Making Sense of our Climate Change: 3. The Science of Global Warming

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Parts 1 and 2 may be accessed here.

The Sun and the earth's atmosphere are the main players in determining the surface temperature of the earth. The primary source of energy is the Sun while the atmosphere acts like a blanket to keep some of the energy trapped on earth and maintain its temperature at about 14C.  Without the atmosphere, the temperature of the earth will be -18C and life on earth will be nearly impossible.  

A complex set of physical processes on earth's surface (land and water) and in atmospheric gases determine the equilibrium temperature of the earth. Earth's temperature will stay constant if total energy received by the Earth equals total energy lost by it. 
It is a question of energy budget - any imbalance will heat or cool the Earth - its climate will change.

Slide 1 shows a simplified picture of the energy budget of the earth 
SLIDE 1:

Energy from the Sun arrives mainly in the form of visible and infra-red (IR - longer wavelengths) radiation and has to pass through the atmosphere before reaching the earth.  Atmospheric gases are largely transparent to visible light and most of it reaches the earth's surface where about 50% of it is reflected back into space, the rest is absobred by water and land. Clouds do reflect some of the solar energy back into space. 
In its turn, the earth radiates energy in the form of IR that is efficiently absorbed by the greenhouse gases (GHG) in the atmosphere (water vapour, carbon di-oxide, methane and nitrous oxide).  GHGs then emit the absorbed  energy isotropically (equally in all directions) and a large fraction of the energy is radiated towards the earth where it is readily absorbed.  It is this 'blanketing effect ' of the atmosphere that keeps the earth at 14C instead of -18C that it would be in absence of the atmosphere.  
Some energy is also transferred to the atmosphere from the earth by water evaporation and warmer gases rising into the atmosphere.  
I have drawn Slide 2 to show the main processes involved in determining the earth's energy budget and slide 3 shows the Greenhouse effect - it makes the earth about 30C warmer:

SLIDE 2: Earth's Energy Budget

Slide 3: The Greenhouse Effect

The Science of Greenhouse Effect:

Let us look at the science behind the processes described in Slide 2 leading to the greenhouse effect.  A large number of science concepts are required - thankfully at the school science level.  We start by stating that the radiation energy from the Sun is in the form of electromagnetic waves with wavelengths in the ultraviolet (UV), visible and infrared (IR) region (Slide 4).

SLIDE 4: 

UV rays are the most energetic and exposure to them over extended periods is harmful.  The ozone layer in the top atmosphere absorbs most of the UV and shields us from their harmful effects.

All bodies radiate energy in the form of electromagnetic waves  -  the energy radiated per second (power) dependes on the surface area and temperature of the body (Stefan-Boltzmann Law) and the range of wavelengths emitted is given by the Planck distribution curve.  Note that the temperature is the absolute temperature of the body in unit of Kelvin and is related to degree centigrade as:  T (Kelvin) = T (C) + 273.  The situation is demonstrated in Slide 5.

SLIDE 5:  

The total energy radiated per second from a given area increases very rapidly as the temperature, T, of the body increases - as fourth power of T.  Doubling the temperture increases the energy emitted by a factor of 16.  Sun's surface is about 20 times hotter than the earth; and per unit area, the Sun radiates 160,000 times more energy.  Also the Sun is much bigger than the earth. 
We also notice that the wavelength at which peak energy is radiated depends on the temperature of the emitting body. There is a simple relation called the Wien's displacement law that tells us where the maximum intensity of the Planck curve is:

Wavelength (max intensity) in microns = 2898/Temperature in Kelvin

Remember:  1 micron = millionth of a meter = 0.001 mm; and T (Kelvin) = T(C) + 273

We can summarise the discussion by plotting the spectral distribution of radiation from the Sun and the earth. In Slide 6, the Sun's radiation intensity has been scaled down to compare its shape with the earth's emitted radiation.  The main take away is that the Sun radiates in regions around the visible wavelengths while the earth's radiation is totally in IR extending from 4 to 70 micron wavelengths. 

SLIDE 6:  

  Earth's Atmosphere

The earth is surrounded by gases to a height of about 100 km.  Most of the mass of the atmosphere is in the first 20 km and this lower part of the atmosphere plays the pivotal role in determining the climate of our planet.  The main constinuents of the atmosphere are gases and some fine particles (aerosols).  I refer you to my lecture on the atmosphere for a detailed discussion (click here) and restrict the current discussion to the way greenhouse gases (GHGs) help to warm the earth.

The main GHGs are water vapour, carbon di-oxide, methane, nitrous oxide and ozone. Gases consist of a large number of molecules which are capable of absorbing electromagnetic radiation of specific wavelengths - each molecule has its own fingerprint called the absorption band. For radiation (from the Sun or the earth) passing through the body of a gas (as in our atmosphere), the gas is transparent to wavelengths which do not lie in the absorption band of this particular gas.  Wavelengths which are in the absorption bands are absorbed by the gas molecules - what fraction of radiation is actually absorbed depends on the number density of the molecules (concentration og the gas in the atmosphere) and on the probability of absorption (called the cross-section). Slide 7 shows how this works:

SLIDE 7: 

We notice that different GHG gases absorb radiation at selected wavelengths.  Earth radiation is centered around 15 micron where carbon di-oxide has strong absorption bands.  Water vapour is also effective at longer wavelengths at the upper end of the earth's radiation. Slide 8 gives a more detailed breakdown of absorption efficiencies of different GHGs and the total for the atmosphere.  For more details click here.

Slide 8:

This completes the objectives of this part.  The role of water vapour is  rather complex as its amount changes regularly and it is expected that this change will continue in the future.  This essentially assumes that water vapour will continue to affect the warming of the earth the same way as it has done in the past.  However, one needs to notice that in the global warming scenario, oceans will evaporate more water and possibly there will be more water vapour that can accelerate global warming.  A counterbalance to this might be provided by increased cloud cover - clouds reflect Sun's radiation and prevent it from reaching the earth that might result in a net cooling effect.  Water is a potent GHG and there is much controversy about its overall effect on global climate. I have discussed the relative roles of water and carbon di-oxide in global warming in a separate publication.

For a discussion of different types of clouds and how they affect solar radiation, you can look at my lecture by clicking here.  
The other important GHGs are carbon di-oxide and methane; for the past 200 years the concentration of these has gone up significantly.  Research has established that they are put into the atmosphere by burning fossil fuels and other human activities.  Similarly much increased use of fertilisers has caused an increase of another potent GHG - nitrous oxide.

In the next part (part 4) we shall look at some of our planet's vital signs that are affected and may even be used to quantify the warming.  

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