Lecture 2 - The Earth's Energy Budget and the Greenhouse Effect
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- Created on: 03-05-21 12:06
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- Lecture 2 - The Earth's Energy Budget and the Greenhouse Effect
- Interaction of radiation with matter
- Transmission (no interaction)
- Allows sunlight and for us to see things
- Scattering
- Radiation changes direction but not wavelength or energy
- Reflection is a special case - radiation originally travelling downwards travels upwards
- Responsible for Earth's albedo
- Leads to reflection, reduction of available energy in the climate system and cooling
- Solar radiation can be scattered by...
- Gas molecules (Rayleigh scattering) - short wavelengths scattered more effectively than long wavelengths
- Clouds - reflect about 20% of incoming radiation (2/3 of total reflection)
- Aerosol particles - naturally through desert dust/sea spray, burning biomass or human activities (only around 1% of planetary albedo)
- Radiation changes direction but not wavelength or energy
- Absorption
- Transforms radiation into heat
- Radiation hits atmospheric molecules, is absorbed and then turns to heat, changing temperature of the atmosphere
- Radiation is emitted according to temperature (> absorption leads to > temperaturesand > admission and readmission of radiation)
- Radiation hits atmospheric molecules, is absorbed and then turns to heat, changing temperature of the atmosphere
- Absorption of radiation by Greenhouse gases is responsible for the Greenhouse Effect
- Transforms radiation into heat
- All processes are wavelength dependent (especially absorption)
- Atmosphere is opaque for some wavelengths and transparent for others
- e.g. hardly any UV radiation reaches the surface - blocked by the ozone, little visible light is absorbed - mostly absorbed at the surface
- Transmission (no interaction)
- Terrestrial radiation
- Main source = surface (around 400W/m2)
- Only 10% is transmitted freely through the atmosphere, contributing to outgoing long wave radiation
- Most comes from elsewhere in the atmosphere e.g. greenhouse gases, clouds
- Greenhouse gases absorb most surface radiation and reflect as back radiation
- Radiation budget is imbalanced
- Net absorption is 0.9W/m2 - responsible for heating up the ocean
- As greenhouse gas levels increase, ocean will take time to take up enough heat to be in equilibrium with atmosphere
- Net absorption is 0.9W/m2 - responsible for heating up the ocean
- Main source = surface (around 400W/m2)
- Greenhouse Effect
- Short wave radiation warms the ground, which then emits long wave radiation
- Longwave can't easily escape the atmosphere (absorbed by gas molecules), so gets trapped, heating up the atmosphere
- Gases responsible include water vapour (0.5% of atmosphere),CO2 (0.04%), methane, nitrous oxide and particles
- Assumptions:
- Incoming solar radiation can be reflected by the surface/atmosphere but absorbed only at surface
- Surface is heated to a certain temperature before emitting radiation
- Absorption within the atmosphere only interacts with terrestrial radiation, not solar
- All radiation emitted from the surface is absorbed by the absorbing layer - no atmospheric window and no radiation can escape directly to space
- Absorbing layer heats up by terrestrial radiation and will emit as much radiation upwards as downwards
- Surface receives both solar and back radiation
- Short wave radiation warms the ground, which then emits long wave radiation
- Interaction of radiation with matter
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