Overview
This lecture covers the main processes, stores, and transfers within the water and carbon cycles at both local and global scales, their interactions, human impacts, and methods of climate change mitigation.
Systems in Geography
- Systems have inputs, outputs, stores, flows, and boundaries.
- Open systems exchange matter or energy with surroundings; closed systems do not.
- Dynamic equilibrium occurs when system inputs equal outputs despite changes.
- Positive feedback amplifies changes; negative feedback reduces them.
- Locally, water and carbon cycles are open; globally, they are closed.
The Water Cycle
- Inputs: Precipitation (convectional, relief, frontal).
- Outputs: Evapotranspiration and streamflow.
- Flows: Infiltration, percolation, throughflow, surface runoff, groundwater flow, stemflow, and streamflow.
- Stores: Soil water, groundwater, river channels, interception, surface storage.
- Water table: upper saturated soil level, indicates drought/wetland health.
- Water balance: Precipitation = Runoff + Evapotranspiration ± Change in Storage.
- Affected by deforestation, storm events, seasonal changes, agriculture, and urbanisation.
- Soil water budget varies seasonally; surplus in winter/spring, deficit in summer.
The Water Cycle: Global Scale and Changes
- Oceans hold 97% of water, glaciers/ice caps 69% of freshwater.
- Water stored in hydrosphere (liquid), lithosphere (rocks), cryosphere (ice), atmosphere (vapour).
- ITCZ drives global rainfall patterns.
- Change caused by natural events (storms, droughts, El Niño/La Niña), cryospheric processes, and human impacts (urbanisation, farming, abstraction).
Flood Hydrographs
- Discharge: volume of river flow, made up of baseflow and stormflow.
- Rising limb, falling limb, lag time, bankfull discharge.
- Flashy hydrographs: short lag, steep limbs, high peak, higher flood risk.
- Subdued hydrographs: long lag, gradual limbs, lower flood risk.
- Influenced by rainfall intensity, antecedent rainfall, geology, drainage density, basin shape, temperature, and land cover.
The Carbon Cycle
- Transfers: Photosynthesis, respiration, combustion, decomposition, diffusion, weathering, burial/compaction, sequestration.
- Carbon sequestration: natural (plants) or artificial (CCS).
- Sere: succession stage ending in a climatic climax (stable ecosystem).
The Carbon Cycle: Global Scale and Changes
- Largest stores: marine sedimentary rocks, oceans, fossil fuels, soil, atmosphere, terrestrial plants.
- Carbon stores and fluxes vary by region and over time.
- Forest loss in the tropics decreases carbon stores; afforestation increases them elsewhere.
- Natural changes: wildfires, volcanic activity.
- Human impacts: fossil fuels, deforestation, farming.
- Carbon budget: balance of carbon inputs and outputs to a store.
Enhanced Greenhouse Effect and Climate Change
- Enhanced greenhouse effect: human-caused increase in greenhouse gases traps more heat, causing global warming.
- COâ‚‚ is the main anthropogenic greenhouse gas.
- Causes: land use change, farming, urbanisation, cement production.
- Mitigation includes renewable energy, carbon capture, and efficiency.
Feedback Loops
- Positive feedback: amplifies change (e.g., permafrost thaw, wildfires, ice melt).
- Negative feedback: reduces change (e.g., increased plant growth, phytoplankton/clouds).
- Feedbacks affect both water and carbon cycles.
Tropical Rainforests: Cycle Interactions
- Intact rainforest: high interception, transpiration, humidity, and carbon sequestration.
- Deforestation: increases runoff, reduces transpiration and carbon storage, raises flood and drought risk.
- Peatland drainage: lowers water table, increases carbon release.
Climate Change Mitigation
- Global: Paris Climate Deal (COP21): aims to limit warming to 2°C.
- Regional: EU 20-20-20 targets for emissions, renewables, efficiency.
- National: UK Climate Change Act 2008 sets legal emission reduction targets.
- Local: insulation, recycling, smart energy use, transport changes.
Key Terms & Definitions
- Dynamic Equilibrium — State where inputs equal outputs in a system.
- Evapotranspiration — Loss of water from soil by evaporation and through plant transpiration.
- Infiltration — Movement of water into soil.
- Percolation — Movement of water from soil into bedrock.
- Flood Hydrograph — Graph showing river discharge over time related to rainfall.
- Carbon Sequestration — Removal and storage of atmospheric carbon.
- Flux — Change in the amount of a substance stored.
- Enhanced Greenhouse Effect — Human-induced intensification of the greenhouse effect.
Action Items / Next Steps
- Review the water and carbon cycle diagrams and processes.
- Learn and practice the key formulas (e.g., water balance).
- Answer practice questions on feedback loops and human impacts.
- Read further on climate change mitigation strategies for case study examples.