6.1F Quaternary period (HL) Notes

Global Warming and the End of the Quaternary Climate Rhythm

1. For the past 2.5 million years, Earth’s climate has been characterised by a repeating pattern of glacial (cold) and interglacial (warm) periods.
2. This geological time period is known as the Quaternary, and its climate cycles have been governed primarily by Milankovitch cycles, natural greenhouse gas fluctuations, and feedback loops such as the ice–albedo effect.
3. However, since the Industrial Revolution, the scale and speed of human-caused warming have begun to push the Earth beyond the natural boundaries of this cycle.
4. Anthropogenic global warming is now moving the planet toward new, hotter climatic conditions, very different from anything observed in Quaternary climate records.

Why This Warming Is Different from Natural Climate Change

1. Natural climate variations in the Quaternary occurred slowly, over tens of thousands of years.
2. The current rate of warming, approximately 1.1°C since the late 19th century, is geologically instantaneous, occurring within a few decades.
3. Atmospheric CO₂ concentrations have risen from 280 ppm (pre-industrial) to over 420 ppm today.
4. This rise is faster, larger, and more abrupt than any natural changes observed in ice-core records covering 800,000 years.

The Anthropocene Epoch

1. Because humans are now the dominant force altering atmospheric chemistry, ecosystems, and climate, many scientists propose a new geological epoch called the Anthropocene.
2. The Anthropocene is characterised by:
   1. Rapid increases in CO₂, CH₄, and N₂O
   2. Massive land-use change
   3. Plastic pollution in sediments
   4. Radioactive isotopes from nuclear testing
   5. Global biodiversity decline
3. These changes act as geological markers, meaning they will be preserved in rock layers for millions of years.

Hothouse Earth Scenario

1. Some researchers warn that Earth might be approaching a “Hothouse Earth” state if warming crosses key thresholds.
2. Features of a Hothouse Earth State:
   1. Global temperatures 4–5°C above pre-industrial levels
   2. Potential rise in sea levels by more than 1 metre
   3. Large-scale collapse of ice sheets
   4. Shift of climate zones, including expansion of deserts
   5. Intensification of heatwaves, cyclones, and floods
   6. Widespread ecosystem disruption

Why it may become irreversible

1. A Hothouse Earth is driven by self-amplifying positive feedback loops, such as:
   1. Melting ice reduces albedo, increasing absorption of heat
   2. Thawing permafrost releases methane, accelerating warming
   3. Warmer oceans release CO₂, rather than storing it
   4. Tropical forests becoming carbon sources, not sinks
2. Once triggered, these feedbacks could push Earth permanently into a hotter climate state.

Evolution of Life and Atmospheric Change

The Prebiotic Atmosphere

1. Earth’s earliest atmosphere developed from volcanic outgassing around 4 billion years ago.
2. It contained:
   1. High CO₂
   2. High CH₄
   3. Nitrogen
   4. Water vapour
   5. Ammonia
   6. No molecular oxygen (O₂)
3. Without oxygen, no ozone layer existed.
4. This exposed early Earth to intense ultraviolet radiation and prevented life from existing on land.

How Life Changed the Atmosphere

1. Cyanobacteria and the Rise of Photosynthesis

1. Around 2.5 billion years ago, cyanobacteria evolved the ability to perform oxygenic photosynthesis.
2. This released O₂ into oceans and the atmosphere.
3. Initially, oxygen did not accumulate because it reacted with dissolved iron.

2. Formation of Iron Oxides (Banded Iron Formations)

1. Oxygen produced by early photosynthesizers reacted with dissolved Fe²⁺ in oceans.
2. This created insoluble iron oxides (e.g., hematite).
3. These oxides settled to the seafloor in layers known as banded iron formations (BIFs).

3. The Great Oxidation Event (GOE)

1. After iron sinks became saturated, O₂ began to accumulate in the atmosphere.
2. This major rise in atmospheric oxygen is known as the Great Oxidation Event (~2.4 billion years ago).
3. GOE transformed Earth’s chemistry and climate.

4. Formation of the Ozone Layer

1. Increased oxygen allowed ozone (O₃) formation in the stratosphere.
2. The ozone layer absorbed UV radiation, enabling life to colonize land.

Why Atmospheric Change Influences Evolution

1. Changes in atmospheric gases alter the availability of energy for organisms.
2. Rising oxygen allowed complex life to evolve, diversify, and occupy new ecological niches.
3. Decreases in CO₂ influenced global climate patterns, including ice ages.
4. The development of the ozone layer enabled life to migrate to terrestrial habitats.