Earths life support systems

• The impact of land-use change and water extraction
• Human impact on the water and carbon cycles


  • Summarised in Table 44
• Urban development reduces permeable surfaces, increasing runoff and flooding
• Forestry reduces above-ground carbon stores
• Ploughing reduces soil carbon storage and exposes organic matter to oxidation
• Harvesting means that only small amounts of organic matter are returned to the soil, further reducing carbon stores
• Rice paddies generate methane
• Livestock release methane gas as a by-product of digestion
• Emissions from tractors increase the level of CO2 in the atmosphere
• Deforestation reduces carbon storage as new growth has fewer leaves and roots, leading to increased overland flow and risk of localised flooding
• Forestry increases carbon stores as forest trees sequester CO2 from the atmosphere for hundreds of years
• Forest trees are only an active carbon sink for the first 100 years or so after planting, so forestry rotations are typically 80-100 years
• Water extraction


  The process of taking water from a surface or ground, temporarily or permanently
• Water extraction can lead to several issues, including damage to wetland ecosystems, sinking water tables if extraction exceeds recharge, and saltwater intrusion in coastal areas
• Aquifers


  Water-bearing rocks, including chalk and sandstone, from which groundwater is abstracted through wells and boreholes
• Artesian aquifers contain groundwater under pressure, which flows to the surface under its own pressure when a well is sunk into the aquifer
• Sedimentary rocks may form a basin shape, trapping an aquifer between impermeable rock layers