D4.3 Climate change

The greenhouse effect is a natural process in which greenhouse gases (e.g. CO₂, CH₄, H₂O vapour) absorb and re-radiate longwave infrared radiation emitted from the Earth’s surface, maintaining a habitable global temperature.

Shortwave solar radiation passes through the atmosphere and is absorbed by the Earth’s surface, which re-emits energy as longwave radiation. Greenhouse gases trap some of this heat, preventing it from escaping into space.

The enhanced greenhouse effect refers to the increase in this process due to anthropogenic emissions of greenhouse gases.

Causes:
= Combustion of fossil fuels → ↑ CO₂
= Deforestation → ↓ carbon sequestration
= Agriculture → ↑ CH₄ (livestock) and N₂O (fertilizers)

Consequences:
=Global temperature increase (global warming)
= Melting ice → sea-level rise
= Increased frequency of extreme weather events
= Ocean acidification due to increased CO₂ absorption

Anthropogenic CO₂ sources:

Fossil fuel combustion (energy, transport, industry)
Deforestation (reduced carbon uptake + release from biomass)

Anthropogenic CH₄ sources:
= Livestock (enteric fermentation)
= Rice paddies (anaerobic conditions)
= Landfills (decomposition of organic waste)
= Thawing permafrost

Fate of methane:
= Methane is a short-lived greenhouse gas (~10 years) that is oxidised in the atmosphere by hydroxyl radicals (•OH), forming CO₂ and H₂O.

Despite its shorter lifetime, CH₄ has a much higher global warming potential than CO₂, making it a significant contributor to the enhanced greenhouse effect.

There is a positive correlation between atmospheric concentrations of CO₂ and CH₄ and global temperature.

Since the Industrial Revolution:
= CO₂ has increased from ~280 ppm to >400 ppm
= CH₄ concentrations have also significantly increased

This rise enhances the greenhouse effect, increasing the absorption of long-wave radiation, resulting in a rise in global mean temperatures (~1.1–1.3°C).

Supporting evidence:
= Ice core data shows historical correlation between CO₂ and temperature
= Recent rapid increases exceed natural variation

Feedback mechanisms amplify warming:
= Reduced albedo from ice melt
= Methane release from permafrost

A positive feedback is a process in which an initial change causes effects that amplify the original change.

Ice–albedo feedback:
Ice melts → ↓ albedo → ↑ heat absorption → further warming
Permafrost thaw:
Warming → thawing → decomposition → release of CO₂ and CH₄ → further warming
Ocean CO₂ release:
Warmer water holds less CO₂ → CO₂ released → increased greenhouse effect
Forest fires/drought:
Warming → more fires → CO₂ release + reduced photosynthesis → further warming
Peat decomposition:
Higher temperatures → faster decomposition → CO₂ release → further warming

A tipping point is a critical threshold at which a small additional change causes a large and irreversible shift in a system.

Under normal conditions, boreal forests act as carbon sinks through photosynthesis.

With climate change:

↑ temperature and drought → ↓ photosynthesis
↑ frequency of wildfires → release stored carbon
↑ decomposition rates

This results in a net release of CO₂, shifting the system from a carbon sink to a carbon source.

Global warming causes:
= Reduced extent and thickness of sea ice
= Earlier seasonal melting
= Increased fragmentation

Ecological impacts:

emperor penguin
= Depend on stable landfast ice for breeding
= Early ice breakup → chick mortality (drowning/hypothermia)
= Reduced reproductive success
walrus
= Use sea ice as resting platforms
= Ice loss → crowding on land → trampling deaths
= Increased energy expenditure to reach feeding grounds

Overall:
Loss of sea ice disrupts food webs and reduces survival and reproduction of specialised species.

Ocean stratification: layering of water based on density differences (temperature/salinity), limiting vertical mixing
Upwelling: movement of cold, nutrient-rich water from deep ocean to the surface
Ocean currents: large-scale movement of seawater driven by wind, temperature, density, and Earth’s rotation

Global warming:

Increases surface temperature → stronger stratification
Reduces vertical mixing and upwelling

Consequences:

↓ nutrient supply to surface
↓ phytoplankton (primary production)
↓ energy transfer through food chains
Reduced fish populations and fisheries productivity

Rising temperatures shift species’ climatic niches, causing:

Poleward migration (toward cooler latitudes)
Upslope migration (toward higher altitudes)

Tropical montane species are vulnerable because:

Narrow thermal tolerance
Limited habitat at higher elevations
Risk of extinction when no higher habitat exists

CO₂ dissolves in seawater forming carbonic acid, which dissociates to release H⁺ ions, lowering pH.

This reduces carbonate ions (CO₃²⁻) needed for calcium carbonate (CaCO₃) formation.

Effect: reduced calcification in corals and shell-forming organisms.

Temperature increase:
= Causes coral bleaching (loss of symbiotic zooxanthellae)
= Reduces energy supply → coral death

Lower pH:
= Reduces carbonate ions
= Decreases calcification → weaker reef structures

Combined effects:
= Reduced growth and increased mortality

Ecosystem consequences:
= Loss of habitat for ~25% of marine species
= Disruption of food webs
= Reduced biodiversity
= Shift to algae-dominated systems

This can lead to ecosystem collapse due to loss of the reef as a structural and functional foundation species.

Carbon sequestration is the long-term storage of CO₂ in biological or geological systems.

Afforestation:
= Planting forests on previously non-forested land
= Increases CO₂ uptake via photosynthesis

Reforestation:
= Restoring degraded forests
= Re-establishes carbon sinks

Peatland restoration:
= Rewetting peat soils
= Restores anaerobic conditions → slows decomposition
= Prevents CO₂ release and allows carbon accumulation

Peatlands are particularly important due to their high carbon storage capacity.