Can Global Warming Cause

[mheslep]

...Atmospheric CO2 has increased due to human activities. Not much question about that.

Atmospheric CO2 is without a doubt a significant absorber of infrared radiation contributing approximately 12% to the global greenhouse effect.

Do you have source for that 12%? I read numbers as low as 2% (Lindzen IIRC).

A doubling of the stuff will impose an additional positive radiative forcing of 3.7W/m^ within the troposphere.

Careful. As stated this implies that is all directly from CO2. The ~4W/m^2 figure is for CO2 and ALL other sources combined, including feedbacks. Radiative forcing directly from increased retention of longwave radiation due to CO2 alone is much less:
http://www.ipcc.ch/graphics/graphics/syr/fig2-3.jpg [Broken]

...The Earth's surface receives nearly twice the warming radiation from it's own atmosphere than it does directly from the Sun.

Huh? 1266 W/m^2 space, nearly 1000 W/m^2 insolation received at the surface, low latitudes. Where is there room for 'twice' more radiation from the atmosphere?

Direct insolation has not increased over the past 50 years, the very period of greatest warming.

I've always wondered over what part of the EM spectrum solar radiation is measured.

Coupled atmosphere/ocean phenomena such as the PDO, ENSO and AMO etc. are examples of internal climate variability rather than external forcings and cancel out over their positive and negative phases.

I don't believe you can isolate the two (internal/external) in a useful way. PDO, AMO, etc can temporarily change factors (i.e. ice and surface albedo) that increase or reduce the heat radiated away from the planet, i.e., which also makes them 'external' forcings (as you use the term), and its not clear to me that the peaks and troughs of the oscillations always 'cancel' each other out rather than having some net radiative effect over time, in particular when they operate over different average temperatures.

 

[WeatherRusty]

Huh? 1266 W/m^2 space, nearly 1000 W/m^2 insolation received at the surface, low latitudes. Where is there room for 'twice' more radiation from the atmosphere?

Averaged globally, day & night, the Earth's receives 342W/m^2 direct solar radiation.
168W/m^2 is absorbed by the surface.

The atmosphere radiates as integrated globally, day and night 324W/m^2 to be absorbed by the surface.

http://asd-www.larc.nasa.gov/SCOOL/energy_budget.html"

 

[WeatherRusty]

When these gases are ranked by their contribution to the greenhouse effect, the most important are:

* water vapor, which contributes 36–70%
* carbon dioxide, which contributes 9–26%
* methane, which contributes 4–9%
* ozone, which contributes 3–7%


Major Greenhouse Gas % of Greenhouse Effect
Water vapor
36% to 66%
Water vapor & Cloud droplets
66% to 85%
Carbon dioxide
9% to 26%
Methane
4% to 9%
Ozone
3% to 7%

Looking for a more convincing source but: http://www.windows.ucar.edu/tour/link=/earth/climate/greenhouse_effect_gases.html&edu=high"

 

[WeatherRusty]

The forcing of a doubling of CO2 is 3.7W/m^2 as measured from the tropopause before feedbacks and should with everything else remaining equal result in a 1.2C increase in global average temp at the surface. Climate sensitivity to that forcing is what is more at issue. How feedbacks react is where most of the uncertainty lies, but is generally considered to about double the effect resulting in ~3C degrees.

 

[mheslep]

Averaged globally, day & night, the Earth's receives 342W/m^2 direct solar radiation.
168W/m^2 is absorbed by the surface.

The atmosphere radiates as integrated globally, day and night 324W/m^2 to be absorbed by the surface.

http://asd-www.larc.nasa.gov/SCOOL/energy_budget.html"

Alright, averaged night and day.

 

[mheslep]

The forcing of a doubling of CO2 is 3.7W/m^2 as measured from the tropopause before feedbacks and should with everything else remaining equal result in a 1.2C increase in global average temp at the surface...

Yes, sorry, I was not careful here - missed the doubling.

 

[John Creighto]

Do you have source for that 12%? I read numbers as low as 2% (Lindzen IIRC).

Here's a good visual:

http://brneurosci.org/spectra.png [Broken]
http://brneurosci.org/co2.html [Broken]

 

[mheslep]

Here's a good visual:

http://brneurosci.org/spectra.png [Broken]
http://brneurosci.org/co2.html [Broken]

Yes, ok, and then CO2 is ~380ppm, 0.04%, and H2O goes up to maybe 3% at saturation, and we're still not accounting for clouds. It is not clear to me how to account for both the absorption bands and the concentrations.

 

[John Creighto]

Yes, ok, and then CO2 is ~380ppm, 0.04%, and H2O goes up to maybe 3% at saturation, and we're still not accounting for clouds. It is not clear to me how to account for both the absorption bands and the concentrations.

For a rough estimate multiply the fraction transmitted for each gas. Of course it should be obvious from this that you can't really say what percentage each gas contributes to the absorptivity. However, I guess you could still define a percentage of the back radiation that comes from each gas.

 

Yes, ok, and then CO2 is ~380ppm, 0.04%, and H2O goes up to maybe 3% at saturation, and we're still not accounting for clouds.

Clouds are not water vapor and their sign could be positive or negative. Water vapor is rare in the upper troposphere. Carbon dioxide has a greater absorption potential at higher altitudes.

 

[mheslep]

Clouds are not water vapor and their sign could be positive or negative. Water vapor is rare in the upper troposphere. Carbon dioxide has a greater absorption potential at higher altitudes.

Yes I understand, first clause was on water vapor, 2nd clause on clouds.

 

Yes I understand, first clause was on water vapor, 2nd clause on clouds.

Clouds are opaque to almost all incoming and outgoing radiation.

Here is a better visual for atmospheric transmittance.

http://www.globalwarmingart.com/wiki/Image:Atmospheric_Transmission_png

 

[Bored Wombat]

Carbon dioxide has a greater absorption potential at higher altitudes.

Why's that?

You get greatest pressure broadening of the absorbance spectrum at lower altitudes.

 

Why's that?

You get greatest pressure broadening of the absorbance spectrum at lower altitudes.

Because at lower altitudes water vapor dominates the absorption spectrum. There is only a small part of the CO² spectrum that is not saturated by water vapor. Higher up where WV is rare there are more available photons of the proper energy to be absorbed.

 

[Bored Wombat]

Because at lower altitudes water vapor dominates the absorption spectrum. There is only a small part of the CO² spectrum that is not saturated by water vapor. Higher up where WV is rare there are more available photons of the proper energy to be absorbed.

So on days or regions with low humidity, the CO₂ at lower altitudes dominates?

Surely radiant energy from the Earth would have to pass through the humid zone anyway to get to the upper atmosphere.

 

[WeatherRusty]

Study this page carefully.

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irwindows.html" [Broken]

You will note that water vapor and CO2 block infrared transmission through the lower atmosphere by absorption at discreet wavelengths to extinction. This is Sky Transparency.

You will also note that the atmosphere radiates in the infrared because of it's particular temperature profile according to Planck's Law. This thermal radiation is Sky Brightness in Infrared.

 

So on days or regions with low humidity, the CO₂ at lower altitudes dominates?

In the absense of water vapor CO² accounts for 36% of a the GE.

Surely radiant energy from the Earth would have to pass through the humid zone anyway to get to the upper atmosphere.

The bulk of what is absorbed by the upper atmosphere is emitted from the atmosphere itself.

 

[WeatherRusty]

The bulk of what is absorbed by the upper atmosphere is emitted from the atmosphere itself.

Correct. In infrared (excluding the IR windows) the surface of the Earth is invisible from the top of the absorbing region. The "view" is clouded both up from the surface and downward toward the surface. The region is opaque to IR. Energy in the form of IR radiation is emitted and absorbed countless times within the absorbing layer. Every new emission is a new photon produced within the gaseous envelope itself directed on average in all directions and carrying no information pertaining to the surface where the energy originated.

 

[John Creighto]

For the physics of global warming this like looks good:

http://patarnott.com/pdf/IRemissionTheory.pdf

which I got from here:
http://patarnott.com/atms749/