Global Warming Topic 8.5

The Sun The sun emits electromagnetic waves (gamma X-rays, ultra-violet, visible light, infra-red, microwaves and radio waves) in all directions.

The Earth Some of these waves will reach the earth (Losses due to inverse squared relationship)

Reflected Around 30% will be reflected by the earth’s atmosphere. This is called the earth’s albedo (0.30). (The moon’s albedo is 0.12) Albedo is the ratio of reflected light to incident light. 30%

Albedo The albedo depends on the ground covering (ice = high, ocean = low), cloud cover etc.

Absorbed by the earth Around 70% reaches the ground and is absorbed by the earth’s surface. 70%

Absorbed by the earth This absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum) Infrared 70%

Absorbed by the earth This absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum) Infrared = HEAT Infrared 70%

Black-body radiation Black Body - any object that is a perfect emitter and a perfect absorber of radiation Object does not have to appear "black" Sun and Earth's surface behave approximately as black bodies

Black-body radiation P = eσAT4 The amount of energy per second (power) radiated from a body depends on its surface area and absolute temperature according to P = eσAT4 where σ is the Stefan-Boltzmann constant (5.67 x 10-8 W.m-2.K-4) and e is the emissivity of the surface (=1 for a black object)

Example By what factor does the power emitted by a body increase when its temperature is increased from 100ºC to 200ºC? Emitted power is proportional to the fourth power of the Kelvin temperature, so will increase by a factor of 4734/3734 = 2.59

Surface heat capacitance Cs Surface heat capacitance is defined as the energy required to increase the temperature of 1 m2 of a surface by 1 K. Cs is measured in J.m-2.K-1. Q = ACsΔT

Example Radiation of intensity 340 W.m-2 is incident on the surface of a lake of surface heat capacitance Cs = 4.2 x 108 J.m-2.K-1. Calculate the time to increase the temperature by 2 K. Comment on your answer. Q = ACsΔT In time t, Q/t = ACsΔT/t Q/tA = CsΔT/t 340 = (4.2 x 108 x 2)/t t = (4.2 x 108 x 2)/340 = 29 days Sun only shines approx 12 hours a day so would take at least twice as long

Remember… The absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum) Infrared 70%

Absorption by the Atmosphere Various gases in the atmosphere can absorb radiation at this longer wavelength (resonance) H O They vibrate more (become hotter) O What happens to the gases when they absorb the radiation? Vibrate more Become hotter No change A & B C H H C H H H O

Greenhouse gases These gases are known as “Greenhouse” gases. They include carbon dioxide, methane, water and N2O. O H O H H C H H C H O

Balance There exists a balance between the energy absorbed by the earth (and its atmosphere) and the energy emitted. Energy in Energy out

Balance This means that normally the earth has a fairly constant average temperature (although there have been big changes over thousands of years) Energy in Energy out

Balance Without this normal “greenhouse effect” the earth would be too cold to live on. Is the greenhouse effect good for us? Why? Yes No Energy in Energy out

Bath balance It’s a bit like trying to add water to a bath with the plug hole open.

Bath balance The water reaches a certain level but no higher because the amount of water leaving eventually equals the amount entering

Bath balance The water reaches a certain level but no higher because the amount of water leaving eventually equals the amount entering Energy entering the earth Earth’s temperature Energy leaving the earth

Greenhouse gases Most scientists believe that we are producing more of the gases that absorb the infrared radiation, thus upsetting the balance and producing a higher equilibrium earth temperature.

Bath with the plug half in! This is like trying to fill a bath, but now with the plug half covering the hole.

A new balance Eventually the water will reach a steady level again, but higher than before. New HIGHER equilibrium temperature

What might happen?

Evidence? Ice core research Weather records Remote sensing by satellites Measurement!