Need Help: Can You Model CO2 as a Greenhouse Gas (Or is This Just Wishful Thinking?)

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  • #26
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Your simple calclation 12/6 is no more accurate than dividing the 500 degree increase on Venus by 12; 500/12=42. (approximately) Also note that the water vapor feedback on Venus is nonexistent.
Straight contribution from CO2 per doubling would be ~1.2C, add the other climate feedbacks and you get 2.0C to 4.5C per doubling.
Isn't Venus closer to the sun? How did you get your figure of 1.2C?
 
  • #27
sylas
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Isn't Venus closer to the sun? How did you get your figure of 1.2C?
The effect of doubling CO2 is for conditions on Earth, where CO2 is a small part of the atmosphere. It's a fairly well constrained result that doubling CO2 in Earth's atmosphere, and holding everything else fixed, will give an additional 3.7 W/m2 of forcing.

You can get approximately the right result here by using a crude estimate of
[tex]Q = \epsilon \sigma T^4[/tex]​
Q here is the energy out the top of the atmosphere, T is the absolute temperature at the surface, σ is the Stefan-Boltzmann constant, and ε is a constant, written here a bit like emissivity, although it is is not actually an emissivity term.

T at Earth's surface is about 298, and Q is about 239 W/m2.

Differentiating, we have
[tex]\frac{dQ}{dT} = \epsilon \sigma 4 T^3 = \frac{4Q}{T} = 3.2[/tex]​

With dQ at as about 3.7, the value dT to restore energy balance is about 3.7/3.2 = 1.16. In practice, this calculation is done over the whole surface of the Earth, and is a bit more complex, but this approximation here gives a good ballpark for "non-feedback" response.

That is, raising the surface temperature by about 1.1 to 1.2 degrees is enough to restore Earth's energy balance in response to the forcing from a doubling of CO2, as long as nothing else changes.

However, other things do change. Ice melts. Vegetation cover varies. Weather patterns and cloud cover alters. The lapse rate shifts in response to a different specific humidity. And so on. All these things in turn have a further effect on temperatures, and the net effect is a positive feedback. But it is not well known precisely what gain is given by this feedback. There have been a range of methods applied to try and constrain it, but so far the best we can say is that it will actually take something from 2 to 4.5 degrees temperature rise to restore the energy balance in practice.

Cheers -- sylas
 
  • #28
Skyhunter


Isn't Venus closer to the sun?
Yes it is, however it has a higher albedo, and therefore absorbs less energy.
 
  • #29
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That is, raising the surface temperature by about 1.1 to 1.2 degrees is enough to restore Earth's energy balance in response to the forcing from a doubling of CO2, as long as nothing else changes.
In MODTRAN it's hard to get values above 0.9 degrees, to get 1.2 degrees one has to keep relative humidity constant, which is already a positive feedback.

All these things in turn have a further effect on temperatures, and the net effect is a positive feedback. But it is not well known precisely what gain is given by this feedback. There have been a range of methods applied to try and constrain it, but so far the best we can say is that it will actually take something from 2 to 4.5 degrees temperature rise to restore the energy balance in practice.
(emphasiz mine)

Typical response characterical differences between positive and negative feedback can be discerned. Positive feedback has a 'persistant' character, pushing an output value like temperature for instance into the direction of the disturbance, while negative feedback does the opposite.

Guess how long we know already that we cannot discern positive feedback behavior in the temperature series?:

O Karner, 2002; On nonstationarity and antipersistency in global temperature series, JOURNAL OF GEOPHYSICAL RESEARCH, doi:10.1029/2001JD002024

...Estimating long-range dependence between the increments reveals a remarkable difference between the two temperature series. Global average tropospheric temperature anomaly behaves similarly to the solar irradiance anomaly. Their daily increments show antipersistency for scales longer than 2 months. The property points at a cumulative negative feedback in the Earth climate system governing the tropospheric variability during the last 22 years...
Of course persistence (positive feedback) or anti-persistence (negative feedback) can be checked on any climate data series, anytime. So I wonder if studies exist, which demonstrate this persistence in any data series.
 
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  • #30
sylas
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In MODTRAN it's hard to get values above 0.9 degrees, to get 1.2 degrees one has to keep relative humidity constant, which is already a positive feedback.
I don't know what you are doing there. I get completely the opposite. Holding relative humidity fixed gives you much greater values than 1.2, as I would expect from the positive feedback of an additional greenhouse effect from the increased specific humidity.

MODTRAN is really geared towards looking at a specific column of atmosphere under a certain profile; to get from that to a value for the whole planet would be a lot of work... and closer to what is really done in practice rather than the simple approximation I presented.

See http://geosci.uchicago.edu/~archer/cgimodels/radiation.html [Broken] at the Uni of Chicago, courtesy of David Archer. The values we are giving here are basically 3.7 / (dQ/dT). You can read off an estimate for dQ/dT from MODTRAN by boosting surface temperature by 1 degree and seeing how much the output radiation changes.

Strictly speaking it is probably best to look down from an altitude of 18 km or so, near the tropopause; but you'll get roughly comparable results with the default 70km, so that doesn't matter much. I'll use the default 70km here, and you can check that 18km would also show the same ball park. You can pick various atmospheres, and hold pressure (specific humidity) or relative humidity fixed. Here are results I obtained with the calculator:
[tex]\begin{array}{cccccc}
\text{atmosphere} & \text{base Iout} & \text{+1C, fix spec h} & \text{Plank Response} & \text{+1C, fix rel h} & \text{Plank Response} \\
\text{Tropical} & 287.844 & +3.674 & 1.01 & +2.198 & 1.68 \\
\text{Std 1976} & 258.862 & +3.297 & 1.12 & +2.229 & 1.66
\end{array}[/tex]​

In other words, using MODTRAN and holding the water vapour pressure fixed gives something close to what I estimated previously with my approximation of a T4 power relation. Holding relative humidity fixed... which corresponds to an increase in specific humidity and a smallish positive feedback... gives you a greater value of about 1.7 or so.

How did you get 0.9?

Typical response characterical differences between positive and negative feedback can be discerned. Positive feedback has a 'persistant' character, pushing an output value like temperature for instance into the direction of the disturbance, while negative feedback does the opposite.

Guess how long we know already that we cannot discern positive feedback behavior in the temperature series?:

O Karner, 2002; On nonstationarity and antipersistency in global temperature series, JOURNAL OF GEOPHYSICAL RESEARCH, doi:10.1029/2001JD002024
Thanks... I've never seen that paper before.

I don't really follow what he is doing there; I've had a quick look but I need to read it more carefully. This result runs counter to all the research, both theoretical and empirical, that I have seen on the matter.

There have been several threads where the literature on constraining feedback has been discussed. We've already shown just above that water vapour should give a positive feedback. There are multiple empirical studies confirming this as a significant effect, as we should expect from basic physics given that water is such a strong greenhouse gas. A recent thread on cloud studies also suggests another strong positive feedback ([thread=327161]Clouds and Global Warming[/thread]). We've discussed the paper by Schwartz, which originally proposed an anomalously small positive feedback, but in response to some criticisms of his method he acknowledged the problems and revised it upwards to be rather stronger. See especially [post=2195419]msg #47[/post] of thread "Estimating the impact of CO2 on global mean temperature". In the [post=2162699]OP of that thread[/post], I describe empirical studies of the response to volcanic eruptions which appears to use a method somewhat similar to your reference; they find that the response to an eruption has a long tail (persistence) indicative of a substantial positive feedback. That post also cites Annan and Hargreaves (2006) showing a range of empirical studies which imply positive feedback. In particular, a negative feedback on climate would make the large temperature swings of the ice ages pretty much inexplicable. The forcing required would be enormous.

However, I grant that it is rather unsatisfactory to merely cite all the evidence (and there's a lot of it!) for positive feedback. If Karner is incorrect, then there's a problem that should be able to be identified in his methods or data; and if that is not known, then we have a legitimate mystery.

I must admit my own bias up front. I am pretty sure Karner can't possibly have a credible case; the case for a positive feedback both empirically and theoretically is very strong and backed up with many different studies. But I admit my bias with a view to recognizing it, so that I can avoid merely prejudging and giving a fair reading as best I can. If you can explain the method in your own words, that may help and I'll be grateful. But I will look at it in any case.

Of course persistence (positive feedback) or anti-persistence (negative feedback) can be checked on any climate data series, anytime. So I wonder if studies exist, which demonstrate this persistence in any data series.
There's the volcano data I mentioned, which shows a strong persistence effect. The reference is Wigley, T. M. L., C. M. Ammann, B. D. Santer, and S. C. B. Raper (2005), Effect of climate sensitivity on the response to volcanic forcing, in J. Geophys. Res., Vol 110, D09107, doi:10.1029/2004JD005557.

You have to compare the temperature time series with the forcing, or you can't even get started. Karner appears to use the solar forcing from the sunspot cycle. It may be instructive to compare with Schwartz' work, mentioned above. But as I say, I'll have to look at it more carefully.

No matter what I think of the matter... thanks for the reference!

Cheers -- sylas
 
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  • #31
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... gives you a greater value of about 1.7 or so.

How did you get 0.9?
From the default http://geosci.uchicago.edu/~archer/cgimodels/radiation.html [Broken] settings ...

(CO2 (ppm> 375
CH4 (ppm) 1.7
Trop. Ozone (ppb) 28
Strat. Ozone scale 1

Ground T offset, C 0
hold water vapor pressure
Water Vapor Scale 1

Locality Tropical Atmosphere
No Clouds or Rain

Sensor Altitude km 70
Looking down
....We get an output of:

Iout, W / m2 = 287.844
Ground T, K = 299.70
Now we change only:

(CO2 (ppm> 750
and the output changes to

Iout, W / m2 = 284.672
Ground T, K = 299.70
So obviously less IR energy reaches the sensor and we have to increase the temperature to get the original / apparantly equilibrium I-out back (287.844 w/m2)

So we put in Ground T offset, C the value +0.89 to see an output of

Iout, W / m2 = 287.844
Ground T, K = 300.59
hence we have to increase the surface temp with 0.89 degrees according to MODTRAN to regain radiation equilibrium.

Now we go back to the defaults and change hold water vapor Rel. Hum. which does not change the output from the basis, however if we double CO2 now, the output is

Iout, W / m2 = 284.672
Ground T, K = 299.70
And now we have to enter 1.48 degrees in ' Ground T offset, C' to regain the equilibrium value again.

For the 1976 standard atmosphere these values are 0.88 and 1.30 degrees respectively

Perhaps it's better to give Karner a dedicated thread as we explore the characteristix of feedback in general.
 
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  • #32


sylas, Michael Tobis had this to say a while back in a RC comment (to Andre actually) on Karner.

33.Re #3, specifically to the references to work by the Estonian statistician, O. Karner.

Karner has been taking single time series of diurnal temperature differences and showing that they act as if they are constrained to return to a fixed value. The statistical properties of this time series are “antipersistent” and may be associated with a feedback in a simple lumped parameter model. This is a purely statistical rather than physical model, and it shows there is a homeostatic process, with a number that can be considered “the feedback”.

Unfortunately, it appears to me that Karner confuses this mathematical property with the H2O amplification of radiative forcing, a physical quantity with which Karner’s feedback constant has only a distant relationship.

Indeed, there is an antipersistence in temperature anomalies on Earth, and the mechanism is well-known: radiative equilibration. In this phenomenon, water vapor plays an important role but it isn;t a soliloquy. Thus, when Karner says things like (see http://www.aai.ee/~olavi/2001JD002024u.pdf )

The revealed antipersistence in the lower tropospheric temperature increments does not support the science of global warming developed by IPCC [1996]. Negative long-range correlation of the increments during last 22 years means that negative feedback has been dominating in the Earth climate system during that period. The result is opposite to suggestion of Mitchell [1989] about domination of a positive cumulative feedback after a forced temperature change

to my reading he is confused. (I am surprised this text passed review at JGR-A.)

His subsequent paper ( http://www.aai.ee/~olavi/cejpokfin.pdf ) seems to show increased awareness on the matter:

Using the H estimates to ascertain the cumulative feedback sign dominating in the Earth climate system for the particular variable. In the present study the term feed-back is used in the sense of total reaction of the variable to customary forcing in the Earth climate system. Such an understanding is unavoidable in statistical analysis of meteorological time series because, as a rule, they are affected by many forcing types including the seasonal and daily cycles in solar radiation. In climatology the term feedback is usually connected to the corresponding feedback loop, e.g ice-albedo feedback [13]. For the whole climate system this means that one has to consider many feedbacks at the same time.

Karner’s methodology does not separate out specific physical mechanisms but is simply a way of characterizing a time series. It in some sense includes but (as I understand it) in no sense measures the impact of water vapor feedback on radiative equilibrium.
My own reading of the Karner paper is that it has little at all to do with attribution or long-term climate feedback, and thus claims as in the abstract are unsupportable. I don't quite understand the statistical end of things in the way Tobis does, but the relation to long term climate feedback is kind of evident.
 
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  • #33
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Chris, the articles of Olavi Karner are peer reviewed and the physics of feedbacks are clear albeit complex.

How about some remark of somebody, challenging that? Is this also peer reviewed? If there is a discussion about this in a formal scientifically recognized magazine then it would be nice to quote that, however there is a strong viewpoint here about the autority of blog discussions.

There are more papers by the way, for instance:

Olavi Kärner 2007 On a possibility of estimating the feedback sign of the Earth climate system, Proc. Estonian Acad. Sci. Eng., 2007, 56, 3, 1–1

..The analysis of the OLR time series indicates that a negative feedback should dominate in the earth climate system....
 
  • #34


One way to think about the no-feedback case is to assume that the emission temperature and surface temperature are linearly related, and so it follows that


[tex]\lambda[/tex] (planck feedback only) =[tex] \left(\frac{\partial ( \sigma T^{4}_{eff})}{\partial T_{s}}\right)

^{-1}[/tex]

Please note that the resultant [tex] (4 \sigma T^{3}_{eff})^{-1}[/tex] requires the emission temperature (i.e., 255 K) as an input, not the surface temperature. This evaluates to roughly 0.27 [tex] K (W m^{-2})^{-1}[/tex]

which says that you get about a quarter degree change in temperature for each Watt per square meter forcing. The forcing for a doubling of CO2 is nearly 4 watts per square meter. This would imply a very stable climate since it would take a 23 W/m2 change in solar constant just to produce a 1 C change in global temperature, about the same as a doubling of CO2. Thus you'd need the equivalent of several doublings of CO2 and/or unreasonable changes in solar irradiance to be consistent with the magnitude of deep-time paleo changes.

Those arguing for net neutral, and especially negative feedbacks are simply not correct.
 
  • #35
sylas
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OK... thanks. I now see what you are doing with MODTRAN. You are actually calculating something rather different to the Planck response; although the reason for this gets a bit subtle.

The forcing for a doubling of CO2 is known to be 3.7 W/m2. This number is well constrained; with about 10% accuracy or so. The major reference for this value is Myhre et al., (1998) New estimates of radiative forcing due to well mixed greenhouse gases, Geophysical Research Letters, Vol 25, No. 14, pp 2715-2718; and similar values are obtained in other work as well. It is not something that should be in serious dispute.

Unfortunately, you can't read this off MODTRAN very well. There are two reasons for this. One is that it depends on the latitude. The second is that it depends on the altitude of the sensor.

Part of the problem is the appropriate definition of a forcing. I describe it, with references, in [post=2162699]msg #1[/post] of "Estimating the impact of CO2 on global mean temperature". It corresponds to a change in energy balance at the top of the tropopause. There's a reason for measuring there rather than at 70km; and it is because of the rapid response of the stratosphere to a forcing... a response that this MODTRAN calculator omits. In [post=2165483]msg #3[/post] of that thread I repeat pretty much the calculation you have given here, but with a detector at 20km. Here is a tabulation of some results:
[tex]\begin{array}{cc|cccc}
\text{Atmosphere} & \text{Altitude, km} & 375 \text{ppm} & 750 \text{ppm} & \text{difference} & \text{extra T reqd} \\
\hline
\text{Tropical} & 20 & 288.378 & 283.856 & 4.522 & 1.385 \\
\text{Tropical} & 70 & 287.844 & 284.672 & 3.172 & 0.89 \\
\text{Std 1976} & 20 & 258.893 & 255.47 & 3.423 & 1.03 \\
\text{Std 1976} & 70 & 258.862 & 256.004 & 2.858 & 0.88
\end{array}[/tex]​

The reason you get a difference at higher altitude is that the atmospheric temperature profile in this calculator is held fixed, and so the calculator actually has stratospheric warming as a response to an increase temperature offset. What happens in reality is that the stratosphere cools; mainly because of the increased emissivity of carbon dioxide which makes it shed heat more rapidly. Furthermore, this cooling response is very rapid, since it is a purely radiative effect. That is why the formal definition of forcing includes settling of the stratosphere, but not of the troposphere. Informally, you can say that the stratosphere response (which has little impact back to ground level) is considered so fast that it is part of the forcing, and not a separate feedback process.

The upshot is that to get a sensible value for the forcing response to doubled CO2, you should really take the lower altitude sensor. Also, you can't have a tropical atmosphere over the whole planet. The value you get will be somewhere between the tropical atmosphere and the standard 1976 atmosphere; and you also need to consider clear sky and cloud as well.

All told, the MODTRAN calculator will get you into the right ball park; but it can't serve as a refutation of the forcing for doubled CO2, which is about 3.7 W/m2 to 10% accuracy or better.

Perhaps it's better to give Karner a dedicated thread as we explore the characteristix of feedback in general.
That's a good idea. I'll let you start it. Furthermore, if I don't join in right away it will be because I am reading, rather than trying to jump in before I understand it more.

Cheers -- sylas

PS. Added in edit. I see I've missed Chris' input since writing this.
 
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  • #36
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The effect of doubling CO2 is for conditions on Earth, where CO2 is a small part of the atmosphere. It's a fairly well constrained result that doubling CO2 in Earth's atmosphere, and holding everything else fixed, will give an additional 3.7 W/m2 of forcing.

You can get approximately the right result here by using a crude estimate of
[tex]Q = \epsilon \sigma T^4[/tex]​
Q here is the energy out the top of the atmosphere, T is the absolute temperature at the surface, σ is the Stefan-Boltzmann constant, and ε is a constant, written here a bit like emissivity, although it is is not actually an emissivity term.

T at Earth's surface is about 298, and Q is about 239 W/m2...

This is tested by experiment? Please cite; I am interested in experimental tests on CO2's greenhouse effect.
 
  • #37
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But these formulas aren't tested in the lab? Can you cite any experimental tests?
 
  • #38
Skyhunter


They are not tested in a lab, they are based on the results of extensive observations of the atmosphere.
 
  • #39
mheslep
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...
While average Antarctic sea ice has increased slightly, this is a function of ozone depletion and it's effect on circulation patterns in the Antarctic, not some alleged global cooling. other areas of the Antarctic, especially the western peninsula, are experiencing a sharp decline in average sea ice extent.
Do you have sources for this, especially the implication that ozone depletion is responsible for Antarctic sea ice creation via currents? Currents and circulation also have a great deal to do with Arctic ice depletion.[1]
Skyhunter said:
...Antarctic total ice mass is decreasing to the tune of about 84 gigatons of ice per year.
That statement would seem to conflict with this
3rd IPCC said:
The Antarctic ice sheet is likely to gain mass because of greater precipitation,
http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/008.htm [Broken]

[1]http://www.nasa.gov/vision/earth/lookingatearth/quikscat-20071001.html
Nghiem said the rapid decline in winter perennial ice the past two years was caused by unusual winds. "Unusual atmospheric conditions set up wind patterns that compressed the sea ice, loaded it into the Transpolar Drift Stream and then sped its flow out of the Arctic," he said. When that sea ice reached lower latitudes, it rapidly melted in the warmer waters.

"The winds causing this trend in ice reduction were set up by an unusual pattern of atmospheric pressure that began at the beginning of this century," Nghiem said.
 
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  • #40


For those interested, spectroscopic databases have been compiled for all the gases in the Earth's atmosphere (e.g., HITRAN) These databases contain line centers and parameters describing line shape as a function of pressure and temperature and provide the amount of absorption to very high spectral resolution.

Before reading too much into much simpler stuff then the approach of modern line-by-line radiative transfer codes and climate models, it may be worth reading some online material on the downfalls of simple experiments, like measuring radiation decay through a tube. See http://www.realclimate.org/index.php/archives/2007/06/a-saturated-gassy-argument-part-ii/ (and part 1) and check out Spencer Weart's site as well.
 
  • #41
Skyhunter


Do you have sources for this, especially the implication that ozone depletion is responsible for Antarctic sea ice creation via currents? Currents and circulation also have a great deal to do with Arctic ice depletion.[1]
That statement would seem to conflict with this
It is in the Antarctic section. (large PDF 15mb)
http://www.ncdc.noaa.gov/oa/climate/research/2008/ann/bams/full-report.pdf

The TAR is a bit outdated and it appears you are confusing Antarctic land ice with Antarctic sea ice, and the Arctic with the Antarctic.

The State of the climate report I cited has the most comprehensive up to date assessment since the 4AR. The sections on the Arctic and Antarctic should help clear up your confusion.
 
  • #42
mheslep
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It is in the Antarctic section. (large PDF 15mb)
http://www.ncdc.noaa.gov/oa/climate/research/2008/ann/bams/full-report.pdf
Thanks, I'll take a look.

The TAR is a bit outdated and it appears you are confusing Antarctic land ice with Antarctic sea ice, and the Arctic with the Antarctic.
No, I was drawing attention to the the way in which you were singling out winds and currents only for the Antarctic explanation; they also factor greatly in explaining Arctic melts as explained by Nghiem.
 
  • #43
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They are not tested in a lab, they are based on the results of extensive observations of the atmosphere.
How do you isolate the effect of CO2? Are you saying the greenhouse effect is too small for exprimental measurement?
 
  • #44
sylas
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How do you isolate the effect of CO2? Are you saying the greenhouse effect is too small for exprimental measurement?
It is isolated by the experiments we have been discussing already, starting with Tyndal in the 19th century.

What you can't measure in an experiment is the total effect on Earth's climate. You can, however, confirm the basic underlying physics which is involved.

We know that carbon dioxide absorbs infrared radiation. We know how it works now in considerable detail, thanks to quantum mechanics; and there is a massive body of experimental work confirming the relevant physics. You can now calculate the absorption spectrum for different gases in considerable detail. The warming effect of this on a planet is consequence of very basic physics indeed. We've not conducted experiments on a planet as such, but experiments on radiation and thermodynamics confirms well beyond any credible doubt that an atmosphere which absorbs infrared radiation will give a higher surface temperature.

This is called the "atmospheric greenhouse effect". You still get people who deny that a greenhouse effect exists at all, but that is pretty much the young Earth creationism of climate science.

On the other hand, there is the effect of changing concentrations of greenhouse gases. This means quantifying the effect, in terms of concentrations; and that is not something you can do directly in a lab either. What you can do in a lab confirms that increasing carbon dioxide will give more absorption of infrared; but you can't just do a simple scale from a gas cell to an atmosphere. To quantify the effect well enough to infer the effect of changing concentrations on a planet is more difficult.

This problem can be broken into two parts; one of which is solved and one of which is not.

We know very well how carbon dioxide and other gases interact with radiation. We measure the spectum of light in the atmosphere (backradiation, radiation out to space, flux at different levels, and so on) and there's a well developed and tested theory associated with that; so that you can calculate to quite good accuracy how much additional energy is available with a change in concentrations.

What is hard is to tell how much the temperature of the surface changes in response to additional energy. Again; the relevant physics is fine, but the problem is the sheer scale of the major cycles and processes interacting in the climate system. You can test various parts of it, but to actually measure the temperature changes due to changing atmospheric composition can't be done directly. You can't separate out the causes and effects as you can in a lab.

Upshot is.

We know, as well as we know anything at all in science, that there's a greenhouse effect. There are all kinds of experiments, some of which we have discussed here, which show that carbon dioxide, water, methane, flourocarbons, and various other gases will absorb thermal radiation and heat up as a result.

We know, to a very high level of confidence, that the effect of a gas like carbon dioxide is logarithmic, and we've got a good handle on the factor. A doubling of CO2 concentrations will give a certain amount of additional energy at the surface of the planet... about 3.7 W/m2. That's the result of a pretty detailed calculation over the surface of the Earth and and though all lines of the spectrum, using well tested physics. You can't measure that number directly; it is a calculation for a whole planet. But there's no credible doubt on the number beyond comparatively small errors of no more than 10%.

We have a rough idea only of how much temperature change that leads to, in the long run. It's something from 2 to 4.5 degrees. That based on empirical and theoretical studies.

Cheers -- sylas
 
  • #45
Skyhunter


No, I was drawing attention to the the way in which you were singling out winds and currents only for the Antarctic explanation; they also factor greatly in explaining Arctic melts as explained by Nghiem.
I was addressing a direct point about the Antarctic, not ignoring the conditions in the Arctic that led to the rapid decrease in Arctic sea ice. I was trying to keep it brief since it is off topic.

The conditions that led to the sharp decline in Arctic sea ice were not unprecedented. The difference in 2007 was the abundance of thin ice that was more suscpeptible to being blown into more temperate waters by the wind.
The development of a relatively younger, thinner ice cover coincided with a strong, persistent positive pattern in the AO from 1989 to 1995 (see Figure A1). These characteristics make the current ice cover intrinsically more susceptible to the effects of atmospheric and oceanic forcing. It is of crucial importance to observe whether the sea ice cover will continue its decline or recover under the recent more neutral AO conditions (Lindsay and Zhang, 2005). http://www.arctic.noaa.gov/report07/seaice.html" [Broken]
http://nsidc.org/arcticseaicenews/" [Broken]sea ice extent there has been no recovery of Arctic sea ice.

http://www.antarctica.ac.uk/press/press_releases/press_release.php?id=838"
 
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  • #46
mheslep
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http://nsidc.org/arcticseaicenews/" [Broken]sea ice extent there has been no recovery of Arctic sea ice.
<shrug>So far in 2009 the Arctic extent is certainly lower than the long term mean, yet 2009 has been an improvement over 2007, and April 2009 almost rejoined the mean.
 
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  • #47
Skyhunter


<shrug>So far in 2009 the Arctic extent is certainly lower than the long term mean, yet 2009 has been an improvement over 2007, and April 2009 almost rejoined the mean.
April 2009 sea ice extent may have nearly rejoined the mean, but sea ice mass (extent x thickness) is steadily declining. 2009 is much less than 2008, and it is declining more rapidly than during the same period in 2007.
N_timeseries.png


August 1, 2008

20080801_Figure2.png


This rapid late season melt is indicative of the thinner ice that is well documented.

http://www.thedailygreen.com/environmental-news/latest/arctic-sea-ice-47011108?src=rss
http://www.nasa.gov/topics/earth/features/arctic_thinice.html
http://www.arctic.noaa.gov/reportcard/seaice.html [Broken]
 
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  • #48
sylas
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<shrug>So far in 2009 the Arctic extent is certainly lower than the long term mean, yet 2009 has been an improvement over 2007, and April 2009 almost rejoined the mean.
When you look at individual years, like 2007, or 2009, you have to consider that there's variation from season to season.

The trend, however, is very strong. Even though 2007 was a major outlier, well below what would be expected from the trend; the trend is still sufficiently high that it is a good bet that there will be a new record low within the next few years; and that the Arctic will have a summer essentially free of sea ice sometime within the lifetime of many of the people reading this thread. Probably within my lifetime, if I make it to the age of my parents.

Picking one month (April) is odd.. unless you have some prior reason for singling out April then it looks a bit like cherry picking; it doesn't mean much. One could as well say that as of now, the ice cover is lower than any year on record except the exceptional 2007 season. But that's no assurance at all that 2009 is going to get to second place for the summer minimum of cover. It might, it might not.

And how can you define a "mean"? The mean over what period? Of the last decade? Sure... but since ice cover as been falling steadily for some time now the idea of a "mean" is rather suspect.

Its worth noting that the Arctic is a region that is not representative of the whole planet. The warming in the Arctic is well above global warming, and is substantially a local effect on top the global warming phenomenon. We had a good thread on this recently: [thread=306202]"Only dirty coal can save the Earth"[/thread] (the title is not a good indication of the implications of the study discussed, but the discussion was interesting).

Cheers -- sylas
 
  • #49
mheslep
Gold Member
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Certainly the Arctic ice reductions are significant. I was replying only in context to Skyhunter's 'no recovery' comment about '2008, 2009' where April stands out. By 'mean' I was referring to 79-2000 mean depicted in the graphs he posted. I wasn't using April to make any broader comment than that.
 
  • #50
Skyhunter


Certainly the Arctic ice reductions are significant. I was replying only in context to Skyhunter's 'no recovery' comment about '2008, 2009' where April stands out. By 'mean' I was referring to 79-2000 mean depicted in the graphs he posted. I wasn't using April to make any broader comment than that.
My no recovery comment is quite valid and not refuted by your comments or examples. Citing the sea ice maximum extent without the context of sea ice thickness is misleading. The surface is expected to refreeze during NH winter. The large open areas actually increases Arctic temperature because of the release of latent heat during the rapid refreeze. Also the rapid refreeze can have a negative impact on sea ice thickness due to the insulating properties of the snow that accumulates on it's surface.
 

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