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The AGW climate feedback discussion

by Andre
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Xnn
#73
Dec14-09, 06:58 PM
P: 555
Trying to stay focused on climate feedbacks...

In a simple radiative model of the Earth, surface temperatures are a function of albedo and emissitivity.

Most people understand albedo fairly well.
Rising temperatures, melt ice.
Ice has a lower albedo than water.
Lower albedo results in higher temperatures.
So, ice water albedo has a positive feedback on surface temperatures.

Vegetation also has an affect on albedo.
Lush forest has a lower albedo than seasonal snow/tundra.
So, warmer moister temperatures result in lower vegatation albedo.

Emissitivity on the other hand is not understood as easily.
Emissitivity is the ability of an object to radiate.
Most of earth's surface has a high albedo.
Water, ice and most vegetation matter is around 0.96.
However, clouds which cover about half of the surface have an emissitivity of about 0.5.
Also, the emissivity of a cloud is a function of it's temperature.
At lower temperatures, clouds do not emit as well as they do at higher temperatures.
Since clouds are usually fairly cold, their emissitivity is <0.5.

Now, it is not totally clear exactly how surface temperatures affect emissitivity.
One might think that rising temperatures could warm up clouds and rise their emissitivity.
However, clouds float and their temperatures probably don't really change all that much.
On the other hand, there will probably be more clouds with warmer temperatures.
Since clouds in general result in a lower planetary emissitivity, this factor would probably drive emissitivty lower. Lower planetary emissitivity results in higher surface temperatures.

So, my suspision is that surface temperature feed back positively by both albedo and emissitivty.

The only negative feedback that I can evision is from the affect of clouds on albedo.
Since I already suspect that rising temperatures will result in more clouds, this would act to rise albedo and thus lower surface temperatures. Maybe this is where the evapotranspiration discussion was heading. More evap = more clouds.

Now, a skeptic might argue that this negative feedback could outweigh or at least equal the positive feedbacks. However, the consensus science is that it doesn't.
More over, I can point to the exaggerated global temperature swings between glacial max and interglacials as an indication that there is an overall amplification (positive feedback) to relatively small forcing of surface temperatures, especially when there is substantial snow and ice on the earth.
mugaliens
#74
Dec15-09, 01:54 AM
P: 595
Quote Quote by vanesch View Post
Just to say that I'm also very interested in learning more about the feedback mechanisms.
Ditto. In light of historical data, it's undeniable the Earth's temp oscillates between brief warming peaks and much longer glaciation periods, as well as that there exist built-in mechanisms which moderate both the regular, periodic warming/cooling cycles and the more cataclysmic effects of seriously large and spikes in vocanic activity and meteor impacts which dwarf our nuclear arsenal many times over.

The fact we're still here, and that millions of years of geological records show nothing more than a single blip or two of departure from the bimodal swings tells me that if we believe mankind's efforts will result in runaway global warming, or somehow permanently destabilize Earth's temperature swings, then we know far less about Earth's climate than we think.

This isn't a statement of abandonment of the idea of AGW. Rather, it's a request for caution, not in the light of what we do know, but because there's so much yet that we don't know.
Bored Wombat
#75
Dec15-09, 02:50 AM
P: 119
Quote Quote by mugaliens View Post
The fact we're still here, and that millions of years of geological records show nothing more than a single blip or two of departure from the bimodal swings tells me that if we believe mankind's efforts will result in runaway global warming, or somehow permanently destabilize Earth's temperature swings, then we know far less about Earth's climate than we think.
If you go back further than the closing of the Panama isthmus, you get a very different climate. (Since that was the start of the ice age).

I think that that shows that the planet can and does go into dramatically different states.
Xnn
#76
Dec15-09, 10:26 AM
P: 555
A few more thoughts about feedbacks; especially negative feedbacks.

While melting ice and snow are strong positive feedbacks, if the earth warms up enough that all of the ice melts than that feedback mechanism disappears. This isn't exactly a negative feedback, but at least there is an upper limit above which it ceases to be as large a factor.

Second, with respect to emissitivity; while most surfaces have very high emissitivities (0.95 to 098) there are some small desert areas that have much lower emissitivities (around 0.6). I believe these are the death valley types which are desicatted.

So, to the extent that warming leads to a growth of these limited deserts, it exerts a negative feedback. This is somewhat counter intuitive, but basically extreme desert areas are better able to radiate heat energy to outer space than most other land areas.
Xnn
#77
Dec15-09, 11:05 AM
P: 555
And a little more on negative feedbacks...

In the extreme cases of warming or cooling, plants react to moderate temperature changes if they are from CO2.

At extremely low CO2 levels, plants don't grow as well so they don't remove as much CO2 from the atmosphere and this helps prevent CO2 levels from falling all they way to zero thereby counter acting the cooling trend.

Likewise, as CO2 levels grow, plants grow faster and this helps limit the CO2 thereby putting something of a brake on the warming.

This feedback works in conjunction with precipitation since high precipitation levels accompany warming which wash more carbon plant and animal material into the oceans where it is sequestered.

So, this is a negative feedback, but only applies to CO2.
Bill Illis
#78
Dec15-09, 01:15 PM
P: 34
Xnn,

I built an Albedo model based on the 10 degree latitude bands and on the different surface reconstruction maps and continental drift maps that have been made available for the planet through history.

The (simple) model builds in an "heroic assumption" however, that the average cloudiness of the Earth remains constant. Is there a methodology to incorporate changing cloudiness levels which is based on the Earth's average Temperature and Albedo.

This picture was taken by Apollo 17 in mid-December 1972. The white in this real picture (clouds and Antarctica) are reflecting between 40% to 80% of the sunlight while the unclouded ocean in the centre of the picture is only reflecting 5%.

Now take Africa and move it to the South Pole where Antarctica is now (and attach South America, Antarctica, India, and Australia to it) as in 443 million years ago, and how much white would show up.

Xnn
#79
Dec16-09, 06:44 AM
P: 555
Bill;

I've looked a little, but can't find a good reference for what the science is for cloud cover as a function of temperature. I'm thinking that it gets more extensive at higher temperatures, but probably not uniformly.

For example, we know that within the past, the Sahara desert was much greener and presumably more cloudy than it is now. But it's not clear to me if this was from a general warming or cooling.

Also, as the continents move around, and larger continental areas are formed, my understanding is that the interiours become deserts. So, I believe the distribution of the continents also plays a role.
Bill Illis
#80
Dec16-09, 09:40 AM
P: 34
Here is a screenshot of this 10 degree latitude band Albedo model. The average Earth Albedo according to Trenberth is 0.298 (although I believe the cloud versus surface estimates he uses for Albedo are off since the math doesn't work - its more like 50% each).

Splitting the Earth up into the different latitude bands along with the average Albedo, surface area and weighted-average solar energy received in each band, we can calculate how each latitude band contributes to the global Albedo number. Putting it all together, have exactly the current Earth Albedo.

Now, as we move through different climate epochs, like the ice ages or Gondwana glaciated over at the South Pole, we can estimate how the Albedo would have varied. Without a high Albedo number like 0.333 for the Last Glacial Maximum, you cannot get even close to the estimated temperature of the time. (GHGs only varied enough to account for 1.7C of the 5.0C decline). The Milankovitch Cycles as well, cannot explain the depth and timing of the ice ages (the timelines only match up to a small extent) - The Ice-Albedo feedback or this case the Albedo driver rather than feedback has to be a self-sustaining, overwhelming factor in the ice ages.

The Cloudiness fraction presents a big problem because it will be a make or break factor.

Including the effects of clouds:

The average Albedo of the Earth is 0.298;

The average Albedo of the Land is approximately 0.344;
Snow-covered glaciers are approximately 0.7;
Non-glaciated Land averages 0.305.

The average Albedo of the Ocean is 0.283;
Sea Ice is approximately 0.5 to 0.65; and
Open Ocean averages 0.267 but is lower at the Equator.

The Albedo is also higher as we move from the Equator to the Poles. The average Albedo at the Poles is 0.685 while the average Albedo at the Equator is 0.240. Take the sea ice and glaciers away from the Poles and replace them with open ocean and the Albedo would drop to about 0.350 (the increased angle means there is more reflection of solar irradiance even in open ocean).

Earth's overall Albedo will not change much at all unless there is a change in cloudiness or more or less ice on the planet (and it only changes by a large amount when the ice and snow moves farther away from the poles like New York). Without ice, the Albedo is going to be between 0.285 and 0.245 (assuming the cloud fraction is constant).



I've rebuilt this table for 11 different climate-continental drift scenarios and the values range from 0.252 (Pangea) to 0.517 (Snowball Earth).
Bill Illis
#81
Dec16-09, 07:38 PM
P: 34
There is a lot of new material above to absorb, but this method can now also help answer the AGW-Albedo feedback issue.

Say, doubling of CO2 increases temperatures by 1.2C, water vapour increases add another 1.2C and we are at +2.4C by 2100.

How does the Earth's Albedo change.

First, the sea ice in the Arctic is now going to melt out earlier. Between 70N to 80N it is going to melt out now in early June instead of early August. Between 80N to 90N, the sea ice is probably going to completely melt out in early August.

The Albedo between 70N to 90N is going to fall slightly (but because there is so little surface area here and the zenith angle of the Sun is so low) it is not going to make much difference. The average Earth Albedo falls to 0.2965 and the Earth warms by 0.15C.

The Greenland glaciers are going to melt back by at least one-third in the next few hundred years. Sustained over a thousand years, the Greenland glaciers are just in the northern areas. Albedo falls to 0.2955 and temperatures increase another 0.1C.

The snow melts a little earlier in the northen hemisphere. temps increase another 0.13C

The sea ice around Antarctica melts a lot earlier, another 0.12C. There is not much change in Antarctica's glaciers but a small decline adds another 0.008C.

The Earth's Albedo has fallen to 0.293 and temperatures are now up 2.9C (2.4 from GHGs and water vapour; 0.5C from Albedo changes over a few hundred years).

Now just as the Earth's Albedo is lower and temperatures are higher, the Milankovitch Cycles start to kick in again. The Axial Tilt is now 23.3 degrees and the sea ice starts refreezing in the northen latitudes and we are going back into another ice age - (the forecast for summer solar insolation in the high northern latitudes doesn't actually decline very much in the millennia ahead so I can't really say this part is going to happen. Just throwing it out there).
Saul
#82
Jan1-10, 08:12 AM
P: 272
Determining whether the planet's feedback response (clouds and related atmospheric responses) is negative or positive is fundamental to all planetary climate research past and future.

It appears the planet's feedback response to a forcing change in the tropics is negative rather than positive.

The recent paper that alleged the planet's feedback response was positive only used the long wave radiation data that is reflected into space. The feedback calculation needs to consider both long wave and short wave radiation. The total sum of long wave and short wave radiation indicates the feedback response is negative rather than positive.

The assumption of positive feedback in the General Climate Models did not make sense based on current and past observations. For example, the planet's response to a step increase such as the cooling associated with a volcanic eruption indicates the feedback it negative rather than positive. (Overdamped response.)

Negative feedback stabilizers systems such that they will naturally resist rather than amplify forcing changes. Almost all physical systems have negative feedback. It seem odd now come to think of it why anyone would assume the planet's response to a forcing change would be to amplify the change. Due to lags in physical systems, a system with positive feedback will be unstable.

If you go the very end of this paper there is a graph that compares the measured feedback response to the feedback response that is used in the climate models.


On the determination of climate feedbacks from ERBE data

By Richard S. Lindzen and Yong-Sang Choi
Program in Atmospheres, Oceans, and Climate
Massachusetts Institute of Technology

Climate feedbacks are estimated from fluctuations in the outgoing radiation budget from the latest version of Earth Radiation Budget Experiment (ERBE) nonscanner data. It appears, for the entire tropics, the observed outgoing radiation fluxes increase with the increase in sea surface temperatures (SSTs). The observed behavior of radiation fluxes implies negative feedback processes associated with relatively low climate sensitivity. This is the opposite of the behavior of 11 atmospheric models forced by the same SSTs.

Therefore, the models display much higher climate sensitivity than is inferred from ERBE, though it is difficult to pin down such high sensitivities with any precision. Results also show, the feedback in ERBE is mostly from shortwave radiation while the feedback in the models is mostly from longwave radiation. Although such a test does not distinguish the mechanisms, this is important since the inconsistency of climate feedbacks constitutes a very fundamental problem in climate prediction.
http://www.leif.org/EOS/2009GL039628-pip.pdf


http://asd-www.larc.nasa.gov/erbe/erbssat.gif
Xnn
#83
Jan1-10, 09:17 AM
P: 555
Quote Quote by Saul View Post
It seem odd now come to think of it why anyone would assume the planet's response to a forcing change would be to amplify the change. Due to lags in physical systems, a system with positive feedback will be unstable.
A positive feedback does not mean that the climate is unstable.

For example, the melting/freezing of ice is a positive feedback mechansim.

As the planet warms, ice melts which reduces the albedo.
Reduced albedo in turn allows more sunlight to be absorbed, which results in more warming.
The additional warming results in more ice melting and so on and so on.
However, at the extreme when all the ice is melted, then that feedback mechanism goes away. Eventually, additional warming will not result in more melting and albedo changes. So, eventually the system will stabilize.

Also, it's not necessary for all the ice to melt in order for the system to stabilize. However, the temperature does have to stabilize before the ice will reach equilibrium.
sylas
#84
Jan1-10, 10:27 AM
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Quote Quote by Saul View Post
The assumption of positive feedback in the General Climate Models did not make sense based on current and past observations. For example, the planet's response to a step increase such as the cooling associated with a volcanic eruption indicates the feedback it negative rather than positive. (Overdamped response.)

Negative feedback stabilizers systems such that they will naturally resist rather than amplify forcing changes. Almost all physical systems have negative feedback. It seem odd now come to think of it why anyone would assume the planet's response to a forcing change would be to amplify the change. Due to lags in physical systems, a system with positive feedback will be unstable.
There's a minor point of terminology to clear up here.

In climate studies, the term "feedback" is usually used only of processes that modify the underlying "Planck response", which is the base temperature response assuming everything else about a system remains unchanged.

However, in control theory the Planck response itself can also be treated as a feedback... a very strong negative feedback. This is described nicely in Appendix A of Bony et al (2006):
It works like this. When you raise the temperature, you start to radiate more energy; and this leads to cooling. Using the notation of Bony et al, appendix A, let ΔR be the change in Earth's energy balance from some given equilibrium condition. Let ΔQ be a "forcing"; an imbalance imposed somehow which leads to a temperature response. Let ΔTs be the change in surface temperature. As a result of the change in temperature, there will be a change in energy balance. Let λ be the amount of energy balance change per unit temperature. This is the climate response.
[tex]\Delta R = \Delta Q + \lambda \Delta T_s[/tex]
Equilibrium is restored once ΔR is back to zero, and the total climate response is the amount of temperature change ΔTs required to compensate for the forcing ΔQ.

The major effect of raising temperature is to emit radiation, in a way that can be estimated from simple radiation physics. This is about -3.2 W/m2 per degree, represented as λp. The negative convention indicates raising temperature lets Earth lose energy; it is a negative feedback and this keeps Earth stable.

There are other factors involved. As temperature increases, so does specific humidity, which gives a positive feedback from the additional greenhouse effect, and a smaller negative feedback from a reduced lapse rate. There is a change in ice cover, which is a positive feedback, as Xnn indicates. There are changes to cloud; which is much harder to determine. Most researchers believe the cloud feedback is a net positive; Lindzen is famous for arguing for a strong negative feedback from cloud responses to temperature. The paper Saul has introduced does not attempt to identify the source of the feedback; it merely attempts to measure it.

The final λ in the energy balance equation can (for small changes, of a few degrees) be approximated quite well as a linear sum
[tex]\lambda = \lambda_P + \lambda_c + \lambda_w + \lambda_i + ...[/tex]
The overall sum is negative; if it was positive then climate would be unstable, just as Saul has said. But when a paper speaks of climate feedback, they invariably mean the sum of all the terms other than the base Planck response. This is what Lindzen and Choi is arguing is negative, and what nearly all other researchers consider to be positive.

Note also; climate models do not make any "assumption" about feedback at all. The feedback is emergent from within the model, as a spontaneous consequence of the interacting processes. This is very clear in Bony et al (2006) which deals with the issue of trying to estimate feedbacks within models. This is quite tricky; because the feedback is not assumed at all.

Lindzen and Choi argue that the models are wrong; which is a point worth considering. But if so, it is because there's some pervasive error in the physics of what they are representing. The most likely candidate for this is cloud effects.

With respect to volcanoes; the response is damped, certainly; that is because the net λ is negative. However, the study of volcanic eruptions indicates that it is not as damped as you would expect from λp acting alone; this is evidence for positive feedbacks on top of the planck response. See Wigley et al (2005)
This is not adequate to refute Lindzen and Choi, of course; neither is Lindzen and Choi adequate to refute Wigley at al. To resolve the discrepancy, one or other of the papers must be fundamentally flawed; and that needs to be identified within the flawed paper itself before the matter can be considered satisfactorily addressed.

Cheers -- sylas
Xnn
#85
Jan1-10, 11:28 AM
P: 555
Quote Quote by sylas View Post
Lindzen and Choi argue that the models are wrong; which is a point worth considering. But if so, it is because there's some pervasive error in the physics of what they are representing. The most likely candidate for this is cloud effects.
I'm not so sure about that, but I'm still trying to figure out exactly what it is that Lindzen and Choi are showing us.

First, their paper is limited to the tropics (20S to 20N) and they are not including any of the land which amounts to about 22% of the tropics.

Second, figure 3 is showing us charts for long wave, short wave and a combination.
Each chart plots Flux/T versus sensitivity and feedback factor.

ERBE data and model results are both plotted along with lines for feedback.

In the LW chart, he shows SW f=0
In the SW chart, he shows LW f=0 and f=1. LW f=1 looks to be a better fit.
In the LW and SW chart, the line isn't labeled, but the ERBE data points to a sensitivty of around 0.5C while the model lines point to around 1.4C.

He also makes the following statement:

Indeed, Fig. 3c suggests that models should have a
range of sensitivities extending from about 1.5C to infinite sensitivity (rather than
5C as commonly asserted), given the presence of spurious positive feedback.
However, response time increases with increasing sensitivity [Lindzen and Giannitsis,
1998], and models were probably not run sufficiently long to realize their full
sensitivity. For sensitivities less than 2C, the data readily distinguish different
sensitivities, and ERBE data appear to demonstrate a climate sensitivity of about
0.5C which is easily distinguished from sensitivities given by models.
Figure 3C would be the LW + SW chart.

So, is Lindzen suggesting that over the long term there could potentially be an infinite climate sensitivity to CO2 based on ERBE data?


Sylas; your help here would be much appreciated!
sylas
#86
Jan1-10, 04:51 PM
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Quote Quote by Xnn View Post
Sylas; your help here would be much appreciated!
I will be taking a bit of time to look at this one. I want to understand the argument better before I attempt to describe it myself. Interestingly, one of the most specific criticisms seems to have been from Roy Spencer, who is also an advocate for very low sensitivities. When I more of a grip on both sides of that, I'll post again.

Cheers -- sylas
Saul
#87
Jan1-10, 05:09 PM
P: 272
Quote Quote by Xnn View Post
I'm not so sure about that, but I'm still trying to figure out exactly what it is that Lindzen and Choi are showing us.



He also makes the following statement:



Figure 3C would be the LW + SW chart.

So, is Lindzen suggesting that over the long term there could potentially be an infinite climate sensitivity to CO2 based on ERBE data?

Xnn,
He is saying that based on the ERBE data there is more SW radiation emitted than LW radiation not emitted when there is an increase in ocean temperature so there is a net reduction of energy top of atmosphere in response to a step change in ocean temperature.

As there is no ice or snow for the tropical case an increase in short wave radiation would indicate that there is either an increase in planetary cloud cover or there is an increase in the albedo of the clouds that do form when the ocean temperature is higher.

Lindzen's comment an infinite response (infinite is too high however his point that positive cloud feedback is not reasonable based on the planet's response to other step changes) is that theoretically if the model feedback is positive that the upper end in the planet's response is likely higher than 5C, if the model is run long term.

Because there are natural lags in all physical systems a system with positive feedback will be unstable.

This is illustrated in Spencer and Braswell's paper. Spencer and Braswell ran multiple model runs using positive feedback for clouds and found the model produced a wide range of responses.

http://www.drroyspencer.com/Spencer-and-Braswell-08.pdf

Potential Biases in Feedback Diagnosis from Observational Data: A Simple Model Demonstration
Xnn
#88
Jan1-10, 09:47 PM
P: 555
Spencers paper demonstrates that nonfeedback top-of-atmosphere radiative flux variations can cause temperature variability, which could result in a positive bias in diagnosed feedbacks. That is fine by me... lots of noise = lots of uncertainty.

However, he does not address the positive feedback of lowering albedo from melting ice.

Lindzen stated that if some of the models were run longer, they might show greater sensitivities. However, he also stated that infinity needs to be included as a potentiality. This is the same thing as stating that we need to consider the possibility that all the snow and ice will melt.

This leads us towards an understanding that the tropics and polar regions behave fundamentally different from each other. Negative feedback over the tropical oceans contrasted against a large positive feedback towards the poles.
Saul
#89
Jan2-10, 08:00 AM
P: 272
Quote Quote by Xnn View Post
Spencers paper demonstrates that nonfeedback top-of-atmosphere radiative flux variations can cause temperature variability, which could result in a positive bias in diagnosed feedbacks. That is fine by me... lots of noise = lots of uncertainty.

This leads us towards an understanding that the tropics and polar regions behave fundamentally different from each other. Negative feedback over the tropical oceans contrasted against a large positive feedback towards the poles.
If there is less snow and ice in high latitude regions that will reduce the amount of sunlight reflected to space. That is however, a separate issue. The general climate models assumed positive feedback for clouds to amplify forcing changes (warm or cold).

If the total cloud feedback (Short wave and Long wave) is negative rather than positive, the amount of warming due to CO2 increases in the atmosphere will be less planet wide.

The paleoclimatic record seems to support Lindzen and Choi's finding. When the planet was warmer the planet's climate was stable. Cloud cover increases to stop the planet from getting too warm and cloud cover decreases to stop the planet from getting too cold. Cloud cover regulates the planetary temperature.

Changing the cloud feedback in the GCM to negative rather than positive reduces the amount of warming due to a CO2 doubling to a range of 0.75C to 1.8C. (rather than 3C to 5C). I believe the 1.2C is no feedback of any kind. (Clouds, ice, or snow.)

The current planetary temperature increase of 0.7C to 0.5C (Depending on what is assumed for the original base and planetary temperature measurement assumptions.) for a 38% increase in CO2 (0.028% to 0.038%) matches what the models predict with no feedbacks of any kind.

As we know snow cover and ice cover is reduced in the Northern Hemisphere in the last decade, that would support the assertion that there is negative cloud feedback.

To get to 0.75C total increase for a doubling of CO2, a significant portion of the 20th century warming would need to have been due to solar affects on planetary albedo (low and/or high level clouds changes. Say 50%.)
Andre
#90
Jan2-10, 08:34 AM
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Quote Quote by Xnn View Post
However, he does not address the positive feedback of lowering albedo from melting ice.
True, that's outside the scope of his study.

However, maybe it is possible to do a guestimate of the importance of feedback in Arctic areas with ice albedo relative to Lindzens feedback, which is restricted to the tropics. We could for instance compare the surface areas that receive the solar insolation. For that we can simple make the Earth two dimensional, as the area of the solar flux hitting earth:



The colored areas roughly depict arctics above ~67 degrees lattitude versus tropics below ~23 degrees lattitude.

Now the formula's for calculating segment areas is here.

If we use the radius as unit we can simply calculate any segment or area below the segment with this simple spreadsheet:



with the formulas as used in row #9. We see that the arctic segment (cell e4) is about 3% of the half circle surface area, while the tropics (cell f6) are representing 48% of the surface.

It seems that a 'unit' of feedback on insolation in the tropics is about 16 times more effective than a similar feedback in the arctic.


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