Archive for Earth Sciences Forum This site is dedicated to the Earth Sciences. We are here for you to discuss issues regarding any aspect of the Earth sciences, at all levels of knowledge. Questions are welcomed, as are open scientific debates. Enjoy!!!
 



       Earth Sciences Forum Forum Index -> Atmosphere, Ocean, and Climate
Andre

positive problems for positive feedback

In the other positive feedback thread we looked at short term feedback in the atmosphere, but how about the feedback of the ice cores.

let's recap

Greenhouse effect is based on radiative properties of certain trace gasses. The basics physics of this is well known. The main point is the presence of those gasses, much more than its concentration. There is some consensus (although disputable) between warmers and sceptics that, purely based on physics, doubling CO2 concentration increases the greenhouse effect with about one degree. So if we would go from a preindustrial 280 ppmv (now ~380ppmv) to 560 ppmv in another 100 years, the pure physics alone would have increased the greenhouse effect with one degree. That's not enough for being catastrophical, is it?

But there are feedbacks, mechanisms that strenghten or weaken the effect. Positive and negative feedback. Melting snow makes the earth darker, it absorpts more light, making it warmer still, that's one example of positive feedback. Warming causes increased evaporation, which cools in the first place but it also forms clouds that reflect sunlight making it cooler, that's an example of negative feedback. The big dispute is which feedback is the strongest.

This question is impossible to solve with models alone, so we have to look back at the climatologic history, which brings us to the ice ages. So we have those ice cores with both temperatures and CO2 doing the same roller coaster ride here:



So a clear solution here? Historical data proofs beyond a doubt that a bit of CO2 changes brings about large temperature changes? Hence large domination of positive feedback? Open and shut case?

Now, suppose that that Mammoth wasn't there to spoil the whole global picture and suppose that all the suppositions about temperature were right, it is still essential to proof a large positive feedback factor to explain the these changes with CO2.

So let's have a real close look at the last spike in the Antarctic ice cores some 20,000 years ago.



These are the last data sets from the EPICA dome C (Concordia). Data here:
http://www.ncdc.noaa.gov/paleo/ic...ctica/domec/domec_epica_data.html
used these, Monnin et al 2004 for the CO2 and these, Stenni et al 2001 for the isotopes, which is supposed to be a proxy for the temperatures.

Now we see two problems. The first problem has been beaten to death as it seems. CO2 reacts several hunderd years later than the temperature (red arrows following the blue arrows). Yes ...yawn...what else is new, we explained that a hunderd times. First the Milankovitch cycles trigger a bit of warming, that warms the oceans which causes CO2 to release from the oceans, which takes over the warming function to create more warming which releases more CO2 from the ocean. Strong positive feedback. Go and have somebody else bored...yawn.. (sorry imitating the real climate team).

But something is very wrong with that explanation for this given sequence, if you know what positive feedback really does.

system feedback is a complicated matter on which 100s of engineers earn their living. Every system had embedded feedbacks, also natural systems with predictable to weird reactions.

Let's go to wikipedia:

Quote:
The end result of a positive feedback is often amplifying and "explosive." That is, a small perturbation will result in big changes. This feedback, in turn, will drive the system even further away from its own original setpoint, thus amplifying the original perturbation signal, and eventually become explosive because the amplification often grows exponentially (with the first order positive feedback), or even hyperbolically (with the second order positive feedback)


So we see the temps go up first, followed a few hundred years later by the CO2 due to system lag, giving another heating input as positive feedback. That lag is crucial because lag works two ways, it delays going up and going down like inertia. A car needs time to accelerate and time to stop. However this ice core "car" stops instantenously halfway without any delay, while the CO2 is still continuing upwards, pulling the temps up, or not?

Instead the CO2 just follows the temperature with the same delay, a follower is not a feedbacker. On the contrary, the enhanced steering away form the stable centre position makes strong positive feedback systems having a strong affiliation with the system extreem values, either low or high with very low sensitivity for natural disturbances halfway.

That's the ugly fact, which slays a beautiful hypothesis, the great tragedy of science (Thomas Huxley)

No strong enough positive feedback here



The standard reaction upon this is but how about strong natural variability? CO2 is not the only factor?

However, a positive feedback system in transit from one system limit to the other limit is practicaly insensitive to other factors besides the positive feedback factor itself i.e. CO2. You can't stop it but yet it happens.

Although the math of feedback is rather complicated, it is surprising easy to simulate with a basic model as I did here with an excel spreadsheet:


Click to download file

here you can see what the graph should have looked like when the strong positive feedback was to be true:



Note that the red temperature response and the orange CO2 never noticed the reversal of trend in the natural "basic forcing" function. They both just jumped up until they hit the roof.

When you tune down the positive feedback considerably then they start noticing the natural trend reversal:



but we still see a very gradual response to the abrupt trend reversal in the "basic forcing" due to the inertia in the delay, cars don't stop and reverse instantaneously. Yet the ice cores do, proving that even this weak positive feedback is too strong.

Hence the ice core strong response to any forcing half way the transition proofs that there cannot be any significant positive feedback from CO2.
Latecommer

Andre, how does the model respond to zero or negative feedback...as a in " a follower is not a feedback"?
Andre

Latecommer wrote:
Andre, how does the model respond to zero or negative feedback...as a in " a follower is not a feedback"?


Doug, I made a long reply, uploaded a lot of graphs and a new excel sheet and then I lost the post due to stupid finger trouble. I hope that billiards knows how to find the uploaded files back, otherwise I'll have to do it again but don't expect surprises.

       Earth Sciences Forum Forum Index -> Atmosphere, Ocean, and Climate
Page 1 of 1
Create your own free forum | Buy a domain to use with your forum
Wordpress Theme|Debt Consolidation|Submit articlesWinunited Bonuses|DJ Store|Debt Consolidation