Atmospheric composition Notes

What Are the Main Gases in the Atmosphere Today?

1. Atmospheric composition means the gases that make up the air around Earth, and their relative proportions.
2. A key idea is that the most abundant gases are not necessarily the most reactive or the ones that most strongly affect climate.

Today, dry air at the Earth’s surface is mainly:

1. Other trace gases include:
   1. Neon (Ne)
   2. Helium (He)
   3. Methane (CH₄)
   4. Krypton (Kr)
2. Water vapour (H₂O) is also an important atmospheric gas, but its percentage varies a lot with location, weather and altitude.
3. It is essential for cloud formation and plays a big role in the greenhouse effect.

How Has the Atmosphere Changed Over Geological Time?

The Earth did not start with the atmosphere we breathe today. Its composition has changed dramatically over billions of years.

Early Earth (about 4.6 billion years ago)

1. Intense volcanic activity released large amounts of:
   1. Carbon dioxide (CO₂)
   2. Water vapour (H₂O)
   3. Nitrogen (N₂)
   4. Small quantities of other gases (e.g. SO₂)
2. There was very little oxygen (O₂) in the atmosphere.

Formation of Oceans and Removal of CO₂

1. As the Earth cooled, water vapour condensed to form oceans.
2. CO₂ began to dissolve in seawater, reducing its concentration in the atmosphere.
3. Over time, marine organisms formed shells and skeletons made of calcium carbonate (CaCO₃).
   1. When these organisms died, their shells were buried and became sedimentary rocks (e.g. limestone) and, eventually, part of fossil fuel deposits.
   2. This process locked away carbon and further reduced atmospheric CO₂.

The Rise of Oxygen – The Great Oxidation Event

1. At first, the atmosphere still contained almost no oxygen. That changed when photosynthetic organisms evolved.
   1. Early autotrophs (e.g. cyanobacteria) used photosynthesis to:
      1. Take in CO₂ and water
      2. Release oxygen as a by-product
2. Over hundreds of millions of years, this led to:
   1. A major decrease in CO₂
   2. A gradual increase in O₂

Today’s atmosphere is the result of this long history of:

1. Volcanic outgassing
2. Ocean formation
3. Rock formation
4. Photosynthesis and respiration

How Do Natural Processes Maintain or Change Atmospheric Composition?

The atmosphere is not static; it is constantly being modified and balanced by natural processes.

The Carbon Cycle – Linking Photosynthesis and Respiration

The carbon cycle is the continuous movement of carbon between:

1. The atmosphere
2. Living organisms
3. Oceans
4. Rocks and soils

Key processes

Photosynthesis

1. Plants, algae and some bacteria absorb CO₂ and use sunlight to make glucose.
2. They release O₂ as a by-product. $$6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \xrightarrow{\text { light }} \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{O}_2$$
3. Result:
   1. Decreases atmospheric CO₂
   2. Increases atmospheric O₂

Respiration and Decomposition

1. Animals, plants and microorganisms respire:
2. They use O₂ to release energy from glucose
3. They produce CO₂ and water
4. When organisms die, decomposers break down their remains, also releasing CO₂.
5. Result:
   1. Increases atmospheric CO₂
   2. Uses O₂

Oceans

1. CO₂ dissolves in the oceans and can:
   1. Form carbonic acid
   2. React to form carbonate ions and carbonate rocks
2. This process also helps control atmospheric CO₂ over long timescales.

Volcanic Activity

1. Volcanoes emit:
   1. Carbon dioxide (CO₂)
   2. Sulfur dioxide (SO₂)
   3. Water vapour and other gases
2. These emissions can:
   1. Add greenhouse gases (e.g. CO₂), which trap heat and contribute to warming.
   2. Release aerosols and ash that can reflect sunlight and cause short-term cooling.

Methane and the Natural Greenhouse Effect

1. Methane (CH₄) is a powerful greenhouse gas, produced naturally by:
   1. Wetlands and swamps
   2. Decomposition of organic matter in low-oxygen conditions
   3. The digestive systems of some animals (e.g. cows, sheep, termites)
2. Even though its concentration is much lower than CO₂, methane has a stronger warming effect per molecule and contributes significantly to the natural greenhouse effect.

The Greenhouse Effect vs Global Warming

Natural Greenhouse Effect (Essential for Life)

1. Greenhouse gases such as:
   1. Water vapour (H₂O)
   2. Carbon dioxide (CO₂)
   3. Methane (CH₄)
2. Absorb infrared radiation emitted by the Earth’s surface and re-emit it in all directions, including back towards the surface.
3. This natural process:
   1. Keeps Earth’s average temperature warm enough to support life.
   2. Without it, the planet would be too cold.

Enhanced Greenhouse Effect (Global Warming)

1. Human activities have increased the concentration of greenhouse gases, especially:
   1. Carbon dioxide from burning fossil fuels (coal, oil, gas) and deforestation
   2. Methane from agriculture, landfills and fossil fuel extraction
2. This leads to:
   1. More heat being trapped
   2. A rise in average global temperature → global warming
   3. Disruption of climate patterns → climate change

Human-Driven Changes in Atmospheric Composition

Human activities are now the dominant cause of rapid changes in atmospheric gases.

Carbon Dioxide from Fossil Fuels and Deforestation

1. Combustion of fossil fuels in:
   1. Power stations
   2. Vehicles
   3. Industry releases large amounts of CO₂.
2. Deforestation:
   1. Fewer trees means less CO₂ absorbed by photosynthesis.
   2. When forests are cleared or burned, additional CO₂ is released.
3. Even small percentage increases in atmospheric CO₂ are enough to:
   1. Strengthen the greenhouse effect
   2. Shift the climate system out of its previous balance

Methane Emissions

1. Methane (CH₄) is more potent than CO₂ as a greenhouse gas, though it has a shorter lifetime in the atmosphere.
2. Major sources:
   1. Natural:
      1. Wetlands and swamps
      2. Termites and other organisms
   2. Human-driven:
      1. Livestock farming (cows, sheep, goats)
      2. Rice paddies
      3. Decomposing waste in landfills
      4. Leakage during oil, coal and natural gas extraction

Aerosols – A Cooling Influence

1. Aerosols are tiny solid or liquid particles in the air.
2. Sources:
   1. Natural: dust, sea salt, volcanic ash
   2. Human-made: soot and particles from burning fossil fuels
3. Aerosols can:
   1. Reflect sunlight back into space → cooling effect
   2. Modify clouds and rainfall patterns

Why Is It Important to Monitor Atmospheric Gases?

Monitoring gases like CO₂ and CH₄ is vital for understanding and managing climate change.

Reliable Evidence of Human Impact

1. Long-term measurements of CO₂ and CH₄ show sharp increases in recent centuries.
2. Ice cores allow scientists to reconstruct past atmospheric composition for hundreds of thousands of years.
3. These records show that:
   1. Current levels of CO₂ and CH₄ are higher than at any time in at least 800,000 years.
   2. The recent rapid rise is closely linked to human activities.

Predicting Future Changes

1. Tracking greenhouse gases helps scientists:
   1. Model future temperature trends
   2. Predict sea-level rise and changes in weather patterns
   3. Provide evidence for international agreements and climate policies
2. Monitoring CO₂ and CH₄ gives some of the best indicators for future global warming.

Identifying Sources and Sinks

1. Atmospheric data help to:
   1. Locate major emission sources (e.g. power stations, agriculture, industry)
   2. Assess how well oceans and forests are acting as carbon sinks
   3. Evaluate the impact of emission-reduction strategies
2. This feedback is essential for managing human impact on the climate.

Consequences of Rising Greenhouse Gases

Increase in Global Temperatures

1. More greenhouse gases → more heat trapped → higher average global temperature
2. Current global temperatures are higher than any recorded in modern human history.
3. This affects ecosystems, food production and water availability.

Ocean Acidification

1. CO₂ dissolves in seawater, forming carbonic acid.
2. This lowers ocean pH, making the oceans more acidic.
3. Acidic water harms:
   1. Coral reefs
   2. Shell-forming organisms (e.g. shellfish, some plankton)
4. This disrupts marine ecosystems and food chains.

Extreme Weather Events

1. Climate change is linked to more frequent or intense:
   1. Heatwaves
   2. Droughts
   3. Floods
   4. Storms and heavy rainfall
2. These events can:
   1. Damage infrastructure
   2. Threaten human health and safety
   3. Impact agriculture and water supplies