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Pielke Sr. and SkS Warming Estimates

Posted on 11 October 2011 by dana1981, Albatross

Dr. Roger Pielke Sr. has written a blog post addressing the disagreement between himself and Skeptical Science (SkS) regarding the contribution of CO2 to the net positive anthropogenic radiative forcing.  Initially Dr. Pielke cited a presentation he gave in 2006 which said (on slide 12):

"The CO2 contribution to the radiative warming decreases to 26.5% using the IPCC framework given in Slide 9"

This "radiative warming" refers to the human plus natural positive radiative forcings ('natural' being solar).  As Dr. Pielke's presentation was given in 2006, before publication of the IPCC Fourth Assessment Report (AR4), his reference to the IPCC is to the Third Assessment Report (TAR) published in 2001.  In his new post, Dr. Pielke also references a previous post on his blog on the same subject, which concludes (emphasis added):

"For all of the human-caused warming radiative forcings, which includes the 0.5 Watts per meter squared value for the shortwave albedo change, and estimating tropospheric ozone as 0.3 Watts per meter squared, the aerosol black carbon direct effect as 0.2 Watts per meter squared, the black carbon on snow and ice as 0.3 Watts per meter squared, the semidirect indirect effect as 0.1 Watt per meter squared, and the glaciation indirect effect as 0.1 Watt per meter squared (with the latter two forcings using a nominal value, since these forcings are very poorly known), the contribution due to CO2 will fall to about 28%."

In this case Dr. Pielke refers to only the human positive radiative forcings, excluding the contribution of solar irradiance.

In short, Dr. Pielke has argued that CO2 contribution to the total positive radiative forcing (since pre-industrial times) is between 26% and 28% (depending on whether solar effects are included), whereas in our previous post, SkS concurred with the AR4 radiative forcing estimates, which put CO2 at approximately 50% of the total positive radiative forcing (nearly twice Dr. Pielke's estimate). 

Below we discuss some problems SkS has identified in Dr. Pielke's estimate, and provide a detailed up-to-date estimate of these values.  The main underlying problem is that Dr. Pielke is relying on an estimate he made in 2006, failing to account for advances in climate research over the past 5 years, and thus his sources are at least 5 years out of date.  Additionally, he appears to have made some mathematical errors in his calculations.

Methane

Dr. Pielke estimates the radiative forcing from methane at 0.8 Watts per square meter (W/m2), which is significantly larger than the IPCC estimate (both TAR and AR4) of 0.48 W/m2.   To support this value, in his 2006 presentation Dr. Pielke references research by "Drew Shindell and colleagues; Keppler et al." (slide 11), and on his blog posts, references Keppler et al. (2006).  Keppler et al. do not estimate the methane radiative forcing in their paper - the 0.8 W/m2 figure is Dr. Pielke's estimate based on Keppler et al.'s results.

However, as we noted in our previous post, both the atmospheric methane concentration and radiative forcing are well-known quantities.  The IPCC TAR and AR4 best estimates of the methane radiative forcing are 0.48 W/m2, 0.49 W/m2 according to Skeie et al. (2011), and 0.504 W/m2 in 2010 according to the NOAA Annual Greenhouse Gas Index (AGGI).  Thus Dr. Pielke's methane forcing estimate appears to be 60% too high.

Additionally, Dr. Pielke appears to have double-counted the methane forcing in his calculations:

"By summing the 0.8 Watts per meter squared for methane and using the total of 2.4 Watts per meter squared of the well-mixed greenhouse gases from the IPCC Report..."

The 0.48 W/m2 methane forcing is included in the 2.43 W/m2 best estimate forcing for well-mixed greenhouse gases in the IPCC TAR (the best estimate is 2.64 W/m2 in the AR4).  Thus, summing Pielke's estimated methane forcing (0.8 W/m2) and the IPCC TAR greenhouse gas forcing (2.4 W/m2) double counts the methane forcing.

Albedo

Dr. Pielke also estimates "0.5 Watts per meter squared value for the shortwave albedo change," which is a forcing not included in the TAR or AR4.  In his presentation (slide 11), Dr. Pielke claims:

"For the period 2000-2004, a CERES Science Team assessment of the shortwave albedo found a decrease by 0.0015 which corresponds to an extra 0.5 W m−2 of radiative imbalance according to their assessment."

However, there are a number of problems with this estimate.  Most importantly, the data in question only cover a period of 4 years.  Changes in the Earth's albedo (reflectivity) over a 4-year period tell us little or nothing about changes in albedo over the past century. It's apples and oranges; one is short-term, the other is long-term. 

Four years is also simply far too short of a timeframe to ascertain a meaningful trend. From Loeb et al. (2007):

"Commonly used statistical tools applied to the CERES Terra data reveal that in order to detect a statistically significant trend of magnitude 0.3 W m−2 decade−1 in global SW TOA flux, approximately 10 to 15 yr of data are needed. "

Additionally, there is significant uncertainty regarding this short-term albedo change (i.e. see Wielicki et al. 2005 and many other papers on the subject).  While the CERES data Dr. Pielke references estimated a decrease in the Earth's albedo  from 2000 to 2004, albedo change estimates over the exact same timeframe using Project Earthshine data found an even larger  increase in albedo from 2000 to 2004 than the CERES-estimated decrease.

Loeb et al. (2007) also used a revised version of the CERES data to show that no statistically significant changes in the Earth’s albedo occurred between 2000 and 2005.  More recently, Palle et al. (2009) conclude:

"Earthshine and FD [International Satellite Cloud Climatology Project flux data] analyses show contemporaneous and climatologically significant increases in the Earth's reflectance from the outset of our earthshine measurements beginning in late 1998 roughly until mid-2000. After that and to date, all three show a roughly constant terrestrial albedo, except for the FD data in the most recent years"

We should also note that an albedo increase/decrease due to increasing cloud cover would also be accompanied by an increased/decreased greenhouse effect, making the net effect on the climate even more uncertain.

But the bottom line is that in order to incorporate an albedo forcing into these estimates, we must use an estimated albedo change from pre-industrial to Present.  We should also investigate the cause of any albedo change to determine if it should be treated as a forcing or as a feedback.  If it's a forcing, then it's not anthropogenic, and Dr. Pielke was incorrect to include it in the anthropogenic forcings.  If it's a feedback, then it should not be included in the calculation of total forcings at all.

Ultimately, for this calculation, Dr. Pielke's 0.5 W/m2 albedo forcing estimate is unjustified and not supported by more recent observations and scientific literature.

Black Carbon

Dr. Pielke cites Hansen and Nazarenko (2004) in estimating the albedo effect of soot on snow and ice at 0.3 W/m2, and the net black carbon forcing at 0.5 W/m2.  However, as the IPCC AR4 noted three years later, the magnitude of the black carbon radiative forcing remains uncertain.  The best estimate of Skeie et al. (2011) of 0.45 W/m2 for the black carbon forcing is in rough agreement with Dr. Pielke's estimate.  Ramanathan and Carmichael (2008) give a best estimate for the black carbon forcing at 0.9 W/m2.

In short, the black carbon forcing remains highly uncertain, but Dr. Pielke's estimate is reasonable.

Tropospheric Ozone

Dr. Pielke's tropospheric ozone forcing estimate is somewhat unclear.  He states that the associated forcing is 0.3 W/m2, but the IPCC TAR estimate is 0.35 W/m2, and Dr. Pielke appears to believe the value should be higher:

"Ozone was responsible for one-third to one-half of the observed warming trend in the Arctic during winter and spring [Drew Shindell]"

This release from NASA GISS appears to be the source, from which, if we are interpreting his presentation correctly, Dr. Pielke estimates an additional 0.3 W/m2 on top of the IPCC 0.35 W/m2 tropospheric ozone radiative forcing. 

However, there are more recent estimates of this forcing, in addition to the IPCC's 0.35 W/m2 (both TAR and AR4).  The best estimate from Skeie et al. (2011) was 0.44 W/m2, and the best estimate from Cionni et al, 2011 (submitted), on which Shindell is a co-author, is 0.23 W/m2.  Thus Dr. Pielke's estimate of 0.65 W/m2 appears to be much too high.

Aerosol Semi-Direct and Indirect Effects

Dr. Pielke also identifies a "glaciation effect" as causing a 0.1 W/m2 forcing, which, in a recent talk, he clarifies as "An increase in ice nuclei increases the precipitation efficiency."    Lohmann et al. (2007) is a very good paper on this subject, and explains the effect, described as the aerosol indirect effect:

"Global climate model studies suggest that if, in addition to mineral dust, hydrophilic black carbon aerosols are assumed to act as ice nuclei at temperatures between 0 and –35°C, then increases in aerosol concentration from pre-industrial to present times may cause a glaciation indirect effect (Lohmann, 2002a). The glaciation effect refers to an increase in ice nuclei that results in a more frequent glaciation of supercooled stratiform clouds and increases the amount of precipitation via the ice phase. This decreases the global mean cloud cover and allows more solar radiation to be absorbed in the atmosphere. Whether or not the glaciation effect can partly offset the warm indirect aerosol effect depends on the competition between the ice nucleating abilities of the natural and anthropogenic freezing nuclei (Lohmann and Diehl, 2006)."

Lohmann et al. (2007) note that the aerosol indirect glaciation effect is negligible.  However,  Perlwitz and Mlller (2010) conclude:

"Despite the high complexity and nonlinearity of the microphysical interaction between aerosols and clouds, modeling studies generally indicate that the net effect of this interaction is to reflect more radiation back to outer space [Forster et al., 2007], although recent results show that aerosols acting as ice nuclei could counteract the cooling effect significantly [Storelvmo et al., 2008]. A few observational studies seem to confirm a relation between soil dust aerosols and cloud cover."

In short, the aerosol indirect glaciation effect remains far from clear.  Dr. Pielke also identifies the aerosol semi-direct effect, which involves tropospheric aerosols absorbing shortwave radiation, as causing a 0.1 W/m2 forcing.  However, the IPCC has not included this as a positive forcing becase

"the semi-direct effect is not strictly considered an RF because of modifications to the hydrological cycle"

Additionally, Lohmann et al. identify the semi-direct effect as most likely causing cooling:

"The semi-direct effect refers to temperature changes due to absorbing aerosols that can cause evaporation of cloud droplets, as was shown in a large eddy model simulation study that used black carbon concentrations measured during the Indian Ocean Experiment (Ackerman et al., 2000). It ranges from 0.1 to –0.5 Wm-2 in global simulations" 

The IPCC AR4 also lists the semi-direct effect as "positive or negative" and "small" potential magnitude, and the indirect effect as "positive" and "medium" potential magnitude, where as Dr. Pielke lists both as positive and equal in magnitude (0.1 W/m2).  In short, the magnitude and roles of the aerosol semi-direct and indirect glaciation effects in terms of radiative forcings remain far from clear.

Carbon Dioxide

Dr. Pielke's estimate for the CO2 radiative forcing (1.4 W/m2) is both outdated and not consistent with the value in the IPCC TAR (1.46 W/m2); it appears that he either rounded the value down or eyeballed the IPCC TAR radiative forcing graphic rather than looking up the precise value.  However, 1.46 W/m2 was the estimated value in 2001, when the TAR was published.  In 2007, when the AR4 was published, the CO2 forcing had already increased to 1.66 W/m2.  More recently, the NOAA AGGI estimated the CO2 forcing at 1.79 W/m2 in 2010, and Skeie et al. at 1.82 W/m2.

In other words, the CO2 radiative forcing has increased 25% over the past decade.  Some of the other forcing estimates (like tropospheric ozone and black carbon) have changed mainly as a result of new research, but the CO2 forcing has changed as a result of rapidly increasing CO2 emissions and atmospheric concentrations. 

As Isaac Held noted,

"I think it is very generally recognized that, for the same global mean forcing, aerosols perturb the mean precipitation field more than do the well-mixed greenhouse gases (WMGGs). So if, up to the present, anthropogenic aerosols and WMGGs have had comparable effects on regional precipitation, say, the WMGG effect will undoubtedly grow and will be essentially irreversible on the time scale of several centuries, in the absence of geoengineering, while the aerosol effect will likely be bounded by its current magnitude, and the WMGGs will dominate."

Estimated CO2 Contribution

Below we summarize various estimates of the CO2 contribution to the net positive radiative forcing. We believe Dr. Pielke has committed two types of errors: mathematical (double-counting and rounding), and using outdated sources.

We believe the first column is a replication of Dr. Pielke's estimates.  The second column corrects Dr. Pielke's math errors by eliminating the double counting of methane, and correcting rounding errors for the CO2 and solar forcings. The third column provides the IPCC TAR estimates which were the basis of Dr. Pielke's estimates, but which, for the most part, we believe are more accurate than Dr. Pielke's suggested values.

The fourth and fifth columns correct for the out-of-date references by using the IPCC AR4 and Skeie et al. (2011) estimates.  Bear in mind we have not included the uncertainty ranges - these are all just best estimates of the respective positive radiative forcings (in W/m2).

ForcingsPielke 2006Pielke Math CorrectedIPCC TARIPCC AR4Skeie 2011
CO2 1.40 1.46 1.46 1.66 1.82
CH4 0.80 0.80 0.48 0.48 0.49
other LLGHGs 1.00 0.49 0.49 0.50 0.51
tropospheric ozone 0.65 0.65 0.35 0.35 0.44
black carbon 0.50 0.50 0.20 0.10 0.45
albedo 0.50 0.50 0 0 0
aerosols (semi-direct+indirect) 0.20 0.20 0 0 0
stratospheric water vapor 0 0 0 0.07 0.07
contrails 0.02 0.02 0.02 0.01 0
solar 0.25 0.30 0.30 0.12 0.12 (AR4)
Total Positive Forcing 5.32 4.92 3.30 3.29 3.90
Total Anthropogenic Forcing 5.07 4.62 3.00 3.17 3.78
CO2 contribution to Total 26.3% 29.7% 44.2% 50.5% 46.7%
CO2 contribution to Anthropogenic 27.6% 31.6% 48.7% 52.4% 48.2%

If we correct for Pielke's double counting and rounding errors, the CO2 contribution to the total net positive forcing increases to approximately 30%.  When we use up-to-date research for all forcings, the CO2 contribution increases to close to 50%, as we originally argued.  We again note that this fraction will continue to increase along with continually increasing human CO2 emissions.

Human Contribution to Global Surface Warming

We are still interested in Dr. Pielke's answer our original question on this subject:

"Approximately what percentage of the global warming (increase in surface, atmosphere, ocean temperatures, etc.) over the past 100 years would you estimate is due to human greenhouse gas emissions and other anthropogenic effects?"

We suggest a back-of-the-envelope answer to this question by applying the probabilistic estimate of transient climate sensitivity by Padilla (2011):

"we find a most-likely present-day estimate of the transient climate sensitivity to be 1.6 K with 90% confidence the response will fall between 1.3–2.6 K" 

We can use this range of transient climate sensitivity (alpha = 0.35 to 0.70 K/Wm-2) and scale the transient climate response (we're currently 49% of the way to the radiative forcing associated with CO2 doubling [~1.8 out of 3.7 W/m2]) to estimate the amount of CO2-caused surface warming:

Where F is the radiative forcing.  Using the Skeie et al. (2011) CO2 forcing best estimate of 1.82 W/m2 for 2010 and the Padilla (2011) range of transient climate sensitivity parameters, this corresponds to a CO2 contribution of 0.64 to 1.28°C, with a best estimate of 0.79°C warming of average global surface temperature.

We can also consider the expected warming for the net anthropogenic forcing, which Skeie et al. estimated at 1.4 W/m2 and the IPCC AR4 estimated it at 1.6 W/m2.  Using these two estimates and the Padilla transient sensitivity range yields a net anthropogenic warming of 0.49 to 1.12°C with a central estimate of 0.65°C warming of average global surface temperature.

Dr. Pielke, would you concur with these estimated ranges of CO2 and anthropogenic warming?

Take-Home Message

The main points here are that CO2 is responsible for approximately 50% of the net positive radiative forcing since pre-industrial times (a percentage which will only continue to increase in the future).  In the absence of negative forcings, CO2 would have contributed 0.79°C of the 0.8°C observed global surface temperature rise, and hence we would expect the total observed rise to be double that.  This tells us that the negative forcings (primarily from human aerosol emissions) have offset approximately 50% of the net positive forcings.

We also found that the net anthropogenic radiative forcing (sum of all positive and negative forcings) accounts for approximately 80% of the observed average surface warming over the past century (~0.65 out of 0.8°C).  The other ~20% is a combination of natural forcings (primarily solar), and perhaps a bit of natural variability.

Another key point is that aerosols have a short atmospheric lifetime, unlike long-lived greenhouse gases.  Thus their large offsetting of close to 50% of the net positive radiative forcing is only temporary, and will decline rapidly if we reduce aerosol emissions.  This is why, as Isaac Held noted in the quote above, we fully expect CO2 and other greenhouse gases to continue as the dominant cause of global warming, and why although we need to address other issues like land use change, CO2 emissions are rightfully the primary target in mitigating climate change.

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Comments 51 to 91 out of 91:

  1. Dr Pielke @ 48... I just read your paper that you provided a link to titled... "Climate Change: The Need to Consider Human Forcings Besides Greenhouse Gases."

    I'm not sure you're going to get a ton of argument on that issue. Quite honestly, you actually might get more argument on much of what is stated in your paper on Anthony Watts' site.

    I think maybe a little perspective is in order here. You have to look at what SkS is doing. SkS tries to address the fact that a large number of people in the general public are not taking a skeptical approach to climate science. SkS is mostly addressing very elemental scientific facts that are being obfuscated by a large number of people out there. (And on top of that, trying to report on what new is coming out in the published research.)

    I can attest to the fact that I, and I'm sure many of the authors here, spend an inordinate amount of time on the internet arguing with people who make claims like: There is no greenhouse effect. AGW contradicts the 2nd law of thermodynamics. It's not actually warming. Volcanoes put out more CO2 than humans. The warming is being caused by the sun. Arctic ice is actually recovering. CO2 is too small a part of the atmosphere to have an effect. And on and on.

    What I think you are approaching has to do with issues of policy and the IPCC. The process the IPCC goes through can always be improved. Policy makers need the best information possible if humanity is going to deal with what could be a very critical issue. But what I think happens is that gets twisted around in the blogosphere into "the IPCC can't be trusted."

    There is so much blatantly wrong and misleading information being propagated out there in relation to climate change that it is extremely concerning to people here. I believe John Cook and all the SkS authors want to make sure that correct information - information that is digestible for the general public - gets voiced.

    When Dana takes on your statement about 28% of forcing I see something totally different than you. I see all the people who take that statement from "Dr Pielke Sr" that "it's only 28% of the forcing" and I see how that gets translated in the real world. It comes out as, "See! Dr Pielke says we don't need to worry about CO2." And I don't think that's the message that you want to project, but that is what gets propagated. That is what SkS is fighting.

    A friend of mine told me once, "In a conversation, always try to listen for context over content." Try to listen to why people are saying what they are saying, not just what they are saying.
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  2. Dr. Pielke, the reason we got 'hung up' on this issue is that you kept coming up with new (but still incorrect) arguments why the CO2 contribution must somehow be significantly less than 50% of the net positive forcing. Every new explanation you come up with requires investigation to determine if the argument has any validity.

    Perhaps you would have been better served to accept the calculations putting the value at ~50%, as opposed to constantly trying to find new reasons to validate your opinion that the value must somehow be lower. Then we could more easily move on to other topics, instead of getting 'hung up'.

    However, at this point I concur that we should move on to other issues. Note that we have moved on to point #2 in the Disagreements post.
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  3. This is a good point by Rob Honeycutt in comment #51, as evidenced by the "skeptic" blog posts focusing on the claim that CO2 is only 26-28% of the net positive forcing, as I noted in comment #47. When you're featured on a site like Junk Science, it should give you great pause. Rob's comment bears reiteration:
    "When Dana takes on your statement about 28% of forcing I see something totally different than you. I see all the people who take that statement from "Dr Pielke Sr" that "it's only 28% of the forcing" and I see how that gets translated in the real world. It comes out as, "See! Dr Pielke says we don't need to worry about CO2." And I don't think that's the message that you want to project, but that is what gets propagated. That is what SkS is fighting."
    This is indeed one reason why we put so much focus into this issue.
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  4. Dr. Pielke writes:

    >>The real substantive issue, however, which no one on this weblog seems to want to debate, is my (and my colleagues) conclusion that

    The substantive issue is that the net positive anthropogenic radiative forcing is much greater than 28%, the topic of this posting and the debate, as SkS made this absolutely clear from the start. If you really felt this topic trivial, that it didn't matter if the fraction amounted to 28% of 48%, you needed to state that from the outset. It would have been simple to do.

    Instead, you engaged in a debate, tried to defend your position, and some 49 posts later, declare that the debate is really about your hobbyhorse, namely, that "The IPCC Has Provided An Inaccurate Narrow Perspective Of The Role Of Humans." Since you have made the claim of 28% multiple times, I would think you would want to stand by it.
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  5. pdt @51, it is true that the rhetoric of global warming tends to center around CO2, in part because it plays so large a role compared to any other single agent, and in part because it is easier to say than "Long lived green house gases". However, legislation (and proposed legislation) to deal with the issue tends to tax all LL GHG in terms of their "CO2 equivalent" levels. Other abatement measures are also focused on other forcing agents. In Brisbane, for example, the CSIRO is developing fodder for cattle that reduces methane emissions. Elsewhere, I believe they are working on dry land rice varieties which will have the same effect (although they have other reasons). Hansen has strongly recommended limiting BC as an early response to global warming both because of its low cost as an abatement measure.

    Given this, it is worthwhile noting that LL GHG represent 72.3% of total forcing since 1750 (Skeie et al), or 55.6% (Pielke's 2006 estimate with mathematical errors corrected). Again this proportion will rise with time if effective emission reduction strategies are not adopted.

    Industrial pollutants (tropospheric ozone and BC) constitute 22.8% of all forcings (Skeie et al), or 24.9% (Pielke corrected). In any terms, these are a substantive proportion and I agree with Hansen that we should target these "low hanging fruit". Having said that, must of these emissions come from China and India (now), and will dissipate in any event when increased prosperity leads to pressure for clean air acts.
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  6. paulhtremblay @54, with respect, while the 28% figure is the topic of this thread, it is not key to the wider debate. Given that many opponents of global warming are willing to take points of of context, the 28% figure needed to be discussed, but given time constraints on the participants, it is perhaps better to move on.

    Having said that:

    1) if Dr Pielke would actually state the argument that his questions about water vapour have been alluding to, rather than fencing around the edges, I believe he raises a substantive (although incorrect) issue. As it stands, his question is purely a hypothetical and not worth further discussion of he will not be plain in his argument.

    2) I for one have several more points I could raise against various of Dr Pielke's comments. I am willing to shelve them for now in the interests of moving to the more substantial point with regard to Dr Pielke's position of Land Cover and Land Use Changes, and the gradient of radiant forcing. However, if others are interested I (and perhaps Dr Pielke) can revisit the issue once we have discussed more substantive matters.
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  7. Rob Honeycutt - You wrote that people will say

    it's only 28% of the forcing" so they will translate "Dr Pielke says we don't need to worry about CO2."

    I do not know how many times I can state it, but I certainly agree that added CO2 is a first order climate forcing (with the biogeochemiccal effect being the larger issue, in my view), and have said so repeatedly in my weblog posts and in my papers. I support my son's approach, as presented in his book "The Climate Fix" as the way forward to promote limiting the increase.

    However, all of the first order climate forcings are important (the global average negative radiative forcings, the regional heterogeneous radiative forcings, and the "non-radiative" climate forcings).

    In my view, the IPCC has failed to adequately consider these other human climate forcings and the fact that CO2 is not even the large majority of the positive radiative forcings underscores this need. Most, if not all, of these other climate forcings will persist at some level for the foreseeable future and, in terms of how they affect atmospheric circulations, could be much more important than the effects on weather patterns from the radiative effect of added CO2.

    This is the perspective that we presented in the 2009 EOS article and in the 2005 NRC report.

    The precise fractional value with respect to preindustrial and 2011, however, is not an important issue. In the biogeochemical perspective, it is the atmospheric concentration that matters, and it is continuing to increase.
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  8. "and the fact that CO2 is not even the large majority of the positive radiative forcings underscores this need. "

    But CO2 _is_ the single largest contributor. Moreover, emissions of CO2 lead to changes in atmospheric concentrations that can last 100s or 1000s of years... many of the other forcings are much more transient (eg, CH4, O3, BC, contrails, etc.).

    So, while I agree that these other forcings are important, and we certainly shouldn't ignore them, CO2 is clearly, at the global scale, the primary contributor to long-term climate change. If, in the next half century, human society realizes that global climate change is a problem, it will be possible at that point to reduce CH4, BC, and O3 concentrations fairly quickly. All the CO2 we've emitted between now and then, though, will be pretty much permanently in the carbon cycle until we figure out air capture...

    I'm also unconvinced that the IPCC hasn't adequately addressed most of these other contributors. I'd certainly admit that the IPCC could do a better job on the causes of weather events, but that is a hard problem, and they are building up to it (In my opinion, interesting recent papers have included interactions of large scale weather with sea ice retreat, solar fluctuations, and historical attribution of southwestern megadroughts to ocean cycles and other factors). But they _do_ make their best attempt at characterizing global forcing from as many sources as possible, and between AR4 and AR5 are probably putting a lot of effort into getting better numbers for the most uncertain of these (such as BC and other aerosols).
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  9. MMM -

    Society and the environment are influenced by regional and local issues, including climate. The use of a global average is of much less value to determine risks.

    In terms of how long each forcing affects these spatial scales land use/land cover change, nitrogen deposition, black carbon deposition, and mineral dust deposition are examples of four other human climate forcings with long residence times in the climate system.
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  10. I have read the 2009 EOS article. Given what it contains, the question which should be asked of Pielke Sr's is not what % of human-caused forcing from CO2 but what % of human-caused forcing from all greenhouse gases.
    EOS 2009 presents three “mutually exclusive” hypotheses (1) Human influence on climate is minimal, (2a) Alongside important natural variation, human influence on climate is significant due to a diverse range of first order climate forcings which includes CO2, (2b) Alongside important natural variation, human influence on climate is significant dominantly due to GHGs, the most important being CO2.
    Two of these hypotheses are then dismissed. “We suggest that the evidence in the peer-reviewed literature (e.g., as summarised by National Research Council (NRC) [2005]) is predominantly in support of hypothesis 2a, in that a diverse range of first-order human climate forcings have been identified.” These non-GHG forcings are “...spacially hetrogeneous and include...” aerosols on clouds, aerosol deposition, reactive nitrogen & changing land use. Their effects are estimated to be multi-decadal and longer.
    If this diverse range of non-GHG forcings are a significant influence on climate, how big is that significance in comparison with human GHG forcing? Pielke Sr appears to be presenting mankind with an additional crisis caused by non-GHG forcings on top of that caused predominantly by GHG emissions stating “global climate models do not accurately simulate (or even include) several of these first-order human climate forcings.
    If EOS 2009 is to be taken seriously, the first step must be to quantify the “significance” of these non-GHG forcings.
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  11. It appears that too many people are getting hung up on the exact figure that CO2 is contributing to the warming. Whether CO2 has contributed 30% or 50% to the observed warming is helpful in making future projections, but is not critical. Considering the uncertainties involved in all the measurements, determining the contribution to within 20% would be amazing.

    What I found surprising was the exclusion of albedo and aerosol contributions. Do people here really believe that they are irrelevant?
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  12. Jonathon: "What I found surprising was the exclusion of albedo and aerosol contributions."

    I think you may have misunderstood the discussion.

    Aerosols (e.g. black carbon) are included. What is in dispute are additional 'knock on' "indirect" and "semi-direct" effects (basically hypotheses that aerosols impact clouds and precipitation) from aerosols. Dr. Pielke apparently believes these are significant while other research suggests they are negligible.

    Similarly for albedo. No one has suggested that it isn't a significant factor. Only that albedo changes due to ice loss are a feedback and thus not relevant to this discussion of human forcing effects. I believe Dr. Pielke's position is that human land use changes have resulted in a significant albedo shift, but again that is a dispute over a particular subset rather than the entire topic of 'albedo'.
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  13. Perhaps this came up before, but I think something useful for this type of analysis would be to estimate uncertainties in each term.

    Also, more useful from a policy perspective would be to estimate forcing from categories of human activity, such as fossil fuel production/use, forest clearing, etc.
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  14. Dr Pielke #32

    Correct me if I have misundertood but the calculation of current radiative forcings is not derived from changes in OHC. The AR4 forcings are largely derived from models and theory.

    Historical forcings and the historical planetary imbalance are important in determining the heat energy accumulated in the oceans since the base date of 1750.

    As you have correctly argued elsewhere, the key metric is changes in heat energy in the oceans (Joules) in measuring the magnitude of the global energy imbalance.

    The point no-one seems to be discussing here is that the sum of all forcings (both positive radiative and climate response negative) is what counts in determining the imbalance.

    For example if Asian aerosols are increasing the negative forcing (which might be linear - who knows?) at the same time as fossil fuel burning is adding to CO2 positive forcing (which is logarithmic), then the net result could be Dr Jim Hansen's net reduction in the forcing imbalance.
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  15. This press release on a research paper regarding the land use effect on the global average radiative flux might be of interest.

    ”World-lead​ing scientists will shed new light on climate-ch​ange mitigation in Garmisch-P​artenkirch​en, Germany, 18-23 September” http://www.ileaps.org/phocadownload/SC2011/SC2011%20press%20release%20080911_NB.pdf

    "Deforestation has resulted in emissions of CO2 into the atmosphere, but clearing the forests may also have cooled the climate because forest is darker and heats up more than agricultural and pasture land.” The increase in atmospheric CO2 caused by human land-cover change became significant long before the Industrial Revolution and by present day its overall effect amounts to about 20 ppm: human changes to land cover appears to explain about one eighth of the observed 1 K global warming."

    This is still a CO2 effect but the basic forcing is land use change. It is, regardless, a separate issue from the role of industrial and vehicular emissions. It does show how complex and interrelated the climate forcings are.
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  16. critical mass #64 - yes, a challenge in this sort of calculation is the overlapping of various effects. We've discussed the warming effects of aerosols and black carbon quite a bit, but the actions which result in their emissions actually have a net cooling effect through the negative aerosol forcing. Personally I prefer to parse it by net forcing by effect (i.e. net warming from GHGs, net cooling from aerosols and black carbon)as opposed to trying to break out net positive and negative forcings.

    Dr. Pielke #65 - yes, we've previously touched on the possibility that human land use change may have impacted the climate during pre-industrial times. I think it's a very interesting theory.

    However, the IPCC does discuss CO2 changes associated with land use changes.
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  17. Hello Dr. Pielke @65,

    Just though that you should know, the press release that you linked to does not contain the quoted text.
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  18. Hi Albatross - Sorry - I sent the wrong url. I cannot find the actual url of the 2nd press release(perhaps someone else can), but I have reproduced the e-mail below where I read it from the e-mail below

    From: Tanja Suni
    Date: Tue, Sep 20, 2011 at 2:27 AM
    Subject: World-leading scientists will shed new light on climate-change mitigation in Garmisch-Partenkirchen, Germany, 18-23 September


    3rd iLEAPS Science Conference Garmisch-Partenkirchen, Germany 18-23 September 2011*


    2nd Press release

    World-leading climate scientists gather in Germany next week to present new research on how Earth’s land areas – forests, soils, permanently frozen regions, deserts, croplands and pastures – affect the global climate.

    The major international conference, organised by the Integrated Land Ecosystems-Atmosphere Process Study, will bring together 400 scientists to shed new light on climate-change mitigation.

    Conference organiser,Academy Professor Markku Kulmala from
    University of Helsinki, Finland said: “Humans have commandeered over 75% of the Earth’s land outside of Greenland and Antarctica. How we manage this land this century will be a major factor in whether we can stabilise global greenhouse gas emissions.” The conference, to take place in the Bavarian town of Garmisch-Partenkirchen, will outline the latest research to combat climate change.

    Although there has been much discussion about greenhouse gas emissions from burning fossil fuels, the next largest source of emissions is how humans have changed the global landscape. Agriculture and deforestation began thousands of years ago. Dr Julia Pongratz from Stanford
    University, USA, explains: “Deforestation has resulted in emissions of CO2 into the atmosphere, but clearing the forests may also have cooled the climate because forest is darker and heats up more than agricultural and pasture land.” The increase in atmospheric CO2 caused by human land-cover change became significant long before the Industrial Revolution and by present day its overall effect amounts to about 20 ppm: human changes to
    land cover appears to explain about one eighth of the observed 1 K global warming.

    “The decision of farmers concerning where to settle and farm in the historical past has enhanced the mitigation potential of future reforestation of this land: because farmers generally chose to deforest land that was more productive than average, reforesting this land can absorb
    large amounts of CO2 and have a strong cooling influence”, Dr Pongratz concludes.

    The press release continues with information on the scheduling.
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  19. dana1981 - The IPCC, however, does not adequately discuss the possible role (and relative contributions) of land use change and other non-GH forcings in allering major atmospheric/ocean circulations. This is why we need more than one metric, as we recommended the 2005 NRC report [which was only superficially referenced by the 2007 IPCC report].
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  20. Dr. Pielke, we are certainly in agreement about the need to use more than one metric - as we have noted, we think it's important to look at all metrics (surface temp, TLT, OHC, ice masses, sea level rise, ocean acidification, etc.). John Cook has a very interesting post/graphic on that subject today.

    Note by the way that we moved onto point #2 in the other discussion thread, if you have any input on the subject.
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  21. Dr Pielke said... "I do not know how many times I can state it, but I certainly agree that added CO2 is a first order climate forcing..."

    If I go and make this exact statement on any number of "skeptic" blogs (including Anthony's) I get skinned alive by a large hoard of commenters. This is the "context" from which most of us are posting.

    I think most people here would also agree that there is still a lot that needs to be better understood in order to form better public policy. And no one harbors any notion that the IPCC is a perfect report on the science.

    The challenge is, as you say, if CO2 is a first order forcing and there are also many other human forcings that need to be addressed... that message is clearly being undermined, or avoided, or even ignored. From what I know of the people on this site, everyone is concerned about exactly this. Climate change has a good likelihood of being the most important issue for humanity for the next century and beyond. This is a critical decade to begin taking action to make changes in order to avoid what could be the worst consequences. That message needs to be conveyed with all its caveats and uncertainties.

    Meanwhile there are a large number of very vocal people out there who are trying to tell the world there is nothing to worry about (which is understandably what people prefer to hear). There are people trying to say the science is unsure or can't be trusted. Worse yet, there are a select number of people, like Chris Monckton, who are trying to tell the world that the scientists who are telling us the bad news are socialists/communists/nazis who are using AGW as a ruse to take over the governments of the world. These messages are literally drowning out the actual science.

    This is exactly what SkS is about. If your message is that the science of climate change needs to be well communicated, we're on the same side. If your message is that human activities are contributing to climate change and need to be addressed (and we believe that is what the science tells us), we are on the same side.
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  22. Hello Dr. Pielke,

    Thank you for the information. I do not think we are in disagreement on this one, and the IPCC AR4 agree too:

    "Understanding land-use and land-cover changes is crucial to understanding climate change. Even if land activities are not considered as subject to mitigation policy, the impact of land-use change on emissions, sequestration, and albedo plays an important role in radiative forcing and the carbon cycle."

    Interestingly the contribution to CO2 levels from land use change that you provide above (~20 ppmv) seems to be lower than the value cited in AR4:
    "Cumulative emissions from historical land-cover conversion for the period 1920–1992 have been estimated to be between 206 and 333 Pg CO2 (McGuire et al., 2001), and as much as 572 Pg CO2 for the entire industrial period 1850–2000, roughly one-third of total anthropogenic carbon emissions over this period (Houghton, 2003)."

    So about a third of a total increase of ~100 ppmv between 1850 and 2005, or ~30 ppmv.

    The take home message is that even when these huge changes to the land scape were taking place since the Industrial Revolution, as you note above, the net contribution to the global temperature may have been about 0.1 C. In contrast, as we showed above, the expected warming from the observed net anthropogenic forcing is ~0.65 C (of the observed 0.8 C). Not to belittle the importance of land use change, but one does not need to invoke the alteration of major atmospheric/ocean circulations by anthro land use change to explain the observed warming. Occam's razor applies here.

    With that said, we cannot continue to lose our forests, and that is one of the many reasons why programs like REDD are so important, and on that I think we all agree.

    Given that we are more or less in agreement on this issue and agree to disagree on the magnitude of the forcings in the above Table (as did RealClimate back in 2006), can I suggest that we please the discussion forward and now discuss the next point outlined on the other thread by Dana. Thank you.
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  23. Rob Honeycutt - I am glad my viewpoint is being more generally appreciated.

    Just a few comments:

    1. Some of the reader's comments on Anthony's weblog may be caustic at times (as they are at SkS; and at Real Climate, where I have also been commenting), but Anthony has the highest level of professional integrity. His posts are very informative, and even if you do not agree with him, you will broaden your prespective. He is as sincere in his views as you are on yours.

    I recommend an open-mind in reading his posts. If one avoids bitter (snarky) comments, you and your colleagues would find that website of considerable value in presenting your views and discussing with those who disagree. I am hoping that more courtesy will catch on, on all of the weblogs.

    2. In terms your statement

    "Climate change has a good likelihood of being the most important issue for humanity for the next century and beyond."

    I would alter this to

    "Climate is a major issue for humanity for the next century and beyond, as it has been in the past. The role of humans in altering the climate is incompletely understood, but is also very important.

    In terms of whether the part of climate that is changing, relative to climate as it has occurred, is the "most important issue", needs, in my view, to be examined using the bottom-up, resource-based persepctive that we present in our paper

    Pielke Sr., R.A., R. Wilby, D. Niyogi, F. Hossain, K. Dairuku, J. Adegoke, G. Kallos, T. Seastedt, and K. Suding, 2011: Dealing with complexity and extreme events using a bottom-up, resource-based vulnerability perspective. AGU Monograph on Complexity and Extreme Events in Geosciences, in press. http://pielkeclimatesci.files.wordpress.com/2011/05/r-365.pdf

    The abstract reads

    We discuss the adoption of a bottom-up, resource–based vulnerability approach in evaluating the effect of climate and other environmental and societal threats to societally critical resources. This vulnerability concept requires the determination of the major threats to local and regional water, food, energy, human health, and ecosystem function resources from extreme events including climate, but also from other social and environmental issues. After these threats are identified for each resource, then the relative risks can be compared with other risks in order
    to adopt optimal preferred mitigation/adaptation strategies.

    This is a more inclusive way of assessing risks, including from climate variability and climate change than using the outcome vulnerability approach adopted by the IPCC. A contextual vulnerability assessment, using the bottom-up, resource-based framework is a more inclusive approach for policymakers to adopt effective mitigation and adaptation methodologies to deal with the complexity of the spectrum of social and environmental extreme events that will occur in the coming decades, as the range of threats are assessed, beyond just the focus on CO2 and a few other greenhouse gases as emphasized in the IPCC assessments."

    In terms of "climate", as contrasted with "climate change", we need to assess what would occur now with the current and projected societal conditions but with the reoccurrence of historical, recent paleo-record, and worse case sequence of weather days. For the global climate models to add value to this they must be able to skillfully predict the changes in the regional weather statistics.

    Your statement that

    "Climate change has a good likelihood of being the most important issue for humanity for the next century and beyond."

    need to be evaluated in this bottom-up framework.

    3. Finally, while we agree that the radiative forcing of added CO2 is "A" first order climate forcing, it is not "THE" only first order climate forcing. I define importance in terms of, for example, how large scale atmospheric and ocean circulations are affected. I have reported on numerous research papers on my weblog that document this issue.

    I will have two questions on my weblog early next week, to continue our constructive discussion.
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  24. Dr. Pielke, for the record, some of us have attempted to present our view on Watts' blog. Let's just say we have not been received kindly. This blog post is a good example of Watts' attitude towards Skeptical Science, and in particular, this comment from Watts towards me (and the one following it). Watts is not very tolerant of opposing viewpoints on his blog, and the causticity of comments on his blog is on a completely different level than on SkS.

    Let's just say we also disagree about the quality of the scientific content on WUWT and leave it at that, since it's off-topic, and since I don't have anything nice to say about it, I probably shouldn't say anything at all.
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  25. Dr. Pielke#68 and #69: There is an older SkS thread regarding land use changes. A USFS graphic posted there shows forested area in the US declined rapidly over the course of the 19th century. Since the early 20th century, there has not been much change.

    Changing land use in the US, as measured by forested area, cannot therefore be a significant contributor to recent warming. I wonder if similar statistics exist to show the timing of land use change in other countries.
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  26. Albatross - Regarding

    "...one does not need to invoke the alteration of major atmospheric/ocean circulations by anthro land use change to explain the observed warming"

    please see Figure 11

    in

    Pielke Sr., R.A., A. Pitman, D. Niyogi, R. Mahmood, C. McAlpine, F. Hossain, K. Goldewijk, U. Nair, R. Betts, S. Fall, M. Reichstein, P. Kabat, and N. de Noblet-Ducoudré, 2011: Land use/land cover changes and climate: Modeling analysis and observational evidence. Wiley Interdisciplinary Reviews: Climate Change, Invited paper, in press.

    The land surface part of the surface temperature record cannot be explained without considering land use/land cover change. Even then there is the complication from aerosols and natutal long term variability (which does involve atmospheric and ocean circulation pattern variations).
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  27. Hello Dr. Pielke @76,

    I was clearly referring to the global mean temperature, and that is what is at issue here, not the land temperature only. It has been shown repeatedly that one cannot model the global mean temperature accurately without including both natural and anthropogenic forcings, and that includes land use change. My point still stands.

    If you wish to discuss this matter further, I encourage you to post future comments on the appropriate thread (here). Thank you.

    And yet another request for participants to please move the discussion forward to our next point. Here is the link. I look forward to hearing Dr. Pielke's thoughts on that particular issue.
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  28. It is interesting that

    'Industrial pollutants (tropospheric ozone and BC) constitute 22.8% of all forcings (Skeie et al), or 24.9% (Pielke corrected). In any terms, these are a substantive proportion and I agree with Hansen that we should target these "low hanging fruit". Having said that, must of these emissions come from China and India (now), and will dissipate in any event when increased prosperity leads to pressure for clean air acts. "

    The implication being scince black carbon and ozone (mostly from NOx chemistry) are local/regional, the local/regional effects are much larger.
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  29. Eli is not so sure that just leaving it at that is a good policy. If someone keeps trying to shove nonsense into a discussion, that someone should not be told, ok, we disagree. This is not a debate with a fool that one can simply walk away from. Something stronger is required.

    The obdurate cynical ploy of never admitting the obvious is a major tactic of rejectionism and it works because it tires people out.

    The 28 vs 50 whatever stuff is important for policy, because policy says that the largest problem should have priority, the attempt to belittle the carbon forcing is simply another tactic.
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  30. "Having said that, must of these emissions come from China and India (now), and will dissipate in any event when increased prosperity leads to pressure for clean air acts."

    Since the pressure in the US is actually on to gut clean air acts and neuter the Environmental Protection Agency, I think the idea that clean air acts and good environmental stewardship are historical necessities is quaint. And wrong.
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  31. Dr Pielke said... "Finally, while we agree that the radiative forcing of added CO2 is "A" first order climate forcing, it is not "THE" only first order climate forcing."

    It rather seems to me that this is the very point of the blog post Dana has done here. In your comment at #4 I believe you accepted Dana's correction which, if I'm not mistaken, would make CO2 "the" first order forcing. If we accept AR4 or Skeie 2011 numbers then CO2 is clearly the first order forcing.

    I don't say this to suggest that other issues shouldn't be addressed. Clearly they should, but as Eli points out policy would suggest that the largest problem should have priority.
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  32. Rob Honeycutt - I do not accept that the global average radiative forcing is the primary climate metric to define first-order climate forcings. I have concluded that, while important, it is not the one we should place most of the emphasis.

    In my view, and from our analyses in our papers, it is the regional/local scale that matters much more to society and the environment. This includes the extent that heterogeneous climate forcings are altering atmospheric/ocean circulations.
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  33. re: 80 and the efficacy of clean air acts and AGW

    It's one of the ironies of life that filthy poisonous air dampens global warming. Were there only the one consequence of burning carbon energy -- an increase in global temperature -- then the devil's bargain of filthy air might seem almost worth it. But ocean acidification is another -- and just as serious -- consequence to carbon burning.
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  34. EliRabbet @78, you have quoted my comments, and I must say that I reject your comments.

    IMO, given the large scale motion of air due to the global circulation, specifically the Hadley and Ferrel cells, the effect of "purely local" forcings is more analogous to holding a magnifying glass to a stream of water than to a piece of paper. This is true even of non-dispersing radiative forcings (such as LC/LUC). It is certainly true of dispersing forcing agents such as aerosols, tropospheric ozone and BC which can disperse up to 12,000 km east/west, and 3,500 km north/south.
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  35. > the regional/local scale that matters much more to
    > society and the environment.

    On the regional/local scale, absorbign damages are of different kinds in different locations, you can absorb a whole lot of losses.

    That works for a while -- while the trading/transportation/financial system holds up, because each area can buy what it can't produce and sell what it overproduces.

    That's the definition of overshoot, ecologically.
    http://www.google.com/search?q=catton+overshoot

    It's not a solution. It's a way of postponing and increasing the damage while hoping the problem doesn't keep adding up.

    Remember the advertising slogan "The solution to pollution is dilution!" from the 1950s? Because impacts could be avoided. For decades.
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  36. I am very suspicious the "regional/local" effects line. No denying that these effects are what matter to us most, but are the local effects due to local causes? I would like some substantial scrutiny of this. The very appealing side of this narrative for Western nations.

    GHG emissions are global in effect. In this way, the historical emission from western nations can have negative climate effects not just for themselves, but also on vulnerable nations with very low emissions. This creates complex questions over rights and responsibility that pure free-market only advocates struggle to solve.

    On the other hand, if you can suggest that local/regional land use change are main or even substantial driver for climate change in the same local region, then there is excuse to simply blame say Pakistan for its own climate problems and sit on your hands when it come to reducing emissions. Very appealing idea but I rather doubt that it is strongly scientifically based.
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  37. I'll just note couple of papers regarding the carbon dioxide - water vapor overlap issue:

    Pielkesr:
    "It is my specific question that an answer is needed for

    "What would be the global annual average radiative forcing change since pre-industrial with CO2 without the water vapor overlap and with the the overlap?"

    I doubt you could find it in a literature search because to my knowledge, it has not been done."


    There is a classic paper by Kiehl & Trenberth (1997), which seems to address this question at least in some form:

    "It is also important to note that different gases can absorb radiation at the same wavelengths; this is called the overlap effect."

    Then, later in the paper:

    "Of this 125 W m-2 clear sky greenhouse effect, we can ask, what is the relative contribution of each atmospheric absorber? A detailed answer to this question is complicated by the overlap among individual gaseous absorption features.
    We calculate the longwave radiative forcing of a given gas by sequentially removing atmospheric absorbers from the radiation model. We perform these calculations for clear and cloudy sky conditions to illustrate the role of clouds to a given absorber for the total radiative forcing. Table 3 lists the individual contribution of each absorber to the total clear sky radiative forcing."


    Table 3 then gives for each included gas the individual and combined effects for both clear and cloudy sky. For carbon dioxide individual clear sky radioative forcing is 29 watts per square meter and combined (with overlap effects) forcing is 32 watts per square meter. There is also separate row for "Overlap H2O–CO2".

    Another paper apparently addressing this issue is Cess et al. (1993). However, I I only have seen the abstract, so I'm not sure how complete analysis this paper has. I just noticed that Google Scholar returns this sentence for this paper:

    "Fig. 3. (A) Scatter plot of LW clear (clear sky) radiative forcing, as generated by the GCMs, with and without overlap of the C02 absorption bands by water vapor absorption."

    So it seems that it might be interesting in this sense. If needed I can dig up more references as there seems to be plenty of research done on this overlap issue.
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  38. Did we ever get to the bottom of Pielke Sr's interpretation of Forcing @40? I.e. "A forcing is ... immediate" and different from "a change in forcing over some time period"
    Immediate or not, a force is still a change from before to after. I did wonder if Pielke Sr was talking of some measure of residual forcing (radiative imbalance) when asking for the forcing for 2011 but @8 he asks for the 2011 forcing and radiative imbalance as well. So I am at a loss at what he is blethering on about.
    Is there some refined climataligical definition of 'forcing' that I am misinterpreting or is this our pet sceptical professor of climatology making yet another 'simple mistake on a fact you would expect him to know well'.?
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  39. Re #88, in #40 Dr. Pielke said: In fact, you are inappropriate mixing a "forcing" from a "change in forcing over some time period". A forcing (such as produces an acceleration, is immediate).

    He explained earlier in the post that some forcings are radiative and some are not, giving an example from the NAP book http://www.nap.edu/openbook.php?record_id=11175&page=40

    In the post he was replying to, #37, Tom Curtis quoted from the AR4 glossary that a forcing is a change in net irradiance "due to a change in an external driver of climate change" such as a CO2 change or TSI change. IMO the only confusion is between the direct forcings of AR4 and the indirect forcings from NAP. But in both cases (and Tom's 37) they are referring to a change in forcing, not the forcing itself (so no inappropriate mixing was done). Nonetheless I did not those particular indirect forcings addressed in any subsequent posts (the ones from the NAP book I linked above) although MA Rodgers also asked about them. It seems appropriate to ask for the direct forcings and the radiative imbalance of direct and indirect forcings as distinct questions.
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  40. @89
    Pielke Sr listed forcings as being (i) direct radiative, (ii) indirect radiative & (iii) non-radiative giving examples of the second sort in the reference. I see no relevance in such distictions within the presented argument.
    AR4, NAP and Tom@37 all refer to changes in forcing (of whatever ilk).
    Where I lose the thread in @89 starts with the talk of "mixing" which I am not clear even whether to read as some philisophical or some climatical mixing. My confusion continues with the second from last sentence which appears to be missing some words and the final sentence is devoid of any mention of "immediate" forcing or the logic for a forcing not being a relative phenomenon.

    In all these replies, the nearest thing to an explanation is @42 where Pielke Sr states "ΔF is the change in forcing ... that does require a base year. The forcing does not and is instantaneous.
    One would never state that "acceleration requires a base time period." Acceleration is the derivative of the velocity at any time. Similarly, radiative forcing is at a specific time although one could time average (e.g. the yearly global averaged radiative forcing).
    "

    This is however muddled thinking. Acceleration represents a rate of change in velocity, velocity a rate of change in distance. Here we consider 'forcing' and 'change in forcing' not a rate of change. There must then surely be a datum to measure from.
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  41. Well said @Rob Honeycutt, nicely distilled. Sometimes it's easy to miss the grain for all the chaff in the air.

    Was going to ask what the motivations were from Pielke Sr for arguing on such a point of contention - is it a need for correctness?

    It seems the motivations for SkS were summed up well by Rob HoneyCutt, though it seems in this thread they have been pretty nicely co-opted in creating more chaff for the doubters to point at (in my [admittedly] humble opinion).
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