Lecture 12: Glaciation Cycles

Glaciation and Quaternary Period

A period of dramatic biogeographic upheaval caused by repeated glacial-interglacial cycles

Glaciation and Quaternary Period is comprised of two epochs:

Pleistocene and the Holocene

When was the Pleistocene?

beginning 2.58 Ma

When was the Holocene?

beginning 11.7 Ka

What did the Pliocene cooling: the more recent epoch consist of?

downward trend in global temperature

When was the pliocene?

5.33 Ma - 2.58 Ma

At the start of the Pliocene, temperatures were much?

warmer than today

What was the overall trend in temperature during the Pliocene?

Downward trend toward drier climates

What did Pliocene cooling climate oscillations consist of?

high frequency-low amplitude oscillations

What happened during Pliocene cooling?

forests declined, grasslands and savannas expanded

epoch ends with shift to high amplitude, longer duration glacial-interglacial cycles until the start of the Pleistocene

Pleistocene glacial-interglacial cycles

glacial periods were long

interglacial relatively short, with warm periods lasting ~10K years

What did the Pleistocene consist of?

Period of great fluxes in:

• Temperature

• Precipitation

• Atmospheric and ocean currents

• Ice distribution

• Sea levels dropped

Who was Louis Agassiz?

A Swiss geologist/biologist that developed the first comprehensive theory on Pleistocene glaciation in 1836

Postulated polar ice masses were highly?

dynamic

In an earlier period, glaciers extended into?

lower latitudes (subarctic and temperate) in the Northern Hemisphere, “Ice Age”

Great impacts on the distributions of?

plants and animals

Major factors influencing climate distilled to factors influencing?

the temporal and spatial variation of solar radiation reaching the Earth

What were the Major factors influencing climate?

Variations in:

• Earth’s orbit

• Absorption and reflectivity atmosphere

• Absorption and reflectivity surfaces

• Solar radiation output from the sun

Other factors

• Positions of land masses

• Area of ice accumulation

• Atmospheric gases

• Feedback interactions

What did Variations in the Earth’s orbit consist of?

affecting where solar radiation strikes the Earth and when it does

What did Variations in the Positions of land masses consist of?

affects where ice accumulates

What did variations in Atmospheric gases consist of?

greenhouse gases affect climate

Glacial-interglacial cycle - high amplitude swings in global temperature

Driven by cyclical changes in earth’s orbit

Predominantly affects where and when solar radiation strikes earth

What are cyclical changes in earth’s orbit driven by?

changes in positions of other planets and their gravitational pull on earth

Who was Milutin Milankovitch?

Serbian geophysicist that developed Milankovitch cycles

When did Milankovitch develope Milankovitch cycles?

published in 1920, 1930

What do Milankovitch cycles predominantly determine?

Earth’s temperature

Three main patters: drivers of Earth’s climate

Eccentricity

Obliquity

Precession

What is Eccentricity?

the shape of Earth’s orbit (elongated eclipse)

How does Eccentricity determines Earth’s climate?

Earth’s elliptical orbit lengthens and shortens in a 96-thousand year cycle, when orbit is more elliptical the perihelion becomes warmer

What is Obliquity?

the tilt of Earth’s axis; sits at a tilt in relation to the sun and wobbles

How does Obliquity determines Earth’s climate?

the tilt of Earth’s axis changes on a 41k year cycle;

when the tilt is greater, more solar radiation is intercepted by polar regions during the summer

What is Precession?

the orientation of the Earth’s axis in relation to the sun: summer/winter;

Earth’s elliptical orbit progresses in space relative to the ecliptic on a 22-thousand year cycle

How does Precession determine Earth’s climate?

Affects whether periods of most direct solar radiation (during summer solstice in the Northern hemisphere) occur when the Earth is at the perihelion (ice melting) or aphelion (ice accumulation), or some intermediate position

What do warmer summers in Northern Hemisphere allow for?

melt snow faster than it can accumulate in the summer

What do colder summers in the Northern Hemisphere allow for?

the accumulation of snow

Changes in Earth’s orbit’ vary independently in cyclical fashions and interact to affect:

Total energy (heat) budget of Earth

Temporal and spatial variation in the global distribution of solar radiation (heat)

Pleistocene Glaciation:

impact of continental land masses (1/3 of the Earth covered in ice)

• 80% of glacial ice

What was the extent of Pleistocene Glaciation?

The Last Glacial Maximum (LGM)

The largest impacts in the Northern Hemisphere

The size of glaciers and impact of their weight

When was the Last Glacial Maximum (LGM)?

26.5 - 19 Ka

What was the Last Glacial Maximum (LGM)?

period of time where 1/3 of land area was covered in ice

What was the largest impact in the Northern Hemisphere?

80% of ice accumulation

What was the size of glaciers and impact of their weight?

• 2-3 km thick

• weight warped the crust underneath

• lowered sea levels 125-134m

Before ~1 Ma: populations

Cold periods: ~25-30 K long

Warm periods: ~10-15 K long

After ~1 Ma:

Cold periods: ~80-90 K long

Warm periods: ~10-15 K long

How many years are left of the Holocene?

~10-15 K years

What were the affects on non-glaciated areas by the LGM?

• Cooler air temperature

• Generally dryer

• Oceans cooler

• Climate zones shifted

Opposite shifts during interglacials (oscillating climates)

What did Cooler air temperature involve during the LGM?

Average air temperatures were 4-8 °C cooler

• Varied greatly temporally and spatially

How was the Earth generally dryer during the LGM?

Through Adiabatic cooling: Water expands out and cools

How were oceans cooler during the LGM?

Ocean surface temperature 2-3 °C cooler; Broadly, but response were more complex

How did climate zones shift during the LGM?

Shifts downward in elevation ~1000 m for tree lines and snowlines

What does reduced evaporation mean?

cold air holds less moisture

What does reduced evaporation result in?

generally less precipitation, expanded aridification

How were climates during glacial periods unstable?

stadials (cold periods)

interstadials (warm periods)

What were D-O events (Dansgaard-Oeschger Events)?

short spikes in temperatures that occur during cold temperatures

What did D-O events involve?

abrupt shifts in climate during last glacial period

shifts may have occurred within decades

How were transitions in climate abrupt?

change from LGM occurred in <3-thousand years

younger Dryas 14-12.9 ka: Northern Hemisphere plunged back into and then out of glacial conditions within decades

Feedback to climate change:

Where greenhouse gases may fit in: earth’s energy budget