Welcome to Topic 6: Atmosphere and Climate Change!

Hello ESS students! This chapter, "Atmosphere and climate change," is arguably one of the most critical topics you will study. It brings together physics, chemistry, biology, and sociology to tackle the biggest challenge facing our planet.

Don't worry if some of the concepts seem large scale—we’re going to break down the atmosphere into simple layers, understand how Earth manages its energy budget, and explore the crucial difference between the natural warmth we need and the extra heat we are adding.

Ready to understand Earth's thermostat? Let's dive in!

6.1 The Atmosphere: Earth's Protective Blanket

The atmosphere is the thin layer of gases surrounding Earth, held in place by gravity. It protects us from solar radiation and regulates temperatures.

Structure of the Atmosphere (Simplified)

We mainly focus on the two lowest layers, as they contain almost all the air and weather systems relevant to climate.

  • Troposphere: (0 to 12 km approx.)
    • This is the layer closest to the ground.
    • Contains 75–80% of the atmosphere’s mass.
    • All weather occurs here.
    • Temperature decreases with altitude.
  • Stratosphere: (12 to 50 km approx.)
    • Temperature increases with altitude because it contains the Ozone Layer (\(O_3\)).
    • The ozone layer absorbs harmful ultraviolet (UV) radiation from the Sun. This is why it heats up!

Memory Aid (Layers from ground up): To Start Making Treats (Troposphere, Stratosphere, Mesosphere, Thermosphere).

Key Takeaway: The troposphere is where our climate operates; the stratosphere contains the protective ozone layer.

6.2 Earth's Energy Budget and the Greenhouse Effect

The Earth's climate is determined by the balance between the energy entering the system (from the Sun) and the energy leaving the system (back into space). This balance is the Energy Budget.

Incoming Solar Radiation (Insolation)
  • Energy arrives as short-wave radiation (high energy, visible light).
  • Not all incoming energy reaches the surface; some is reflected.
Albedo: The Reflective Factor

Albedo is the reflectivity of a surface. It is measured on a scale from 0 (perfect absorption) to 1 (perfect reflection).

  • Surfaces with high albedo (e.g., fresh snow, ice, bright clouds) reflect a lot of energy, keeping the area cool.
  • Surfaces with low albedo (e.g., dark soil, oceans, forests) absorb a lot of energy, leading to warming.

Analogy: Think about wearing a black t-shirt (low albedo) versus a white t-shirt (high albedo) on a sunny day. The black shirt absorbs more heat!

The Natural Greenhouse Effect

The Greenhouse Effect is a natural and essential process. Without it, Earth’s average temperature would be around \(-18^\circ C\), and life as we know it would not exist.

Step-by-Step Process:

  1. The Sun emits short-wave radiation to Earth.
  2. Earth absorbs this energy, heating the surface.
  3. The warm Earth re-radiates energy back towards space as long-wave radiation (infrared heat).
  4. Certain gases in the atmosphere, called Greenhouse Gases (GHGs), absorb this long-wave radiation.
  5. These gases then re-radiate the heat back down to the surface, effectively trapping heat and warming the planet.
Major Greenhouse Gases (GHGs)

These gases differ in their concentration, ability to absorb heat, and lifespan in the atmosphere.

  • Water Vapour (\(H_2O\)): Most abundant natural GHG. Its concentration is highly variable and linked to temperature (a feedback mechanism).
  • Carbon Dioxide (\(CO_2\)): Most significant anthropogenic (human-caused) contributor. Released mainly through burning fossil fuels and deforestation.
  • Methane (\(CH_4\)): Much higher warming potential than \(CO_2\), but shorter lifespan. Sources include livestock farming, rice cultivation, and melting permafrost.
  • Nitrous Oxide (\(N_2O\)): High GWP; often released from agriculture (fertilizers) and industrial processes.
  • Halocarbons (e.g., CFCs, HCFCs): Powerful GHGs, though their primary issue was ozone depletion.

Quick Review: GHGs don't trap the sun's energy (shortwave); they trap the Earth's re-radiated heat (longwave).

6.3 The Enhanced Greenhouse Effect and Anthropogenic Drivers

The problem isn't the natural greenhouse effect; it's the Enhanced Greenhouse Effect, caused by increasing the concentration of GHGs above natural levels due to human activity.

Comparing Greenhouse Gases: Global Warming Potential (GWP)

Not all GHGs are created equal in terms of their warming power.

Global Warming Potential (GWP) measures how much energy a gas absorbs over a specific time period (usually 100 years), compared to the absorption of the same mass of carbon dioxide (\(CO_2\)).

  • \(CO_2\) is the baseline: GWP = 1.
  • Methane (\(CH_4\)) has a GWP of approximately 28–36 (meaning 1 tonne of methane warms the planet 28 times more than 1 tonne of \(CO_2\) over 100 years).
  • Nitrous Oxide (\(N_2O\)) has a GWP of about 265.

Key Drivers of GHG Increase:

  1. Industrialisation and Energy: Burning fossil fuels (coal, oil, gas) for electricity, heat, and transport releases massive amounts of \(CO_2\).
  2. Agriculture: Large-scale livestock farming produces methane (\(CH_4\)). The use of synthetic fertilizers releases nitrous oxide (\(N_2O\)).
  3. Land Use Change (Deforestation): Forests act as carbon sinks. When trees are cut down and burned, stored carbon is released as \(CO_2\), and the sink capacity is reduced.

Did you know? Carbon dioxide is long-lived. While methane breaks down relatively quickly (around 12 years), a significant portion of the \(CO_2\) released today will stay in the atmosphere for centuries.

6.4 Climate Change Mechanisms: Feedback Loops (SL & HL)

The climate system is complex. Changes in one area often trigger further changes, which can either amplify or reduce the original warming. These are called feedback loops.

Positive Feedback Loops (Amplify the Change)

Positive feedback loops make the initial change worse, leading to further destabilisation of the system.

Example 1: Ice-Albedo Effect

  1. Rising temperatures melt ice and snow (high albedo surfaces).
  2. Melting exposes darker land or ocean (low albedo surfaces).
  3. Darker surfaces absorb more solar energy.
  4. Increased absorption leads to further warming and more melting (loop repeats).

Example 2: Melting Permafrost

  1. Rising temperatures melt arctic permafrost (frozen ground).
  2. Melting releases huge quantities of trapped methane (\(CH_4\)) and \(CO_2\).
  3. Methane and \(CO_2\) are powerful GHGs, causing increased atmospheric warming.
  4. Increased warming leads to more permafrost melt (loop repeats).

Negative Feedback Loops (Dampen the Change)

Negative feedback loops counteract the initial change, helping to stabilise the system.

Example 1: Increased Plant Growth (Carbon Sink)

  1. Increased atmospheric \(CO_2\) stimulates global rates of photosynthesis.
  2. Plants grow faster and absorb more \(CO_2\) from the atmosphere.
  3. This absorption reduces the atmospheric concentration of \(CO_2\).
  4. The reduction in \(CO_2\) slows down the enhanced greenhouse effect.

Note: Scientists are still debating how long this negative feedback loop can keep pace with emissions, as factors like nutrient availability and temperature stress limit plant growth.

Key Takeaway: Positive feedback loops are terrifying because they create runaway change, making temperature increase accelerate beyond human control.

6.5 Impacts and Strategies: Mitigation vs. Adaptation

Climate change impacts are observed globally, affecting ecosystems, human societies, and infrastructure. Understanding these impacts is crucial for developing effective responses.

Impacts of Climate Change
  • Sea Level Rise: Due to thermal expansion of water and melting glaciers/ice sheets, threatening low-lying coastal areas and Small Island Developing States (SIDS).
  • Changes in Weather Patterns: Increased frequency and intensity of extreme weather events (heatwaves, droughts, intense rainfall, floods).
  • Changes to Biomes/Species Distribution: Altered growing seasons, species migrating toward poles or higher altitudes, leading to loss of biodiversity.
  • Ocean Acidification: Oceans absorb excess \(CO_2\), decreasing the pH, which harms organisms with calcium carbonate shells (e.g., coral reefs, shellfish).
Strategies to Address Climate Change

We generally categorize our responses into two types: Mitigation (tackling the cause) and Adaptation (tackling the effects).

A. Mitigation Strategies (Reducing the Source)

Mitigation aims to reduce the inputs of GHGs into the atmosphere.

  1. Reduce Energy Consumption: Improving energy efficiency (better insulation, more efficient appliances).
  2. Decarbonisation of Energy Supply: Transitioning from fossil fuels to renewable energy sources (solar, wind, hydroelectric).
  3. Carbon Capture and Sequestration (CCS): Technologies that capture \(CO_2\) from power plants or industrial sources and store it underground.
  4. Regulation and Policy: Implementing carbon taxes, cap-and-trade schemes, and international agreements (like the Paris Agreement) to limit national emissions.

Encouragement: Mitigation often requires difficult, immediate changes but offers long-term, global benefits by slowing the rate of warming.

B. Adaptation Strategies (Reducing Vulnerability to Impacts)

Adaptation strategies acknowledge that some warming is unavoidable and focus on reducing the harm caused by climate impacts.

  1. Coastal Defenses: Building sea walls, levees, or restoring mangroves to protect against sea level rise and storm surges.
  2. Water Management: Developing desalination plants or improving rainwater harvesting to cope with drought and changing precipitation patterns.
  3. Agricultural Adjustments: Planting drought-resistant crops or switching to different farming methods better suited for new climate conditions.
  4. Relocation: In extreme cases, moving populations away from high-risk areas.
C. Geoengineering (HL Focus - Evaluation)

Geoengineering strategies are large-scale, deliberate interventions in the Earth’s natural systems to counteract climate change. These are often highly controversial due to potential unknown risks.

  • Solar Radiation Management (SRM): E.g., injecting aerosols into the stratosphere to reflect sunlight back into space (mimicking a large volcanic eruption).
  • Carbon Dioxide Removal (CDR): E.g., Iron fertilisation of the ocean to stimulate phytoplankton growth (which absorbs \(CO_2\)).

Warning: Geoengineering methods often face ethical debates. While they might reduce global temperatures quickly (SRM), they do not address the root cause (GHG concentration) or issues like ocean acidification.

Final Key Takeaway: Effective climate action requires a combination of Mitigation (to slow the problem) and Adaptation (to live with the changes already locked in).