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How does Earth cool itself?

  1. Aug 20, 2010 #1
    So we humans are burning fuel releasing heat and energy into the atmosphere. (Let's not bring problems about global warming into the topic) And, so there is the major source of energy from the sun's radiation which hits on Earth by adding more energy and heat into the system.

    I know that some of that sun's radiation is gets reflected away by clouds and when it hits Earth's surface. However, there is a certain amount of energy that is transferred to the Earth when it hits the surface which is not reflected. Excluding the part of global warming where clouds impede the reflected radiation to come out (which makes Earth harder to cool down), how Earth cools itself out?

    We might even skip the part that we humans are releasing energy to the surroundings since the amount of energy supplied by humans might be almost insignificant compared to the sun's radiation i still don't get how the Earth cool itself down since most of the reactions that happens in earth is Exothermic and not Endothermic.

    Edit: I read that Earth cools itself down by melting the ice on the poles. But doesn't that mean the total energy in Earth will just increase over time regardless?
     
    Last edited: Aug 20, 2010
  2. jcsd
  3. Aug 20, 2010 #2

    phyzguy

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    The Earth is cooled by radiating energy into space. Since the surface temperature of the Earth is ~300K, most of this radiated energy is in the form of infrared radiation. Look up blackbody radiation. You can even calculate (roughly) Earth's surface temperature by calculating how much energy is received from the sun, and assuming that an equal amount is radiated away. If you assume that the Earth is a blackbody ( it isn't, but this is a good first assumption), you can use the Stefan-Boltzmann law to calculate what the temperature needs to be in order for the energy absorbed from the sun to equal the energy radiated away. Why don't you try the calculation?
     
  4. Aug 20, 2010 #3
    So the idea is that outer-space is mostly dark compared to the Earth specially at night so the space becomes the black body in which Earth radiates to right?
     
  5. Aug 20, 2010 #4
    I don't think space can be called a black body, as it is essentially nothing. Thermal radiation doesn't require anything to radiate to. EM radiation is emitted in all directions from the Earth at all times day and night. That EM radiation carries away with it all the energy that was gained from the Sun (or any other source).
     
  6. Aug 21, 2010 #5

    vanesch

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    Although Dale is right in that strictly speaking "outer space" is not a black body as such (not a set of radiators in thermal equilibrium with a heat reservoir), it can actually be considered as such in exactly that way. The cosmic microwave background makes outer space actually equivalent to a black body with a temperature equal to something like 4 Kelvin.
     
  7. Aug 26, 2010 #6
    The black body temperature of Earth is -18C.

    Water Vapor, methane, CO2 and the like conspire to trap some of the Solar radiation allowing the planet to actually be inhabitable by other than Walruses and Reindeer. Excess heat is radiated into space. The Earth is not a closed system.
     
  8. Sep 6, 2010 #7
    Keep in mind that much of the radiation reaching earth is not converted into heat directly because plants store it in the form of the electrical bonds that hold complex carbon molecules together. Animals or combustion may eventually convert part of this energy into heat.

    Cold blooded animals convert solar heat into the kinetic energy used to move them around.

    The ground and water transfer heat energy through conduction to the air which converts heat energy into potential energy as it rises. This potential energy generally is generally not converted back into heat energy when it becomes kinetic energy in the form of down drafts.

    The heat energy conducted to the air substantially reduces the amount of energy that can be radiated into space. The equation uses the temperature in degrees Kelvin to the 4th power. At a temperature of 301 K (28 C) reducing the temperature by only one degree to 300 K reduces the value of the energy converted to radiation by over 100m ( a little over 4 times the lower number cubed - 100 cubed is a million) The difference in radiation between normal earth ground temperature and what the temperature could be increases rapidly as the temperature gets larger. Without an atmosphere the ground temperature might reach the same temperature as the surface of the moon which reaches over 373 K. In short, the atmosphere doesn't reduce the energy radiated into space by blocking or reflecting that radiation, but by reducing the amount of energy converted to radiation.

    Bodies of water (along with wet ground and plants and animals) releases heat energy into the atmosphere through evaporation. Most of this heat energy is in the form of the heat of vaporization which is at least 540 times more heat than what is required to raise the temperature of the same amount of water 1 C.

    Some of this energy is converted back into heat in the form of lightning, including the blue jets above clouds, but most is only converted into kinetic energy. Some of this kinetic energy may be converted into heat after being converted to electricity by hydroelectric power plants.
     
    Last edited: Sep 6, 2010
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