6.2A Climate and anthropogenic GHG emissions Notes

1. Climate refers to the long-term average and extremes of atmospheric conditions such as temperature, precipitation, humidity, wind, and air pressure.
2. These conditions must be measured for at least thirty years to be considered climate data.
3. Climate represents the overall atmospheric behaviour of a region and includes the full range of variation and extreme events.
4. Climate is distinguished from weather, which describes short-term changes over hours, days or weeks.

Key Physical Processes That Determine Climate

Solar radiation distribution

1. The equator receives more intense solar energy than the poles because of Earth’s curvature and axial tilt.
2. This uneven heating drives atmospheric circulation systems and influences regional climate patterns.

Atmospheric circulation systems

1. The global circulation cells (Hadley, Ferrel and Polar cells) transport heat and moisture around the planet.
2. Rising warm air and sinking cold air create predictable climatic zones such as tropical, temperate and polar regions.

Convection and uplift

1. Warm air rises because it becomes less dense.
2. As it rises, it cools and leads to condensation and cloud formation, especially in tropical and monsoon regions.

Cloud formation and condensation

1. Clouds reflect incoming sunlight and trap outgoing longwave radiation.
2. The balance between reflection and trapping influences temperature.

Precipitation cycles

1. Rain, snow, sleet and hail form from condensed water droplets or ice crystals.
2. Seasonal patterns of rainfall directly influence vegetation, soil moisture, water availability and ecosystem distribution.

Evaporation and humidity

1. Heat causes evaporation from oceans, lakes and soils.
2. This contributes to humidity levels and affects cloud formation.

The natural greenhouse effect

1. Greenhouse gases such as carbon dioxide, methane and water vapour trap heat that would otherwise escape into space.
2. This process maintains temperatures warm enough to support life.

Seasonal Variations as Drivers of Climate

1. The Earth’s axial tilt causes variations in temperature and day length throughout the year.
2. Seasonal differences influence rainfall patterns, winds, ecosystem productivity, vegetation changes and water availability.
3. Climate includes these seasonal cycles, such as monsoons or Mediterranean dry summers.

Anthropogenic Carbon Dioxide Emissions and Their Acceleration

Historical Increase in Atmospheric Carbon Dioxide

1. Before industrialisation, atmospheric CO₂ levels were approximately 280 ppm, and remained relatively stable for about 6,000 years.
2. During the Industrial Revolution in the late 18th century, coal burning, mechanised manufacturing and urbanisation increased CO₂ emissions.
3. By the early 20th century, levels had risen to approximately 300 ppm.
4. The global rate of emissions accelerated sharply after 1950, reflecting increased industrial output, fossil fuel consumption, globalisation and population growth.
5. CO₂ levels reached 315–316 ppm in the late 1950s and surpassed 420 ppm in the early 2020s.
6. This represents a 50 percent increase from pre-industrial values.

Why Emissions Increased So Rapidly

1. Industrialisation: Expansion of manufacturing, energy use and transportation dramatically increased fossil fuel combustion.
2. Population growth: A rising global population increased the demand for energy, food, goods and land.
3. Land-use change: Deforestation reduces the number of trees available to absorb CO₂ through photosynthesis. Burning forests releases stored carbon.
4. Fossil fuel dependence: Coal, oil and natural gas became the dominant energy sources for electricity, heating, transport and industry.

Example

The Keeling Curve, recorded at Mauna Loa since 1958, shows a clear upward trend in CO₂ along with a seasonal pattern caused by Northern Hemisphere vegetation growth and decay.

How Ice Cores, Tree Rings, and Sediments Reveal Climate History

1. Ice cores, tree rings and sediment layers preserve physical and chemical signals of past climates.
2. These records provide information on past temperatures, atmospheric carbon dioxide levels, methane concentrations, vegetation types and volcanic activity.
3. They allow scientists to reconstruct climate conditions extending hundreds of thousands of years into the past.

Ice Cores: Frozen Time Capsules

1. Ice cores drilled in Antarctica and Greenland contain air bubbles that trap samples of ancient atmospheres.
2. These bubbles provide direct measurements of past carbon dioxide and methane levels.
3. Ice core records show that:
   1. carbon dioxide and methane levels today are higher than at any time in the past 800 000 years
   2. temperature and greenhouse gas concentrations follow a strong positive correlation
   3. natural glacial and interglacial cycles correspond closely with long-term variations in these gases
4. The pace of carbon dioxide increase in recent decades is far faster than changes observed in past ice age transitions.

Tree Rings: Nature's Annual Record

1. Tree rings grow wider in warm, wet years and narrower in cold or dry years.
2. Tree-ring studies from regions such as Alaska, Mongolia and Siberia indicate recent warming over the past century.
3. Some tree-ring records show inconsistencies because growth can be influenced by factors other than temperature, such as nutrients, pests or elevation.

Sediment Cores: Layers of History

1. Layers of sediment accumulate on lake beds and ocean floors, trapping pollen grains, shells, minerals and chemical isotopes.
2. Pollen preserved in sediments indicates past vegetation types, which reflect temperature and precipitation patterns.
3. Isotopes in shells provide information on past ocean temperatures.
4. Sediment evidence shows major climatic shifts such as past wetter conditions in regions that are now deserts.

Correlation Between Carbon Dioxide and Temperature

1. All three evidence sources show consistent alignment between carbon dioxide levels and global temperature trends.
2. Higher carbon dioxide concentrations coincide with warmer conditions.
3. Lower concentrations correlate with glacial periods.
4. Human-driven increases in carbon dioxide are far faster than natural changes.
5. The same amount of carbon dioxide released naturally over one thousand years is now released in less than two decades.

Key Findings from Proxy Data

1. CO₂ and temperature have risen and fallen together over glacial cycles.
2. Glacial periods had CO₂ levels near 180 ppm.
3. Interglacial periods had levels near 270 to 300 ppm.
4. Modern CO₂ levels (~420 ppm) far exceed any natural values in the last 800,000 years.
5. Current warming is much faster than the natural warming that occurred at the end of glacial periods.

The Enhanced Greenhouse Effect: A Human-Made Problem

1. The natural greenhouse effect keeps Earth at a habitable temperature by trapping some outgoing longwave radiation.
2. Human activities have increased the concentrations of greenhouse gases, leading to an enhanced greenhouse effect.
3. Enhanced warming results in long-term changes to global temperatures, climate patterns, sea levels and extreme weather events.

Major Anthropogenic Sources of Greenhouse Gases

1. Carbon dioxide (CO₂): Produced primarily by burning fossil fuels, deforestation and industrial processes.
2. Methane (CH₄): Emitted by livestock digestion, rice paddies, fossil fuel extraction and melting permafrost.
3. Nitrous oxide (N₂O): Released by synthetic fertilisers, industrial emissions, manure and combustion processes.
4. Fluorinated gases: Industrial chemicals with high global warming potential and long atmospheric lifetimes.

Consequences of Enhanced Greenhouse Gas Emissions

1. Higher global temperatures
2. Increased frequency and intensity of heatwaves
3. Changing precipitation patterns and altered monsoons
4. Melting glaciers and polar ice caps
5. Rising sea levels
6. Ocean warming and acidification
7. More extreme weather events such as storms and floods
8. Shifts in ecosystems and biodiversity loss