Equilibrium Climate Sensitivity

[nflwxman]

In an effort to provoke a higher level discussion on one of the most hotly debated topics in climate science and clean up subsequent threads, I figure we could start here. I'd love to get some hard thoughts and projections from the regulars here. My personal belief is that the actual ECS value is around 2.5K. Keep in mind, we will likely reach 560ppm by the late 21st century, which would equal a doubling from pre-industrial times. Remember, ECS does not occur immediately after a particular level of CO2 is reached (thus it will be hard to validate in the future). It could take several decades for the system to theoretically erase any forcing imbalance. A few points below to kick it off:

1) We know that CO2 has a net forcing that would heat atmosphere 1.2K if no other major feedbacks were present.

2) Paleoclimate sensitivity estimates are generally higher overall than recent empirical data estimates. Sometimes shockingly so (4-6K)

3) Recent empirically based estimates have placed ECS around 1.5-3K

4) Climate sensitivity estimates seem to vary widely based on the human made aerosal negative feedback assumptions in the mid 20th century.

The definition is below:

The equilibrium climate sensitivity (ECS) refers to the equilibrium change in global mean near-surface air temperature that would result from a sustained doubling of the atmospheric (equivalent) carbon dioxide concentration (ΔT_x2). As estimated by the IPCC Fifth Assessment Report (AR5) "there is high confidence that ECS is extremely unlikelyless than 1°C and medium confidence that the ECS is likely between 1.5°C and 4.5°C and very unlikely greater than 6°C.

This is a great resource for papers on the topic:

http://agwobserver.wordpress.com/2009/11/05/papers-on-climate-sensitivity-estimates/

Why it matters?

[PhillipS]

Just a small point:  pre-Industrial Revolution CO2 concentration is generally given as 280 ppmm so a doubling would be an increase to 560 ppm, not 460 ppm.  So far, the anthropogenic increase to 400 ppm represents a 43% increase in CO2.

 

In addition to ECS you may also want to discuss Transient Climate Response (TCR), those faster responding climate aspects.  I've seen varying definitions of TCR but they all seem to include air, sea surface and land surface; and generally exclude the deep ocean and ice sheets, both of which take centuries to equilibriate.  The value for TCR, and its response time, will be be a percentage of ECS, but what percentage it is might be someting to discuss.  I've seen estimates that TCR is 2/3rds to 3/4ths of ECS and its response is decades rather than centuries, but I don't know of any peer-reviewed papers on it.

 

As the BEST project reported, the global temperature has risen about 0.8 C since the early 1900s.  Just for a ball-park calculation I'll attribute that to AGW.  So TCR = 0.8 / 0.43 = 1.86 C/ doubling.  If true then ECS should be in the range [1.86 / 0.75] < ECS < [1.86 / 0.66] or roughly 2.5 C < ECS < 2.8 C.  Now, before y'all tear into me, I understand that the observed warming is split between AGW and natural variability, but I also understand that there is additional warming in store because we are continuing to dump gigatons of GHGs into the air and we are seeing an energy imbalance at TOA. 

[ORH_wxman]

Just a small point:  pre-Industrial Revolution CO2 concentration is generally given as 280 ppmm so a doubling would be an increase to 560 ppm, not 460 ppm.  So far, the anthropogenic increase to 400 ppm represents a 43% increase in CO2.

 

In addition to ECS you may also want to discuss Transient Climate Response (TCR), those faster responding climate aspects.  I've seen varying definitions of TCR but they all seem to include air, sea surface and land surface; and generally exclude the deep ocean and ice sheets, both of which take centuries to equilibriate.  The value for TCR, and its response time, will be be a percentage of ECS, but what percentage it is might be someting to discuss.  I've seen estimates that TCR is 2/3rds to 3/4ths of ECS and its response is decades rather than centuries, but I don't know of any peer-reviewed papers on it.

 

As the BEST project reported, the global temperature has risen about 0.8 C since the early 1900s.  Just for a ball-park calculation I'll attribute that to AGW.  So TCR = 0.8 / 0.43 = 1.86 C/ doubling.  If true then ECS should be in the range [1.86 / 0.75] < ECS < [1.86 / 0.66] or roughly 2.5 C < ECS < 2.8 C.  Now, before y'all tear into me, I understand that the observed warming is split between AGW and natural variability, but I also understand that there is additional warming in store because we are continuing to dump gigatons of GHGs into the air and we are seeing an energy imbalance at TOA. 

 

 

Your ECS calculation based on the TCR calculation should theoretically account for this....that is why you have a greater ECS number than TCR because there is "assummed warming in the pipeline". Otherwise, TCR would equal ECS which we are almost certain is not true.

 

The differences in ECS ultimately come down to what type of feedbacks are calculated. The timeline for ECS is also a point that has begun to be debated a bit. I don't have the papers at my fingertips (but I'll try and find them later and post them here), but some pretty recent literature argued that a significant chunk of ECS could take thousands of years versus many estimates that think most of the ECS value is realized after a few hundred.

[PhillipS]

My point, which I should have expressed better, is that since even TCR may take decades to fully manifest we haven't seen all of the warming yet from the radiative changes of the 1990s or 2000s, much less more recent years.  We're nowhere near equilibrium today, nor will we be for the foreseeable future.  We'll be able to determine a solid value for TCR after we've had stable CO2 concentration for, say, 30 years.  Something I don't expect to see in my lifetime.

 

I think you're right that the full ECS will take millenia to reach - but that's not good news.  It just means that however bad things get in our lifetimes, it will be worse for our descendants.  That's a pretty louusy legacy, IMO.

[ORH_wxman]

My point, which I should have expressed better, is that since even TCR may take decades to fully manifest we haven't seen all of the warming yet from the radiative changes of the 1990s or 2000s, much less more recent years.  We're nowhere near equilibrium today, nor will we be for the foreseeable future.  We'll be able to determine a solid value for TCR after we've had stable CO2 concentration for, say, 30 years.  Something I don't expect to see in my lifetime.

 

I think you're right that the full ECS will take millenia to reach - but that's not good news.  It just means that however bad things get in our lifetimes, it will be worse for our descendants.  That's a pretty louusy legacy, IMO.

 

 

No, this is not what TCR is.

 

TCR as defined by Cubasch et al 2001 is what the warming is once you get to 560ppm (or doubling of CO2) in a roughly 1% increase per year scenario...that is why it is called "transient climate response". Any warming that takes place after this period is beyond TCR and gets covered by ECS.

[nflwxman]

Thank you for your edits Phillip.  I was a bit tired when I started this thread and apparently forgot how to do math.  I agree that TCR likely carries more implications for society during our lifetimes.  We should be able to free discuss both here.  While your back of the book calculation seems rudimentary, I believe it carries some aspect of truth to it.  Perhaps the simple empirical model is the way to go (particularly for TCR).  

 

Forcing estimates from several papers up to 2013 suggest Solar forcing from the start of global temperature measurements (1750 or so) to present is close to zero due to the recent TSI slowdown.  Several papers claim from top to bottom of the 11 year solar cycle can cause a 0.1K change in global temperature (all else the same).  Equal forcing is about +/-0.2 Watts/m2.  This is interesting as the sun has become more of negative forcing on the climate in the last decade.

 

https://www.youtube.com/watch?v=aqmw46Q4LdE#t=15.

 

http://www.atmos-chem-phys.net/13/2045/2013/acp-13-2045-2013.pdf

 

[nflwxman]

No, this is not what TCR is.

 

TCR as defined by Cubasch et al 2001 is what the warming is once you get to 560ppm (or doubling of CO2) in a roughly 1% increase per year scenario...that is why it is called "transient climate response". Any warming that takes place after this period is beyond TCR and gets covered by ECS.

ORH, thank you for the clarification. 

[PhillipS]

No, this is not what TCR is.

 

TCR as defined by Cubasch et al 2001 is what the warming is once you get to 560ppm (or doubling of CO2) in a roughly 1% increase per year scenario...that is why it is called "transient climate response". Any warming that takes place after this period is beyond TCR and gets covered by ECS.

 

Thank you, ORH, for the info.  I haven't read that paper so I'll track it down and learn more about TCR.  My limited knowledge comes from various articles and blog posts.  Clearly I have much to learn.

[Sophisticated Skeptic]

The thread kind of only points towards warming and not cooling.

 

What about cooling and the coolest night of the summer here so far last night.  44.4

 

What about the quiet sun the last several years, and it's possible beginning effects of cooling.

 

Leaves changing color in August

 

http://pittsburgh.cbslocal.com/2014/08/15/experts-cold-summer-leads-to-changing-leaves-in-august/

 

 

Experts: Record Cold Summer Leads To Changing Leaves In August

PITTSBURGH (KDKA)- Pittsburgh is dealing with one of the coldest summers in history, and it’s having an effect on the trees.

Friday morning temperatures fell into the 40s in Western Pennsylvania.

Meteorologists say these cold temperatures are leading to trees changing colors in the middle of August.

 

[ORH_wxman]

The thread kind of only points towards warming and not cooling.

 

What about cooling and the coolest night of the summer here so far last night.  44.4

 

What about the quiet sun the last several years, and it's possible beginning effects of cooling.

 

Leaves changing color in August

 

http://pittsburgh.cbslocal.com/2014/08/15/experts-cold-summer-leads-to-changing-leaves-in-august/

 

 

Regional temperature variation is quite great, but it doesn't apply to the topic in this thread. For every area that sees a record cold snap, someone will see record warmth...and whole underlying trend is warming on a global scale scale.

 

 

There isn't any highly compelling evidence that the sun is the primary driver of temperature variation right now. If we all of the sudden cool quite a bit in the next few years? then perhaps that could be revisited...but everything we've seen until now has pointed toward only a minor role played by the sun.

[nflwxman]

An interesting paper from the mid 2000s using empirical data from recent volcanic eruptions to try and get a measure of climate sensitivity.  It arguably carries too much uncertainty using a few events to calculate this value, but it's an novel approach nonetheless.

 

http://onlinelibrary.wiley.com/doi/10.1029/2004JD005557/pdf

 

Abstract:

 

The results from 16 coupled atmosphere/ocean general circulation model (AOGCM)

simulations are used to reduce internally generated noise and to obtain an improved

estimate of the underlying response of 20th century global mean temperature to volcanic

forcing. An upwelling diffusion energy balance model (UD EBM) with the same forcing

and the same climate sensitivity as the AOGCM is then used to emulate the AOGCM

results. The UD EBM and AOGCM results are in very close agreement, justifying the use

of the UD EBM to determine the volcanic response for different climate sensitivities.

The maximum cooling for any given eruption is shown to depend approximately on the

climate sensitivity raised to power 0.37. After the maximum cooling for low-latitude

eruptions the temperature relaxes back toward the initial state with an e-folding time of

29–43 months for sensitivities of 1–4C equilibrium warming for CO2 doubling.

Comparisons of observed and modeled coolings after the eruptions of Agung, El Chicho´n,

and Pinatubo give implied climate sensitivities that are consistent with the

Intergovernmental Panel on Climate Change (IPCC) range of 1.5–4.5C. The cooling

associated with Pinatubo appears to require a sensitivity above the IPCC lower bound of 1.5C.

[Sophisticated Skeptic]

<------ Democrat, who doesn't believe in the warming....yet. 

[HailMan06]

<------ Democrat, who doesn't believe in the warming....yet.

Can you please stick with the science?

[nflwxman]

An abundance of emphasis has been placed on the PDO and climate sensitivity recently.  Many speculate that the PDO can act as a natural internal forcing either amplifying or dulling the warming.  Below are some papers that address that topic (Bluewave also had a nice thread going before).  Obviously, this has large implications for climate sensitivity using short term empirical observations.  Personally, I believe the PDO acts as change in immediate baseline.  Roughly speaking, when the PDO turns to it's negative phase, global temperatures are dropped immediately, but the increase after the initial drop would be similar to that if the PDO were in it's positive phase.  While it's not entirely black and white, I believe that initial temperature drop in this phase was in 2008, and the onging forcing since has allowed the temperatures to "recover" in recent years.  By 2025 the 15 year trend of warming should be in the 0.2C/decade level predicted by many climate models.  How this affects ECS remains unknown.

 

 

http://agwobserver.wordpress.com/2011/02/10/papers-on-pacific-decadal-oscillation/

 

Notice the intial drop in temperatures in 1945 associated with the PDO flip and aerosal levels.

 

[sokolow]

One thing to add to the array of ECS estimates given in the OP that I think is helpful is out of Roe and Baker's 2007 paper ("Why Is Climate Sensitivity So Unpredictable?") and which was reiterated in Roe and Bauman's 2013 paper ("Climate sensitivity: should the climate tail wag the policy dog?") is that the space for ECS between ~1.5-4.5C has been stable, reproduced in repeated studies for decades.

So the figure given in the 2007 paper presents the array of estimates in the OP like so:

Figure 1: Probability distribution of climate sensitivity to a doubling of atmospheric CO2, from Roe and Baker (2007)

Roe and Bauman observe that the big success for climate scientists on this question is hammering down the lower bound & establishing that its "kinda really unlikely" to fall below 1.5C. But they also note that the big ol' tail of high values is intractably persistent in the results. In fact, Roe and Baker (2007) argued that the long tail of improbable-but-plausibly-not-zero values is going to appear as an intrinsic feature of attempts to model ECS.

As to that, the Roe and Bauman 2013 paper opens by starightup saying: don't expect that to change because "It will require improbably large reductions in uncertainties about the radiative forcing the planet has experienced—or, equivalently, in our uncertainty about physical feedbacks in the climate system—to substantially remove the skewness" and that even given attempts to constrain that long tail, if you have to bet on it, its going to be "prudent" to assume that estimated space for climate sensitivity "will not change substantially for the foreseeable future."

So Roe and Bauman say, well, fine. Change the question around and ask instead: if we were to realize one of these doomsday scenarios how fast, actually, could we cook the planet. That's a question that they argue hangs on how efficiently the oceans can act as a heat sink, i.e., there's going to be a hard geophysical limit to the timescale within which that can manifest. This has implications for research priorities, as in, there is a reason why all of a sudden everyone should care about what the oceans are up to.

Roe and Bauman also kicks the can down the road to the policy jerks and economics sorcerers saying in effect, look, climate science has their business together such that the imponderbales about ECS are now less of a problem than the limitations of impact modeling:

The welfare impacts of climate change ultimately depend on the interplay between three factors: the temperatures resulting from a given carbon emissions profile, the damage functions resulting from those temperatures, and discounting issues that in our view are closely tied to assumptions about baseline economic growth. Fix any two of these and vary the third, and the results are likely to span the range of policy prescriptions from “don’t worry about it” to “drastic action required immediately”.

... such that, basically, there's no point to arguing over the possibility of 10+C worth of warming: tinkering with ECS is unlikely to help tell us what to do. Instead we need to care about "what might actually happen, irl" to assorted crucial socio-environmental systems at meaningful scales with 3-6C of warming and query whether our toy econ models and the implicit values they incorporate can grasp those outcomes.

[PhillipS]

No, this is not what TCR is.

 

TCR as defined by Cubasch et al 2001 is what the warming is once you get to 560ppm (or doubling of CO2) in a roughly 1% increase per year scenario...that is why it is called "transient climate response". Any warming that takes place after this period is beyond TCR and gets covered by ECS.

 

Well, I've had a chance to read Cubasch et al 2001 and I feel a bit better informed about TCR.  I had to smile though when I read the following:

 

The range of TCR for current AOGCMs is +1.1 to +3.1°C with an average of 1.8°C.

 

In my earlier ballpark calculation above I came up with an estimate for TCR of 1.86 C - so I don't feel I was completely out to sea.

 

Here is the full definition of TCR given:

 

TCR − Transient climate response

The temperature change at any time during a climate change integration depends on the competing effects of all of the

processes that affect energy input, output, and storage in the ocean. In particular, the global mean temperature change which

occurs at the time of CO2 doubling for the specific case of a 1%/yr increase of CO2 is termed the “transient climate response” (TCR) of the system. This temperature change, indicated in Figure 9.1, integrates all processes operating in the system, including the strength of the feedbacks and the rate of heat storage in the ocean, to give a straightforward measure of model response to a change in forcing. The range of TCR values serves to illustrate and calibrate differences in model response to the same standardised forcing. Analogous TCR measures may be used, and compared among models, for other forcing scenarios.

 

I was puzzled at first about why he defines TCR only for a specific point in time (reaching 560 ppm CO2), a fixed rate of CO2 increase (1% / year), and assumes CO2 levels stabilize at the moment doubling is reached.  Correct me if I'm wrong but my understand is that he imposed those constraints in order to allow apples-to-apples comparisons of model outputs.  If I'm correct I don't see any reason TCR can't be used in a more general sense.  For example, if TCR is 1.8 C / doubling then our 43% increase in CO2 is responsible for about 1.8C * 0.43 or about 0.77C of the observed warming of 0.8C.  Similarly, if and when CO2 concentration reaches 420 ppm (a 50% anthropogenic increase) we should see about 0.9C of warming on top of natural variability.  Of course, I'm assuming that TCR is roughly linear in the short term.

[ORH_wxman]

 

Well, I've had a chance to read Cubasch et al 2001 and I feel a bit better informed about TCR.  I had to smile though when I read the following:

 

The range of TCR for current AOGCMs is +1.1 to +3.1°C with an average of 1.8°C.

 

In my earlier ballpark calculation above I came up with an estimate for TCR of 1.86 C - so I don't feel I was completely out to sea.

 

Here is the full definition of TCR given:

 

TCR − Transient climate response

The temperature change at any time during a climate change integration depends on the competing effects of all of the

processes that affect energy input, output, and storage in the ocean. In particular, the global mean temperature change which

occurs at the time of CO2 doubling for the specific case of a 1%/yr increase of CO2 is termed the “transient climate response” (TCR) of the system. This temperature change, indicated in Figure 9.1, integrates all processes operating in the system, including the strength of the feedbacks and the rate of heat storage in the ocean, to give a straightforward measure of model response to a change in forcing. The range of TCR values serves to illustrate and calibrate differences in model response to the same standardised forcing. Analogous TCR measures may be used, and compared among models, for other forcing scenarios.

 

I was puzzled at first about why he defines TCR only for a specific point in time (reaching 560 ppm CO2), a fixed rate of CO2 increase (1% / year), and assumes CO2 levels stabilize at the moment doubling is reached.  Correct me if I'm wrong but my understand is that he imposed those constraints in order to allow apples-to-apples comparisons of model outputs.  If I'm correct I don't see any reason TCR can't be used in a more general sense.  For example, if TCR is 1.8 C / doubling then our 43% increase in CO2 is responsible for about 1.8C * 0.43 or about 0.77C of the observed warming of 0.8C.  Similarly, if and when CO2 concentration reaches 420 ppm (a 50% anthropogenic increase) we should see about 0.9C of warming on top of natural variability.  Of course, I'm assuming that TCR is roughly linear in the short term.

 

 

 

Well the definition of TCR was to determine how much warming would occur by the time the doubling was reached...it is a useful estimate for policy action because it is relatively short term. We should reach 560ppm of CO2 before the end of this century, therefore we should reach the TCR temperature before the end of this century.

 

I suppose someone could make another metric that is basically "temperature of the earth 25 years after a doubling" or something...but its really not that different than estimating the TCR itself. The TCR has a lot of debate in the literature with a lot more papers recently coming in values lower than the original ones that were closer to 2C...more in the 1.1-1.4C range.

 

Essentially, the more data we get with actual global temps, we should get a more accurate estimate of TCR. So our estimate in 2025 should be better than our estimate in 2014....just as our estimate in 2014 is better than in 2003. But that still doesn't mean there are not important uncertainties at the moment in these calculations.

[chubbs]

Well the definition of TCR was to determine how much warming would occur by the time the doubling was reached...it is a useful estimate for policy action because it is relatively short term. We should reach 560ppm of CO2 before the end of this century, therefore we should reach the TCR temperature before the end of this century.

 

I suppose someone could make another metric that is basically "temperature of the earth 25 years after a doubling" or something...but its really not that different than estimating the TCR itself. The TCR has a lot of debate in the literature with a lot more papers recently coming in values lower than the original ones that were closer to 2C...more in the 1.1-1.4C range.

 

Essentially, the more data we get with actual global temps, we should get a more accurate estimate of TCR. So our estimate in 2025 should be better than our estimate in 2014....just as our estimate in 2014 is better than in 2003. But that still doesn't mean there are not important uncertainties at the moment in these calculations.

 

 

Much sooner than that if you include all man-made GHG. According to NOAA (http://www.esrl.noaa.gov/gmd/aggi/aggi.html) forcing from all man-made GHG was equivalent to 478 ppm of CO2 in 2013 an increase of 4 ppm over 2012, At recent rates of increase,  forcing from all ghg will be equivalent to twice the pre-industrial CO2 level in roughly 20 years.

[ORH_wxman]

Much sooner than that if you include all man-made GHG. According to NOAA (http://www.esrl.noaa.gov/gmd/aggi/aggi.html) forcing from all man-made GHG was equivalent to 478 ppm of CO2 in 2013 an increase of 4 ppm over 2012, At recent rates of increase,  forcing from all ghg will be equivalent to twice the pre-industrial CO2 level in roughly 20 years.

 

 

CO2 right now is near 400ppm...that is what matters when talking about climate change. Not how much was manmade and how much was absorbed by carbon sinks like plants and the ocean. What matters is what is in the atmosphere for temperature projections.

 

Based on this, we should reach 560ppm sometime around 2070 given we are around 2ppm increase per year...accelerating now, but projected to decelerate toward mid-century.

[nflwxman]

CO2 right now is near 400ppm...that is what matters when talking about climate change. Not how much was manmade and how much was absorbed by carbon sinks like plants and the ocean. What matters is what is in the atmosphere for temperature projections.

 

Based on this, we should reach 560ppm sometime around 2070 given we are around 2ppm increase per year...accelerating now, but projected to decelerate toward mid-century.

Right that is the crux of climate sensitivity.  Actual levels, since the biosphere has differing abilities in the future to absorb CO2, natural or man made.

[nflwxman]

More research related to the PDO and hiatus decades- loosely related to ECS.

 

The linked reference indicates that for high rates of GHG emissions (such as we have now) "… there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption.  We further demonstrate that most non-volcanic hiatuses across CMIP5 models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase."  As I believe that we are now entering a positive phase of the IPO, this research implies that we may never see another negative phase of the IPO (in the foreseeable future):

Nicola Maher, Alexander Sen Gupta and Matthew H. England, (2014), "Drivers of decadal hiatus periods in the 20th and 21st Centuries", Geophysical Research Letters, DOI: 10.1002/2014GL060527


http://onlinelibrary.wiley.com/doi/10.1002/2014GL060527/abstract

Abstract: "The latest generation of climate model simulations are used to investigate the occurrence of hiatus periods in global surface air temperature in the past and under two future warming scenarios. Hiatus periods are identified in three categories, (i) those due to volcanic eruptions, (ii) those associated with negative phases of the Interdecadal Pacific Oscillation (IPO) and (iii) those affected by anthropogenically released aerosols in the mid 20th Century. The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption. We further demonstrate that most non-volcanic hiatuses across CMIP5 models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase."

[chubbs]

CO2 right now is near 400ppm...that is what matters when talking about climate change. Not how much was manmade and how much was absorbed by carbon sinks like plants and the ocean. What matters is what is in the atmosphere for temperature projections.

 

Based on this, we should reach 560ppm sometime around 2070 given we are around 2ppm increase per year...accelerating now, but projected to decelerate toward mid-century.

 

You need to account for CH4, N2O, CFCs, HFCs and other ghg gases that are increasing due to man. The increased forcing from all man-influenced GHGs is currently equivalent to 478 ppm of CO2 and will be equivalent to 560 ppm CO2 in roughly 20 years

[ORH_wxman]

You need to account for CH4, N2O, CFCs, HFCs and other ghg gases that are increasing due to man. The increased forcing from all man-influenced GHGs is currently equivalent to 478 ppm of CO2 and will be equivalent to 560 ppm CO2 in roughly 20 years

 

 

I'm not sure what you are agruing...the TCR is based on CO2 doubling (and the associated feedbacks which include methane release, etc). If you are arguing that TCR should be calculated based on a doubling of all GHGs, then it actually argues for a TCR much lower than anything published since we are already almost there.

[chubbs]

I'm not sure what you are agruing...the TCR is based on CO2 doubling (and the associated feedbacks which include methane release, etc). If you are arguing that TCR should be calculated based on a doubling of all GHGs, then it actually argues for a TCR much lower than anything published since we are already almost there

 If TCR is defined as the response to a 1% annual increase in CO2 until CO2 doubles. Then it dioesn't matter whether the forcing is from CO2 or an equivalent forcing from other ghg and CO2. Its easy to vary CO2 alone in a model but in the real-world CO2 can't be separated from other GHG so estimates from historical data would have to be based on CO2-equivalent i.e including all ghg.

[The_Global_Warmer]

Thank you for your edits Phillip.  I was a bit tired when I started this thread and apparently forgot how to do math.  I agree that TCR likely carries more implications for society during our lifetimes.  We should be able to free discuss both here.  While your back of the book calculation seems rudimentary, I believe it carries some aspect of truth to it.  Perhaps the simple empirical model is the way to go (particularly for TCR).  

 

Forcing estimates from several papers up to 2013 suggest Solar forcing from the start of global temperature measurements (1750 or so) to present is close to zero due to the recent TSI slowdown.  Several papers claim from top to bottom of the 11 year solar cycle can cause a 0.1K change in global temperature (all else the same).  Equal forcing is about +/-0.2 Watts/m2.  This is interesting as the sun has become more of negative forcing on the climate in the last decade.

 

https://www.youtube.com/watch?v=aqmw46Q4LdE#t=15.

 

http://www.atmos-chem-phys.net/13/2045/2013/acp-13-2045-2013.pdf

 

 

 

The power of methane is quite disturbing.   

[chubbs]

A very rough "poor mans" TCR estimate can be made by regressing annual average GISS ST against the NOAA greenhouse gas index (http://www.esrl.noaa.gov/gmd/aggi/aggi.html). The NOAA ghg index, with the acronym AGGI, is a linear measure of manmade GHG forcing. AGGI is zero for pre-industrial conditions, 1 in 1990, 1.34  in 2013 and 1.86 for forcing equivalent to doubled CO2 (560 ppm).

 

The chart below plots annual average GISS vs AGGI with a single point for each year since 1880. There is a roughly linear relationship and a strong correlation between AGGI and GISS ST. AGGI explains 86% of the variation in the GISS series since 1880. 

 

According to the regression trend line, the GISS 2013 average anomoly of 0,61 is 1.04C above the pre-industrial value (at zero AGGI) of -0.43C. Note that AGGI had already increased over pre-industrial values at the 1880 start of the GISS series. The regression predicts .a GISS anomoly of 1.01C at AGGI equivalent to doubled CO2 giving a TCR estimate of 1.44. As discussed in the post above, at recent rates of increase, AGGI equivalent to doubled pre-industrial CO2 could be reached around 2035. GISS needs to increase around 0.18C per decade to reach the predicted 1.01C GISS anomoly value at doubled CO2-equivalent. This rate of increase is similar to the GISS increase over the past 40 years.

 

Of course this method ignores many important factors including natural variation, response lags, and aerosals. Still the TCR estimate of 1.44 is within the range of published values.

[PhillipS]

Going back to Cubasch et al 2001, it talks about the issue of aggregate GHGs as compared to CO2 alone:

 

The 1%/yr rate of increase for future climate, although larger than actual CO2 increase observed to date, is meant to account for the radiative effects of CO2 and other trace gases in the future and is often referred to as “equivalent CO2” (see discussion in Section 9.2.1). This rate of increase in radiative forcing is often used in model intercomparison studies to assess general features of model response to such forcing.

 

And in the referrenced section 9.2.1:

 

Equivalent CO2

The radiative effects of the major greenhouse gases which are well-mixed throughout the atmosphere are often represented in

GCMs by an “equivalent” CO2 concentration, namely the CO2 concentration that gives a radiative forcing equal to the sum of the forcings for the individual greenhouse gases. When used in simulations of forced climate change, the increase in “equivalent CO2” will be larger than that of CO2 by itself, since it also accounts for the radiative effects of other gases. 

 

1%/yr increasing CO2.

A common standardised forcing scenario specifies atmospheric CO2 to increase at a rate of 1%/year compound until the concentration doubles (or quadruples) and is then held constant. The CO2 content of the atmosphere has not, and likely will not, increase at this rate (let alone suddenly remain constant at twice or four times an initial value). If regarded as a proxy for all greenhouse gases, however, an “equivalent CO2” increase of 1%/yr does give a forcing within the range of the SRES scenarios.  

 

   This forcing prescription is used to illustrate and to quantify aspects of AOGCM behaviour and provides the basis for the analysis and intercomparison of modelled responses to a specified forcing change (e.g., in the SAR and the CMIP2 intercomparison).  The resulting information is also used to calibrate simpler models which may then be employed to investigate a broad range of forcing scenarios as is done in Section 9.3.3. Figure 9.1 illustrates the global mean temperature evolution for this standardised

forcing in a simple illustrative example with no exchange with the deep ocean (the green curves) and for a full coupled AOGCM (the red curves). The diagram also illustrates the transient climate response, climate sensitivity and warming commitment.

 

Bottom line, as I understand this, is that the definition of TCR is for the Equivalent CO2, or AGGI if preferred, and not for CO2 alone.

[nflwxman]

Going back to Cubasch et al 2001, it walks about the issue of aggregate GHGs as compared to CO2 alone:

The 1%/yr rate of increase for future climate, although larger than actual CO2 increase observed to date, is meant to account for the radiative effects of CO2 and other trace gases in the future and is often referred to as “equivalent CO2” (see discussion in Section 9.2.1). This rate of increase in radiative forcing is often used in model intercomparison studies to assess general features of model response to such forcing.

And in the referrenced section 9.2.1:

Equivalent CO2

The radiative effects of the major greenhouse gases which are well-mixed throughout the atmosphere are often represented in

GCMs by an “equivalent” CO2 concentration, namely the CO2 concentration that gives a radiative forcing equal to the sum of the forcings for the individual greenhouse gases. When used in simulations of forced climate change, the increase in “equivalent CO2” will be larger than that of CO2 by itself, since it also accounts for the radiative effects of other gases.

1%/yr increasing CO2.

A common standardised forcing scenario specifies atmospheric CO2 to increase at a rate of 1%/year compound until the concentration doubles (or quadruples) and is then held constant. The CO2 content of the atmosphere has not, and likely will not, increase at this rate (let alone suddenly remain constant at twice or four times an initial value). If regarded as a proxy for all greenhouse gases, however, an “equivalent CO2” increase of 1%/yr does give a forcing within the range of the SRES scenarios.

This forcing prescription is used to illustrate and to quantify aspects of AOGCM behaviour and provides the basis for the analysis and intercomparison of modelled responses to a specified forcing change (e.g., in the SAR and the CMIP2 intercomparison). The resulting information is also used to calibrate simpler models which may then be employed to investigate a broad range of forcing scenarios as is done in Section 9.3.3. Figure 9.1 illustrates the global mean temperature evolution for this standardised

forcing in a simple illustrative example with no exchange with the deep ocean (the green curves) and for a full coupled AOGCM (the red curves). The diagram also illustrates the transient climate response, climate sensitivity and warming commitment.

Bottom line, as I understand this, is that the definition of TCR is for the Equivalent CO2, or AGGI if preferred, and not for CO2 alone.

Thinking a little deeper into this, it would be difficult to get a fair estimation of TCR just for CO2 without considering other GHGs (natural and anthropogenic). AGGI would seem to be a reasonable way to go here.

Did anyone read that paper above about hiatus decades? Curious if there are any comments.

[chrisf97212]

 If TCR is defined as the response to a 1% annual increase in CO2 until CO2 doubles. Then it dioesn't matter whether the forcing is from CO2 or an equivalent forcing from other ghg and CO2. Its easy to vary CO2 alone in a model but in the real-world CO2 can't be separated from other GHG so estimates from historical data would have to be based on CO2-equivalent i.e including all ghg.

 

 

I'm not sure what you are agruing...the TCR is based on CO2 doubling (and the associated feedbacks which include methane release, etc). If you are arguing that TCR should be calculated based on a doubling of all GHGs, then it actually argues for a TCR much lower than anything published since we are already almost there.

 

I'm a little lost.  The other GHGs' absorb energy at different wavelengths, so I don't see how they are interchangeable (if that's what the point was).  Rather I thought the other GHG's were feedback mechanisms that were already included in the TCR.  Its been the feedback mechanisms that I considered to be the least understood and weakest link in GCM.

[The_Global_Warmer]

Does anyone have a link to those that Russian teams ftp site that has their methane data on it?

 

I lost it.  Or it's no longer updated.