Week 6.1 orbital scale climate change

ORBITAL SCALE CLIMATE CHANGE

Week 6.1: 29th August

Orbital Scale Climate Change

Earth’s Orbit has changed over time

1. Tilt

2. Eccentricity

3. Precession

The Milankovitch Cycles

Milankovitch Cycles: Glaciation

Orbital Scale…?

Tectonic Scale Climate Change: The time period where the Earth’s tectonic make-up was the most important climate driver. From the formation of the Earth until the breakup of the last supercontinent (Pangea) and the formation of the current plate configuration.

Orbital Scale Climate Change: The time period when orbital movements become an important climate driver. From approx. 30 mya, or the late Cretaceous period.


Orbital-scale climate change is generally focused over the last 3 million years. Why?

Continents and Oceans were reaching current positions.

Climate records are more easily available for this time period.

We can see cause and effect relationships between orbital cycles and changes in Earth’s climate.

How do we study Orbital Scale Climate Change?

Ocean sediments: radiometric dating, magnetic reversals,

Ice cores (from 400,000 years)

Magnetic Reversals

Magnetic reversals can tell us about the positions of lithospheric plates.

What are they?

It starts with: A SPREADING RIDGE

Magnetic Reversals

Magnetic Reversals

As the ridge spreads, molten rock from the asthenosphere rises up through the rift and solidifies once it hits the cool ocean on the top of the lithosphere.

The molten rock solidifies as Basalt, and this basalt holds a slight magnetic dipole.

This dipole will align with the Earth’s magnetic field at the time of cooling.

Magnetic Reversals

We know from celestial observations that the Earth’s magnetic dipole switches periodically.

We can see this when we look at the magnetic direction of ocean floor Basalt off spreading ridges.

The results look like strips of basalt aligned to opposite poles.

Magnetic Reversals

Magnetic Reversals

Spreading ridges were an important finding in the study of plate tectonics because they could prove the movement of lithospheric plates.

Using newer dating methods (radiocarbon dating, sediment analysis) the reversal of the poles has been put into a time sequence.

Spreading ridges from all oceans have been compared to study the speed of plate movement.

Other dating methods of rock especially, are now compared to the pole reversals sequence to help confirm other dating methods.


Earth’s Orbit has changed with time

The tilt of Earth’s axis

The shape of Earth’s path around the sun

The positions of the seasons on Earth’s path

These changes follow cycles and can be used to calculate the amount of insulation arriving on Earth at any latitude and season. Insulation is an important driver of climate processes.

Key Terms: axis of rotation, tilt, orbit, elliptical, perihelion, aphelion, cycles, millennial, insolation, solar radiation, eccentricity


Earth’s orbit around the sun is not permanent.

The tilt of Earth’s Axis, the shape of it’s yearly path around the sun, and the changing positions of Earth’s seasons along that path, have changed over time in cycles ranging from 20,000 to 40,000 years.

We know the timing of these cycles with relative accuracy for the last several million years.

These orbital cycles have corresponded with records of many climatic responses on Earth.


Variations in the eccentricity of the orbit cause changes in the annually averaged amount of sunlight hitting Earth.

Variations in the tilt (obliquity variations and the precession of the tilt) do not affect the averaged amount of solar radiation to the Earth.

The tilt variations affect seasons.

Earth’s Orbit: Tilt

We experience earth’s tilt in our seasons.

When a hemisphere is tilted directly towards the sun, it receives the more direct radiation of summer.

When tilted away from the sun, the hemisphere receives less direct radiation of winter.

Note: Also called Obliquity!


Assume the Earth has a perfectly circular orbit around the Sun.

With no tilt, incoming solar radiation is always directed straight at the equator throughout the year.

With no tilt, no seasonal changes occur in solar radiation received at any latitude.

As a result, solstices and equinoxes do not even exist

NO SEASONS!


http://www.harding.edu/lmurray/113_files/html/b_historical%20development/sld081.htm


Earth’s rotational spin axis is currently tilted at an angle of 23.5° away

from a line perpendicular to the plane of its orbit around the sun.

• 90°- 23.5° = 66.5° Hence the 66.5° latitude of the arctic and Antarctic

circles. Full sun in summer, no sun in winter.

• Also explains the tropics. Cancer at 23.5° N and Capricorn at 23.5° S.


Earth’s Orbit: The Shape of the current orbital path

Earth’s orbit is elliptical. This means that at points along the orbit, Earth is closer or further from the sun.

On average Earth lies 149.6 million km from the sun, but the distance ranges from 147 to 152 million km.

Perihelion: position in which Earth is closest to the sun (147 million km).

Aphelion: position in which Earth is furthest from the sun (152 million km).

Earth’s Orbit: The Shape of the Orbital path

This image is slightly exaggerated.

Winter radiation in the Northern Hemisphere and Summer radiation in the Southern hemisphere are slightly higher than they would be in circular orbit and vice versa.

The above effect on the seasons is minimal.

https://manoa.hawaii.edu/exploringourfluidearth/media_colorbox/2291/media_original/en

Earth’s Orbit: Tilt Change

The angle of Earth’s tilt varies over time between 22.2 degrees and almost 24.5 degrees.

These variations are caused mainly by the gravitational tug of large planets, such as Jupiter.

Cyclic changes in tilt occur over a period of about 41,000 years.

Present day tilt (23.5 degrees) is near the middle of the range, and is decreasing.


http://www.museum.state.il.us/exhibits/ice_ages/tilt_graph.html

Ruddiman, W. F. (2008) Earth’s Climate: past, present and future.


Changes in tilt cause long term variations in seasonal solar insulation received on Earth.

The largest changes are at high latitudes.

Larger tilt angles the poles more towards and away from the sun, increasing and reducing the amount of solar radiation received in summer and winter.

Decreases in tilt diminish the amplitude of seasonal differences.

Earth’s Orbit: Eccentricity

The shape of the Earth’s orbit around the sun has varied in the past. Sometimes becoming more circular, and at other times more elliptical than today.

This is described by the term Eccentricity (є). Or degree of departure from a normal circular orbit.

𝜖 =(𝑎2−𝑏2)

𝑎

𝜖 =(𝑎2 − 𝑏2)

𝑎

http://www.mathsisfun.com/geometry/eccentricity.html


http://earthobservatory.nasa.gov/Features/OrbitsCatalog/


Eccentricity increases from zero to one with increasing elliptical nature.

Earth’s orbital eccentricity has ranged from 0.005 to 0.0607.

Todays is 0.0167, todays orbital shape is close to circular.


http://agron-www.agron.iastate.edu/courses/Agron541/classes/541/lesson07b/7b.2.html


Changes in the eccentricity of Earth’s Orbital cycles have been more irregular than changes in tilt, but three cycles show up in data.

100,000 year cycle. Four smaller cycles blended into one.

413,000 year cycle. 100,000 year cycles tend to cluster and alternate between larger and smaller amplitutes.

2.1 million years cycle. Much weaker in amplitude.


https://sureshemre.wordpress.com/2014/05/03/difference-between-the-precession-of-the-equinoxes-and-the-precession-of-

earths-axis/

Earth’s Orbit: Axial Precession

The Earth has a long term wobbling motion. This is called axial precession.

This motion means that the solstices and equinoxes gradually shift position with respect to Earth’s orbit, and with respect to the perihelion and aphelion positions.

The distance from Earth to the Sun has varied with time for each of the seasons.

These changes have produced changes in solar radiation received on Earth.


Axial precession is the term used to describe how the Earth’s spinning axis gradually leans in different directions through time.

Tilt describes the amount of leaning of the axis (the angle of tilt)

Axial precession describes the direction of leaning.

Earth’s axial precession is caused by the gravitational pull of the sun and moon on the slight bulge in Earth’s diameter at the equator.

Axial precession can be described as one turn of the Earth’s axis around a circular path every 25,700 years.


Look Here if you are confused!

http://www.sciencecourseware.org/eec/globalwarming/tuto

rials/milankovitch/

Earth’s Orbit: Precession

The Precession of the ellipse is a motion where the Earth’s orbit itself rotates, with the long and short axes of the ellipse turning slowly in space. This movement is slower than the axial procession.

The Precession of the ellipse, and the axial precession combine into a movement called the precession of the equinoxes.

This describes the absolute motion of the solstices and equinoxes in reference to the universe.

The precession of the equinoxes has two cycles. The strongest of the two is the 23,000 year cycle.

Earth’s Orbit: Eccentricity-modulated precession

Because of Earth’s eccentric orbit, the precession of the equinoxes causes equinoxes and solstices to have orbital positions at varying distances from the sun.

These changes vary the amount of solar radiation received on Earth.

The gradual change in eccentricity, combined with the precession of the Equinoxes also causes changes in the amount of solar radiation received on Earth.

We can combine these factors, using some complicated mathemantics, into Eccentricity-modulated precession. This helps when looking at climate change.

Earth’s Orbit: Milankovitch Cycles

We know that every one of the factors we have just discussed causes effects on solar insolation.

The combined effect of these orbital movements are called the Milankovitch Cycles.

Earth’s Orbit: Milankovitch Cycles

Milankovitch Cycles

Quick summary: the Milankovitch cycles are:

Tilt cycles

Precession cycles

Eccentricity cycles

These two combine into eccentricity modelled precession

(the precessional index)

Tilt cycles and the precessional index can explain the amount of insolation to arrive at

Earth at any latitude and season.

Insolation is a large driver of climate processes.

Note: The Milankovitch cycles explain the seasonality and location of solar energy

around the Earth, impacting the contrasts between the seasons, and not directly

the amount of solar radiation

Milankovitch cycles

Tilt and the precessional index have differing effects on insolation at different latitudes.

Tilt has the greatest influence on insolation at higher latitudes (poles). (41,000 year cycles)

Precession has greater influence on insolation in lower and mid latitudes. (roughly 23000 year cycles, with modulation at 100,000 and 413,000 years)

Milankovitch cycles: Glaciation

A good example of the Milankovitch cycles as a climate change driver is glaciation.

http://www.alaska.org/advice/glaciers-in-alaska


Continental ice sheets occur when the rate of snow fall and ice formation is higher or equal to the rate of ablation (ice loss).

Ice sheets occur at high elevation or high latitudes, where ice can be present during the summer season.

Ice (snow) accumulates at mean annual temperatures below 10° C. the rate of ice accumulation stays below 0.5 m/yr. This occurs because at higher temps, ice falls as rain, and at lower temps, there is little water vapour in the air to merge the snow into ice.

Ablation of glacial ice accelerates rapidly in warm temperatures. Melting begins at mean annual temperatures of -10° C and can reach rates of 3 m/yr.


Ice forms in high latitudes and altitudes during the winter periods.

High latitudes always have relatively low solar radiation, and ice formation is not as critical to glaciation as ablation.

Therefore, the key season to look at orbital scale effects on glaciation is summer.

Low summer insolation is the critical factor that cools the climate enough to allow snow and ice to persist from one winter to the next.

Milankovitch Theory

Milankovitch theory attempted to explain ice sheet changes in the Northern Hemisphere.

Proposed that ice sheets grow (ice age) during times when summer insolation is low in high northern latitudes.

This occurs when Earth’s tilt is small, when the northern hemisphere’s summer solstice is furthest from the sun and when the orbit is highly eccentric.

Ice melts during the opposite orbital configuration.

Milankovitch theorised that the last point of latitude of ice formation and melting was the most critical for solar insolation (about 65° N).

Milankovitch Theory

Milankovitch Theory

From Milankovitch Theory, we expect that orbital scales will have had a large influence on past glaciation and ice ages. How do we find out?

The best evidence is in ocean sediments.

Note: We also expect an overall effect on glacial cycles to shift towards a cooler climate due to atmospheric CO2 declining over the last several million years (tectonic effect).

Ocean Sediment analysis

The formation of ocean sediments is relatively uninterrupted and it is easy to follow successive layers, when they haven’t been disturbed.

Evidence of past glaciations consist of: ice-rafted debris. Course and fine sediments delivered to the ocean by melting icebergs.

Δ18 O records from the shells of foraminifera

These sediments are dated by comparing them to the shifting patterns of magnetic reversals contained in core samples.

Ocean Sediment Analysis

Ocean sediment analysis has shown a strong correlation between glacial periods and orbital cycles up until 0.9 Myr ago

Cycles of 41,000 years (tilt cycle) until 0.9 Myr ago.

At 0.9 Myr ago, an unknown 100,000 year cycle starts to show a larger effect on glaciation.

Note shift due to dropping CO2



A closer look shows that the 41,000 yr (tilt) and 23,000 yr (precession) cycles still evident, but overwhelmed by 100,000 yr cycle.

Ice sheets persisted for longer intervals and grew larger.

This image shows what are called large scale glaciations and interglacial periods.

Note that we are currently in a period of interglaciation.


Ruddiman 2008

Interglaciation

How did this effect us?

Each glacial minima (interglaciation period) would have brought sea level rises due to the ice sheet melts.

Each glacial maxima would have caused drought and desertification, but also created land bridges for people, animals and plants to move across.

E.G. about 14-12,000 years ago, the world saw sea level rise of up to 24 meters!

This rise separated PNG, and Tasmania from mainland Australia.

What is creating the “new” 100,000 year cycle?

This is where it gets messy! There are many factors that are likely to be playing a part in this new 100,000 year glacial cycle:

CO2 levels decreasing slowly over large time scales.

The shape and feedback effects of large ice sheets (thin ones spread further and melt faster than thick sheets)

Oceanic and atmospheric currents bringing heat and CO2, creating compounding effects.

Summary

From about 30mya, orbital variations and movements become a highly influential climate driver.

The shifts in the orbit are called the Milankovitch cycles, and are made up of:

o Tilt cycles

o Precession cycles

o Eccentricity cycles

The Milankovitch Cycles influence glacial cycles (ice ages).

Note: They also have an affect on atmospheric CO2 and the monsoon. I

References

Ruddiman, W. F. (2008) Earth’s Climate: past, present and future.

http://solarsystem.nasa.gov/planets/sun/facts

http://study.com/academy/lesson/how-earths-orbit-tilt-impacts-climate-change.html

http://www.giss.nasa.gov/research/briefs/gornitz_09/

http://solarsystem.nasa.gov/planets/sun/facts

http://study.com/academy/lesson/how-earths-orbit-tilt-impacts-climate-change.html

http://www.giss.nasa.gov/research/briefs/gornitz_09/

Science

Week 6.1 orbital scale climate change