1.2 Factors Influencing the Earth’s Climate Since its Formation

The Seven Factors at a Glance

When an exam question uses the command verb identify, you do not need an essay. You need the factor named together with its single distinguishing feature. Keep these seven summary statements ready:

• Volcanic eruptions: Explosive eruptions inject sulphur dioxide into the stratosphere, forming reflective sulphate aerosols that cool the surface for 1 to 3 years; sustained volcanic outgassing releases $CO_2$ that warms the planet over millions of years.
• Formation of the oceans: Liquid water dissolved huge quantities of primordial atmospheric $CO_2$ into carbonate sediments, while its high heat capacity stores and moves thermal energy across latitudes, steadying global temperatures.
• Tectonic plate activity: Continental drift changes where land sits relative to the poles, opens and closes oceanic gateways, and builds mountain ranges whose chemical weathering strips $CO_2$ out of the atmosphere over millions of years.
• Evolution of life: Photosynthetic organisms restructured the atmosphere, most dramatically during the Great Oxidation Event roughly 2.4 billion years ago, which broke down atmospheric methane and triggered global cooling.
• Changes in sunlight intensity: The Sun has brightened by roughly 25% to 30% over 4.6 billion years, and it fluctuates weakly on an 11-year magnetic cycle, altering total energy entering the Earth system.
• Slow changes in Earth's orbit (Milankovitch cycles): Variations in eccentricity, obliquity, and precession redistribute seasonal sunlight across latitudes, pacing ice ages over tens to hundreds of thousands of years.
• Formation of fossil fuels: Rapid burial of ancient organic matter in anoxic environments locked carbon into rock for hundreds of millions of years, cooling ancient Earth by drawing down atmospheric $CO_2$.

Climate Framework: Forcings, Feedbacks, and Core Definitions

To make sense of climate history, we separate the initial push on the climate from the system's reaction.

A climate trigger (forcing) is an external or initial driver that alters the planet's energy balance. Examples include orbital shifts, solar brightening, or massive volcanic events.

A climate feedback is an internal process that responds to that initial push. A positive feedback amplifies the initial change, driving temperatures further in the same direction. A negative feedback pushes back against the initial change, stabilising the system.

Essential Definitions

Before examining the factors, master three foundational terms:

• Albedo is the fraction of incoming solar radiation that a surface reflects straight back into space, measured from 0 to 1 (or 0% to 100%). Fresh snow has an exceptionally high albedo of over 0.80, reflecting more than 80% of sunlight. Dark open ocean has a low albedo of roughly 0.06, absorbing nearly all of it. Anything that makes Earth brighter raises its planetary albedo and cools the surface.
• Insolation is the amount of incoming solar radiation received on a given surface area over a given time.
• A glacial is a cold period during an ice age when continental ice sheets expand. An interglacial is a warmer interval between glacials, such as our present Holocene epoch.

Systems Thinking: Transfers Between Earth Spheres

Every natural factor represents an exchange of energy or matter between Earth systems:

• Volcanic eruptions: geosphere $\rightarrow$ atmosphere
• Formation of the oceans: atmosphere $\rightarrow$ hydrosphere
• Tectonic activity: geosphere $\rightarrow$ hydrosphere $\rightarrow$ atmosphere
• Evolution of life: biosphere $\rightarrow$ atmosphere
• Solar intensity: external energy input into the whole Earth system
• Orbital cycles: alters energy distribution across the cryosphere, ocean, and land
• Fossil fuel formation: biosphere $\rightarrow$ geosphere

Volcanic Eruptions: Short-Term Cooling and Long-Term Warming

Volcanoes act as a direct pathway between the geosphere and the atmosphere, but their climate impact depends on whether you look at individual explosive blasts or deep geological time.

Short-Term Surface Cooling (1 to 3 Years)

When gas-rich magma produces an explosive eruption, it blasts out plumes of pulverised rock (ash) and sulphur dioxide ($SO_2$) directly into the stratosphere. Because this sits above the troposphere, rain cannot wash the gases out.

Ash particles are relatively heavy, so they settle out within days or weeks. The sustained cooling comes from $SO_2$. In the stratosphere, $SO_2$ reacts with ambient water vapour to produce microscopic droplets of sulphuric acid ($H_2SO_4$), known as sulphate aerosols:

These pale aerosols scatter incoming solar radiation back into space, raising Earth's albedo and cooling the surface.

• Mount Pinatubo (Philippines, 1991): Injected roughly 20 to 22 million tonnes of $SO_2$ into the stratosphere, lowering global average surface temperatures by roughly 0.5 °C for 18 to 24 months.
• Mount Tambora (Indonesia, 1815): The most powerful eruption in recorded history lowered global mean surface temperatures by 0.5 °C to 0.7 °C. In parts of Western Europe and eastern North America, regional summer temperatures dropped by up to 3 °C, causing widespread crop failures and the historic 'year without a summer' in 1816.

Long-Term Greenhouse Warming (Millions of Years)

Over geological timescales, volcanism warms the planet. Steady volcanic outgassing along mid-ocean ridges, continental rifts, and subduction zones releases $CO_2$ and water vapour from the mantle into the atmosphere. During catastrophic volcanic episodes, such as the Siberian Traps eruptions roughly 252 million years ago, immense and sustained greenhouse gas emissions caused severe global warming and mass extinction.

Formation of the Oceans: Planetary Heat Buffer and Geochemical Sink

During Earth's accretion 4.6 billion years ago, heat from radioactive decay and meteorite bombardments kept the surface molten. As the crust cooled below 100 °C, water vapour released through primordial volcanic outgassing condensed. Rains fell over millions of years, filling crustal depressions to assemble the first oceans between 4.4 and 4.0 billion years ago.

Once the oceans existed (the hydrosphere, encompassing all water on Earth), they altered planetary climate in three ways:

1. Geochemical Carbon Sink

Liquid water dissolved huge amounts of $CO_2$ from the primordial atmosphere. Marine chemical reactions precipitated this dissolved carbon into carbonate sediments on the seabed. Without the oceans removing vast amounts of $CO_2$ from the early air, Earth could have suffered a runaway greenhouse effect like Venus, where surface temperatures exceed 400 °C.

2. Planetary Thermal Buffer

Water has a very high specific heat capacity. The oceans absorb and release vast quantities of thermal energy while changing temperature only slightly—a property called thermal inertia. This buffers maritime landmasses against harsh seasonal extremes.

3. Latitudinal Heat Transport

Ocean currents function as a giant conveyor belt, moving surplus heat from the tropics toward the poles. A key example is the North Atlantic Drift, which carries warm water toward north-western Europe. It keeps Irish winter temperatures markedly milder than places at equivalent latitudes, such as coastal Labrador in Canada.

Tectonic Plate Activity: Continental Drift, Ocean Gateways, and Weathering

Earth's outer shell is cracked into rigid plates (the lithosphere). These slide very slowly over a hotter, softer layer beneath them (the asthenosphere), typically at about 1 to 10 centimetres per year—roughly the speed human fingernails grow. Over tens of millions of years, this motion alters planetary climate through three distinct mechanisms:

1. Latitudinal Continental Drift

Plates carry landmasses into different climate zones. Continental crust has a lower heat capacity than water and cools quickly. When continents cluster over polar latitudes, winter snow survives the summer and builds into permanent ice sheets. This triggers the ice-albedo feedback: white ice reflects incoming solar radiation, cooling the planet further.

Irish Geological Evidence: Around 340 to 330 million years ago, tectonic drift placed the landmass that is now Ireland in warm, shallow equatorial seas. Skeletons and shells of marine organisms accumulated to build the Carboniferous limestone that underlies much of the Irish midlands. Later in the Carboniferous, roughly 320 to 300 million years ago, sea level fell and the same landmass—still near the equator—was covered in hot, humid swamp forests. The buried remains of those trees formed the coal seams mined at Arigna, Co. Roscommon, and Castlecomer, Co. Kilkenny.

2. Opening and Closing of Ocean Gateways

Tectonic movement can open or seal passages between ocean basins, redirecting global heat flow. Roughly 34 million years ago, the separation of South America from Antarctica opened the Drake Passage (and around the same time, Australia drifted clear of Antarctica). With no land barriers left, water circled the continent continuously, creating the Antarctic Circumpolar Current. This current isolated Antarctica from warm tropical waters, allowing its permanent continental ice sheets to develop.

3. Mountain Uplift and Silicate Weathering

When continental plates collide, mountain ranges rise. Uplift fractures fresh silicate rocks, exposing them to chemical weathering by rainwater. Rain absorbs atmospheric $CO_2$ to form weak carbonic acid ($H_2CO_3$), which breaks down silicate minerals:

Rivers carry these dissolved ions into the oceans, where organisms incorporate them into calcium carbonate ($CaCO_3$) shells. When they die, their shells settle on the sea floor, locking atmospheric carbon into limestone for millions of years.

Silicate weathering acts as Earth's natural thermostat—a negative feedback. Chemical weathering reactions speed up in warm, wet climates. If atmospheric $CO_2$ builds up and warms the planet, weathering accelerates, pulling $CO_2$ out of the air faster and cooling the climate back down. If the planet turns cold, weathering slows, allowing volcanic outgassing to replenish $CO_2$ and warm the surface. This negative feedback has kept Earth habitable for billions of years, but it operates over hundreds of thousands to millions of years—far too slowly to clean up human emissions today.

Evolution of Life: Oxygenation and Atmospheric Restructuring

Living organisms are not passive passengers; through metabolic processes like photosynthesis, life has actively reshaped the atmosphere and changed climate across geological time.

The Great Oxidation Event and Methane Collapse

Earth's early atmosphere was anoxic (oxygen-free) and rich in methane ($CH_4$). Roughly 2.4 billion years ago, primitive single-celled cyanobacteria began oxygenic photosynthesis on a global scale:

At first, this released oxygen was absorbed by dissolved ferrous iron in primordial oceans, precipitating out as sedimentary layers called Banded Iron Formations (BIFs). Once oceanic iron sinks were exhausted, free oxygen began escaping into the atmosphere.

This produced an abrupt climatic cooling. Free oxygen reacted with atmospheric methane, oxidising it to carbon dioxide and water vapour. Because methane traps far more heat per molecule than $CO_2$ does—roughly 28 times as much over a 100-year period—replacing methane with carbon dioxide collapsed the atmospheric greenhouse effect. This plunged Earth into the Huronian Glaciation, an extreme ice age lasting millions of years.

Accumulating oxygen also formed the stratospheric ozone layer ($O_3$), shielding the surface from harmful ultraviolet radiation and allowing life to eventually colonise land.

Land Plant Colonisation

Between 400 and 300 million years ago (the Devonian and Carboniferous periods), the evolution of deep-rooted land plants accelerated chemical rock weathering and drew down enormous quantities of atmospheric $CO_2$ into plant tissue, driving down temperatures and sustaining late Paleozoic glaciations.

Sunlight Intensity: The Faint Young Sun and Modern Solar Irradiance

Solar radiation is the primary energy source powering Earth's climate system. Changes in solar output operate across two different timescales:

The Faint Young Sun Paradox

Models of how stars evolve show that when Earth formed 4.6 billion years ago, the Sun had converted much less of its hydrogen into helium. Its core was less dense and slightly cooler, fusion ran more slowly, and the Sun shone with roughly 25% to 30% less energy than it does today.

Under such low solar output, early Earth should have been completely frozen. Yet geological evidence—such as 3.8-billion-year-old pillow basalts formed underwater—proves that liquid oceans and early life existed. This paradox is resolved by atmospheric chemistry: intense early volcanic outgassing filled the atmosphere with high concentrations of greenhouse gases ($CO_2$ and $CH_4$). This potent natural greenhouse effect trapped enough heat to keep the oceans liquid despite the dimmer Sun.

Modern Solar Cycles

On shorter timescales, the Sun's magnetic activity varies across an approximate 11-year solar cycle, tracked by sunspot counts. Between solar minimum and solar maximum, total solar irradiance changes by only about 0.1% (around $1.3 \text{ W/m}^2$), altering global temperatures by less than 0.1 °C.

Satellites have measured the Sun's output directly since 1978. Over that time, solar irradiance has stayed flat or slightly declined while global surface temperatures have climbed sharply. The Sun cannot be the cause of modern warming.

Milankovitch Cycles: Orbital Pacing of Ice Ages

Milutin Milankovitch calculated that predictable shifts in Earth's orbital geometry alter how solar radiation (insolation) is distributed across latitudes and seasons. Over the Quaternary period (the last 2.6 million years), these cycles have paced the regular rhythm of glacial and interglacial periods.

The Glaciation Trigger and Amplifying Feedbacks

Milankovitch cycles do not change Earth's total annual incoming sunlight significantly; they redistribute it. The critical trigger for starting an ice age is cool summers at high northern latitudes (~65° N). If summer sunlight is weak, winter snow in Canada and northern Eurasia does not completely melt. Unmelted snow survives the summer, thickens year-on-year, and gradually builds into continental ice sheets.

Two self-reinforcing positive feedbacks then take over and amplify that small initial nudge:

1. Ice-Albedo Feedback: Expanding white ice replaces darker land and sea surfaces. Surface albedo jumps from ~0.06 (open water) to over 0.80 (snow and ice). More sunlight is reflected straight back to space, causing further cooling and allowing ice sheets to grow wider.
2. Carbon Cycle Feedback: Cooler ocean waters dissolve more atmospheric $CO_2$. Antarctic ice cores prove that atmospheric $CO_2$ dropped to roughly 180 ppm during glacial maxima and rose to roughly 280 ppm during warm interglacials. The weakened greenhouse effect deepens the global freeze.

Formation of Fossil Fuels: Deep Geological Carbon Sequestration

Before fossil fuels became an industrial energy source, their formation was one of Earth's most effective natural carbon sinks.

The Five Conditions for Formation

Transforming dead biological tissue into fossil fuel requires five specific environmental conditions:

1. Large volumes of organic matter (trees, swamp vegetation, marine plankton)
2. Rapid burial by sediments
3. Anoxic (oxygen-depleted) conditions to halt decay
4. Sustained heat and geological pressure
5. Millions of years of time

Coal Formation and Coal Ranks

During the Carboniferous period (360 to 300 million years ago), vast equatorial swamp forests produced abundant plant debris. Trees collapsed into stagnant, waterlogged bogs where anoxic water prevented aerobic microbes from breaking down tough lignin. As thick sediment accumulated overhead, heat and pressure compressed the peat through progressive coal ranks:

Oil and Natural Gas Formation

In ancient oceans, microscopic plankton and algae died and settled onto muddy sea floors under anoxic conditions. Over millions of years, deep burial heated this kerogen-rich mud. Moderate heat cooked the organic material into liquid petroleum (crude oil), while higher temperatures and pressures cracked the molecules into natural gas (principally methane, $CH_4$).

Geological Cooling versus Modern Warming

By burying billions of tonnes of carbon in sedimentary rock for hundreds of millions of years, fossil fuel formation drew huge amounts of $CO_2$ out of the active carbon cycle, cooling ancient Earth.

Modern fossil fuel combustion does the reverse: it extracts carbon that took hundreds of millions of years to bury and returns it to the atmosphere in a couple of centuries. This rapid carbon release drives the modern enhanced greenhouse effect.

Evidence and Timescales: How We Know and How They Compare

Thermometers have recorded weather for only about 170 years. Everything we know about ancient climates comes from proxies—natural systems that record past physical and chemical conditions.

Comparing Natural Timescales

These seven factors operate on wildly different timescales. Knowing which is which is the fastest way to distinguish them in an exam:

Why This Topic Matters for the Course

Every natural factor listed here is still active today: volcanoes still erupt, tectonic plates still creep, the Sun still runs its 11-year cycle, and Earth's orbit still wobbles. Because scientists can calculate the exact radiative forcing of each of these natural factors, we know that none of them account for the rapid warming of roughly 1.2 °C observed over the last 150 years. Understanding the natural baseline is what makes human attribution scientifically certain.

Key terms

Natural Climate Factor

Any non-human astronomical, geological, chemical, or biological process that alters Earth's climate by changing incoming solar radiation, planetary albedo, greenhouse gas concentrations, or oceanic heat transport.

Albedo

The fraction of incoming solar radiation reflected back into space by a surface, expressed as a decimal from 0 to 1 or as a percentage.

Insolation

The total amount of incoming solar radiation reaching a given area of Earth's surface over a given period.

Sulphate Aerosols

Microscopic droplets of sulphuric acid (H₂SO₄) formed in the stratosphere when volcanic sulphur dioxide (SO₂) reacts with water vapour, reflecting incoming sunlight and cooling the planet for 1 to 3 years.

Volcanic Outgassing

The continuous release of dissolved gases, primarily water vapour and carbon dioxide, from magma into the atmosphere along tectonic boundaries and vents.

Thermal Inertia

The property of large water bodies like oceans to absorb, store, and release massive quantities of heat with minimal temperature change, due to water's high specific heat capacity.

Silicate Weathering

The chemical breakdown of silicate rocks by carbonic acid in rainwater, which draws carbon dioxide out of the atmosphere and deposits it as carbonate rocks on the seafloor.

Great Oxidation Event

A major planetary transition roughly 2.4 billion years ago when cyanobacterial oxygen accumulated in the atmosphere, oxidising atmospheric methane and causing global cooling (the Huronian Glaciation).

Banded Iron Formations

Marine sedimentary rock strata made of alternating bands of iron oxide and chert, formed when early photosynthetic oxygen reacted with dissolved ferrous iron in the oceans.

Faint Young Sun Paradox

The problem of how early Earth maintained liquid oceans 4.6 to 3.8 billion years ago despite the Sun being 25% to 30% dimmer, resolved by high primordial atmospheric greenhouse gas concentrations.

Milankovitch Cycles

Predictable, cyclical variations in Earth's orbital eccentricity, axial tilt (obliquity), and axial precession that redistribute incoming solar radiation across latitudes and seasons.

Ice-Albedo Feedback

A positive feedback loop where expanding ice cover reflects more incoming solar radiation back to space, lowering temperatures and driving further ice growth.

Carbon Sink

Any natural biological, oceanic, or geological reservoir that absorbs and stores more carbon from the atmosphere than it releases.

Coal Ranks

The progressive stages of coal formation as increasing geological heat and pressure compact organic matter: peat → lignite → bituminous coal → anthracite.