Greenhouse Effect

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Notas de aula

Albedo

Albedo (a) is the ratio of the total scattered (reflected) power to the total incident power of radiation on a surface.
The albedo of a planet is the ratio between the total scattered radiation and the total incident radiation of that planet.
Earth's average albedo is about 0.3, meaning 30% of incoming solar radiation is reflected back into space.
An albedo of 1 represents a surface that scatters all incident radiation; an albedo of 0 means all radiation is absorbed.
Albedo has no units because it is a ratio of powers.
Albedo values for common surfaces: fresh asphalt 0.04, bare soil 0.17, green grass 0.25, desert sand 0.40, new concrete 0.55, ocean ice 0.50–0.70, fresh snow 0.85.
Earth's albedo varies daily due to cloud cover, season, latitude, terrain and the incident angle of radiation.

Emissivity

Emissivity (e) is the ratio of the power radiated per unit area by a surface to that of a black body at the same temperature.
It is calculated using: e = (power radiated by an object) / (power emitted by a black body).
For a perfect black body, emissivity equals 1.
Calculations of emissivity assume the black body is at the same temperature and has the same dimensions as the object.
The Stefan-Boltzmann law for a non-black body is P = eσAT⁴, where P is power (W), e is emissivity, σ is the Stefan-Boltzmann constant, A is surface area (m²) and T is absolute temperature (K).

The Solar Constant

The solar constant (S) is the intensity of the Sun's radiation arriving perpendicularly to the Earth's atmosphere when the Earth is at its mean distance from the Sun.
Its average value is 1.36 × 10³ W m⁻² (often quoted as 1.4 kW m⁻²).
The solar constant varies year-round because the Earth's orbit is elliptical (distance changes) and the Sun's output varies by about 0.1% over its 11-year sunspot cycle.
Calculations of the solar constant assume radiation is incident perpendicular to the Earth's surface and the Earth is at its mean distance from the Sun.
The intensity of solar radiation received by a planet depends on its distance from the Sun; for example, Venus receives a higher intensity than Earth.
The mean radiative power per unit area received by a planet is S/4, because the planet intercepts radiation over a cross-sectional area πr² but distributes it over its surface area 4πr².
The solar constant can be calculated from the Sun's total power output P using S = P / (4πr²), where r is the distance from the Sun.

Greenhouse Gases

The main greenhouse gases, in order of decreasing contribution, are water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).
All greenhouse gases have both natural and human-generated origins.
A greenhouse gas absorbs long-wave (infrared) radiation re-emitted by the Earth's surface, trapping it in the atmosphere.
Ozone (O₃) absorbs nearly 100% of incoming ultraviolet radiation and also absorbs infrared between 9 μm and 10 μm, but it is not a significant greenhouse gas because its concentration is small.
Carbon dioxide strongly absorbs infrared radiation with a wavelength of 15 μm and is a good absorber between 1.5–30 μm; its increasing concentration makes it a major contributor to the greenhouse effect.
Water vapour is the best absorber of infrared radiation, absorbing wavelengths between 0.8–35 μm; its concentration increases as air becomes warmer.
The atmosphere is mostly transparent to incoming visible radiation, which is absorbed by the Earth's surface.
The relative significance of a greenhouse gas depends on its concentration and how strongly it absorbs specific wavelengths.

The greenhouse effect: sunlight reaching Earth's surface and infrared radiation being absorbed and re-emitted by greenhouse gases

The Greenhouse Effect

Incoming solar radiation is primarily short-wavelength (ultraviolet and visible); the Earth re-emits it as long-wavelength infrared radiation.
About 25% of incoming solar radiation is absorbed by the atmosphere on its way to Earth, while about 80% of the re-emitted infrared radiation is absorbed on its way back out.
Greenhouse gases absorb infrared radiation and re-emit it in all directions, including back towards the Earth's surface.
This trapped heat keeps the Earth at a habitable temperature.
The molecular energy level model explains the effect: greenhouse gas molecules have natural frequencies in the infrared region, so they absorb infrared light and resonate, heating up.
The higher the concentration of greenhouse gases, the more infrared radiation remains trapped in the Earth–atmosphere system, increasing average global temperatures.

The greenhouse effect

The Enhanced Greenhouse Effect

The enhanced greenhouse effect is the increase in average global temperature due to human activity.
Human activities increase greenhouse gas concentrations: CO₂ from burning fossil fuels, burning wood and deforestation; methane from decay of organic matter (manure, landfill, crops); nitrous oxide from artificial fertilisers and burning fossil fuels.
Deforestation reduces the removal of CO₂ from the atmosphere because fewer trees carry out photosynthesis.
Atmospheric CO₂ levels have increased by more than 100 ppm to about 420 ppm in 2020.
Average global temperatures have increased by over 1°C since pre-industrial times.
The natural greenhouse effect arises from natural causes, while the enhanced greenhouse effect arises from human activity.

Energy Balance Problems

Earth's energy balance compares incoming solar energy with outgoing energy; if they are equal, the Earth's temperature remains constant.
Climate models help predict temperature changes based on current and increased greenhouse gas concentrations.
The simplest model uses a one-layer atmosphere above the Earth's surface.
In simplified climate models, the Earth's surface and atmosphere are often assumed to act as black bodies (emissivity = 1) and to remain at constant temperature.
The intensity absorbed by the Earth's surface equals the solar radiation reaching the surface plus the intensity radiated by the atmosphere.
The power per unit area emitted by a body is given by I = eσT⁴.
Example: with an atmospheric emissivity of 0.720, albedo 0.280 and solar intensity 344 W m⁻², a 6 K increase in atmospheric temperature leads to about a 2 K increase in surface temperature.

Diagram of Earth's energy balance, showing solar radiation absorbed, thermal radiation into space, and greenhouse gas absorption in watts per square metre

Slides

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Questões de prática

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1.Which of the following contributes to the enhanced greenhouse effect?
Easy
- ABurning fossil fuels
- BDestruction of the ozone layer
- CCarbon dioxide from active volcanoes around the world
- DIncrease in ultraviolet radiation penetrating the Earth's atmosphere
2.Which equation is used to calculate emissivity?
Easy
- Apower radiated by an object / power emitted by a black body
- Bpower emitted by a black body / power radiated by an object
- Ctotal scattered power from an object / total incident power from a black body
- Dpower absorbed by an object / power absorbed by a black body
3.The average albedo of desert sand is 0.4. What is the ratio of power absorbed by desert sand to power reflected by desert sand?
Medium
- A0.4
- B0.67
- C1.5
- D4.0
4.Which of the following describes the role of carbon dioxide in the greenhouse effect?
Easy
- AIt absorbs incoming radiation from the Sun.
- BIt reflects incoming radiation from the Sun.
- CIt absorbs outgoing radiation from the Earth's surface.
- DIt reflects outgoing radiation from the Earth's surface.
5.The solar constant is the incoming radiative power of the Sun at
Easy
- Athe surface of the planet
- Bthe surface of the Sun
- Cthe average distance between the Sun and the planet
- Dthe top of the planet's atmosphere
6.Which factors affect the amount of solar power incident on a given point on the surface of the Earth? I. Weather conditions II. Latitude III. Position of the Earth in its orbit of the Sun
Medium
- AI and III only
- BI and II only
- CI, II and III
- DIII only
7.The solar constant is quoted as an average rather than an absolute value. Which statements correctly explain this? I. The Earth follows an elliptical orbit around the Sun II. The Earth rotates on an axis which is tilted at 23.5° to the plane of its orbit III. The energy output of the Sun varies according to an 11-year cycle
Medium
- AI only
- BII and III only
- CI and III only
- DIII only
8.Certain gases, for example carbon dioxide and methane, are categorised as greenhouse gases because they:
Medium
- ATransmit incoming radiation from the Sun and then absorb outgoing radiation from the Earth
- BAbsorb incoming radiation from the Sun and also absorb outgoing radiation from the Earth
- CReflect incoming radiation from the Sun
- DReflect outgoing radiation from the Earth

Mais perguntas

9.Some of the energy incident on the surface of the Earth is emitted as infrared radiation. Why does this cause a 'greenhouse effect'?
Medium
- AThe radiation becomes trapped in the troposphere
- BThe radiation heats the upper atmosphere
- CThe radiation is absorbed by the atmosphere and is re-radiated in all directions
- DThe radiation is absorbed by the upper atmosphere then all re-radiated back to the surface of the Earth
10.Which two assumptions are made when calculating the solar constant?
Easy
- AI. The Earth is in an elliptical orbit around the Sun II. The Sun's output varies during its 11-year sunspot cycle
- BI. Radiation from the Sun is incident on the Earth for one second II. Radiation from the Sun is incident on one square meter of the Earth
- CI. Radiation is incident on the Earth, parallel to its surface II. The Earth is at its maximum distance from the Sun
- DI. Radiation is incident perpendicular to the Earth's surface II. The Earth is at its mean distance from the Sun
11.The diagram shows a simplified model of the energy balance of the Earth. What is the albedo of the Earth according to this model? (Diagram: incoming solar radiation 212 units; reflected 42 units; absorbed 170 units.)
Medium
- A0.20
- B0.25
- C0.30
- D0.80
12.Procyon is a similar star to the Sun, but has twice the radius and a surface temperature which is about 15% higher. The current average temperature of Earth's atmosphere, near the surface, is 288 K. If the Sun were replaced by Procyon, how much would the Earth's average temperature increase by? Assume the mean distance between the Earth and Procyon remains the same as the mean distance between the Earth and the Sun.
Hard
- A103 K
- B131 K
- C288 K
- D419 K
13.The diagram shows a simplified model of the energy balance of the Earth's surface. The atmosphere and the surface of the Earth can be modelled as two black body radiators, each at constant temperature. The surface of the Earth receives both solar radiation and radiation emitted from the atmosphere. The average equilibrium temperature of the Earth's surface is TS and the average equilibrium temperature of the atmosphere is TA = 252 K. Using the black-body model for the Earth's surface, what is the value of TS?
Hard
- A238 K
- B256 K
- C280 K
- D295 K
14.The average temperature of the surface of the Earth is 290 K, while the average surface temperature of the Moon varies from 145 K to 435 K, depending on the side that is illuminated by the Sun. The emissivities of the surfaces of the Earth and the Moon are about the same. The radius of the Moon is approximately 4 times smaller than the radius of the Earth. The average intensity radiated by the Earth is I. What are the maximum and minimum values of the intensity radiated by the Moon?
Challenge
- Amaximum = I × (1.5)⁴, minimum = I × (0.5)⁴
- Bmaximum = I × (4)² × (1.5)⁴, minimum = I × (4)² × (0.5)⁴
- Cmaximum = I × (1.5)⁴ × (4)², minimum = I × (0.5)⁴ × (4)²
- Dmaximum = I × (1.5)⁴ / (4)², minimum = I × (0.5)⁴ / (4)²
15.The atmosphere of Venus can be modelled in a similar way to the Earth. The Earth's orbital radius around the Sun is 1.5 times greater than that of Venus. The solar constant for the Earth is 1360 W m⁻². In a simple climate model for Venus, the following data is used: Average equilibrium temperature of Venus's atmosphere = 250 K, Emissivity of Venus's atmosphere e = 0.85, Albedo of Venus's atmosphere a = 0.75. The observed mean surface temperature TS of the planet Venus is about 750 K, which is much higher than the average equilibrium temperature Te of Venus's surface as predicted by the simple model. To account for the temperature difference, the model can be modified by introducing 'layers of atmosphere'. As radiation passes through each layer, the temperature of the atmosphere steadily increases. This temperature difference is represented by the equation TS = (N + 1)^(1/4) × Te, where N is the number of atmospheric layers, and each layer can be assumed to absorb and emit all of the long-wave radiation it receives. How many atmospheric layers are needed to account for the difference between the observed and predicted temperatures?
Challenge
- A18
- B40
- C46
- D63
16.A perfect black body has an emissivity of 1.
Easy
True or false?
17.The albedo of a surface has units of watts per square metre (W m⁻²).
Easy
True or false?
18.Which of the following are greenhouse gases? (Select all that apply.)
Medium
- ACarbon dioxide (CO₂)
- BMethane (CH₄)
- COxygen (O₂)
- DWater vapour (H₂O)
- ENitrogen (N₂)
19.Which of the following statements about the solar constant are correct? (Select all that apply.)
Medium
- AIt is the intensity of solar radiation at the top of the Earth's atmosphere.
- BIt is measured at the Earth's surface.
- CIt assumes the Earth is at its mean distance from the Sun.
- DIt is constant throughout the year.
- EIts average value is about 1.36 kW m⁻².
20.Match each greenhouse gas with its primary human source.
Medium
- Carbon dioxide
- Methane
- Nitrous oxide
- Burning fossil fuels
- Decay of organic matter in landfills
- Artificial fertilisers
21.Place the steps of the greenhouse effect in the correct order, starting with incoming solar radiation.
Medium
- Solar radiation reaches the Earth's surface.
- The Earth's surface absorbs the radiation and warms up.
- The Earth's surface re-emits infrared radiation.
- Greenhouse gases absorb the infrared radiation and re-radiate it in all directions.
- Some of the re-radiated infrared radiation returns to the surface, warming it further.
22.Which of the following best describes the 'enhanced greenhouse effect'?
Medium
- AAn increase in global average temperature due to natural causes
- BA decrease in global average temperature due to human activity
- CAn increase in global average temperature due to human activity
- DA decrease in global average temperature due to natural causes
23.Which of the following statements about the absorption of radiation by the atmosphere is correct?
Medium
- AAbout 25% of incoming solar radiation is absorbed by the atmosphere, and about 80% of outgoing infrared radiation is absorbed.
- BAbout 80% of incoming solar radiation is absorbed by the atmosphere, and about 25% of outgoing infrared radiation is absorbed.
- CThe atmosphere absorbs all incoming ultraviolet and visible radiation.
- DThe atmosphere is transparent to both incoming solar and outgoing infrared radiation.
24.Which of the following is the primary reason that the greenhouse effect warms the Earth's surface?
Hard
- AGreenhouse gases absorb incoming ultraviolet radiation and re-emit it as visible light.
- BGreenhouse gases absorb outgoing infrared radiation and re-emit it in all directions, including back towards the surface.
- CGreenhouse gases reflect incoming solar radiation back into space.
- DGreenhouse gases increase the albedo of the Earth's surface.
25.The diagram shows a simplified model of the energy balance of the Earth's surface. The atmosphere and the surface are modelled as two black body radiators at constant temperature. The surface receives solar radiation and radiation emitted from the atmosphere. If the atmosphere's temperature increases by 6 K, what happens to the surface temperature?
Hard
- AIt increases by less than 6 K because the surface absorbs additional radiation from the atmosphere.
- BIt increases by exactly 6 K because both are black bodies.
- CIt decreases because the atmosphere radiates more energy to space.
- DIt remains constant because the Earth's albedo increases.
26.Match each greenhouse gas with the wavelength of infrared radiation it most strongly absorbs, according to the absorption data.
Medium
- Carbon dioxide (CO2)
- Ozone (O3)
- Water vapour (H2O)
- 15 μm
- 9–10 μm
- 0.8–35 μm
27.Which of the following statements about the absorption of radiation by the atmosphere are correct? (Select all that apply.)
Medium
- AAbout 25% of incoming solar radiation is absorbed by the atmosphere.
- BAbout 80% of the long-wave radiation re-emitted by the Earth is absorbed by the atmosphere.
- CThe atmosphere strongly absorbs incoming visible radiation.
- DOzone absorbs almost all incoming ultraviolet radiation.
- EThe atmosphere is transparent to microwave radiation.
28.Which of the following are human activities that increase the concentration of greenhouse gases in the atmosphere? (Select all that apply.)
Hard
- ABurning fossil fuels
- BDeforestation
- CUse of artificial fertilisers
- DVolcanic eruptions
- ERespiration by animals
29.The enhanced greenhouse effect is caused by natural variations in the Sun's output.
Easy
True or false?
30.Which of the following best explains why the greenhouse effect increases the Earth's average surface temperature?
Medium
- AGreenhouse gases absorb incoming solar radiation and re-emit it towards the surface.
- BGreenhouse gases absorb outgoing infrared radiation and re-emit some of it back towards the surface.
- CGreenhouse gases reflect incoming solar radiation back into space.
- DGreenhouse gases prevent convection currents from carrying heat away from the surface.
31.The solar constant is 1360 W m⁻². What is the average solar power incident per unit area on the Earth's surface, assuming the Earth intercepts radiation over a cross-sectional area of πr² and distributes it over a surface area of 4πr²?
Hard
- A340 W m⁻²
- B680 W m⁻²
- C1360 W m⁻²
- D5440 W m⁻²
32.A surface has an albedo of 0.55. What fraction of the incident radiation is absorbed by the surface?
Medium
- A0.55
- B0.45
- C0.82
- D1.82
33.A planet has radius r and is at a distance d from its star. The star's radiation is incident on a cross-sectional area πr², but the planet's total surface area is 4πr². Which expression gives the average intensity of solar radiation absorbed per unit area of the planet's surface, assuming no reflection?
Medium
- AS
- BS/2
- CS/4
- D4S
34.A one-layer atmosphere model has an atmosphere at 252 K and a surface at 288 K. The atmosphere has emissivity 0.72 and albedo 0.28. If the atmosphere warms by 5 K, which of the following is the best estimate of the new surface temperature? Use σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴.
Medium
- A288 K
- B290 K
- C293 K
- D298 K
35.Which of the following statements about the energy balance of the Earth are correct? (Select all that apply.)
Medium
- AIf incoming and outgoing energy are in balance, Earth's temperature remains constant.
- BThe Earth's surface absorbs both solar radiation and radiation emitted by the atmosphere.
- CThe atmosphere is completely transparent to infrared radiation.
- DThe solar constant is the intensity of solar radiation at the top of the atmosphere.
- EIncreasing greenhouse gas concentration increases the amount of infrared radiation escaping to space.
36.The albedo of a surface can be greater than 1.
Easy
True or false?
37.Match each greenhouse gas with the wavelength range it most strongly absorbs, according to the absorption data.
Medium
- Ozone (O₃)
- Carbon dioxide (CO₂)
- Water vapour (H₂O)
- 0.8 – 35 μm
- 9 – 10 μm
- 1.5 – 30 μm
38.Place the steps of the enhanced greenhouse effect in the correct order, starting with human activity.
Medium
- Human activity increases greenhouse gas concentration.
- More infrared radiation is absorbed by the atmosphere.
- Less long-wave radiation escapes to space.
- Average global temperature increases.
39.Which of the following is the best definition of the solar constant?
Medium
- AThe intensity of the Sun's radiation arriving perpendicularly to the Earth's atmosphere when the Earth is at its mean distance from the Sun
- BThe total power of the Sun's radiation arriving at the Earth's surface when the Earth is at its mean distance from the Sun
- CThe intensity of the Sun's radiation arriving parallel to the Earth's atmosphere when the Earth is at its maximum distance from the Sun
- DThe total power of the Sun's radiation arriving at the top of the Earth's atmosphere at any distance from the Sun
40.Which of the following assumptions are made when calculating the solar constant? (Select all that apply.)
Medium
- ARadiation is incident perpendicular to the Earth's surface
- BThe Earth is at its mean distance from the Sun
- CThe Earth is in an elliptical orbit around the Sun
- DThe Sun's output varies during its 11-year sunspot cycle
- EThe Earth's atmosphere absorbs all incoming radiation
41.In an experiment looking at solar energy, the total incident power is 1500 W. The albedo of green grass is 0.25. Calculate the total scattered power when this light is incident on green grass.
Medium
42.Complete the definition of albedo.
Easy
Albedo is the ratio of the total ____ power to the total ____ power.
43.Complete the sentence about the solar constant.
Medium
The solar constant is the intensity of the Sun's radiation arriving perpendicularly to the Earth's ____ when the Earth is at its ____ distance from the Sun.
44.What is the value of the solar constant? Give your answer in W m⁻².
Easy
45.The average albedo of fresh snow is 0.85. Calculate the ratio: energy absorbed by fresh snow divided by energy reflected by fresh snow. Give your answer to 2 decimal places.
Medium
46.A planet receives a solar intensity of 2000 W m⁻² at the top of its atmosphere. Its albedo is 0.30. Calculate the intensity absorbed by the planet's surface, in W m⁻².
Medium
47.The solar constant at Earth is 1360 W m⁻². Mars is 1.52 times further from the Sun than Earth. Calculate the solar constant at Mars, in W m⁻², to 3 significant figures.
Medium
48.Name the greenhouse gas that is the best absorber of infrared radiation between 0.8 and 35 μm.
Easy
49.Complete the sentence about the molecular mechanism of the greenhouse effect.
Medium
Greenhouse gases have a natural frequency in the ____ region, so they absorb infrared radiation and begin to ____.
50.State the term for the ratio of the total scattered power to the total incident power of radiation reflected by a surface.
Easy
51.In an experiment looking at solar energy, the total incident power is 1500 W. The albedo of green grass is 0.25. Calculate the total scattered power when this light is incident on green grass. Give your answer in W.
Medium
52.Complete the definition of the solar constant.
Easy
The solar constant is the intensity of the Sun's radiation arriving perpendicularly to the Earth's atmosphere when the Earth is at its ____ distance from the Sun.
53.Define the solar constant.
Medium
54.The solar constant varies year-round for two main reasons. State the two reasons.
Medium
55.Climate change scientists are looking to reduce the number of greenhouse gases in the atmosphere. State the gases that are considered greenhouse gases.
Medium
56.Explain how the molecular structure of greenhouse gases leads to the greenhouse effect.
Hard
57.The Earth's surface has an average temperature of 288 K and an emissivity of 0.95. Calculate the power radiated per unit area by the Earth's surface. Use σ = 5.67 × 10⁻⁸ W m⁻² K⁻⁴.
Hard
58.In an experiment looking at solar energy, the total incident power is 1500 W. The albedo of green grass is 0.25. Calculate the total scattered power when this light is incident on green grass.
Medium
- A337 W
- B413 W
- C375 W
- D451 W
59.The average albedo of fresh snow is 0.85. Calculate the ratio: energy absorbed by fresh snow divided by energy reflected by fresh snow. Give your answer to 2 decimal places.
Medium
- A0.18
- B1.18
- C2.18
- D-0.82
60.A planet receives a solar intensity of 2000 W m⁻² at the top of its atmosphere. Its albedo is 0.30. Calculate the intensity absorbed by the planet's surface, in W m⁻².
Medium
- A1400 W m⁻²
- B1260 W m⁻²
- C1680 W m⁻²
- D1540 W m⁻²
61.The solar constant at Earth is 1360 W m⁻². Mars is 1.52 times further from the Sun than Earth. Calculate the solar constant at Mars, in W m⁻², to 3 significant figures.
Medium
- A589 W m⁻²
- B530 W m⁻²
- C707 W m⁻²
- D648 W m⁻²
62.In an experiment looking at solar energy, the total incident power is 1500 W. The albedo of green grass is 0.25. Calculate the total scattered power when this light is incident on green grass. Give your answer in W.
Medium
- A451 W
- B337 W
- C413 W
- D375 W

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