Earth's climate history and major states

Earth's climate is dynamic and has undergone significant shifts over millions of years. These changes influence everything from sea levels to the formation of ice masses.

Greenhouse and icehouse conditions compared

Greenhouse conditions and icehouse conditions represent the two main climate states. Greenhouse conditions occur when the planet warms, leading to the retreat of ice. Icehouse conditions happen when cooling causes ice to expand.

Earth is currently in an icehouse phase but experiencing a warmer period within it.

The Quaternary period and its epochs

The Quaternary period marks the most recent geological time frame, encompassing major climate fluctuations.

Key features of the Quaternary

• The Quaternary period spans from about 2.6 million years ago to today.
• It includes two epochs: the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago) and the Holocene epoch (from 11,700 years ago to the present).
• During the Pleistocene, global temperatures fluctuated, creating cycles of colder and warmer phases.

Some scientists propose the term Anthropocene for the last 100–200 years, when human activities have become the main driver of climate change.

Glacial and interglacial periods in the Pleistocene

Within the Pleistocene epoch, Earth's climate cycled between extreme cold and relative warmth.

Characteristics of glacial and interglacial periods

• Glacial periods - Colder times when glaciers advanced (grew and spread), and sea levels fell due to water being trapped in ice. These typically lasted around 100,000 years.
• Interglacial periods - Warmer intervals when ice retreated (melted and shrank), and sea levels rose from meltwater. These usually lasted 10,000–15,000 years.

Earth is presently in an interglacial period.

The last glacial maximum and its impacts

The Last Glacial Maximum occurred about 21,000 years ago, when ice sheets reached their greatest extent. This event dramatically altered global environments.

Key effects of the last glacial maximum:

• Polar ice sheets covered much of the UK, northern Europe, Canada, and parts of Russia.
• Southern Europe was largely periglacial (regions near ice sheets experiencing frequent freeze-thaw cycles).
• Mountain ranges like the Himalayas and Rockies had extensive alpine glaciers (ice masses in high-altitude areas).
• Vast areas around ice sheets were periglacial, while other regions remained ice-free.

Long-term causes of climate change: Milankovitch cycles

Long-term climate changes, spanning thousands of years, are primarily driven by variations in Earth's orbit and tilt. These are explained by Milankovitch cycles, named after the scientist who proposed them. They affect how much solar radiation reaches Earth, triggering shifts between glacial and interglacial periods.

Eccentricity cycle

• Eccentricity refers to changes in the shape of Earth's orbit around the Sun.
• The orbit shifts from circular to more elliptical (oval-shaped) over time.
• A more elliptical orbit increases the variation in Earth's distance from the Sun, affecting the amount of solar radiation received at different points in the year.
• This cycle repeats every 100,000 years, aligning closely with glacial-interglacial patterns.

Obliquity cycle

• Obliquity describes variations in the tilt of Earth's axis.
• The axis tilts between 22.1° and 24.5° (currently at 23.4°).
• A smaller tilt (22.1°) reduces seasonal differences, allowing snow and ice to persist through summer and build up over time.
• A larger tilt (24.5°) increases seasonal temperature contrasts, making ice melt more likely in warmer months.
• This cycle lasts 41,000 years.

Axial precession

• Axial precession involves a wobbling motion of Earth's axis.
• The wobble causes different hemispheres to face toward or away from the Sun at varying times.
• This alters season lengths; longer winters allow more snow and ice to accumulate.
• The cycle completes every 26,000 years.

How Milankovitch cycles drive further changes

Milankovitch cycles initiate climate shifts, which then amplify through feedback mechanisms (processes that enhance or reduce the initial change).

Key feedback examples:

• Albedo effect - Ice and snow reflect solar radiation back to space due to their high albedo, cooling the surface and allowing more ice to form.
• Cloud cover - During interglacials, increased evaporation leads to more cloud cover, which reflects radiation and cools the Earth further.

Short-term causes of climate change

While Milankovitch cycles operate over millennia, some climate changes happen over much shorter periods, like decades or centuries. These are often linked to variations in solar activity or geological events.

Solar output variations

• Solar output refers to the energy emitted by the Sun, which can fluctuate and influence Earth's climate.

• Intense solar flares coincide with sunspot activity, where sunspots are dark spots on the Sun's surface.

• Higher sunspot activity increases solar radiation, warming Earth; low activity reduces it, causing cooling.

• Sunspot cycles occur approximately every 11 years.

Historical examples linked to solar output:

• The Medieval Warm Period (900–1300) featured high solar activity and warmer temperatures.
• The Little Ice Age (detailed later) aligned with minimal sunspot activity.

Volcanic eruptions

• Volcanic eruptions release materials into the atmosphere that can alter global temperatures.

• Eruptions eject ash, water vapour, carbon dioxide, and sulphur dioxide.

• Greenhouse gases like carbon dioxide and water vapour can trap heat long-term, but the dominant effect is cooling.

• Ash blocks sunlight, reducing insolation.

• Sulphur dioxide reflects radiation from the upper atmosphere, further cooling the planet.

Effects from a typical eruption last only a few years, making this a short-term driver.

Examples of past climate events

The Little Ice Age

The Little Ice Age was a period of cooling from 1300 to 1850, mainly in Europe and North America. It was caused by a mix of low sunspot activity and increased volcanic eruptions.

Key details:

• Average temperatures were 0.5–1°C cooler than post-1900 levels.
• Impacts included frozen rivers (e.g., the River Thames), expanded Arctic sea ice, advancing alpine glaciers that destroyed settlements, and reduced crop yields due to shorter growing seasons, leading to food shortages.

The Loch Lomond Stadial

The Loch Lomond Stadial was a brief return to ice age conditions around 12,700 years ago, lasting about 1,000 years during the late Pleistocene. A stadial is a period of falling temperatures and glacier expansion.

Key details:

• Triggered by sudden atmospheric cooling in the Northern Hemisphere.
• Ice caps grew rapidly in UK mountainous areas.
• Affected regions included western Scotland, Snowdonia, and the Lake District, with significant winter temperature drops.