Estimating the impact of CO2 on global mean temperature

[sylas]

The transparent windows do not move, they are fixed wavelengths lying between the bands absorbed by CO2, Water Vapor and other GHG's.
The surface temperature, and thus the emission spectrum, varies, so the percentage of freely escaping outgoing IR for any given concentration of GHG is not a constant (even before considering dynamic processes like convection.)

Oops. Quite right, of course. Thanks for picking that up.

I tend to think of velocity as relative! (I must have been thinking of the relativity threads I'm engaging... I'm aware that the bands are fixed, but I phrased things there using the spectrum peak as a reference frame.)

If you consider the spectrum as a reference point, then the bands move; but of course you are quite right that it is actually the peak of the emission spectrum which is moving and I should have phrased it the other way around. I'm not actually thinking that there is a change in the absolute frequency of the transmission and absorption bands! But did you look at the diagrams that show just how little shift there is in the emission spectrum, for a temperature increase of 10 degrees?

The percentage difference as a result of the movement in the peak of the emission spectrum is trivial; and note that the tropopause spectrum doesn't actually move much at all, unless maybe in the other direction! In either case, what matters for climate is absolute quantities of emission radiation... the forcings. The calculation of absolute radiation values already incorporates the shift of spectrum with temperature. It falls out naturally from the fact that all this is done with valid physics. If you try to single out the movement of the emission spectrum towards higher frequencies, you are looking at a completely trivial effect, which is already fully a part of the calculations of total impact.

For example, wouldn't more vegetative cover result in a darker surface and thus a shorter (warmer) wavelength IR emission from the surface? Land use satellites monitor these color changes regularly, and globally.

Yes. This is a part of climate models also, in the most recent generation of models. The net effect is small, but it can matter with regional forcings.

You are correct that the physics of the greenhouse effect is not hacked science (although the term "greenhouse" is something of a misnomer). The basic labaratory physics are well known and demonstrable.

The application of this solid physics to a dynamic fluid system in order to formulate extremely long term projections appears to be where the "hacked guesswork" lies.

Grins...

Shrug... I take your point; except that hacked guesswork is not a good description IMO. Fluid mechanics is not the major problem. For a better example of where models are a long way short of thorough physical modeling, consider cloud effects. A cloud is much smaller than a grid cell in a climate model, and so the models use abstractions, that summarize broad aspects of cloud cover in a region; percentage cover, altitude distributions, composition etc. There's a lot of work in making and testing such abstractions. So while it is certainly true that they are a long way short of a complete physical model, it is a lot better than guesswork.

Another point where models lack the resolution to capture the physics in detail is ocean transport of heat. Small scale eddy effects are unclear and have to be represented with parametrized abstractions... and how those change over time as the planet heats up is another uncertainty.

As I have said recently in another forum... an appropriate degree of scientific skepticism is important when looking at a complex subject like this. What's proper is the normal practice of working scientists right now. They don't make grand claims of perfection where there is real uncertainty, and papers are typically hedged throughout with explicit recognition of problems and uncertainties. As a body of literature, climate studies are full of open debate and disagreement, and something like the IPCC assessment reports has uncertainties and alternatives strongly emphasized throughout. It's also important to look at the impact of uncertainties. Some aspects of climate are fairly robust in the face of uncertainty in other facets.

Where skepticism goes off the rails is when it turns into a head-in-the-sand refusal to accept anything at all until it's all perfect. I'm not saying that's you, by the way! But it is certainly common in the "denialist" literature, and it is so blatant that a phrase like "denialist" is appropriate. It's not skepticism any more at that point, in my opinion.

Some things are known as well as we know anything in physics... like the absorption bands of CO2 and increased thermal absorption with increasing concentrations, which can be studied in a lab as you note. And yet it's all grist to the mill for the hard core denialist, who will even seize upon pseudoscience like the recent Gerlich and Tscheuschner paper denying that the atmospheric greenhouse effect works at all, cited recently in this thread.

With this thread I'm not trying to solve the whole climate problem, but to help explain greenhouse gases, and carbon dioxide in particular, necessarily stands as a major impact for shifting climate in recent decades. That's a fact, as much as anything is a fact.

Cheers -- sylas

PS. You're right that the word greenhouse is not perfect... but it's not that bad either. Both an atmospheric greenhouse and a glasshouse work by trapping heat and inhibiting a vertical transport of heat up from the surface. The main factor in a glasshouse is blocking convection, and the main factor in the atmospheric greenhouse by blocking radiant emission, so physically the process is somewhat different. But the net effect is similar and for much the same reason. The surface has to heat up more to shed the solar energy it is receiving.

 

[skypunter]

Your last post puts us in agreement at least as to the nature, if not the degree, of uncertainty in climate models.
I agree that scientists are generally aware of the uncertainties. It's the mainstream media and the political body that it feeds which fail to take them into account.
Your point is well taken that this thread is concerned only with the impact of CO2. Please excuse the divergence.
Sincere thanks for the lively discussion!

 

[joelupchurch]

Your last post puts us in agreement at least as to the nature, if not the degree, of uncertainty in climate models.
I agree that scientists are generally aware of the uncertainties. It's the mainstream media and the political body that it feeds which fail to take them into account.
Your point is well taken that this thread is concerned only with the impact of CO2. Please excuse the divergence.
Sincere thanks for the lively discussion!

I think there may also be issues about how scientists present their findings about climate models to the lay community. There is an interesting paper about it: Seductive Simulations? Uncertainty Distribution Around Climate Models

http://www2.geog.ucl.ac.uk/~mdisney/teaching/1006/papers/lahsen_gcm.pdf"

There is also the issue that the underlying process may be chaotic and not predictable even in theory.

 

[skypunter]

There is also the issue that the underlying process may be chaotic and not predictable even in theory.

I cannot resist.

This link describes an attempt at practical use of fluid dynamic modelling. It might be viewed as a microcosm of the climate modeling dilemma.

http://www.lassc.ulg.ac.be/bibli/MinetHeyen-2001.pdf

Many blast furnace designers have returned to designing furnaces the old fashioned way, trial and error.

Another apology for another side-track.

 

[Skyhunter]

Here is http://globalchange.mit.edu/pubs/abstract.php?publication_id=1974"

Abstract

The MIT Integrated Global System Model is used to make probabilistic projections of climate change from 1861 to 2100. Since the model's first projections were published in 2003 substantial improvements have been made to the model and improved estimates of the probability distributions of uncertain input parameters have become available. The new projections are considerably warmer than the 2003 projections, e.g., the median surface warming in 2091 to 2100 is 5.2°C compared to 2.4°C in the earlier study. Many changes contribute to the stronger warming; among the more important ones are taking into account the cooling in the second half of the 20th century due to volcanic eruptions for input parameter estimation and a more sophisticated method for projecting GDP growth which eliminated many low emission scenarios. However, if recently published data, suggesting stronger 20th century ocean warming, are used to determine the input climate parameters, the median projected warning at the end of the 21st century is only 4.1°C. Nevertheless all our simulations have a much smaller probability of warming less than 2.4°C, than implied by the lower bound of the IPCC AR4 projected likely range for the A1FI scenario, which has forcing very similar to our median projection. The probability distribution for the surface warming produced by our analysis is more symmetric than the distribution assumed by the IPCC due to a different feedback between the climate and the carbon cycle, resulting from the inclusion in our model of the carbon-nitrogen interaction in the terrestrial ecosystem.

Full article available here: http://dx.doi.org/10.1175/2009JCLI2863.1

 

[Skyhunter]

Many blast furnace designers have returned to designing furnaces the old fashioned way, trial and error.

Not wishing to derail the topic, but do you have any reference or source to support that assertion?

And what do you mean by many? A majority? A significant minority? More than three?

 

[bellfreeC]

the hard core denialist, who will even seize upon pseudoscience like the recent Gerlich and Tscheuschner paper denying that the atmospheric greenhouse effect works at all...

The surface has to heat up more to shed the solar energy it is receiving.

Still, the muddled thinking I see...

The surface does no such thing. I don't even think convection is the real key difference. Most of the so-called global warming occurs at night over the Arctic. How far North now can one build a successful Greenhouse that rarely needs extra heat by artifical means? I doubt that has changed much. The loss of glaciers have a more albedo {as well as a key process transpiration} cause then from a trace gas called carbon dioxide yet vital as plant food and therefore vital to humans. This is not to say drinking water isn't important but in the Arctic circle or just below, this is hardly a concern compared to the bitter cold!

MrB.

 

[skypunter]

The dipole physics of the CO2 molecule are well known.
The premise of this thread, "Estimating the impact of CO2 on global mean temperature" seems moot without discussing the extent of the mitigating or extenuating effect of dynamic processes such as albedo change, circulation and variable insolation, to name a few.
Otherwise the percentage of CO2's effect may only be compared against the known effect of other gases "in a jar".
Such a partial equation is of little practical benefit.

 

[sylas]

The dipole physics of the CO2 molecule are well known.
The premise of this thread, "Estimating the impact of CO2 on global mean temperature" seems moot without discussing the extent of the mitigating or extenuating effect of dynamic processes such as albedo change, circulation and variable insolation, to name a few.
Otherwise the percentage of CO2's effect may only be compared against the known effect of other gases "in a jar".
Such a partial equation is of little practical benefit.

What premise do you mean? I'd really like to know. I have deliberately kept this thread pretty basic, WITHOUT reliance on premises other than what is very solidly established science. Go back to the OP and look at the six steps for how I have tried to be clear about what I am assuming.

I'm not trying to solve the whole climate problem. I am trying to help explain one issue which stands as one of the most solidly confirmed discoveries of climate science, and yet which remains a widespread focus of poorly informed public "skepticism".

CO2 has a straightforward direct impact on temperature, by absorbing thermal radiation. The effect is very well quantified, and not in any scientific doubt whatsoever. It is a large effect. It is not based on dubious inferences or indirect correlations. When you add substantial amounts of greenhouse gases to the atmosphere, you are bound to be increasing temperatures as surely as if you increased solar luminosity. It's THAT basic.

To call this of "little practical benefit" is just surreal!

The "mitigating" processes you mention are no such thing.

Albedo effects are either part of the sensitivity of climate, and add up to a net increase in sensitivity and a stronger impact for ANY forcing, or else (mainly as part of a cloud impacts) they are another forcing... and a negative one at that, and so quite impossible to displace the main conclusion of this thread, that there is a very straightforward physical reason for emphasizing greenhouse effects and CO2 in particular as the major driving factor for heating up the planet in recent decades.

Circulation is about redistribution of energy. It can alter the rate at which the climate system comes to equilibrium, because it affects the rate at which heat is taken up into the ocean. But it is not a source of energy, and does not drive a trend in global net increase or decrease in temperatures. It is a major complexity in climate; but it is not a premise that makes a blind bit of difference for the main conclusions of this thread. I am not trying to make a climate model or calculate the rate at which temperatures will change or regional distributions of effects.

Variable insolation is another forcing that can be quantified... and it is much less than the greenhouse forcing, by far.

Look, there's no question that the whole scientific problem of understanding climate is difficult and involves many factors. I can appreciate that there are some skeptics who understand the basic physics of greenhouse effects and focus on genuinely open questions in climate science. But most popular climate "skepticism" is credulous naivety over points that have long since been well confirmed basic scientific discoveries. Most popular skepticism has all the validity and rigor of creationism or intelligent design in biology, and there's a place to help explain some of the really basic stuff.

In particular, many people still think that the whole carbon dioxide and greenhouse link to climate is dubious. It isn't. It is basic applied physics, and a foundation for all the real open questions.

Here at physicsforums we have an audience that is mostly reasonably clued up and interested in physics, and able to follow some of the details of WHY greenhouse effects are scientifically so uncontroversial as the major cause of global heating in recent decades.

Cheers -- sylas

 

[skypunter]

What premise do you mean?
Cheers -- sylas

The premise that one can assess or estimate the impact of one factor without considering all factors. The ratio of one factor to an unknown whole is an unknown.

 

[skypunter]

I have a salad which contains six ounces of carrots.
I double the amount of carrots.
I remove six cucumber slices.
What is the ratio of carrots to cucumbers by weight?
What percentage of the entire salad do the carrots represent?

 

[mheslep]

...
... But most popular climate "skepticism" is credulous naivety over points that have long since been well confirmed basic scientific discoveries. Most popular skepticism has all the validity and rigor of creationism or intelligent design in biology, and there's a place to help explain some of the really basic stuff.
...
Here at physicsforums we have an audience that is mostly reasonably clued up and interested in physics, and able to follow some of the details of WHY greenhouse effects are scientifically so uncontroversial as the major cause of global heating in recent decades.
...

I would prefer that the social insights on skeptics, the pronouncements upon who is clued up or not, and where the consensus lies all go to another subforum, and that just the science remain here.

 

[sylas]

The premise that one can assess or estimate the impact of one factor without considering all factors. The ratio of one factor to an unknown whole is an unknown.

There are two problems I have with that.

1. Scientists DO consider all possible factors.
2. It's wrong anyway. You CAN determine that the impact of one factor without knowing the impact of others, and you can ALSO determine that it is necessarily significant if that impact is comparable in magnitude to what is required to give the total observed effect.

On the first point, of course scientists are looking for all possible factors, and there is definitely nothing that comes close to greenhouse as a positive forcing. Other forcings are typically much less accurately known, but there's no prospect whatever for anything to be a strong positive addition on top of what you already have as a large greenhouse forcing. You can't hide something that big!

On the second point, consider the method I used in this thread. The impact of CO2 as a forcing is completely independent of what other forcings may exist. Furthermore, you can show that this impact is necessarily significant if the temperature change expected for that forcing is of a comparable magnitude to the observed effect of measured temperature increases. It is... and therefore CO2 is necessarily significant, no matter WHAT else is involved.

Note that I am NOT attempting to calculate a "ratio". That's a red herring. I'm simply doing what the thread title says... estimating the impact of CO2 on global mean temperature.

Consider a case where water levels are dropping in a reservoir. In investigating possible causes, you find that there's a leak, and that that further investigation suggests something between 2 and 5 Mlitres/day will be lost in the leak. The water level drop indicates about 4 Mlitres/day is being lost.

You've not shown that the leak is all that matters. You HAVE shown that the leak is significant.

This is what I have done with CO2 in this thread. The CO2 forcing is well known basic physics. The total impact is large. It is necessarily significant, no matter what other forcings are involved, because it is of a comparable magnitude to the forcing required for observed global temperature increases.

It would be possible to go into a lot more detail, and look at the work in quantifying, with uncertainty limits, all the various forcings involved. Such a study would show that CO2 represents a bit over half the total greenhouse impact, and that non-greenhouse forcings are much less well known, but negative, or else an order of magnitude smaller. So scientifically speaking, there's still lots of open questions and uncertainty, but the fact that CO2 has a significant impact on present climate changes is about as solid as anything ever gets in science.

Note that my original post did not claim that CO2 is the largest single forcing. In fact, it is, quite easily, but to show that would have required a longer and more detailed argument. So I kept it at an even more basic level. I'm trying to keep this as simple as possible, because so much popular misconception is at this really fundamental disconnect from basic physics.

Cheers -- sylas

 

[chriscolose]

The premise that one can assess or estimate the impact of one factor without considering all factors. The ratio of one factor to an unknown whole is an unknown.

But we do consider other factors, and we do have a clue as to what the important factors are. If we knew nothing about the time-evolution of all factors which affect global radiative balance except CO2 then global temperature prediction would not be a good idea, but fortunately we have pretty good constraints on those things from the Holocene. For instance it is very implausible that the sun will dim a full 21 W/m2 or so that would be needed to offset a doubling of CO2. If a big asteroid hit the Earth then there's no longer any use for current projections over this century, but that is why they are projections. There's also a lot of research going into feedbacks or sensitivity as a whole, with constraints coming from modelling, paleoclimate records, and observations. So we do have very good insight into how the global temperature will evolve over the century as CO2 increases rapidly.

 

[skypunter]

You can't hide something that big!
Cheers -- sylas

Like dark matter?

Furthermore, you can show that this impact is necessarily significant if the temperature change expected for that forcing is of a comparable magnitude to the observed effect of measured temperature increases. It is... and therefore CO2 is necessarily significant, no matter WHAT else is involved.

The change expected does not meet observation.
We could have an ice age with high CO2 levels. The CO2 would cause a retention of a certain baseline of heat, thankfully, but the cause of the cooling would be something else altogether.

Note that I am NOT attempting to calculate a "ratio". That's a red herring. I'm simply doing what the thread title says... estimating the impact of CO2 on global mean temperature.

I would take that to mean the impact in relation to other causes. You must be measuring against something or you have no measurement. That relationship is a ratio.

Consider a case where water levels are dropping in a reservoir. In investigating possible causes, you find that there's a leak, and that that further investigation suggests something between 2 and 5 Mlitres/day will be lost in the leak. The water level drop indicates about 4 Mlitres/day is being lost.
You've not shown that the leak is all that matters. You HAVE shown that the leak is significant.
Analogies are fun, helpful and instructive.
Consider that a porus strata may be absorbing some of the water.

This is what I have done with CO2 in this thread. The CO2 forcing is well known basic physics. The total impact is large. It is necessarily significant, no matter what other forcings are involved, because it is of a comparable magnitude to the forcing required for observed global temperature increases.

How do we account for the decreases?

It would be possible to go into a lot more detail, and look at the work in quantifying, with uncertainty limits, all the various forcings involved. Such a study would show that CO2 represents a bit over half the total greenhouse impact, and that non-greenhouse forcings are much less well known, but negative, or else an order of magnitude smaller.

Such a study does not exist.

So scientifically speaking, there's still lots of open questions and uncertainty, but the fact that CO2 has a significant impact on present climate changes is about as solid as anything ever gets in science.

CO2 is certainly an important "greenhouse gas" which helps the Earth retain heat. That much is solid. Its impact on "climate changes" is probably less understood than the still mysterious science of aerodynamics.

Note that my original post did not claim that CO2 is the largest single forcing. In fact, it is, quite easily, but to show that would have required a longer and more detailed argument. So I kept it at an even more basic level. I'm trying to keep this as simple as possible, because so much popular misconception is at this really fundamental disconnect from basic physics.

Sometime a basic level of discussion is the best course, so long as the conclusions are justified by the argument. I see no proof in basic physics that CO2 is the largest single forcing.

Roquefort, Bleu Cheese or Italian dressing on your salad?

 

[sylas]

Can I suggest we stick with normal quoting conventions? I have replaced the red font with quote tags as appropriate.

Like dark matter?

Yes. Dark matter in the solar system can serve as another example where unknown factors cannot possibly refute the plain significance of known factors.

We know for a fact that there's more normal matter than dark matter within the solar system. It doesn't matter that we can't measure it directly... we CAN measure the orbits of planets, and there's no WAY that there's enough dark matter around the solar system to compare with the mass of the Sun or major planets, because if there WAS then we'd notice the effect on orbits.

Clear so far?

In precisely the same way we know that there isn't some large "dark forcing" on Earth's climate. We can calculate the forcings of known effects very well, and they are large. The known forcings are ample to account for what we observe in climate changes. There is room for some unknown forcings to be involved; but not by much! Scientists ALREADY consider and quantify as many forcings as they can.

Don't mix up dark matter in the whole galaxy with climate! The difference is like night and day! On galactic scales, dark matter is the larger factor, because gravitational effects of known visible matter are much too small to account for observed motions. That's not remotely like climate. So-called "skeptics" may like to suggest that the known forcings for climate are inadequate, but that's flatly false, of course. There's no need for any "dark forcing" at all to account for observations. You can't rule out new discoveries, but there's no credible prospect of major unknown forcings as large as the ones already known and studied. It's a bit like the solar system where there may be dark matter influences yet to be discovered, but there's no credible prospect of such unknowns being as large as the major gravitational forces known so far.

The forcing of carbon dioxide since pre-industrial times is 1.7 W/m². That's really fundamental physics, and known to high accuracy, with errors of 10% or less. You get it from 3.7 W/m² per doubling, and an increase from 280ppm to 385ppm.

1.7 = 3.7 \times \log_2(385/280)​

The sensitivity of climate is less well known, but it has to be something between 0.5 and 1.2 degrees per W/m². (References in msg #1.) So the forcing from carbon dioxide alone has to be worth something from 0.8 to 2.0 degrees. And the temperature rise since pre-industrial times? About 0.75 (+/- 0.2) degrees.

You can't make a direct match of these numbers, because there ARE lots of other factors. There are other forcings, both positive and negative, and there is also a delay in total response. Something from 0.25 to 0.75 W/m² of forcing is so far directed into the flux of energy into the oceans, and represents warming which will be realized as the ocean comes up into an equilibrium again. (See the thread [thread=311982]Ocean Heat Storage[/thread] for more detail.) All this represents open research questions; but it is science being built on known physics and empirical data -- data that shows that the impact of carbon dioxide is necessarily significant.

Furthermore, you can show that this impact is necessarily significant if the temperature change expected for that forcing is of a comparable magnitude to the observed effect of measured temperature increases. It is... and therefore CO2 is necessarily significant, no matter WHAT else is involved.

The change expected does not meet observation.
We could have an ice age with high CO2 levels. The CO2 would cause a retention of a certain baseline of heat, thankfully, but the cause of the cooling would be something else altogether.

Reference please -- and a coherent account of what change you mean and what observation. It looks like an outright error or misunderstanding of the state of observation and expectation.

In fact, observed changes DO match well within the range of what is expected. This is empirical science, and scientists are working hard to constrain all the various uncertainties; but the basic expectations considered in this thread, of the large CO2 forcing and the empirically constrained climate sensitivity, are all entirely in accord with observations. They are based on observations.

Your undefended assertion about ice ages and high carbon dioxide levels is physically impossible... given the prevailing conditions on Earth. Of course, if you are going back hundreds of millions of years, with continents drastically rearranged and a younger dimmer Sun, then everything becomes a lot less clear. The only ice ages since then have been in the Quaternary period (the last 2 million years or so) and these ice ages are always linked with greatly reduced CO2 levels. Always.

Note that I am NOT attempting to calculate a "ratio". That's a red herring. I'm simply doing what the thread title says... estimating the impact of CO2 on global mean temperature.

I would take that to mean the impact in relation to other causes. You must be measuring against something or you have no measurement. That relationship is a ratio.

Still a red herring.

I DID give the magnitude of the carbon dioxide forcing in relation to other measurements... measurements of temperature change. That's enough for what I have shown in the thread. No matter what other forcings are involved, we KNOW that the forcing of carbon dioxide is of a similar magnitude to the total effect of observed increasing temperatures. That means it is necessarily significant.

The relationship to other causes could be used as well, which would make carbon dioxide stand out even more. But keep it simple. You can compare the expected impact of carbon dioxide with the observed consequence of all forcings whatever they may be. That's what I did in the original post.

Do you have any problem with the sequence of steps? It should be pretty straightforward! Here again is the comparison, quoted from message #1.

For example, over recent decades the rate of increase of CO₂ has been around about 2ppm/year, on top of about 385ppm. The corresponding contribution of CO₂ to rising temperature is about S_e*Ln(387/385), which is in the range 0.011 to 0.034 C/year, with a best estimate of 0.022 C/year.

Direct measurements indicate that globally averaged surface temperatures on Earth are increasing in recent decades, at around about 0.02 C/year. (Brohan et al, 2005)

I'll give the other method below, using comparisons with other forcings, just for completeness.

Consider a case where water levels are dropping in a reservoir. In investigating possible causes, you find that there's a leak, and that that further investigation suggests something between 2 and 5 Mlitres/day will be lost in the leak. The water level drop indicates about 4 Mlitres/day is being lost.
You've not shown that the leak is all that matters. You HAVE shown that the leak is significant.

Analogies are fun, helpful and instructive.
Consider that a porus strata may be absorbing some of the water.

Right. Now THINK. No matter what other factors may be involved, the magnitude of the leak shows that it HAS to be a significant part of the total reservoir losses. You DON'T NEED to know about the porus structure to figure that out. A study of the magnitude of the leak shows for any reasonable person that the leak is significant. There may well be other significant effects, but a comparison of the impact of the leak with the magnitude of total losses is enough to establish that the leak is necessarily a significant part of the whole picture. OK?

Now pay real close attention here. I've said this throughout the thread and just maybe you'll get it this time.

I am NOT trying to solve the whole climate problem. I know there are other factors than carbon dioxide. I am trying to address one point which is a matter of popular confusion and uncertainty. This is basic basic science; and not dubious in the slightest. Carbon dioxide is necessarily an important factor driving increasing temperature in recent decades.

This is what I have done with CO2 in this thread. The CO2 forcing is well known basic physics. The total impact is large. It is necessarily significant, no matter what other forcings are involved, because it is of a comparable magnitude to the forcing required for observed global temperature increases.

How do we account for the decreases?

What decreases do you mean? As I have said throughout this thread, climate is not a completely solved problem. I hope this isn't yet another red herring to into some other issue. We are NOT trying to solve the whole climate problem here. Even if we had no idea what causes short term variations, it would make no difference to the basic scientific demonstration of the significant of carbon dioxide, that is the topic of this thread.

But let me guess -- because in fact we do have some reasonable indications of the causes for short term decreases.

• There's a small decrease in global temperatures in the middle of the twentieth century. A significant part of that is mostly likely driven by aerosol forcings.
• There's a lot of short term variation in the climate record, driven by a number of factors. Volcanic eruptions have given some sharp dips in the record, and recently there's been a small impact from the extended solar minimum. There is also a larger impact from the ENSO oscillation, which I have mentioned previously in the thread, and which is the major cause for 1998 being well above the normal trend and 2008 substantially below. The overall trend still remains positive over this time. It is also easy -- and silly -- to cherry pick data over a few years only and find decreases just from unforced variation.

None of this makes a blind bit of difference to the straightforward demonstration of the significance of the carbon dioxide impact, which is necessarily a crucial factor no matter what other impacts are involved.

It would be possible to go into a lot more detail, and look at the work in quantifying, with uncertainty limits, all the various forcings involved. Such a study would show that CO2 represents a bit over half the total greenhouse impact, and that non-greenhouse forcings are much less well known, but negative, or else an order of magnitude smaller.

Such a study does not exist.

Of course it does! This is a whole field of investigation with hundreds of papers. There's been a heck of a lot of work in quantifying all the various forcings involved. Here's a summary of what is known, along with uncertainty bars. (Figure 2.20 from the IPCC 4AR, WG-1, chapter 2.)

Points to note. The carbon dioxide forcing is about 1.7 W/m², as I have calculated. This is the largest single positive forcing. All other greenhouse gases combined give a similar positive forcing on top of the CO₂ contribution. The largest negative forcing is from aerosols, and the uncertainty is large. A distribution of possible total forcing is shown at the bottom.

You can give an independent estimate of the total forcing based on the total temperature change and ocean heat flux, and you get the same basic range of what is scientifically credible... the net forcing is somewhere from 0.6 to 2.4 W/m². CO₂ gives about 1.7 of that, and so stands as the largest single heating influence, though of course the other lesser forcings are crucial when attempting to get a complete picture.

There are hundreds of papers involved in this kind of study, so it's a bit of dilemma to know what to cite! But here are two, both of which give a kind of review of the field:

• Shine, K.P. (2000) Radiative Forcing of Climate Change, in Space Science Reviews, Vol 94, No 1-2, pp 363-373.
• Joos, F. and Spahni R. (2008) http://www.pnas.org/content/105/5/1425.abstract, in PNAS Vol. 105 no. 5 pp 1425-1430

Note that my original post did not claim that CO2 is the largest single forcing. In fact, it is, quite easily, but to show that would have required a longer and more detailed argument. So I kept it at an even more basic level. I'm trying to keep this as simple as possible, because so much popular misconception is at this really fundamental disconnect from basic physics.

Sometime a basic level of discussion is the best course, so long as the conclusions are justified by the argument. I see no proof in basic physics that CO2 is the largest single forcing.

As I told you, I have made no attempt to give such a proof. The word "proof" is a problem, because science does not really deal in "proof", but evidence.

To keep this thread simple, I stuck with calculation of forcing and comparison with empirical observations of temperature change and sensitivity. This demonstrates for anyone with minimal literacy in science that the carbon dioxide impact is necessarily a significant factor for recent global warming... but it is not strictly "proof".

In the same way, you can take the large body of literature on forcings, summarized in the above diagram, as an overview of the state of scientific knowledge so far. Carbon dioxide does indeed stand out as the largest single effect for heating of the planet at this time. There are still large error bars there, and of course the whole vexed issue of sensitivity to forcing, time scales, regional distributions of change and so on; but there's really no credible prospect at all of some other positive forcing being as big as the CO₂ forcing. That's why scientists conventionally talk about "anthropogenic global warming", even while working away at all the stuff that remains unknown.

What makes popular dispute on climate stand out from real scientific dispute is that so many people are fixated on stuff that is really not in any credible doubt. That's why I am trying to focus on some of the basics here.

Cheers -- sylas

 

[Skyhunter]

Roquefort, Bleu Cheese or Italian dressing on your salad?

Just a little oil and vinegar thank you.

 

[Patriot Vet]

Originally Posted by sylas
It would be possible to go into a lot more detail, and look at the work in quantifying, with uncertainty limits, all the various forcings involved. Such a study would show that CO2 represents a bit over half the total greenhouse impact, and that non-greenhouse forcings are much less well known, but negative, or else an order of magnitude smaller.

Such a study does not exist.

Of course it does! This is a whole field of investigation with hundreds of papers. There's been a heck of a lot of work in quantifying all the various forcings involved.

Yes, but that is only for anthropogenic sources, not all of them. Water vapor is a forcing, as well as a feedback. To exclude the major forcing / warming constituent is not entirely fair in their calculations. It also appears that contrary to popular opinion, water vapor is the largest single forcing.

• Kiehl, J. T., and K. E. Trenberth, 1997: http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/KiehlTrenbBAMS97.pdf" . Bull. Amer. Met. Soc., 78, 197-208.

Like dark matter?

Exactly, even though we can't see it we know it there because of it's gravitational influence.

It could be http://arxiv.org/PS_cache/astro-ph/pdf/0608/0608602v1.pdf" . It is good that they are still encouraging skepticism until they have definitive proof. But this is off topic, isn't it?

known forcings

Is it possible that there are forcings that are not accounted for? Or do we know all of them already? And, do we know, to enough certainty, how all affect one another?

Is it possible that the impact could be lowered http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B757C-48C7BXC-S&_user=10&_coverDate=12%2F31%2F1984&_rdoc=1&_fmt=high&_orig=browse&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f14b9b1ed259e7171efa86b6aa09577f", and not raised?
http://www.ejournal.unam.mx/atm/Vol03-1/ATM03101.pdf" .

 

[chriscolose]

Patriotvet,

Water vapor cannot be thought of as a forcing under Earth's current climate regime because it's condensible at Earth-like pressures and temperatures, and is thus controlled by the underlying climate state. The atmosphere has an essentially infinite source of water vapor (the oceans) yet the upper limit of its concentration is constrained by Clausius-Clapeyron. Nothing in Kiehl and Trenberth should give you a different impression. On the other hand, CO2 concentration is not set by temperature or circulation, but rather by sources and sinks, and it will not condense and precipitate out under a climate like Earth. As such, CO2 can continue to build up in the atmosphere, and it can do so independently of temperature.

Given the uncertainty in aerosol forcing as well as climate sensitivity, there is still plenty of wiggle room for possible forcings which are not yet taken into account, although it's very unlikely that they can be significant (in the sense that they are comparable to aerosol or methane changes). There are only 2 ways to change the global radiative balance of the planet, which is to change the incoming energy (either by changing the solar intensity or the planet's albedo), or the outgoing energy, so it's a lot harder to miss something important than people might think. In any case, as sylas pointed out, it is easily shown that CO2 is very significant and that impact is independent of other climate variables.

 

[mheslep]

... On the other hand, CO2 concentration is not set by temperature or circulation, but rather by sources and sinks,

CO2 concentration is not set (in part) by temperature? Ocean uptake and release in particular is dependent on temperature.

 

[Skyhunter]

CO2 concentration is not set (in part) by temperature? Ocean uptake and release in particular is dependent on temperature.

http://www.learner.org/courses/envsci/visual/animation.php?shortname=anm_geocarboncycle

The carbon cycle on geologic time scales.
http://www.learner.org/courses/envsci/visual/vis_bytype.php?type=animation

 

[mheslep]

http://www.learner.org/courses/envsci/visual/animation.php?shortname=anm_geocarboncycle

The carbon cycle on geologic time scales.
http://www.learner.org/courses/envsci/visual/vis_bytype.php?type=animation

Thank you

 

[chriscolose]

CO2 concentration is not set (in part) by temperature? Ocean uptake and release in particular is dependent on temperature.

Of course, although this impacts sources and sinks and not CO2 concentration directly, and the corresponding changes in CO2 generally occur over much longer timescales. This is a function of the underlying biogeophysical boundary conditions, and it generally happens that changing the climate will change the chemistry of the atmosphere (through ocean solubility or weathering, etc on longer timescales), although there is no physical law which mandates it to do such. We live in a very fortunate circumstance with oceans and other processes which can keep CO2 well constrained in its atmospheric concentration. In the case of water vapor which has a very short residence time in the atmosphere, changing the global temperature will result in a roughly exponential change in the saturation vapor pressure allowing the H2O to condense and precipitate out once it builds up enough. In the case of CO2, there is really no limit (aside from fossil fuel reserves and economic activity) as to how much we can release and build up its atmospheric concentration, and that concentration will rise even if the temperature doesn't change beforehand. The increase in atmospheric water vapor is dictated by the Clausius-Clapeyron relation, which is a well-founded principle of physics.

 

[skypunter]

"Dark Matter" and "Dark Energy" are basically a placeholders for an unknown factor in the prominent theory of the universe, the big bang. Astrophysicists are quite honest about this unresolved discrepancy.
That is why the analogy is appropriate in terms of the "forcing" equation.

 

[chriscolose]

http://www.learner.org/courses/envsci/visual/animation.php?shortname=anm_geocarboncycle

The carbon cycle on geologic time scales.
http://www.learner.org/courses/envsci/visual/vis_bytype.php?type=animation

Interesting site, thanks.

 

[skypunter]

None of the radiative forcing arguments address the fact that the atmosphere circulates. There are two each of the Hadley, Polar and Ferrell cells of circulation. Heat laden air is constantly being carried aloft, above the majority of greenhouse effect, where it freely radiates IR into space. Not to mention the tremendous energy carried aloft by tropical cyclones and thunderstorms. These mechanisms are dynamic and vary according to temperature.
http://ess.geology.ufl.edu/ess/Notes/AtmosphericCirculation/7-11.jpeg

The radiative forcing graphic in post 66 is often presented today as a global energy balance equation. It's probably less than half of the story.

the magnitude of the leak shows that it HAS to be a significant part of the total reservoir losses.
(Sorry, haven't figured out how to quote multiple times.)

Consider that other porus strata may be feeding the lake additional water.

 

[mheslep]

None of the radiative forcing arguments address the fact that the atmosphere circulates. There are two each of the Hadley, Polar and Ferrell cells of circulation. Heat laden air is constantly being carried aloft, above the majority of greenhouse effect, ...

I think this is backwards. My understanding is the CO2 greenhouse effect of interest happens primarily at higher altitudes, as the high water vapor content at the surface makes the surface atmosphere nearly opaque to infrared, or a very effective water vapor dominated greenhouse effect if you will. So near the surface a major heat transfer mechanism is in fact the convection you mention. Convection moves heat up to higher altitudes, and it is there that the greenhouse effect due to CO2 can have its impact.

 

[chriscolose]

Actually this kind of heat transfer was considered even in early radiative-convective models since at least Manabe

 

[sylas]

(Sorry, haven't figured out how to quote multiple times.)

It's the same as using color. You just add quote tags rather than color tags. To do your quoting, simply put tags around quote text as follows (but using square brackets, of course):
{QUOTE=author}...text...{/QUOTE}

If you are using the "advanced" message editor, then look for the little button that looks like a speech balloon: https://www.physicsforums.com/Nexus/editor/quote.png .[/URL] That will put QUOTE tags around selected text.

If you are composing posts off-line (my preferred method) then you have to add the tags yourself directly, but you do need to watch that every {QUOTE} tag is followed by a {/QUOTE} tag at the end of the quoted material. It's not necessary, but you can also add the "=author" if you like, or even "=author;$#" where$# is the post number you are quoting. You get this for free if you just copy and paste of the quote tags given automatically when you first hit the "QUOTE" button to start your reply.

None of the radiative forcing arguments address the fact that the atmosphere circulates.

That is because this is not a source of energy, but a matter of how energy distributes.

It is most definitely considered in climate models; but you are now mixing up apples and oranges to a completely absurd degree. The circulation of the atmosphere makes no difference whatsoever to the simple physical fact that carbon dioxide is necessarily a crucial forcing driving current increases in global temperature.

Circulation shows up when you want to look at the consequences of temperature change and shifting climate patterns, and it is needed to model feedback processes within a dynamical system, and it is needed for looking at rates of change in response to forcing. But it doesn't do a damn thing for sorting out what forcings are driving the changes currently underway.

This is the red herring to end all red herrings. I am NOT trying to model climate here. I am simply giving a basic physical fact of the large impact of carbon dioxide. The effect of circulation is totally irrelevant to that topic. It's important for understanding climate. It is NOT a source of energy to force increasing temperatures. Stick to the topic.

The radiative forcing graphic in post 66 is often presented today as a global energy balance equation. It's probably less than half of the story.

It's a LOT less than half the story, if by the story you mean every last aspect of climate. That's why it only shows up in one chapter of the WG-1 report. But it is pretty much the whole story known at present if we are looking at what is forcing the changes in climate -- which is the topic of this thread.

You appear to be completely confused about following the different parts of this problem. No wonder you are so unable to distinguish what is known to high confidence from what is open research questions with high uncertainty.

the magnitude of the leak shows that it HAS to be a significant part of the total reservoir losses.

Consider that other porus strata may be feeding the lake additional water.

Still a red herring. It DOESN'T MATTER what other factors are involved, either positive or negative. If you have found one factor which has a total impact comparable to the total effect, then you HAVE to take that factor into account to get anywhere close to an explanation for the effect being considered. The comparison of magnitudes with the total effect is sufficient to show that the leak is necessarily significant. Stop trying to invent spurious analogies. It's a distraction. The role of an analogy is to help explain some concept with a related simpler example. It is NOT to invent new associations out of thin air and draw conclusions. It's for explanation, not for inference.

Can't you see that if you have an effect of about 5 Megalitres per day, and some factor which has an impact of about that magnitude, then this factor HAS to be significant? It's not a proof of being the "most" significant -- which is why the argument set out for discussion in this thread makes no attempt to prove CO2 is most significant.

Now in fact, as I have shown, if you are willing to go further and look at actual honest science attempting to consider at all the forcings involved, then you DO find that carbon dioxide is the largest single heating influence, by a substantial margin.

That's a fact as much as anything is a fact. It is not impacted in the slightest by irrelevant distractions such as circulation, or pushing analogies into something completely divorced from what we SEE when we honestly look at climate itself.

Cheers -- sylas

 

[sylas]

CO2 concentration is not set (in part) by temperature? Ocean uptake and release in particular is dependent on temperature.

Yes, that is true, but it is a rather long time scale effect. Chris Colose (in [post=2216203]msg #73[/post]) has given a good account of how time scales matter. The animations linked by skyhunter (in [post=2216169]msg #71[/post]) take this even further to the extremely long scales of the geological carbon cycle.

Here's a bit more detail, looking at a couple of examples that have shown up in the thread.

Climate is so complicated because there are a lot of interacting processes and they work on all kinds of different time scales. Speaking of the "equilibrium" makes a certain amount of good sense; but when you have a perturbation in the system, some things come to equilibrium faster than others.

(1) Very fast: stratospheric response

The temperature of the stratosphere responds very rapidly indeed to any change in radiant energy balance. There's not much circulation or heat capacity to complicate things. Hence, the formal definition of "radiative forcing" is a change in energy balance after the stratosphere has come to equilibrium. (See [post=2199572]msg #69[/post] of thread "Physics of Global Warming" for the formal definition and references.)

(2) Fast: water vapour

The humidity of the atmosphere depends largely on temperature. Industry adds a lot of water vapour to the atmosphere, and this doesn't actually have all that much effect; nothing like the effect of carbon dioxide -- even though the carbon dioxide is a weaker greenhouse gas. That is because when water levels are raised much above, or below, the natural equilibrium level, you rapidly get the equilibrium restored, as water evaporates back into the atmosphere or is precipitated out again.

This is the key to why water vapour is not treated as a "forcing" at all. When you add water vapour directly, it rains out again too quickly to have any extended climate effect. On the other hand! If you raise temperatures by some other means, then you change the natural equilibrium level of specific humidity... and the ocean adds the water vapour to match; and because this change is persistent, the additional water vapour contributes to the extended increase in temperature. That is -- this is a feedback process, not a forcing.

(3) Slow. The carbon cycle in the biosphere.

Just like there is a natural equilibrium of water vapour, so too there is a natural equilibrium for carbon dioxide, between atmospheric and oceanic carbon levels. The time it takes for atmosphere and ocean to relax back to equilibrium, however, is measured in many centuries. If this process was as fast as the water cycle, then all our industrial CO2 emissions would have only a small effect on atmospheric CO2 levels, because about 99% of what we added to the atmosphere would end up absorbed into the ocean.

What happens in practice is that about half of all the CO2 we have added since the development of industry has ended up in the ocean or other carbon sinks; and about half has ended up in the atmosphere. If we stopped adding CO2 tomorrow, most of the elevated CO2 levels would gradually relax back down into the ocean... but this would take at least a thousand years. There are multiple processes involved in restoring this equilibrium, each with their own characteristic time constant, and that makes the net relaxation time a rather complicated mathematical function.

This is where there is an important temperature impact. The natural equilibrium between ocean and atmosphere is temperature dependent. Now at present the atmosphere is a long way out of balance with the ocean; and so there is a steady net flux of CO2 into the oceans, at about half the rate of the flux of CO2 into the atmosphere from human industry. The temperature effect in the present, therefore, is mainly about the rate at which the ocean takes up carbon, and not about the equilibrium level, since it be at least another thousand years before there's any equilibrium.

For climate studies of interest to human society, therefore, carbon dioxide is treated as a forcing; and you estimate atmospheric carbon dioxide levels based on emissions and on models of how carbon is flushed back out into other sinks.

If someone wanted to make a very long scale model of climate for the ice ages of the quaternary period (time span of a million years or so, and time steps of a century or so) then carbon dioxide would show up as a feedback rather than a forcing; much like water vapour shows up as a feedback on scales of interest to us in the present. The difference between "feedback" and "forcing" is not hard and fast, but depends on the scale of interest.

(4) Insanely slow. The geological carbon cycle

This is what skyhunter's link was talking about. On really long time scales, from around millions of years to hundreds of millions of years, what counts is the transfer of carbon in and out of geological reserves, which are enormously more than what is seen in oceans or atmosphere. These cycles are too slow even to explain the cycles of ice ages in the quaternary period; but they become critical for explaining changes between "greenhouse" and "icehouse" conditions on very long times scales of hundreds of millions of years, and can involve much larger levels of atmospheric carbon than anything considered for climate in modern times or the foreseeable future.

The most drastic example of this is "Snowball Earth" theory, which by now is pretty much mainstream. There have been episodes in Earth's long history (the most recent of which was about 650 million years ago) in which we had ice ages of such intensity that the entire Earth was frozen, right into the tropics. Such a condition is self-perpetuating, because ice and snow are so reflective, and with most of sunlight being reflected, there is not enough energy coming into melt the ice.

In this condition, the processes discussed in Skyhunter's link become important. Weathering is much reduced, but outgassing is not. The result is a steady increase in levels of carbon dioxide, up to levels many times greater than what we have at present. Eventually -- and this can take a long long time -- the greenhouse effect becomes so strong that ice can begin to melt around the tropics. In this condition, a runaway feedback process occurs, because as ice melts, the albedo rises, and you start to get more absorbed sunlight. Over a geological eyeblink (as little as a thousand years) ALL the ice melts, and the Earth flips over into a "greenhouse" state, with very high carbon dioxide levels and a temperature rise from the "showball" state of as much as 50 degrees. It would have been the mother of all climate shifts. From there, of course, carbon dixoxide levels begin to fall again... rapidly at first, and then slowly, slowly... as carbon is taken up into the geological reserves once more.

For more details on this fascinating idea, see the website Snowball Earth, and in particular the FAQ question How did the snowball Earth's end?. There is now an extensive scientific literature on this. See, for example:

• Hoffman, P.F. et. al. (1998) A Neoproterozoic Snowball Earth, in Science Vol 281, 28 Aug 1998, pp 1342-1346.

Note that coming out of the snowball Earth condition may not occur until CO2 levels are as much as 350 times current levels, as described in the abstract of the above paper. That’s an atmosphere of about 12% carbon dioxide. From there, once the ice melted, deposition of carbon into geological reserves would begin, quite rapidly at first. This is the focus of the paper by Hoffman et al.

There's a lot of ongoing work with modeling the geological carbon cycle on long time scales like this, but the broad picture is now fairly solid, of a snowball Earth state in the Neoproterozoic, ending with a rapid transition to a hot greenhouse state with enormously elevated atmospheric carbon dioxide levels, followed by a return of carbon into geological reserves as carbonates precipitate out of the warmer ocean and a corresponding decline of temperatures -- although still a hot greenhouse state much warmer than prevailing conditions in the present, and well beyond anything predicted as a result of anthropogenic global warming.

I have in mind a new thread sometime in which I look at a really simple toy model that illustrates some of the basic ideas of feedback and hysteresis as they apply for snowball Earth. In the meantime, here's a diagram of how it works, from the snowball Earth site:

Basically, there is a kind of runaway albedo feedback that occurs as you move into and out of the snowball state, moving the Earth between two different stable equilibrium conditions. This effect is called hysteresis.

Cheers -- sylas

 

[skypunter]

Sorry, I'll be on my way now.

 

[sylas]

Sorry, I'll be on my way now.

And my apologies in turn for allowing myself to get a bit frustrated! Sorry! I'm glad to have had you in the thread, and you are welcome back anytime.

Since I am mainly interesting in contributing to basic education on particular points where there is a lot of public confusion, I need to watch myself more and not be rude to people who are making a sincere attempt to follow along. I was too rude to you just now, and I apologise.

I still stand by all the substantive remarks, of course. There's nothing in climate that sensibly corresponds to dark energy or dark matter in cosmology. That analogy only confuses the state of play; the nature of what is unknown in climate is not unknown forcings, but hard to model consequences.[*] Atmospheric circulation is important, but it really doesn't make any meaningful difference for sorting out the the forcings. It's part of the complexity of climate modeling... though actually one of the parts we can manage quite effectively. If you want to look at where we have much less of an idea of what is going on, look at circulation in the ocean, not the atmosphere! This has a major impact on the rate at which climate responds to forcings, and can give very strong effects on short term temperature variability. In some respects the ocean sometimes looks a bit like a forcing, because of the large heat capacity involved.

Cheers -- sylas

 

[sylas]

This thread seems like a good place to leave this note. A recent letter to Nature has proposed a simple metric: the "climate carbon response" (CCR). Basically, this is the the temperature rise per unit carbon emissions. This will depend on both carbon cycle and climate sensitivity estimates, both of which are uncertain; and so the value of the CCR is also uncertain. But it can be estimated with uncertainty bounds, and the number gives a convenient number for quantifying the number that was the topic of this thread. However, in the thread I have been comparing CO2 in the atmosphere to temperature; this new measure is relating carbon in emissions to temperature, which is potentially a more useful number for those who want a quick way to estimate to effects of changes to emissions.

• Matthews, D.H. et. al. (2009) The proportionality of global warming to cumulative carbon emissions, in Nature, 459, 829-832 (11 June 2009) | doi:10.1038/nature08047

Extract:

From observational constraints, we estimate CCR to be in the range 1.0–2.1 °C per trillion tonnes of carbon (Tt C) emitted (5th to 95th percentiles)[/color]​

What I found most interesting about this proposal is that it suggests a way to avoid a problem with "equilibrium sensitivity" and "transient response sensitivity". Basically, if you increase CO2 levels, then it may take a long time for the climate to respond. So there is a "transient" response (which is what you get when CO2 is increased gradually to a final level) and the "equilibrium" response (which is what you get when you keep waiting after CO2 has stabilised until the temperature come to equilibrium. The equilibrium response is larger than the transient response, by these definitions.

However, if you are interested in emissions, then as you wait the atmospheric CO2 levels also start to decay. It's rather artificial to simply hold atmospheric CO2 fixed and let temperature equilibriate, because there is at the same time an equilibriation of the carbon cycle.

Lets compare with the expected value you might get from considerations in this thread. I've proposed about 3 degrees per doubling, which would be 3/Ln(2) = 4.3 degrees per natural log, and at present the atmosphere contains about 8.2*10¹¹ tons of carbon, or 0.82 trillion tons. Assuming the logarithmic relation, we have dT/dC=4.3/C which works for any unit of carbon content C in the atmosphere.

Now if we just consider carbon in the atmosphere, the value is about 5.2 degrees per trillion tons carbon. But this is the equilibrium response, and appropriate for looking at the long term effect of a given atmospheric concentration.

On the other hand, if we are specifically interested in anthropogenic factors, then we can try to look at emissions rather that atmospheric concentrations. The results of this paper suggest that we can do this by using the transient sensitivity.

Transient sensitivity also called transient climate response (TCR) is about 1 to 3 degrees per doubling, with a best value of around 2. Using 2/Ln(2) we get about 2.9 degrees per natural log.

We also need to consider how much of emissions actually end up in the atmosphere. Much of it gets cycled into the ocean and other reservoirs of the carbon cycle. Off the top of my head I believe we are in the right ball part to assume about half of emissions actually end up in the atmosphere.

Using this approximation we have about 1.6 trillion tons of emissions equivalent, in the atmosphere, and the CCR would be about 2.9/1.6 = 1.8 degrees per trillion tons emission. This estimate was really crude, but I've ended up inside the bounds of 1.0 to 2.1 quoted in the published letter.

The letter emphasizes that there is a lot of uncertainty in the magnitude of this number. What is impressive is that the value is comparatively insensitive to how rapidly this is emitted or when! This gives a much more clearly understandable basis for people interested in policy or mitigation proposals focused on carbon footprints.

Cheers -- sylas

 

[joelupchurch]

Frankly, I have some trouble believing that the fundamental constraint isn't the concentration of CO₂ in the atmosphere. They seem to be arguing that even if we stabilized CO₂ at say 450ppm, that the temperature would continue to increase if we emitted any CO₂. This seems to violate some pretty basic physics.

BTW, didn't Nature publish a rather similar paper in April?

"Warming caused by cumulative carbon emissions towards the trillionth tonne"
Nature 458, 1163-1166 (30 April 2009) | doi:10.1038/nature08019; Received 25 September 2008; Accepted 25 March 2009
Myles R. Allen, David J. Frame, Chris Huntingford, Chris D. Jones, Jason A. Lowe, Malte Meinshausen & Nicolai Meinshausen
https://regtransfers-sth-se.diino.com/download/f.thompson/migrated_data/EandH/nature08019.pdf"

 

[sylas]

Frankly, I have some trouble believing that the fundamental constraint isn't the concentration of CO₂ in the atmosphere. They seem to be arguing that even if we stabilized CO₂ at say 450ppm, that the temperature would continue to increase if we emitted any CO₂. This seems to violate some pretty basic physics.

I don't understand the comment. There's no physical problem here.

All that matters for temperature, physically, is what carbon is in the atmosphere; but it still takes time to get the response. Suppose we stabilise at 450ppm. In that case, what we emit or not is beside the point; the premise of the comment is that the atmosphere has been stabilised, and that is all you need to know for the temperature estimates.

In the event that the atmosphere is stabilised at a certain concentration, the temperatures will indeed continue to increase. The reason for this is that there is a large time lag in the climate system, as a consequence of the heat sink in the ocean.

Think of it like this. Image the atmosphere suddenly jumps to 450ppm overnight. This will result, almost immediately, in an excess of energy being received at the surface, and the surface will start to heat up. The surface will continue to heat up until it gets to an equilibrium of the energy balance. Now the main reason the surface does't heat up in a month is the ocean. It takes a long long time for the ocean to heat up; and until this occurs, there is a flux of energy from the surface going down into the ocean. Once the ocean temperature has come to the equilibrium, this net flux is gone, and the surface has to be in balance with the top of the atmosphere again.

There's a long approach of temperature to the equilibrium value.

The Earth at present has an excess of energy flowing into the ocean. It's not clear how much this is. It is almost certainly less than 1 W/m². An estimate of 0.5 is probably close, but it could be less; and is unlikely to be more IMO. If the atmosphere remains fixed at the present composition, right now, then this excess of 0.5 W/m² will gradually be realized as a temperature increase at the surface, which is probably in the ball park of 0.4 degrees. This is often called temperature rise "in the pipeline".

There are some larger estimates for this published. In particular, a recent paper in Science proposed 0.85 W/m². I've commented before on why I think the smaller estimates are a bit more accurate. See [post=2186640]msg #3 of "Ocean Heat Storage" thread[/post]. Nailing this down is an open question as well, of course.

Suppose we put a pulse of CO2 into the atmosphere. If you wait a long time, then most of that pulse will come back out of the atmosphere, since the largest reservoirs of carbon in our carbon cycle are in the ocean. In the meantime, temperature will take a long time to come up to the equilibrium value. One point of this paper is to argue that these two opposing effects nearly cancel.

It's physically sensible; and the hypothesis seems very credible.

BTW, didn't Nature publish a rather similar paper in April?

"Warming caused by cumulative carbon emissions towards the trillionth tonne"
Nature 458, 1163-1166 (30 April 2009) | doi:10.1038/nature08019; Received 25 September 2008; Accepted 25 March 2009
Myles R. Allen, David J. Frame, Chris Huntingford, Chris D. Jones, Jason A. Lowe, Malte Meinshausen & Nicolai Meinshausen
https://regtransfers-sth-se.diino.com/download/f.thompson/migrated_data/EandH/nature08019.pdf"

Thanks for the reference! Great catch. I've had a quick look, and I agree. They are very closely related. Matthews et al cite this paper, and credit the authors in the acknowledgments as people who have provided useful commentary and discussion on the work. The citation you have given for Allen et al likewise references the paper by Matthews et al, though it is marked as "in press" as Allen et al came out a few months earlier.

Cheers -- sylas

 

[chriscolose]

I think the Hansen et al number of 0.85 W m^-2 was based on the year 2005 relative to some pre-industrial baseline, not a long-term value.

 

[skypunter]

Pardon me if this is a stupid question.
Does this formula take into account the logarithmic reduction in the effect of additional CO2 in the atmosphere?
For example, it takes a doubling to increase temperature a certain amount, but it takes another doubling of the new base to increase temperature the same amount as the first doubling. That is logarithmic, correct?
This simple formula does not appear to have a logaritmic component, and that makes me skeptical.

 

[skypunter]

"Now if we just consider carbon in the atmosphere, the value is about 5.2 degrees per trillion tons carbon."

Here is another stupid question.
Why are CO2 emissions referred to as "carbon" emissions, when the chemical contains more oxygen atoms than carbon. Shouldn't CO2 emissions be referred to as "Oxygen" emissions?

 

[sylas]

Three replies in one here; to chris and skypunter.

I think the Hansen et al number of 0.85 W m^-2 was based on the year 2005 relative to some pre-industrial baseline, not a long-term value.

The value was based on a model; and even in the 2005 paper it is apparent that the model value is greater than what is obtained from ocean data. A later lecture by Hansen uses smaller values for model based estimate, and clearly distinguishes the ocean data based estimates. The estimates in the 2005 paper were based on the decade 1993-2003. Here is the content of a chart in a lecture he gave earlier this year.

Chart 14:

Modeled Imbalance: +0.75 +/- 0.25 W/m²
Ocean Data Suggest: +0.5 +/- 0.25 W/m²​

Now, the ultimate question: can we stabilize climate? We would need to restore the planet’s energy balance. The underlying imbalance (averaging over short-term fluctuations) is probably close to 0.5 W/m².

—Air Pollutant Climate Forcings within the Big Climate Picture, Talk given by J. Hansen at the Climate Change Congress, “Global Risks, Challenges & Decisions”, Copenhagen, Denmark, March 11, 2009​

​

This is not an "anomaly" in the sense that it is measured with respect to a baseline of any kind. It is an absolute value for a total energy flux. The flux will vary from year to year, so you can certainly look for averages over a time span. The very long term average is effectively zero, because there's no significant source of energy in the ocean; it is almost all ultimate a redistribution of energy from the Sun.

I discuss this in more detail in [post=2194788]msg #31[/post] of thread "Ocean Heat Storage". In my opinion, this is a quantity where we are likely to get better estimates in time. I've stuck my neck out in that post to suggest that something a bit less than 0.5 is probable; but that's just my guess. 0.5 works for back of the envelope approximations.

Pardon me if this is a stupid question.
Does this formula take into account the logarithmic reduction in the effect of additional CO2 in the atmosphere?
For example, it takes a doubling to increase temperature a certain amount, but it takes another doubling of the new base to increase temperature the same amount as the first doubling. That is logarithmic, correct?
This simple formula does not appear to have a logaritmic component, and that makes me skeptical.

The answer to this is yes and no. You are quite right that it is not consistent with the logarithmic relation in the sense that you couldn't use this number over a very wide range of concentrations. For example, if you calculate this value again in a condition of substantially greater concentrations, you'd get a smaller value, for precisely the reason you identify.

However it is consistent in the sense that the underlying mathematical models used to calculate the number do indeed have this logarithmic relationship, and the number given works for estimating impacts in the present. Current CO2 values are approaching 400ppm. This number is a guide for the effects emissions on temperature in this case. There are substantial uncertainties in the number (the range is 1.0 to 2.1 at the 5th and 95th percentiles) and the consequences of the logarithmic relation are not particularly significant in this range.

Here is figure 2 from Allen et al (2009). What we are looking at here is temperature on the vertical axis, being the peak in warming over a pre-industrial average; and total carbon emissions on the horizontal axis. Currently we are at a bit over 0.4 trillion tons. The white crosses are best fit values, where each cross is a difference scenario. The grey shading represents a likelihood distribution.

You can see the logarithmic relation pretty clearly in how the white crosses lie. If you go over to 3 or 4 trillion tons, then the effect is clearly dropping off, as you should expect from the logarithmic relation of atmospheric carbon to temperature. But for total emissions of up to 1 trillion tons (basically emit in the future a bit more than what we've emitted since the start of the industrial revolution), the value proposed works well. It's not bad over higher values up to 1.5 or (yeesh) 2 trillion.

Note that this is only looking at carbon dioxide effects. This is one of the largest factors, but there are many other significant anthropogenic factors involved with industrial emissions as well. This is also noted in the papers cited.

Why are CO2 emissions referred to as "carbon" emissions, when the chemical contains more oxygen atoms than carbon. Shouldn't CO2 emissions be referred to as "Oxygen" emissions?

That's a point well worth emphasizing when looking at numbers. Numbers that get thrown around are sometimes for carbon, sometimes for CO₂, sometimes for mass and sometimes for volume. The conversions are not hard, but I've tripped up before this by mixing up the actual quantities being used in some report.

We don't refer to oxygen emissions because the oxygen involved comes from the atmosphere anyway. Burning of carbon based fuels takes oxygen out of the air, and carbon out of the fuel, and returns CO2 to the air.

It's useful to focus on the carbon, because what matters is the carbon content of the various fuels we use. Also, the Earth's carbon cycle involves various chemical reactions where carbon moves in and out of different compounds. The one common factor is the carbon; and so we speak of the carbon cycle and the carbon content of various reservoirs, without worrying about whether the carbon is there as CO₂, or (C₆H₁₀O₅)_n (cellulose, in wood), or H₂CO₃ (carbonic acid, in the ocean), or any number of other forms.

Cheers -- sylas

 

[skypunter]

this new measure is relating carbon in emissions to temperature, which is potentially a more useful number for those who want a quick way to estimate to effects of changes to emissions.

Such as the press.