3. Our Changing Planet Week 3

Lecture 5. The late glacial and Holocene and Lecture 6. Climate of the last millennium

What time period does the late glacial cover?

From the Last Glacial Maximum (~21,000 years ago) to the start of the Holocene (~11,700 years ago)

How was the transition from glacial to interglacial climate?

It involved abrupt climatic changes

What types of changes occurred during the late glacial?

Temperature changes - mid/high latitudes

Moisture changes - low latitudes

Ocean circulation shifts

What does “sub-Milankovitch periodicity” mean?

Climate changes happening faster than orbital cycles

What do Greenland ice cores provide and how?

A precise timescale of past climate changes, by counting layers

How are oxygen isotopes plotted in ice cores?

Against depth, not time

What changed rapidly at the start of the Holocene?

Temperature increased sharply

What is a stadial?

A short-lived cold period

What is an interstadial?

A short-lived warm period

What is the Younger Dryas?

A major cold stadial

What are the Bølling and Allerød?

Warm interstadials

What does the dip in longterm temperature graphs represent?

The Younger Dryas cold event

How can insects be used to reconstruct past temperatures?

Different species tolerate specific temperature ranges

What does pollen data indicate?

Vegetation and therefore air temperature

What trend in temperatures occurred during the late glacial?

Increasing summer temperature, decreasing winter temperatures

Did orbital forcing causes abrupt changes?

No, insolation changed gradually

What is insolation?

short for "incoming solar radiation"

Total amount of solar energy that reaches the Earth's atmosphere and surface

Measured in watts per square meter, it varies by latitude, season, and time of day

What are meltwater pulses?

Rapid increases in sea level from ice sheet melting

Why do meltwater pulses occur?

because the melting of ice sheets and glaciers is not a steady process.

climate warming causes massive ice sheets to reach "tipping points," leading to sudden, rapid collapses

+ a "stepped" increase in global ocean volume

Why is meltwater important to understand in climate?

It disrupts ocean circulation, not just adds water

What happens when freshwater enters the North Atlantic? and why is this important?

It reduces salinity and density

this can shut down thermohaline circulation

What is thermohaline circulation?

‘global conveyor belt’

(a slow, large-scale deep-ocean circulation driven by density differences in water, controlled by temperature (thermo) and salinity (haline)")

What is the consequence of thermohaline circulation shutdown

Reduced heat transport → rapid cooling

Name 2 key drainage routes of meltwater

Mississippi River and Hudson River

How is methane measured in ice cores?

From gas bubbles trapped in ice

What does a drop in methane indicate?

Drying in the tropics

Why do the tropics control methane levels?

They are major methane sources

What likely caused the Younger Dryas?

Freshwater disrupting ocean circulation

Why is the Younger Dryas impact hypothesis (YDIH) unlikely?

Similar events occur repeatedly in glacial cycles

What is the Younger Dryas Impact Hypothesis (YDIH)?

A comet/asteroid fragmented and exploded over North America around 12,900 years ago,

triggering an abrupt 1,300-year cold snap, massive wildfires, megafaunal extinctions, and the collapse of the Clovis culture.

What is the Holocene? when did it start? and what marine isotope stage (MIS) is it?

The current interglacial period

~11,700 years ago

MIS 1

Why were sea levels low in the early Holocene?

Large ice sheets still existed

When did sea levels reach near-present levels?

Mid-holocene

What major process occurred in early holocene

Deglaciation of Northern Hemisphere

What was the 8200 year event?

sudden, intense cooling trend that lasted 2-4 centuries

What caused the 8200 year event?

collapse of Laurentide ice sheet

drainage of Lakes Agassiz and Ojibway

How did the 8200 year event affect climate?

caused widespread dry and cold climate, particularly in the Northern Hemisphere (Disrupted North Atlantic circulation)

What is ice rafting?

Icebergs transporting debris into the ocean

As the ice drifts and eventually melts, it deposits this debris onto the floor of lakes or oceans, sometimes thousands of kilometers from its source.

What cycle duration is observed in the Holocene? and what is it linked to

~1500-year cycles

linked to solar variability

What happened to methane levels during the Holocene?

High at start → drops → fluctuates

What controls large methane changes?

Monsoon strength + tropical wetness

What time period does the “last millennium” refer to?

The last ~1000 years of climate history

Why is the last millennium important in climate studies?

It provides high-resolution records to compare natural vs human-driven change

What is the key takeaway from millennium-scale temperature graphs?

Climate varies naturally but within limits

What are climate reconstructions of the last millennium based on?

Proxy data (e.g. tree rings, ice cores, sediments)

What is the advantage of proxy records in the last millenium?

High temporal resolution (annual to decadal)

What major historical event is linked to a CO₂ dip? and how?

European colonisation of the Americas

Massive population decline due to disease

Abandoned farmland reverted to forest → reforestation increased carbon uptake

What type of climate variability dominates the last millennium? and how does this compare to glacial-interglacial changes?

Short term (decadal to centennial) variability

Much smaller in magnitude

What is North Atlantic Oscillation (NAO)

Pressure difference between Icelandic Low and Azores High

Negative NAO → small pressure difference

• Weaker Atlantic jet stream

• Northern Europe cold + dry

• Southern Europe warm + wet

Positive NAO → large pressure difference

• Stronger Atlantic jet stream

• Northward shift of storm tracks

• Northern Europe Warm + wet

• Southern Europe Cold + dry

What is solar forcing/variability

Changes in solar output → changes the quantity of solar radiation reaching the top of the

atmosphere

What is

1. Natural forcing

2. Anthropogenic forcing

1. Natural forcing → solar variability, volcanic eruptions,

‘pre-industrial’ greenhouse gases,

ocean circulation

2. Anthropogenic forcing → emissions of greenhouse gases

Mann et al. (1999)

Temperature reconstruction of Northern Hemisphere temperature for the past millennium

(large-scale temperature change)

Much smaller amplitude of change compared to the Holocene and the last glacial cycle, but

still interesting intervals with shifts in climate

Resembles an ice hockey stick laid horizontally, with the blade representing the sharp rise in

temperature in the 20th-21st century

Record derived from a range of proxy data sources + instrumental and documentary records

Uncertainties increase with age