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As I understand it, greenhouse gases absorb infrared radiation from the sun. Much of that radiation would otherwise continue and be absorbed on the planet surface in the ocean or on land.

If CO2 increases are causing the oceans to warm, does that happen mainly by convection? It's counter intuitive to me to think about air convection having that much effect on ocean temperatures. Why wouldn't greenhouse gases' soaking up of infrared instead cool the oceans, that would otherwise be warmed by the radiation?

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No, greenhouse gases do not absorb infrared radiation from the sun... the Earth is really the source of infrared. The amount of infrared energy from the sun that reaches Earth is insignificant. Visible light from the sun heats the Earth, NOT infrared light. The visible light passes through the atmosphere and is absorbed by the surface of the Earth. Then, the Earth re-radiates that energy back into the atmosphere in the form of heat (aka infrared radiation). The infrared radiates back into space, but if there are infrared absorbers in the atmosphere, the energy is not lost to space. That has a net warming effect. In fact the Earth needs greenhouse gases, or we would all be frozen. It is a delicate balance between GHG concentrations and climate effects on the hydrologic cycle.

It is important to note that ocean water expands in response to temperature increase... so even just a modest increase in ocean temperature can lead to significant sea level changes. Human civilization is thus directly affected by oceanic temperature increase due to sea-level rise.

As a side note, the absorption of carbon into the ocean is really of more environmental concern than the actual heating of the ocean, in terms of acidic effects on the marine ecosystems. The ocean absorbs roughly half the carbon emitted into the atmosphere and there are areas already seeing the negative effects of acidification.

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  • $\begingroup$ Very clearly explained, thank you. Only part that makes me scratch my head is your statement that the atmosphere gets more infrared from the earth than it gets from the sun. Is that also true in any given area during the daytime? I found an image online that looks like what you're talking about: eesc.columbia.edu/courses/ees/slides/climate/blackbody.gif -- however since the sun's blackbody radiation spectrum has a factor of 10^-6 I don't know how to compare the two parts. $\endgroup$
    – WolfRevokCats
    Mar 27, 2015 at 20:35
  • $\begingroup$ I (partly) remember a physics calculation we had to do at school: If there was not CO2, what would the average temperatur be? IIRC, that was somewhere around -10 Centigrade $\endgroup$
    – Jan Doggen
    Mar 28, 2015 at 10:40
  • $\begingroup$ @WolfRevokCats all objects with heat emit thermal radiation. en.wikipedia.org/wiki/Thermal_radiation the hotter the object the higher the wavelength range. Infra-red is also a very broad range. pro-therm.com/images/infrared_basics_figure2_large.gif The sun almost certainly hits the earth with more short-wave infra-red than the Earth emits, but the Earth emits a whole lot more long-wave infrared than it's hit by from the sun. In fact, we probably get more long wave infra-red from the Moon than the Sun. $\endgroup$
    – userLTK
    Nov 14, 2015 at 22:44
  • $\begingroup$ this is actually why it is called a green house effect, glass lets visible light through but tends to reflect infrared. so the energy from incoming light passes through glass and warms the soil inside. the soil gives off infrared which is reflected by the glass keeping the soil warm. $\endgroup$
    – John
    Feb 21, 2017 at 1:37
  • $\begingroup$ @John. It is a very common misconception that a real greenhouse (glasshouse) works because of a one way diode type effect in the glass with respect to I.R. This is nonsense and has been verified as such. A glasshouse gets hot because the physical structure inhibits convective loss, absolutely NOTHING to do with the frequency response of a glass sheet. $\endgroup$
    – user7733
    Apr 17, 2017 at 0:03
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As I understand it, greenhouse gases absorb infrared radiation from the sun.

That's not correct. The atmosphere is more or less transparent to the incoming solar radiation. About 29% of the incoming solar radiation is reflected back into space (that's the Earth's albedo). The remaining 71% is absorbed. Clouds and the atmosphere are responsible for a bit less than a third of that absorption. The other two thirds is absorbed by the Earth's surface.

This absorption is not where the greenhouse gases come into play.

The greenhouse gases instead come into play as a multi-layered blanket that keeps the surface of the Earth from cooling off. By way of analogy, suppose you went camping in the desert. While deserts can get quite hot during the day, they get surprisingly cool at night. A blanket protects you against those cool desert nights. While the blanket doesn't generate heat, it very much does slow down the heat transfer. It does this by emitting half of its thermal energy upward, half downward. This back-radiation makes your body remain warm. Add more blankets and you get even more protection.

While our atmosphere is more or less transparent in the visible range, the greenhouse gases make it opaque in thermal infrared. It is so opaque in the thermal infrared that our atmosphere acts as the equivalent of a multi-layer blanket. The solar radiation absorbed by Earth's surface is a bit less than 1/3 of the total energy received by the surface. The other two thirds is back-radiation from the Earth's atmosphere.

This topic is the subject of the Earth's radiation budget. Thanks to satellites, atmospheric scientists have been investigating the radiation budget for about half a century.

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  • $\begingroup$ Just making it clear (yet again unfortunately0 that the Trenberth diagram above of radiative flux in w/m^2 are NOT a HEAT or ENERGY transfer diagram but a Radiative Flux diagram. There is a big difference. People are often confused by this. For example the "back radiation" above is not transfering heat from the cold atmosphere to the warm surface. Heat transfer is only one way (hot to cold). Radiative fluxes flow in every direction from everything at every temperature. It is NET radiative flux in specific direction that determines HEAT flow. $\endgroup$
    – user7733
    Apr 17, 2017 at 0:08
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    $\begingroup$ @user7733 -- What you wrote is incorrect. You are correct that NET heat transfer is always from hot to cold. However, a cooler object can transfer heat to a warmer object so long as the heat transfer rate from the warmer object to the cooler exceeds that from the cooler object to the warmer one. $\endgroup$
    – David Hammen
    Apr 17, 2017 at 6:00
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If CO2 increases are causing the oceans to warm, does that happen mainly by convection, then? It's counter intuitive to me to think about air convection having that much effect on ocean temperatures. Why wouldn't greenhouse gases' soaking up of infrared instead cool the oceans, that would otherwise be warmed by the radiation?

In a nutshell, and as pointed out above, CO2 and the greenhouse effect primarily warms the lower atmosphere. After that it's all circulation. Warm air over the colder ocean is the cause for heat transfer into the ocean. The same thing happens every summer, when the warm air gradually warms up the ocean - which is why the ocean is colder in June than September. Warm air warms the ocean - but it takes time. Cold air cools the ocean - this also takes time. The ocean is a very effective heat sink.

It's the heat transfer between the air and the ocean that causes most of the heat from climate change to go into the ocean. It's not correct to say the ocean is warming more, cause it's actually warming much more slowly, but it takes about 1000 times as much heat to warm the top half of the ocean than it does to warm the troposphere. So the ocean ends up trapping 90% of the heat trapped from man made climate change.

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  • $\begingroup$ Hi, thanks for engaging in your answer on what was to me the most interesting aspect of this, which was asking how the extra heat from CO2's infrared absorption gets into the ocean. I think you are saying that it is mostly by convection, correct me if I'm wrong. All the answers had helpful info and corrected misunderstanding(s) on my part, but I have to pick one, so picking yours. $\endgroup$
    – WolfRevokCats
    Mar 28, 2015 at 23:14
  • $\begingroup$ Temperature transfer between atmosphere and ocean is partially convection and partially evaporation/condensation. Also, the Strengh of the ocean current plays an important role. El Nino / La Nina. El Nino is associated with a warmer current on the top of the mid-pacific. La Nina, a colder current. A colder top of the pacific will take more heat from the air, while a warmer one will take less. That's why El Nino years are hotter than La Nina years - so the ocean current plays a key role in how much heat the ocean takes in, but yes, it is primarily convection that transfers the heat. $\endgroup$
    – userLTK
    Mar 29, 2015 at 1:54
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The global mean SST is 16.1 °C (60.9 °F) while the global mean surface air temperature is 14°C (57°F) so the net heat flux is from the ocean to the air. Since the surface air temperature has been rising faster than the sea surface temperature the rate of heat flux from ocean to air has decreased. Since the sun continues to heat the oceans at the same rate then the total heat in the oceans increases.

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Water vapor certainly does absorb solar infrared radiation in the near infrared spectrum and mostly by water vapor - the graph shows this clearly.

GHG's heat the cool skin layer of the ocean - they cannot penetrate any more than a hairsbreadth so they simply promote evaporation and cooling via latent heating of the atmosphere.

http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png

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  • $\begingroup$ The picture you linked has nothing to do with the 2 sentences you wrote. $\endgroup$
    – userLTK
    Apr 15, 2017 at 3:03

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