulary and labels
  4. Provide a scientific mechanism for what you are claiming
  5. Be based on real data

Your task:

Analyze the orbital factor data β†’ Predict the effect on 65Β°N insolation and ice sheets β†’ Compare predictions to the actual glacial-interglacial graph

Period 1: 144 kya β†’ 130 kya

Factor 144 kya 130 kya Change
Eccentricity min 145.6 / max 153.6 Mkm min 143.8 / max 155.4 Mkm ↑ Higher (more elliptical)
Tilt 22.6Β° 24.25Β° ↑ Increased
Precession NH toward Sun when closer (βˆ’) NH toward Sun when closer (βˆ’) No major change

What should happen?

  • Tilt increased β†’ more direct radiation at 65Β°N in summer
  • Eccentricity increased β†’ amplifies seasonal differences
  • Both factors increase summer insolation at 65Β°N

144 kya β†’ 130 kya: Prediction & Check

Prediction:

  • Summer insolation at 65Β°N should increase
  • Ice sheets should shrink (melting > accumulation)
  • Expect a transition from glacial β†’ interglacial

Does the data match?

βœ… Yes! The July insolation graph shows increasing insolation, and the temperature record shows the transition from glacial to interglacial conditions during this period.

This is the onset of the Eemian interglacial β€” the last warm period before our current one.

Period 2: 130 kya β†’ 113 kya

Factor 130 kya 113 kya Change
Eccentricity min 143.8 / max 155.4 Mkm min 143.6 / max 155.6 Mkm Remained high
Tilt 24.25Β° 22.25Β° ↓ Decreased
Precession NH toward Sun when closer (βˆ’) NH away from Sun when closer (+) Shifted

What should happen?

  • Tilt decreased β†’ less direct radiation at 65Β°N in summer
  • Precession shifted β†’ NH summer now at aphelion (farther from Sun)
  • Eccentricity still high β†’ amplifies the precession shift
  • All factors decrease summer insolation at 65Β°N

130 kya β†’ 113 kya: Prediction & Check

Prediction:

  • Summer insolation at 65Β°N should decrease
  • Ice sheets should grow (accumulation > melting)
  • Expect a transition from interglacial β†’ glacial

Does the data match?

βœ… Yes! The July insolation graph shows decreasing insolation, and the temperature record shows the transition back to glacial conditions.

This is the end of the Eemian interglacial and the beginning of the last ice age.

Period 3: 25 kya β†’ 9 kya

Factor 25 kya 9 kya Change
Eccentricity min 147.36 / max 151.84 Mkm min 146.61 / max 152.59 Mkm Slightly ↑ (still low)
Tilt 22.3Β° 24.25Β° ↑ Increased significantly
Precession NH away from Sun when closer (+) NH away from Sun when closer (+) No major change

What should happen?

  • Tilt increased significantly (22.3Β° β†’ 24.25Β°) β†’ much more direct radiation at 65Β°N
  • This is the dominant factor β€” tilt drives the warming
  • Eccentricity is low, so precession effects are muted

25 kya β†’ 9 kya: Prediction & Check

Prediction:

  • Summer insolation at 65Β°N should increase (driven by tilt)
  • Ice sheets should shrink
  • Expect a transition from glacial β†’ interglacial

Does the data match?

βœ… Yes! This corresponds to the end of the last ice age and the transition into our current interglacial period (the Holocene).

The Laurentide ice sheet that once covered much of North America melted during this period, raising sea levels by ~120 meters.

Summary: Orbital Factors and Glacial Cycles

The three Milankovitch cycles β€” eccentricity (~100 kyr), obliquity (~41 kyr), and precession (~26 kyr) β€” primarily change the distribution of solar energy across latitudes and seasons.

When their effects combine to increase summer insolation at 65Β°N β†’ ice sheets melt β†’ interglacial

When they combine to decrease summer insolation at 65Β°N β†’ ice sheets grow β†’ glacial

The ~100,000-year glacial cycle matches eccentricity's timescale, but it is the combined effect of all three that drives the cycle.

EXPLAIN β€” Part 2

Testing Our Explanation with a Computational Model

The Vostok Ice Core

  • A collaborative ice-drilling project (Russia & U.S., 1998)
  • Drilled at Vostok Station in East Antarctica
  • Reached a depth of 3,623 meters
  • Trapped air in ice reveals changes in atmospheric composition
  • Provides a 400,000-year temperature record

Key Vocabulary

Term Definition
Perihelion Point in orbit where Earth is closest to the Sun
Aphelion Point in orbit where Earth is farthest from the Sun

Current Earth-Sun Configuration

For today's orbital configuration:

  • Summer solstice (NH summer) occurs near aphelion β€” Earth is farther from the Sun
  • Winter solstice (NH winter) occurs near perihelion β€” Earth is closer to the Sun
  • Earth's axis tilts toward the Sun during aphelion
  • Earth's axis tilts away from the Sun during perihelion

This means Northern Hemisphere summers are currently less intense than they could be if summer occurred at perihelion. Precession is currently in an unfavorable configuration for strong NH summers.

Tilt and Orbit in New York

How do tilt and orbit determine solar radiation in New York (~41Β°N)?

Summer (NH tilted toward Sun):

  • Sunlight strikes at a more direct angle
  • Days are longer (more hours of sunlight)
  • More energy delivered per square meter

Winter (NH tilted away from Sun):

  • Sunlight strikes at a lower, more oblique angle
  • Days are shorter (fewer hours of sunlight)
  • Less energy delivered per square meter

Currently Earth is closest to the Sun in January (winter), which slightly moderates NH seasonal extremes.

Testing Each Factor Against Vostok Data

Using the computational model, we overlay each orbital factor against the Vostok temperature record:

Factor Alone Correlates with Temperature?
Eccentricity only ❌ No β€” peaks/valleys don't align well with temperature
Precession only ❌ No β€” ~26 kyr cycle is too short to match ~100 kyr glacial cycle
Tilt only βœ… Partially β€” better match than either alone, but not perfect
All three combined βœ…βœ… Best match β€” most closely tracks temperature over 400,000 years

Key Finding: Combined Factors

When all three Milankovitch cycles are combined, the resulting pattern most closely matches the Vostok temperature record over 400,000 years.

No single factor alone can explain glacial-interglacial cycles. It is the interaction of all three that produces the pattern.

But it's not perfect...

There are areas where the orbital factor line and temperature don't match perfectly. This tells us that other factors also influence temperature:

  • Greenhouse gas feedbacks (COβ‚‚, CHβ‚„)
  • Ice-albedo feedbacks
  • Ocean circulation changes

These will be investigated in upcoming 5E sequences.

From Correlation to Causation

We can now say we have established a causal link:

  1. Mechanism: Physical models showed how each orbital factor changes radiation at 65Β°N
  2. Correlation: Computational model showed the combined orbital factors track with temperature data
  3. Prediction: We successfully predicted glacial/interglacial transitions for three specific time periods using orbital data

Correlation β€” We see the patterns match.
Causation β€” We understand the mechanism (changing radiation distribution β†’ ice sheet response) that explains why they match.

ELABORATE

How well does activity from the Sun correlate with glacial-interglacial cycles?

Sunspots and Solar Radiation

Sunspots are dark regions on the Sun's surface associated with increased ultraviolet radiation output.

  • More sunspots β†’ more UV radiation emitted
  • Fewer sunspots β†’ less UV radiation emitted

The number of sunspots changes in a regular cycle:

Solar Cycle β€” Regular changes in sunspot number and solar radiation output (~11-year cycle)

Solar Maximum β€” Peak sunspot activity β†’ most radiation

Solar Minimum β€” Lowest sunspot activity β†’ least radiation

Solar Radiation and Temperature

Does solar radiation affect Earth's temperature?

Yes β€” more radiation from the Sun means more energy input to Earth's climate system.

Are sunspot activity and temperature always correlated?

No! Looking at the 10,000-year record:

  • Sometimes both rise together
  • Sometimes solar activity increases while temperature decreases
  • Sometimes temperature changes without a matching solar change

Why the mismatch?

Other factors also influence temperature: orbital cycles, volcanic eruptions, ocean circulation, greenhouse gases, ice-albedo feedbacks.

The Timescale Problem

Cycle Length
Solar cycles ~11 years
Glacial-interglacial cycles ~100,000 years

The timescales are off by a factor of ~10,000.

Solar cycles cannot explain glacial-interglacial cycles.

The ~11-year solar cycle is far too short and the variation in radiation output is too small (~0.1%) to drive the massive, long-term glacial-interglacial transitions.

Milankovitch cycles (orbital factors) remain the best explanation for glacial-interglacial cycles.

Solar Output: What It Can and Can't Do

What solar cycles CAN do:

  • Cause small, short-term temperature fluctuations
  • Contribute to decade-scale climate variability
  • Influence upper atmosphere chemistry

What solar cycles CANNOT do:

  • Drive glacial-interglacial cycles (wrong timescale)
  • Explain current rapid warming (output has been flat/declining)
  • Override long-term orbital forcing

EVALUATE

How do we know that orbital factors are not causing climate change today?

The Rate of Current Warming

Looking at the historical temperature record:

Warming Event Time to Rise ~1.25Β°C
Current climate change ~100–150 years
Next fastest natural event ~1,000–2,000 years
Post-glacial warming (20 kya) ~5,000–10,000 years

Current warming is 50–100Γ— faster than any natural warming event in the geological record.

Natural cycles simply cannot produce warming this rapid.

Where Are Orbital Factors Now?

Comparing 9,000 years ago to today:

Factor 9 kya Today Trend
Tilt 24.25Β° 23.4Β° ↓ Decreasing
Precession NH toward Sun when closer (βˆ’) NH away from Sun when closer (+) Shifted unfavorably
Eccentricity 146.61 / 152.59 Mkm 147.06 / 152.14 Mkm Low, nearly circular

What do orbital factors predict?

  • Tilt is decreasing β†’ less direct summer radiation at 65Β°N
  • Precession is unfavorable β†’ NH summer at aphelion (farther)
  • Both factors should be reducing summer insolation at 65Β°N

The Critical Contradiction

Orbital factors predict Earth should be COOLING.

But temperatures are RAPIDLY RISING.

This is one of the strongest pieces of evidence that current climate change is NOT caused by natural orbital cycles.

Based on Milankovitch cycles alone, Earth should be slowly moving toward the next glacial period. Instead, we observe the fastest warming in the geological record.

Something else must be driving the warming.

What About the Sun?

Looking at Total Solar Irradiance vs. Global Temperature since ~1880:

  • Solar output has been relatively flat or slightly declining since the 1960s
  • Global temperatures have increased sharply over the same period
  • The two lines have diverged completely since ~1980

If the Sun were driving current warming, temperatures should follow solar output. They don't.

Solar variation is eliminated as a cause of recent climate change.

Putting It All Together

Why scientists are sure natural cycles are NOT causing current climate change:

1. Orbital factors predict cooling, not warming

  • Tilt is decreasing, precession is unfavorable for NH summers
  • Earth should be slowly heading toward a glacial period

2. Solar output is flat or declining

  • No increase in solar radiation to explain rising temperatures
  • Solar cycles (~11 yr) operate on the wrong timescale

3. The rate of warming is unprecedented

  • Current warming is 50–100Γ— faster than any natural cycle
  • Natural processes simply cannot produce this rate of change

So... What IS Causing Current Climate Change?

Natural cycles cannot explain the current warming.

The evidence points to factors we will investigate next:

  • 🏭 Greenhouse gas emissions from human activity
  • 🌑️ Changes in atmospheric composition (COβ‚‚, CHβ‚„, Nβ‚‚O)
  • πŸ”„ Feedback mechanisms that amplify warming

These are the questions we'll tackle in our next investigation.

Key Vocabulary Review

Vocabulary Reference

Term Definition
Milankovitch Cycles Three orbital variations that affect Earth's climate over thousands of years
Precession Wobble of Earth's axis; changes which hemisphere faces Sun at perihelion (~26 kyr)
Obliquity Change in the degree of Earth's axial tilt between 22.1°–24.5Β° (~41 kyr)
Eccentricity Change in the shape of Earth's orbit from circular to elliptical (~100 kyr)
Perihelion Point where Earth is closest to the Sun
Aphelion Point where Earth is farthest from the Sun

Vocabulary Reference (continued)

Term Definition
Insolation Amount of solar radiation reaching a surface (e.g., at 65Β°N)
Glacial period Cold period with expanded ice sheets (ice age)
Interglacial period Warm period with reduced ice sheets (like today's Holocene)
Correlation Two variables appear related β€” they change together
Causation One variable directly causes the other through a mechanism
Solar cycle ~11-year cycle of sunspot activity and solar radiation output
Sunspots Dark spots on the Sun linked to changes in UV radiation output