Human activities altering land use

Human actions significantly modify the carbon cycle, which is the process by which carbon moves between the atmosphere, land, oceans, and living organisms. These changes often stem from growing populations and economic development, which increase the demand for resources like food and fuel. As societies become wealthier and more urbanised, land is repurposed to meet these needs, altering how carbon is stored and transferred.

Key drivers of land use change

• Population growth - As the number of people increases, more land is needed for housing, infrastructure, and resource extraction, particularly in developing countries where populations are expanding rapidly.
• Economic development - Rising living standards lead to higher consumption, such as increased demand for meat, which requires converting land for livestock grazing rather than crops.
• Urbanisation and industrialisation - The shift towards cities creates a need for built-up areas, reducing natural vegetation and changing carbon storage patterns.

Main types of land use changes

• Deforestation - This involves clearing forests and grasslands for agriculture, logging, or urban developments. In developing countries, it also occurs when locals collect wood for fuel or produce charcoal for domestic use.
• Farming adaptations - Wealthier populations consume more meat, leading to more land being used for pasture (grazing livestock), which is less productive than crops.
• Afforestation - This is the process of planting trees on previously non-forested land, often to restore vegetation. However, it may involve planting single species in large numbers, which stores less carbon than diverse, natural forests where trees vary in growth rates and heights.

These changes disrupt the natural balance of carbon fluxes, which are the movements of carbon between different stores like the biosphere (living organisms) and atmosphere.

Impacts on carbon stores from land use changes

Converting natural land to agricultural or urban use reduces the biosphere's capacity to store carbon and absorb carbon dioxide (CO₂) from the atmosphere. Vegetation acts as a carbon sink by taking in CO₂ during photosynthesis, so its removal releases stored carbon and limits future uptake.

General effects of land use conversion

• Loss of vegetation - Clearing land for buildings or farms removes plants, decreasing carbon storage in the biosphere and reducing CO₂ removal.
• Soil and water changes - Preparing soil for crops can lead to moisture loss; urban impermeable surfaces (like concrete) increase runoff and decrease evapotranspiration (the process where water evaporates from land and plants into the atmosphere).
• Reduced shade and drier soils - Shorter vegetation from forest-to-farmland conversion exposes soil to more sunlight, increasing evaporation rates and making soils drier.

Specific impacts of deforestation

• Carbon release - Deforestation, often through burning, directly releases carbon from the biosphere as CO₂ into the atmosphere by destroying plant matter.
• Soil erosion - Cleared land becomes prone to wind and water erosion, washing away humus (organic matter in soil) into oceans, which depletes soil carbon content.

Effects of farming practices

• Livestock emissions - Animals release CO₂ during respiration and methane (a potent greenhouse gas) during digestion.
• Soil disturbance - Ploughing exposes and releases CO₂ stored in the soil.
• Crop-specific impacts - Growing rice in flooded paddies produces methane as organic matter decomposes underwater.

These practices increase atmospheric greenhouse gases, which trap heat and contribute to global warming.

Influences of climate change on carbon stores

Human activities, especially burning fossil fuels (coal, oil, and gas), have increased atmospheric CO₂ by over 40% since 1750. This enhanced greenhouse effect, where higher greenhouse gas levels cause more heat retention, leads to rising temperatures and alters carbon stores across different environments.

Impacts on land carbon stores

• Permafrost melting - Permafrost is permanently frozen ground in Arctic and alpine regions. Warmer temperatures cause it to thaw, releasing stored carbon into the atmosphere as CO₂ or methane.
• Changes in vegetation - Higher temperatures can reduce overall carbon storage.

Impacts on atmosphere and climate

• Extreme weather events - Increased frequency of storms, floods, and droughts can destroy forests, releasing their stored carbon.
• Shifts in ecosystems - Polar ice caps may shrink, and arid (dry) zones on continents could expand, reducing the size of carbon-rich areas like forests.

Impacts on ocean carbon stores

• Decline in phytoplankton - These microscopic marine plants use CO₂ for photosynthesis. Warmer waters reduce their numbers, leading to less CO₂ absorption.
• Reduced CO₂ solubility - Warmer ocean water holds less dissolved CO₂, leaving more CO₂ in the atmosphere and exacerbating warming.

Oceans act as major carbon sinks by absorbing CO₂, but climate change limits this role.

Case study: Amazon drought events

The Amazon rainforest is a vital carbon store and regulator of climate through processes like evapotranspiration, which releases moisture into the air, influencing humidity and rainfall. Climate change has triggered severe droughts in the region, disrupting these functions.

Key events and impacts

• Drought occurrences - Major droughts hit the Amazon in 2005 and 2010, linked to altered rainfall patterns from global warming.
• Effects on species - Many plants and animals, adapted to moist conditions, died during dry spells.
• Slowed carbon uptake - Photosynthesis rates dropped, meaning less CO₂ was removed from the atmosphere.
• Increased wildfires - Dry conditions led to more fires, releasing additional CO₂ and raising local temperatures.
• Long-term threats - Repeated droughts could cause species extinctions and impair the forest's overall health, affecting carbon and water cycling within the ecosystem.

These events highlight how climate change can turn a carbon sink into a source of emissions.

Ocean acidification and its consequences

Ocean acidification occurs when absorbed CO₂ reacts with seawater to form carbonic acid, lowering the ocean's pH (a measure of acidity). Increased atmospheric CO₂ from fossil fuel burning accelerates this process, harming marine ecosystems.

Process and effects on marine life

• Dissolved CO₂ creates carbonic acid, increasing ocean acidity over time.
• Acid dissolves calcium carbonate, needed by creatures like shellfish for building shells, leading to thinner or weaker structures.
• Acidity hinders marine plants' ability to absorb minerals, potentially reducing populations of primary consumers (organisms that eat plants).

Specific impacts on coral reefs

• Coral reefs are particularly vulnerable, as acidity removes carbonate ions essential for their calcium carbonate skeletons.
• Even slight acidity increases make skeletons harder to form, resulting in fragile corals prone to breakage.
• Beyond a certain acidity level, reefs cannot recover, leading to permanent dissolution.
• Warmer waters stress corals, causing them to expel colourful algae (their food source), turning them white, which is called coral bleaching. Prolonged heat prevents algae return, leading to coral death.

Ecosystem services provided by coral reefs

Ecosystem services are benefits that natural environments provide to humans, and coral reefs support around 1 billion people globally.

These services provided by coral reefs include:

• Tourism - They attract visitors for diving and boat trips, generating income for local businesses.
• Fishing - Reefs serve as productive habitats, providing fish for markets that supply both locals and tourists.

Loss of reefs through acidification and bleaching threatens these services, affecting livelihoods and economies.