Greenhouse Effect and the Global Energy Balance

Transcription

1 Greenhouse Effect and the Global Energy Balance

2 Energy transmission ( a a refresher) There are three modes of energy transmission to consider. Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion. Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy. Radiation: the propagation of electromagnetic waves through space.

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4 Convection

5 ELECTROMAGNETIC RADIATION how the earth receives the sun s energy

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7 Blackbodies if an object has a temperature greater than 0K, it has a blackbody curve Temperature tells us 1. How much energy the body emits 2. At what wavelength of radiation most of the energy is emitted E = σt 4 λ max = 2897/T Sun s blackbody curve Blackbody curve for the sun

8 Balancing Act ABSORPTION, EMISSION, and EQUILIBRIUM the idea of a blackbody The earth absorbs all incident radiation and reemits at longwave radiation (thermal) Wien's Law -- curves the wavelength of energy emitted the most

9 Balancing Act ABSORPTION, EMISSION, and EQUILIBRIUM the idea of a blackbody The earth absorbs all incident radiation and reemits at longwave radiation (thermal) This occurs at equal rates (think day and night) As such, earth is said to be at radiative equilibrium temperature 255K (-18C, 0F) Earth s observed average surface temperature is actually 288K (15C, 59F)! WHY? Lets track the incoming radiation

10 The atmosphere selectively absorbs and emits radiation

11 Atmospheric composition PERMANENT GASES

12 VARIABLE GASES Atmospheric composition

13 A blackbody without an atmosphere All incoming (incident) radiation is absorbed an equilibrium temperature is reached and equally, outgoing energy is emitted at a different wavelength

14 A blackbody with an atmosphere. Note the increased average surface temperature which gases are responsible?

15 Atmospheric composition VARIABLE GASES and GREENHOUSE GASES

16 Recall incident radiation, as it passes through the atmosphere, is both reflected and absorbed

17 Of the solar radiation absorbed by the surface and the atmosphere, there exists a net gain Absorbed by greenhouse gases NET GAIN Because the earth radiates day and night

18 We can observe that modes of energy transfer play a role in the warming of the atmosphere Earth s surface receives 147 units from the sun and the atmosphere Earth s surface radiates 117 units net gain = 30 units The atmosphere receives 130 units 19 units from the sun and 111 units from the earth The atmosphere loses 160 units net loss = 30 units That loss is the gain, i.e., the warming of the atmosphere 30 units = 7 units (conduction, convection) + 23 units (latent heat)

19 THE ATMOSPHERIC GREENHOUSE EFFECT How the atmosphere absorbs energy from the sun and the earth The y-axis is the percent of radiation absorbed by the atmosphere The x-axis is the wavelength of radiation absorbed short to long wave Other contributions

20 THE ATMOSPHERIC GREENHOUSE EFFECT How the atmosphere absorbs energy from the sun and the earth If you add those individual contributions together you get

21 THE ATMOSPHERIC GREENHOUSE EFFECT How the atmosphere absorbs energy from the sun and the earth SUN s ENERGY EARTH S ENERGY

22 THE ATMOSPHERIC GREENHOUSE EFFECT So, let s review, sequence of steps: 1. Solar radiation absorbed by earth s surface. 2. Earth gives off infrared radiation. 3. Greenhouse gases absorb some of the Earth s infrared radiation. 4. Greenhouse gases (water and CO 2 ) give off infrared radiation in all directions. 5. Earth absorbs downward directed infrared radiation Result: warmer surface temperature

23 Putting it together Activity: Drawing the Earth s energy balance

24 Remember incident radiation, as it passes through the atmosphere, is both reflected and absorbed Let s examine the reflected part

25 Taking a closer look at incoming solar radiation Each beam of incoming sunlight can be either: Reflected back to space: ALBEDO Clouds Atmosphere Surface Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth s surface. Activity time

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