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Can longwave radiation heat the oceans?

  1. Dec 12, 2014 #1
    Thats my simple question!
     
  2. jcsd
  3. Dec 12, 2014 #2

    Bystander

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    Is there some reason it shouldn't?
     
  4. Dec 12, 2014 #3
    Hi bystander - I read that infra red could not penetrate the ocean but caused evaporation on the surface skin. If that is true then infra red would not be able to heat the ocean like short wave radiation does. Is that true? thanks
     
  5. Dec 12, 2014 #4

    Bystander

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    The enthalpy of vaporization has to come from somewhere --- you don't suppose IR absorption by water might be the source?
     
  6. Dec 12, 2014 #5
    Absolutely is the source in this example. But the energy from the incoming IR is not retained by the ocean as heat - its released through evaporation into the atmosphere. So is it correct to think that IR cannot heat (i.e. increase the temperature) of the ocean?
     
  7. Dec 12, 2014 #6

    Doug Huffman

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    No. IR does heat water but at a very low rate, with not much energy compared to temperature coefficient of water.
     
  8. Dec 12, 2014 #7

    Bystander

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    Consider also, conduction of heat from the "warm" surface to cooler water layer below the surface. To quote a faculty member from grad school days, "Every calorie looks the same once it's off the bus." Assigning origins and destinations to energies can be misleading.
     
  9. Dec 12, 2014 #8
    Thanks Doug. The IPCC estimates that a doubling of CO2 in the atmosphere would increase radiative forcing by 3.7W/m2 assuming clear sky conditions. This could only heat water in the top few molecules initially if water is as impermeable to infra red as I understand. Then as Bystander says any heating at the surface which does not evaporate surface skin molecules could be conducted to (or just mixed with) deeper levels. Could you point me in the direction of calculations as to what the magnitude of this heating might be? I completely understand that measuring this through real world observations must be very difficult as the calories are off the bus by then, but someone must have modelled this or tested this in a controlled environment?
     
  10. Dec 12, 2014 #9

    Doug Huffman

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    The IPCC spins at best and lies at worst. Impeached.
     
  11. Dec 12, 2014 #10

    Bystander

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    Quick and dirty demonstration? Two liter soda bottle filled with water; measure T in the morning; leave in sun all day; measure T in late afternoon.
     
  12. Dec 12, 2014 #11
    But sunlight is Short wave radiation ! Bystander - the experiment you suggest would only tell me what I already know that: short wave radiation heats water. I want to understand if infra red can heat water!!
     
  13. Dec 12, 2014 #12

    Doug Huffman

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  14. Dec 12, 2014 #13
    Bystander - I found the following passage on a CAGW skeptic's website (http://climaterealists.com/index.php?id=4245)
    "However the effect of downwelling infrared is always to use up all the infrared in increasing the temperature of the ocean surface molecules whilst leaving nothing in reserve to provide the extra energy required (the latent heat of evaporation) when the change of state occurs from water to vapour. That extra energy requirement is taken from the medium (water or air) in which it is most readily available. If the water is warmer then most will come from the water. If the air is warmer then most will come from the air. However over the Earth as a whole the water is nearly always warmer than the air (due to solar input) so inevitably the average global energy flow is from oceans to air via that latent heat of evaporation in the air and the energy needed is taken from the water. This leads to a thin (1mm deep) layer of cooler water over the oceans worldwide and below the evaporative region that is some 0.3C cooler than the ocean bulk below."
    The last sentence does seem to be validated with this paper: http://www.nature.com/nature/journal/v358/n6389/abs/358738a0.html
    But is the rest fair?
     
  15. Dec 12, 2014 #14
    Doug - I'm not sure I'm reading your graph correctly but it does seem to show that sunlight at sea level is a mixture of UV, visible and infra red wavelengths. If so - putting a bottle of water in the sunlight and measuring the temp change over a day isn't going to tell me anything about the effectiveness of IR in heating water
     
  16. Dec 12, 2014 #15

    Bystander

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    The shortwave (visible) is going right on through. That's observation one. A day in the sun will bring the bottle up to 60-70 C. That's observation two. From the Planck radiation law 75% of the energy in sunlight is transmitted at wavelengths longer than the peak intensity wavelength, 500 nm (red).
     
  17. Dec 13, 2014 #16
    Dear Bystander - I appreciate your patience with me on this and I am grateful. That said, a bottle in the sunlight "experiment" is a blind alley if we're trying to understand the oceans. Low evaporation rates, the conduction of heat through the plastic of the bottle and diffraction of the light through the curved surfaces of the bottle all make such an experiment an extremely poor way to model the ocean temperature / IR relationship. Furthermore your comment on Planck radiation law calculations does not address this question if incoming IR causes evaporation in the ocean skin layer rather than an increase in the temperature of the ocean.

    To get back on track do you agree that this graph is representative of the absorption spectrum of liquid water?
    http://en.wikipedia.org/wiki/Electr.../File:Absorption_spectrum_of_liquid_water.png

    If so - its clear that IR does not penetrate below 1cm and the wavelengths of back radiation from the atmosphere penetrate much less than that (e.g. 1/10^5 metres).

    As per the nature article I referenced above (http://www.nature.com/nature/journal/v358/n6389/abs/358738a0.html) the top 1mm of the ocean is typically 0.3C cooler than the bulk mixed layer. Forgive me for being slow - but if most of the total IR radiation and all of the back radiation from the atmosphere penetrates less than the depth of the ocean skin layer that is cooler than the water below - how can IR increase the temperature of the ocean?

    These observations suggest to me that almost all of IR radiation incident on the ocean causes evaporation rather than an increase in temperature of the ocean. Where am I going wrong?
     
  18. Dec 13, 2014 #17

    Bystander

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    ... not "wrong," into a "semantic ditch," perhaps. If you're going to give me all the solar radiation that penetrates further than 1 mm by the Kebes plot (shorter than 2 μm), you've given me 80 - 90% of the IR. If you define IR as only that radiation that is absorbed in 1 mm or less, and ignore the 0.8 - 2 μm gap between visible and IR acknowledged by a specific argument, you're losing a lot of energy.
     
  19. Dec 14, 2014 #18
    Thanks Bystander. I think I get where you're coming from. I'll give you whatever IR you want! Thanks to this discussion I think I can now refine my original question a little more clearly:

    Can an increase in Atmospheric back radiation (from say increases in atmospheric concentrations of greenhouse gases) lead to increases in ocean temperatures?

    For anyone else that is interested in this topic I found this set of articles (and associated comments) really useful.

    As with most other discussions in climatology the answer isn't simple . . .
     
  20. Dec 14, 2014 #19

    Bystander

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    What is "back radiation?"
     
  21. Dec 14, 2014 #20
    its downward longwave radiation. IR radiating from the atmosphere down to the surface of the earth. Not IR direct from the sun.
     
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