Environmental Issues (Global Issues)

Greenhouse Effect And Global Warming (from Environmental Issues)

The greenhouse effect is a natural process that warms the Earth's surface. When the Sun's energy reaches the Earth's atmosphere, some of it is reflected back to space, and the rest is absorbed and re-radiated by greenhouse gases.

Greenhouse Effect:

• Natural Process: Greenhouse gases (GHGs) like water vapour ($H_2O$), carbon dioxide ($CO_2$), methane ($CH_4$), nitrous oxide ($N_2O$), and ozone ($O_3$) in the atmosphere trap some of the Sun's heat, preventing it from escaping into space. This natural warming process keeps the Earth's average temperature at a habitable level (around 15°C).
• Mechanism: Solar radiation (short wavelength) passes through the atmosphere and heats the Earth's surface. The Earth then radiates heat back as infrared radiation (long wavelength). Greenhouse gases absorb this infrared radiation and re-emit it in all directions, including back towards the Earth's surface, thus warming the lower atmosphere.

Enhanced Greenhouse Effect and Global Warming:

• Human Influence: Human activities, particularly since the Industrial Revolution, have significantly increased the concentration of greenhouse gases in the atmosphere, intensifying the natural greenhouse effect.
• Major Greenhouse Gases and Sources:
  • Carbon Dioxide ($CO_2$): Burning of fossil fuels (coal, oil, natural gas), deforestation, cement production. It is the most significant contributor to the enhanced greenhouse effect.
  • Methane ($CH_4$): Natural gas leaks, livestock digestion (enteric fermentation), rice paddies, decomposition of organic waste in landfills, biomass burning.
  • Nitrous Oxide ($N_2O$): Agricultural activities (use of nitrogen fertilizers), fossil fuel combustion, industrial processes.
  • Halocarbons (CFCs, HFCs): Industrial refrigerants, solvents, aerosol propellants. Though concentrations are lower, they are potent GHGs and also contribute to ozone depletion.
  • Water Vapour ($H_2O$): While the most abundant GHG, its concentration in the atmosphere is largely controlled by temperature (feedback mechanism) rather than direct human emissions.
• Global Warming: The increase in the Earth's average surface temperature due to the enhanced greenhouse effect.
• Effects of Global Warming:
  • Rising Global Temperatures: Leading to heatwaves and changes in weather patterns.
  • Melting of Glaciers and Ice Sheets: Contributing to sea-level rise.
  • Sea-Level Rise: Due to thermal expansion of oceans and melting ice, threatening coastal areas.
  • Changes in Precipitation Patterns: More intense rainfall in some areas, droughts in others.
  • Extreme Weather Events: Increased frequency and intensity of storms, floods, heatwaves, and droughts.
  • Ocean Acidification: Absorption of excess $CO_2$ by oceans leads to increased acidity, harming marine life (especially coral reefs and shell-forming organisms).
  • Impacts on Ecosystems and Biodiversity: Shifts in species ranges, habitat loss, and increased extinction rates.
  • Impacts on Agriculture: Changes in crop yields, water availability.
• Mitigation and Adaptation: Reducing GHG emissions (transitioning to renewable energy, improving energy efficiency, sustainable land management) and adapting to the impacts of climate change.

Ozone Depletion In The Stratosphere (from Environmental Issues)

The ozone layer, located in the stratosphere, plays a vital role in protecting life on Earth by absorbing most of the harmful ultraviolet (UV) radiation from the sun.

Ozone ($O_3$) Formation and Destruction in the Stratosphere:

• Formation: Ozone is continuously formed and destroyed in the stratosphere through photochemical reactions involving oxygen molecules and UV radiation.
  • $O_2 \xrightarrow{UV \text{ (short wavelength)}} O + O$ (Oxygen atoms)
  • $O + O_2 \rightarrow O_3$ (Ozone)
• Natural Destruction: Ozone is also naturally destroyed by reacting with oxygen atoms.
  • $O_3 + O \rightarrow 2O_2$
• Balance: Normally, the rate of ozone formation and destruction maintains a dynamic equilibrium, resulting in a stable ozone layer.

Ozone Depleting Substances (ODS):

• Human-Made Chemicals: Certain synthetic chemicals, when released into the atmosphere, reach the stratosphere and catalytically destroy ozone molecules faster than they are formed.
• Chlorofluorocarbons (CFCs): These are the most significant ODS. They are stable in the troposphere but decompose in the stratosphere under UV radiation to release chlorine free radicals ($Cl\cdot$).
• Mechanism of Ozone Destruction by Chlorine Radicals:
  • $CFCl_3 \xrightarrow{UV} Cl\cdot + CFCl_2\cdot$
  • $Cl\cdot + O_3 \rightarrow ClO\cdot + O_2$
  • $ClO\cdot + O \rightarrow Cl\cdot + O_2$
  • Net Reaction: $O_3 + O \xrightarrow{Cl\cdot} 2O_2$
  A single chlorine atom can destroy thousands of ozone molecules before being deactivated.
• Other ODS: Halons (containing bromine), carbon tetrachloride ($CCl_4$), methyl chloroform ($CH_3CCl_3$), and hydrochlorofluorocarbons (HCFCs). Bromine radicals are even more effective at destroying ozone than chlorine radicals.

Ozone Depletion and its Effects:

• Ozone Hole: Significant thinning of the ozone layer, particularly over Antarctica during spring, where the ozone concentration drops dramatically.
• Increased UV-B Radiation: Depletion of the ozone layer allows more harmful UV-B radiation to reach the Earth's surface.
• Effects of Increased UV-B Radiation:
  • Human Health: Increased risk of skin cancer (melanoma), cataracts, and immune system suppression.
  • Terrestrial Ecosystems: Damage to crops, reduced plant growth, and disruption of forest ecosystems.
  • Aquatic Ecosystems: Harm to phytoplankton (the base of the marine food web), fish larvae, and other aquatic organisms.

Control Measures:

• Montreal Protocol (1987): An international treaty designed to phase out the production and consumption of ODS, including CFCs and halons.
• Transition to Alternatives: Development and use of ozone-friendly alternatives like hydrofluorocarbons (HFCs) – although HFCs are potent greenhouse gases and are being phased down under separate agreements.
• Continued Monitoring: Ongoing research and monitoring of the ozone layer and ODS concentrations.