Greenhouse Effect and Climate Change

Solar Radiation

Not all incoming solar radiation reaches the Earth's surface. Approximately 26% is reflected back into space by clouds and the atmosphere. Another 19% is absorbed by the atmosphere and clouds, then radiated into space and back down to Earth. The rest reaches the Earth's surface where it is either absorbed or reflected depending on the surface's albedo.

Dark surfaces have low albedo, absorbing sunlight and converting the energy into infrared radiation (warmth). Lighter surfaces have high albedo, reflecting sunlight back into space or towards clouds and greenhouse gases.

The Greenhouse Effect

The greenhouse effect is crucial for life on Earth. Gases in the atmosphere trap heat from the sun and radiate it back to the surface. Without this effect, Earth would be too cold to support life.

Solar radiation (UV and visible light) heats the Earth's surface. Greenhouse gases absorb the infrared radiation emitted from the Earth's surface, trapping the heat and regulating the temperature. Some of this heat is radiated back down to Earth, while some escapes into space.

Greenhouse Gases

  • Carbon Dioxide (CO_2):
    • Primary source: Fossil fuel combustion, decomposition, deforestation. Human activities are a significant factor.
  • Methane (CH_4):
    • Sources: Natural gas extraction and combustion, volcanic activity, animal agriculture (livestock), and anaerobic decomposition (e.g., permafrost thaw, landfills).
  • Nitrous Oxide (N_2O):
    • Source: Agricultural soils, particularly from flood irrigation and over-fertilization.
  • Chlorofluorocarbons (CFCs), Hydrochlorofluorocarbons (HCFCs), Hydrofluorocarbons (HFCs):
    • CFCs were replaced by HCFCs (still ozone-depleting and greenhouse gases), then HFCs (no longer ozone-depleting but still greenhouse gases), and finally Hydrofluoroolefins (HFOs), which are neither ozone-depleting nor greenhouse gases.
    • Sources: Refrigerants, aerosol propellants, and blowing agents.
  • Water Vapor (H_2O):
    • Source: Evaporation and transpiration from plants.
    • Technically a greenhouse gas, but its concentration is driven by temperature change, not vice versa.

Global Warming Potential (GWP)

GWP measures a molecule's potential to warm the planet relative to CO_2. It depends on:

  • Residence time (how long the molecule stays in the atmosphere)
  • Infrared absorption (how the gas absorbs and radiates IR)

CO_2 has a GWP of 1 (the baseline).

  • Methane: Higher GWP than CO_2 due to greater heat absorption, stays in the atmosphere for approximately 12 years.
  • Nitrous Oxide: GWP of 300, remains in the atmosphere for around 115 years.
  • CFCs: GWP ranges from 1,600 to 13,000, with a residence time of 50-500 years.

Impacts of Increased Greenhouse Gases

  • Sea Level Rise:
    • Thermal expansion: Water molecules move further apart when heated.
    • Melting ice sheets and glacial ice (land-based ice) contribute to sea level rise. Melting icebergs (already in the ocean) do not significantly raise sea levels.
  • Consequences of Sea Level Rise:
    • Flooding of coastal ecosystems (estuaries, mangroves, salt marshes) and communities.
    • Threats to species in Arctic and tundra ecosystems.
    • Loss of thaw-freeze cycles, depriving ecosystems and communities of water.
  • Human Impacts:
    • Relocation of coastal populations.
    • Increased flood frequency, leading to higher insurance and repair costs.
    • Saltwater intrusion contaminating freshwater reserves and irrigation water.
    • Potential refugee crises due to displacement.
  • Disease Vectors:
    • Warmer temperatures allow insect-transmitted diseases to spread to previously colder regions.
    • Examples: Malaria may increase in the southern United States, Central America, parts of Asia, and Eastern Europe.

Climate Change and Earth's History

  • Earth's climate has varied over geologic time due to variations in its orbit around the sun.
  • Milankovitch Cycles:
    • Eccentricity: Changes in the shape of Earth's orbit (more or less ovular).
    • Obliquity: Variations in the tilt of Earth's axis (every 40,000 years), affecting sun exposure at different latitudes.
  • Evidence of Past Climate:
    • Form and infra shells in ocean sediments indicate past temperature ranges.
    • Air bubbles in ice cores contain ancient atmospheric gases.
    • Oxygen-18 isotopes: Higher temperatures correlate with more oxygen-18.

Modern Climate Change

Carbon dioxide levels are strongly correlated with temperature. A significant spike in carbon dioxide levels began around the 1800s (Industrial Revolution).

  • Effects of Climate Change:
    • Rising temperatures, habitat and species loss, drought, soil desiccation, heat waves, increased precipitation in some regions, rising sea levels.
    • Melting of permafrost, which releases methane and CO_2, creating a positive feedback loop.
    • Coral bleaching.
  • Impacts by Region:
    • Arctic region highly affected.
    • Coastal flooding, river flooding, crop yield reduction, heat-related health issues.
    • Property loss and potential relocation.

Effects on Oceans and Marine Ecosystems

  • New marine habitats may form due to rising sea levels, but existing ecosystems (estuaries, intertidal zones) will be lost.
  • Some ocean areas may become too deep to receive sunlight, impacting the photic zone and coral reefs.
  • Polar amplification: Polar regions are warming faster due to less land/water to absorb heat, melting ice (reducing albedo), and thermal haline circulation.
  • Arctic Sea Ice Loss: Leads to habitat loss and reduced biodiversity, as algae growing on the bottom of the ice are the base of the Arctic food web.
  • Dirty ice accelerates melting: Soot and other particulates lower albedo.