Greenhouse effect Flashcards
Intensity formula
Intensity = power/Area
- units = W/m2 = Wm-2
Intensity = e(sigma)(T^4)
Area of a sphere
A = 4 * pi * r2
- we assume that e.g. the sun is a sphere
The solar constant
- represents the amount of solar energy received per unit area at the top of Earth’s atmosphere
Albedo
The ratio of scattered (reflected) to total incident power
- formula: total scattered P/total incident P
chat: Albedo is the measure of how much light or radiation is reflected by a surface compared to how much it receives. A surface with high albedo reflects most of the incoming light.
Emissivity
The ratio of the energy radiated by a body to the energy radiated by a perfect black body at the same temperature is called the emissivity.
- adapted equation for the power radiated:
P = eAsigmaT4
The emissivity of a black body is 1; other materials will have emissivity less than 1
Emissivity is the ratio of actual thermal energy emitted to that which would be emitted by a black body. We can express this in terms of energy or power by the equation:
emissivity = P radiated per unit area / sigmaT4
What does it mean when something is incident to the surface?
When something is incident to the surface, it means that it is striking or arriving at that surface. In physics, this often refers to waves, rays, or particles coming into contact with a surface at a certain angle.
Thermal equilibrium
to maintain a constant global temperature, the Earth must emit the same amount of energy that it absorbs
Greenhouse effect
- gas molecules absorb and reemit infrared radiation: This happens because the shape of these molecules means that they have natural vibration frequency that matches the frequency of infrared waves
If there was no atmosphere, the earth would experience a net loss of energy and reach equilibrium at an average temperature about 30°C colder than it is currently.
Ranking of absorption of infrared E (greenhouse gases)
- Water vapor
- Carbon dioxide
- Methane
- Nitrogen oxides
- Oxygen/Ozone
- first 3 are top greenhouse gases
Why do CO2 levels flactuate yearly?
Seasons
Earth without atmosphere
- perfectly black Earth without atmosphere
- radiation only lands on 1 side of the Earth (absorbs like a disk)
- As the temperature of the Earth increases, it will emit radiation which, according to the Stefan-Boltzmann law, will be proportional to the fourth power of the temperature in kelvin. So when the temperature is T, the power radiated per unit area is given by: power per unit area = sigma*T4
Earth with atmoshphere
- more realistic because not all the radiation is absorbed by the ground: some is scattered by the atmosphere (particularly clouds) and some is reflected off the surface
We can see that the atmosphere has two competing effects: the clouds raise the albedo, resulting in less power reaching the Earth, but the greenhouse gases make the emissivity lower, resulting in a higher overall temperature.
The conservation of energy principle
- states that energy cannot be created or destroyed, only transferred or transformed from one form to another
crucial to understanding the greenhouse effect in physics as: - Fossil Fuel Combustion: Converts chemical energy into heat and CO₂ emissions.
- Solar Radiation Absorption: Earth absorbs light energy and transforms it into thermal energy.
- Infrared Radiation: Earth re-emits heat, but greenhouse gases trap some of it, preventing full escape.
- Energy Balance Disruption: More greenhouse gases → More trapped heat → Increased global temperatures.
- No Energy Loss, Only Redistribution: Extra heat remains within Earth’s system, altering climate patterns.
Earth’s albedo
- varies daily and is dependent on cloud formation and latitude (some light is scattered by the clouds)
Average intensity
The incoming radiative power is dependent on the projected surface of a planet along the direction of the path of the rays, resulting in a mean value of the incoming intensity being S/4
Main greenhouse gases
CH4, H2O (vapour), CO2, and N2O (nitrous oxide)
- they have origins that are both natural and created by human activity and they absorb infrared radiation
- these molecules then re-emit radiation in random directions so some of it returns to the Earth
Molecular energy levels and the resonance model
Resonance:
- main molecules in the atmosphere can be made to vibrate by infrared radiation, but they only absorb the frequency of radiation that is the SAME as their natural frequency. Once the molecule starts to vibrate, it will re-emiy the radiation in random direction (including toward space), which results in less radiation reaching the Earth.
Molecular Energy Levels:
- Greenhouse gases (GHGs) absorb and emit infrared (IR) radiation due to vibrational and rotational transitions.
- Vibrational transitions occur when IR radiation matches the energy gap between molecular vibrational states.
- Rotational transitions occur alongside vibrations and require lower energy (microwave region).
The enhanced greenhouse effect
Human caused: There is a lot of evidence that the burning of fossil fuels, which produces carbon dioxide at a faster rate than the living trees remove it, has led to an increase in the percentage of CO2 in the atmosphere. This has enhanced the greenhouse effect, causing more energy to be radiated back to Earth. If more radiation returns to the Earth, then more must be emitted, resulting in a higher average temperature. This effect is made worse by deforestation because fewer trees are available to store carbon dioxide.
Molecular energy levels
- greenhouse gases have molecular energy levels that typically differ by 1 meV
Solar panel vs. solar cell
Solar Cell – A single photovoltaic (PV) device that converts sunlight directly into electricity. Made of semiconductor materials like silicon.
Solar Panel – A collection of multiple solar cells connected together to generate more electricity. Panels are used in solar power systems to produce usable energy for homes, businesses, and grids.
Some of the energy incident on the surface of the Earth is emitted as infrared radiation.
Why does this cause a ‘greenhouse effect’?
The radiation is absorbed by the atmosphere and is re-radiated in all directions.
Certain gases, for example carbon dioxide and methane, are categorised as greenhouse
gases because they:
Transmit incoming radiation from the Sun and then absorb outgoing radiation from
the Earth
Which two assumptions are made when calculating the solar constant?
I. Radiation is incident perpendicular to the Earth’s surface
II. The Earth is at its mean distance from the Sun
