Briefing Note 1 of 4 Basics of Global Warming

Dr. John Hollins, past Chair CACOR Board of Directors,  provides a basic overview of the physics of global warming.

What we know from 150 years of the study of physics

•   The average temperature of the surface of planet Earth is determined by the amount of radiant energy (sunshine) that it absorbs from the sun and the amount of energy that it sends back to space.
  • Energy can neither be created nor destroyed, so the relationship between incoming and outgoing energy is like a financial account that must balance – deficits and surpluses are not allowed!

• The energy from the sun arrives in the form of electromagnetic radiation. The spectrum of the radiation is determined by the temperature of the surface of the sun (about 5,500 °C); it spans a wide range of “colour”: infrared radiation, visible light, ultraviolet radiation, and X-rays.o   The amount of energy arriving at the top of the atmosphere of the Earth varies slightly because the sun is a turbulent ball of energetic particles and the orbit of the Earth is not a perfect circle.o   The amount of incoming energy absorbed at the surface of the Earth depends on the amount of incoming radiation that is reflected back to space by clouds, aerosols (particles from volcanoes and pollution), and the surface of the Earth; examples:
  • arctic ice reflects most of the incoming sunshine;
  • forested areas in summer absorb most of it.
• The energy sent back to space from the surface of the Earth is determined by two factors:

1. The temperature of the surface of the Earth (average now about +15 °C) determines the amount and the spectrum (colour) of the radiation (energy) emitted.

• The spectrum of the Earth’s outgoing radiation is in the infrared — no visible light or more energetic radiation.

2. Some of the outgoing infrared radiation can be absorbed and then re-emitted by compounds known as greenhouse[1] gases: When radiation heading straight out to space from the surface of the earth is absorbed by a molecule of a greenhouse gas in the atmosphere, the energy absorbed “excites” the molecule.

• The molecule subsequently “relaxes” by re-emitting the same amount of energy, but in all directions:
  • half continues out to space; the other half returns to the surface of the Earth,
  • thereby increasing the amount of energy arriving at the surface of the Earth, raising its temperature.
• The naturally occurring greenhouse gases in the atmosphere of the Earth are carbon dioxide and water vapour.
  • The natural carbon dioxide was originally emitted — and still is from time to time — by volcanoes[2]. This natural carbon dioxide controlled the greenhouse effect before the advent of the industrial age.
• Without the greenhouse effect of the natural carbon dioxide in the atmosphere, the average temperature at the surface of the planet would be -18 °C:
  • all the water on the planet would be frozen — oceans and lakes — and there would be no water vapour in the atmosphere to add to the greenhouse effect of the carbon dioxide.
  • Life as we know it today would not exist.
• By way of comparison, the surface temperature of the moon is -23 °C:
  • it has no atmosphere, so no greenhouse effect; in addition,
  •  its surface reflects more of the incoming energy back to space than Earth’s living surface.

Conclusion: The greenhouse effect is real; it is controlled by the total amount of carbon dioxide in the atmosphere, derived today from both natural sources and human activity.

 

[1] “Greenhouse” gas is not a particularly good metaphor. Glass does allow sunshine to pass though and it does absorb some of the infrared radiation emitted by the warm interior of the greenhouse on its way out. But glass is also a physical barrier to the movement of warm air, which is not the case for the atmosphere. The term “heat-trapping” gases is sometimes used, but, imperfect or not, “greenhouse gas” is the term that is in general use

[2]  Humankind emits nowadays about 100 times as much carbon dioxide as volcanic activity.