B2 greenhouse effect

stefan-boltzmann law

power radiated by object is directly proportional to the fourth power of its surface temperature in absolute scale

emissivity

ratio (not %) of the total power radiated by an object to that emitted by a black body at the same temperature

(water, ice and vegetation have high emissivity of around 0.95)

albedo

fraction of the radiation received by a planet that is reflected back into space.

= total reflected power / total incident power

S

incident solar power per unit area (Wm^-2) aka solar intensity

solar constant depends on weather conditions and latitude (since different parts of earth’s surface receive different amounts of solar radiation). constant in db is for ideal case where solar radiation is incident perpendicular to earth.

see q7b(ii) of timed practice

energy balance climate model

at equilibrium temperature,

incoming solar energy = outgoing solar energy

power (intensity) absorbed by earth’s surface = power (intensity) radiated by sun

• used to estimate temperature of earth’s surface

• assumes no radiation from earth’s atmosphere → calculated temp is lower than actual

greenhouse effect on ice

• ice melts

• less power reflected

• albedo decreases from 0.9 to 0.06

• temperature increases

• less ice

molecular resonance

• greenhouse gas molecules in atmosphere have highly flexible structures → absorb infrared radiation and vibrate

• natural frequencies of vibration are same as infrared radiation frequencies → infrared radiation emitted when vibrate

• therefore can absorb and re-emit infrared radiation (molecular resonance)

• trap heat, greenhouse effect

greenhouse effect of greenhouse gases

• natural resonant frequency same as infrared, molecules will vibrate at that frequency

• readily absorb infrared radiation radiated from earth

• re-radiate the energy in random directions back to earth

absorption spectrum

shows frequencies/wavelengths of infra-red radiation absorbed. absorptivity against wavelength.

solutions to enhanced greenhouse effect

• more efficiency of power production

• replace coal and oil with natural gas

• use combined heating and power systems

• increase use of renewable energy sources and nuclear power

• use electric vehicles

• carbon dioxide capture and storage

wien’s law

power absorbed by earth

power = S(1-α) π r²

NOT IN DATA BOOKLET

where α is the albedo of earth, S is the solar intensity, r is the radius of earth

derive from power = intensity x area

intensity = average solar intensity incident on earth’s surface

incident area = area of disc

average solar intensity incident on earth’s surface

= ¼ S(1-α)

where α is the albedo of earth, S is the solar intensity, r is the radius of earth

NOT IN DATA BOOKLET

since only the circle surface (1/4 of total surface area)

timed practice q4

timed practice q2

power radiated by earth

= e 4pi r² sigma T⁴

note the A used is area of sphere since whole sphere radiating

average temperature of earth

NOT IN DATA BOOKLET

which feature of the graph explains greenhouse effect?

• IR region is sheet 0.78 (IR has low frequency, high wavelength)

• sharp troughs at certain points of graph → absorbed at specific wavelengths