Lecture 2 - Earth's Energy Budget and the Greenhouse Effect (2)
- Created by: sikemi__
- Created on: 03-05-21 14:34
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- Lecture 2 - Earth's Energy Budget and the Greenhouse Effect
- Greenhouse Effect (2)
- Presence of one I.R. absorbing layer
- Back radiation doubles amount of energy received at the surface (increasing equilibrium surface temperature by about 20%)
- Multiple I.R. absorbing layers
- Something that a lot of people haven't recognised and have used as an argument against global warming
- Not the same as global warming
- Greenhouse effect increases surface temperature by 33K, necessary to support life on Earth
- Global warming is the additional increase and strengthening of the greenhouse effect over the last 100 years, likely due to anthropogenic emissions
- Anthropogenic contribution
- CO2 is important (54%) but not the only anthropogenic GHG
- CH4 is a better absorber than CO2 (18% but already has large effect)
- Presence of one I.R. absorbing layer
- Radiative forcing
- Artificial imbalance between outgoing and incoming radiation
- +ve = warming
- -ve = cooling
- Calculated from externally imposed perturbation in Earth's energy budget e.g. adding CO2
- No feedbacks included, simply increases the amount of GHG so reducing amount of radiation that can escape to space
- Neglect back radiation
- Leave surface temperature constant at Ts=288K
- Anthropogenic greenhouse forcing (approx 3.3W/m2) is small compared to atmospheric greenhouse forcing (approx 150W/m2)
- However, this has led to warming by 1 degree Celsius
- Aerosols cool the atmosphere (total anthropogenic forcing approx 2.3W/m2)
- However, this is larger than the 0.5W/m2 in 1950
- Artificial imbalance between outgoing and incoming radiation
- Feedback
- Mechanism that amplifies (positive) or weakens (negative) a signal in the climate system
- Examples:
- Ice albedo effect (+ve) - warmer climate = less snow and ice coverage = less reflection of solar radiation and enhanced warming
- Thermostat effect (-ve) - warmer climate = more evaporation = more water vapour = more clouds = more reflection of solar radiation = cooling
- However, it has been proven that clouds don't provide -ve feedback, they provide +ve
- Strongest -ve feedback is Stefan Boltzmann Law - small increase in warming and temp. = strong increase in radiation emitted
- Temperature response - change in temp after reaching new equilibrium, including all feedbacks after applying forcing
- Note that CO2 emissions and CO2 concentration aren't the same - concentration should be considered
- Clouds
- Thermal - cools surface by 17W/m2
- Because most sunlight is absorbed at the surface, so the surface becomes warmer than the atmosphere
- Heat conduction occurs at the surface, turbulence in lower atmosphere picks heat up and transports it further into atmosphere
- Evapotranspiration occurs over water and land
- Energy needed for this creates latent heat
- Evapotranspiration occurs over water and land
- Heat conduction occurs at the surface, turbulence in lower atmosphere picks heat up and transports it further into atmosphere
- Because most sunlight is absorbed at the surface, so the surface becomes warmer than the atmosphere
- Reflect 20% of incoming solar radiation, emit 26% of outgoing terrestrial radiation
- Radiative forcing of -20W/m2
- Redistribute energy within climate system due to latent heat transport
- Cloud feedbacks are the largest uncertainty in global warming estimates
- Cirrus - warming (greenhouse > albedo)
- Cumulus - neutral (greenhouse = albedo)
- Stratus - cooling (albedo > greenhouse)
- Thermal - cools surface by 17W/m2
- Greenhouse Effect (2)
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