Arguments
Why is the IPCC AR5 so much more confident in human-caused global warming?
Posted on 27 September 2013 by dana1981
Skeptical Science has launched a new feature, a collection of debunkings of the most popular myths about the IPCC. This post has been adapted into a rebuttal of the myth "IPCC human-caused global warming attribution confidence is unfounded". The short URL for this new resource is http://sks.to/ipcc
The fifth Intergovernmental Panel on Climate Change (IPCC) report states with 95 percent confidence that humans are the main cause of the current global warming. Many media outlets have reported that this is an increase from the 90 percent certainty in the fourth IPCC report, but actually the change is much more significant than that. In fact, if you look closely, the IPCC says that humans have most likely caused all of the global warming over the past 60 years.
Spot the Differences
Here is the relevant statement from the fourth IPCC report in 2007:
"Most of the observed increase in global average temperatures since the mid-20th century is very likely [90 percent confidence] due to the observed increase in anthropogenic greenhouse gas concentrations
Now here is the statement from the fifth IPCC report:
"It is extremely likely [95 percent confidence] more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together."
Did you spot the differences? The 2007 IPCC statement focused on human greenhouse gas emissions, while the 2013 statement pertains to all human influences on the climate. This includes the cooling effect from human aerosol emissions (pollutants that scatter sunlight).
Cooling from human aerosol emissions offsets about one-third of the warming from human greenhouse gas emissions. The new IPCC statement says that even taking that aerosol cooling effect into account, humans are still the main cause of the global warming over the past 60 years.
Current Global Warming Caused by Greenhouse Gases, Not Nature
The IPCC elaborates further.
What's causing global warming: human greenhouse gas emissions.
"The best estimate of the human-induced contribution to warming is similar to the observed warming over this period ... The observed warming since 1951 can be attributed to the different natural and anthropogenic drivers and their contributions can now be quantified. Greenhouse gases contributed a global mean surface warming likely to be in the range of 0.5°C to 1.3 °C over the period 1951−2010, with the contributions from other anthropogenic forcings, including the cooling effect of aerosols, likely to be in the range of −0.6°C to 0.1°C."
What's not causing global warming: natural external factors like solar activity, and natural internal factors like ocean cycles.
"The contribution from natural forcings is likely to be in the range of −0.1°C to 0.1°C, and from internal variability is likely to be in the range of −0.1°C to 0.1°C."
We've observed about 0.6°C average global surface warming over the past 60 years. During that time, the IPCC best estimate is that greenhouse gases have caused about 0.9°C warming, which was partially offset by about 0.3°C cooling from human aerosol emissions. During that time, natural external factors had no net influence on global temperatures. For example, solar activity has been flat since 1950.
Annual global temperature change (thin light red) with 11 year moving average of temperature (thick dark red). Temperature from NASA GISS. Annual Total Solar Irradiance (TSI; thin light blue) with 11 year moving average of TSI (thick dark blue). TSI from 1880 to 1978 from Krivova et al (2007). TSI from 1979 to 2009 from PMOD.
As for the natural internal variability of the Earth's climate system, short-term noise averages out to zero over long time frames. Warm and cool ocean cycles cancel each other out, and thus internal variability has no long-term influence on average global temperatures.
Put it all together, and the IPCC is 95 percent confident that humans have caused most of the observed global surface warming over the past 60 years. Their best estimate is that humans have caused 100 percent of that global warming.
IPCC is Summarizing the Scientific Research
The IPCC does not conduct any original research; it's a summary report, and these statements accurately reflect the body of climate science research. For example, last year climate scientists Tom Wigley and Ben Santer published a paper concluding that human climate influences were responsible for 50 to 150 percent of the observed warming from 1950 to 2005.
Like this new IPCC statement, they found that humans have caused at least half the observed warming since 1950, and most likely all of it. It's also possible that humans have caused more warming than has been observed because natural factors may have had a net cooling effect. The Wigley and Santer results are consistent with the body of scientific research on the causes of global warming.
Net human and natural percent contributions to the observed global surface warming over the past 50-65 years according to Tett et al. 2000 (T00, dark blue), Meehl et al. 2004 (M04, red), Stone et al. 2007 (S07, light green), Lean and Rind 2008 (LR08, purple), Huber and Knutti 2011 (HK11, light blue), Gillett et al. 2012 (G12, orange), Wigley and Santer 2012 (WS12, dark green), and Jones et al. 2013 (J12, pink).
The 'fingerprints' of climate change are also all consistent with what we expect to see as a result of human-caused global warming, for example changes in the atmosphere, as another paper by Ben Santer recently concluded.
Summary of observational evidence that human carbon dioxide emissions are causing the climate to warm.
What About the Naysayers?
A few naysayers like Judith Curry from Georgia Tech have disputed the IPCC confidence on this question, for example in an interview with the reliably inaccurate David Rose.
However, while Curry is a climate scientist, she doesn't research the causes of global warming. She also has a history of exaggerating climate uncertainties. Her comments are inconsistent with the body of scientific research on the subject. Put simply, she is speaking outside her area of expertise, like a podiatrist giving advice on open heart surgery.
The 97 Percent Consensus is Evidence-Based
This is why there's a 97 percent consensus amongst climate experts and in the climate science literature that humans are causing global warming. The scientific evidence on this question is overwhelming.
Many commenters have noted that the expert consensus is itself not scientific evidence for human-caused global warming. That's true. The expert consensus is however based on the scientific evidence. The fact that 97 percent of climate experts agree on this subject also demonstrates the strength of the scientific evidence on human-caused global warming. And the strength of the evidence is why the IPCC is able to say with 95 percent confidence that humans are the main cause of the current global warming.
Check out more debunkings of IPCC myths at http://sks.to/ipcc
One category of claims I've seen that I can't wrap my head around is that the IPCC statement is weakening the former statements, e.g. from "most" to "more than half" here, or from "warmest in 500 years" to "warmest in 800 years" in another case. Posts predicated on such misreadings predictably generate 50+ credulous comments at WUWT. It's rather remarkable.
Thanks Dana for the very useful distinction between "GHG-induced warming" in AR4 and "total human influence on climate" in AR5.
I would disagree about portraying Judith Curry as incompetent, ala " podiatrist on heart surgery". Given her areas of reaserch: hurricanes, remote sensing, atmospheric modeling, polar climates, air-sea interactions, etc., she looks perfectly competent to me as climate change spokeperson. But the fact that she does not understand the basic statistics (i.e. the calculation of uncertainty that I've learned as undergrad) while performing all of that research at Georgia Tech, is beyond my comprehension. She is a "heart surgeon" you're talking about (at least holds the required qualifications) but a bad one who should not have received the qualifications or has forgotten all skills since then. She's not a "naysayer" but an outlier among climate experts.
Note that other disciplines also have such outliers, perhaps more than 3% of experts; the outliers are just not as loud because no one listen to them, as there is no need to rationalise public mind against those other disciplines if a discipline are not as "inconvenient" as climate science. 97% consensus is very high number and it's very hard to beat.
In statistics, 95% confidence interval (so lower than climate science consensus and coincidentaly the same as AR5 confidence on AGW) is assumed as typical confidence interval for a good reason: otherwise the inherent uncertainty of your observations would preclude the confirmation of your hypothesis (i.e. the null hypothesis would always be true). Too much talking about null hypothesis starts hindering the advancement of your theory. So you'd be stuck in denial of many laws of physics just like climate science denialists are stuck today.
I felt that your negative portrayal of Judith, even if justified, was unnecessary. I would think it would be sufficient to point out that her opinions were just that, opinions, and not science.
As noted, Curry is a climate scientist, but has no attribution expertise. Hence the analogy is accurate.
"Put simply, she is speaking outside her area of expertise, like a podiatrist giving advice on open heart surgery."
Dana, 2 things:
1) Would you dismiss Curry's opinion this way if she was emphasizing the certainties of the science rather than the uncertainties?
2) You yourself are not a climate scientist yet you write articles as if you are an expert on every subfeild of climate science. It seems odd to me that you think Curry shouldnt blog outside of her subfeild but you think you are allowed to blog about every subfeild.
Dana,
I understand that the latest IPCC report forecasts a temperature increase at equilibrium of between 1.5C to 4.5C for a doubling of CO2 in the atmosphere.
Judith Curry (26 Sep-13) has reported a calculation that the heating in the 0-2000 meters ocean layer for the past 45 years is only some 0.065C. The implication is that there is an incompatability between this calculation and any forecast of potentially damaging climate change.
I would be grateful for any comments that you may have (not immediately expected, as the calculations do not seem to be trivial!), in particular:-
1. Is the calculation correct?
2. If the calculation is correct, does this mean that the IPCC forecasts for future global warming need to be modified to allow for the possibility of a lower level of warming? Or, alternatively, that the calculation simply proves that it will take a very long time for temperatures to reach equilibrium?
3. Is there any method for cross checking the calculation against an independent indicator? The average annual sea level rise seems an obvious candidate.
ptbrown31@5 - I always support what I write with references to peer-reviewed literature. Curry has no supporting evidence, she's merely giving her 'gut feeling'. If you're going to ask people to trust your gut, you'd better at least be an expert in the field you're talking about. Curry isn't. I don't expect people to trust my gut, instead I reference the expert scientific literature.
Ali G @6 - same problem as ptbrown. You're placing your "faith" in someone's "gut" and ignoring the peer-reviewed scientific literature in the process.
trevor @7 - I believe you're confusing surface warming (1.5 to 4.5°C for doubled CO2) with ocean warming. The warming of the sub-surface oceans doesn't tell us much about the warming of the surface air. Very different warming rates.
I have just come across this comment by John Stoker, who is apparently co-chair of the IPCC working group.
"There are not sufficient observations of the uptake of heat, particularly into the deep ocean, that will be one of the possible mechanisms that would explain this warming hiatus,"
http://www.theguardian.com/environment/2013/sep/27/ipcc-world-dangerous-climate-change
My interpretation of this commentis that there is currently no reliable measurement of heat in the deeper levels of ocean; however it was my understanding that, for some time now, we did have reliable and accurate measurements through the argo network, and that these measurements clearly showed increased warming. I would be really grateful if someone could explain the discrepancy; have I misundersstod the John Stoker comment in some way. Thanks.
Far be it for me to suggest...
But there have been no reliable historic temperature measuerments of 2/3 of the planets surface.
As for what goes on below 2/3 of the surface of the planet's surface.
Over and out!
.
"But there have been no reliable historic temperature measuerments of 2/3 of the planets [sic] surface."
[PW] Ali G, please provide a credible source of data for this statement.
Curry has made numerous claims that are demonstrably false. Her walk back of her own involvement in the BEST study and denial of its results being the most obvious example. If anything, the description of her in the article above is overly generous. Making incorrect statements in areas outside your experience is foolish. Making incorrect statements of basic fact and logic is inexcusable.
Zen, the Argo network is now providing much better data for ocean warming down to about 2000 meters. The average depth of the worlds' is about 4267 meters... so we are currently measuring heat accumulation for less than half of the water in the oceans. This is the 'deep ocean' that Stoker was referring to.
ptbrown31@6
I think Dana's simile is quite apt: podiatrists and cardiologists are medical doctors, with different areas of expertise. He didn't say Curry had no right to share her opinions, and he identified her as a climate scientist. The problem with an outlier like Curry is that someone unfamiliar with the way scientific disciplines are broken into many different sub-specialities or areas of expertise might well draw the conclusion that her qualifications as a climate scientist make her an equally well-qualified expert in all areas of climate science.
Using another medical analogy, I go to my general practicioner for routine checkups and for intitial consultations, but my general practicioner refers me to experts when I have a problem outside his area of expertise.
Curry should be acting like a general practicioner, but instead she often operates quite differently. More often than not, she acts as if she is a better expert on any given sub-discipline of climate science than the scientists in that sub-discipline. If Curry were a medical doctor, she would be the general practicioner who rarely if ever referred anyone to a cardiologist, because she believed almost all cardiologists were misguided or dangerous.
As for Dana, he, like other non-climate scientist authors of reports or summaries on Skeptical Science, generally does two things: 1) render scientific publications into language more suitable for lay people to understand, while remaining true to the thrust of the original source documents; and 2) use published climate science to rebut denier positions. This means that Dana and Skeptical Science writers represent the orthodox point of view where climate science in 2013 is concerned.
We can easily see the difference between what Skeptical Science authors do and what those on the other side do when we think about a site like WUWT. On that site, authors take an emphatic heterodox stance against most of the published science. In effect, the opinions found on WUWT are almost always written by non-climate scientists who act as if they are experts on various subfields of climate science who possess the ability to see that effectively everything published by climate scientists working in any given sub-discipline is wrong.
Expressed as an analogy, going to WUWT for climate science opinions is like going to an auto mechanic for a second and consistently contrarian opinion on the bypass operation the cardiologist recommended.
So, to sum things up, Skeptical Science writer seek to pass along the simplified but otherwise unchanged diagnoses of climate science specialists, Judith Curry often poses as the arch climate scientist, and WUWT writers are delusional.
Ali G
You're welcome to suggest....
But i should imagine having reliable historic temperature measurements of 1/3 of the planets surface, given a good spread of coverage, is more than adequate to make broadly accurate conclusions.
CBunkerson
Thanks for the clarification
Ali G @11
Further to Zen and assuming your figure is correct.
If we have accurate historical global temperature records of 1/3, and they have recorded a warming trend and are scattered throughout the 2/3, what do you think is happening in the 2/3?
Don9000 @14 - well said, +1.
Zen @10 - honestly I think Stoker was probably tired when he said that (having been up all night working on the final SPM). It's hard to know what he means by 'deep' oceans. We certainly have faily good estimates for the 700-2000 meter layer, which I consider the most important part of the 'deep' ocean (as layers deeper than that don't accumulate much heat). There's no question that deep ocean storage is one mechanism causing slowed surface warming, the question is how much it's contributing vs. how much reduced radiative forcings may be increasing. There's also been some really good recent research published on that, which unfortunately missed the IPCC cutoff, so Stoker may not have been aware of it.
Satire alert.
Dana--you or anyone at Skeptical Science is more than welcome to delete this for being inappropriate, too long, or just plain absurd.
Taken to the extreme, Ali G's theory is fascinating. I think I can expand on it.
I have a thermometer in my room that has an internal column diameter of less than one millimeter. For the sake of the argument, let's assume the cross-section of the glass thermometer tube occupies a 1 mm x 1 mm square. Well, it turns out that to cover just a single square meter of the earth's surface with similar vertically positioned thermometers, we would need one million similarly sized thermometers.
Now, the earth's surface area is 510,072,000 square kilometers, or 5.1 x 1014 m2, or 5.1 x 1020 mm2. My calculations--done without a government grant, and non-peer-reviewed I might add, like the "science" on WUWT--indicate that two-thirds of this area is approximately 3.4 x 1020 mm2. This number also gives us the number of thermometers necessary to cover the same area with thermometers (I believe this correlation is true because of Lord Kevin's Fifth Law of Thermometers, but don't quote me on that). Also, while I'm not a mathematician, I think the number translates into 340 quintillion thermometers. Admittedly, that's a somewhat large number of thermometers to reach even the two-thirds level of coverage, but if we want to be reasonably certain global warming is really happening, we surely shouldn't quibble over a few hundred quintillion thermometers. We can probably order them on Amazon and get a quantity discount and free super saver shipping.
Now, I need to put the current dire situation into context. Shockingly, NOAA NOAA link, one of the world's most reliable sources of global surface temperature data, currently uses only 1500 weather stations around the world to monitor the earth's surface temperature! With all this in mind, I can really see why some skeptics are concerned about the lack of data. Even if we dare to assume that each of these 1500 stations has two thermometers, each of which occupies a square millimeter in cross-section, this still only lets NOAA monitor the earth's temperature on the equivalent of slightly less than a 55 mm2 section of the earth's surface! For the non-scientific in the audience, that is actually a smidge less than a third of the earth's surface by my calculations. Why, the area of my main vegetable garden covers approximately 96,000,000 mm2! Despite this shockingly inadequate coverage, a bunch of crazy climate scientists are telling us to cut our carbon emissions! What nerve!
Clearly, the world's nations shouldn't do anything rash until they have rectified this unacceptable situation. Surely it isn't too much to ask that governments cover a mere two-thirds of the planet's surface with simple thermometers, and then take the earth's temperature for thirty years so we have enough data to find a clear trend.
Of course, as we can all see, and before a skeptic feels the need to raise the point, it wouldn't be a complete and accurate record if we didn't take the temperature from each and every one of those 3.4 x 1020 thermometers almost constantly for those thirty years. I'd say one reading per thermometer per second would satisfy most skeptics that enough data was being recorded, though I suppose some hardcore WUWT scientists might want to record the temperature a hundred times a second, or a million times, just to be on the safe side--as they might point out, you never know when the temperature is going to drop a fraction of a degree after all, and it might just turn out that, because we are not sampling continuously we have missed some kind of colossal hidden temperature decrease due to our faulty non-continuous sampling methods which may mean we have all been frozen solid for the last thirty years and are just imagining that global warming is happening. That could be true. I dunno.
Still, I want to be practical, so I'm going to stick with the 1 sample per second option. That would work out to (pardon me if my math is a bit off) 3.2 x 1029 data points at the end of the thirty years of data gathering. Such a robust data set surely is necessary to have before we actually do anything crazy like cut global emissions of greenhouse gases on the word of a few thousand scientists, who, by the bye, if they have an average cross-sectional area of 594 cm2 each, only occupy a tiny fraction of the earth's surface . . . but I digress.
Next, scientists would have to analyze the data. They should obviously do this without the aid of computers, since we all know that the models are all wrong, and Al Gore has programmed all computers running Microsoft Windows to yield data supporting his theories (he also gets a penny every time you access the Internet, but that's another story).
By my admittedly rough estimate, scientists should have the definitive answer on whether or not the earth is warming in approximately 5.34 x 1045 years, assuming they work 40 hour weeks and take only two weeks of vacation per year.
I could go on, but I'm sure I've made some kind of point already!
And so what theory did I propose?
I have walked away - but am still listening!
Ali G - how about having an honest conversation and responding in kind to the points made to you by Brian and Zen
First, given their chosen name, it is very likely that Ali G is simply winding us up. People using as a net name, the name of a well known satiricist should not be given the benefit of any doubt as to whether or not they are trolling IMO.
Second, contrary to Ali G's claims, there are reliable temperature records over much of the ocean for far longer than is the case over land. This is due to the practice of ships taking multiple daily temperature readings and recording them in their logs, thereby giving a consistent temperature record along major shipping lanes since the 1850s, whereas reliable temperature records on land from that period are largely restricted to Europe and the North East US. For what it is worth, here is the record of HadCRUT4 coverage (annual means) since the start of the record:
Further to my post @22, I would like to draw attention to this plot of global temperature using just 85 rural stations, with an average annual coverage of 50 stations:
The rural stations were selected by an algorithm ensuring only that they were dispersed so as to not generate biases in location, and that they were the longest records available in their region. Details are here. As Caerbannog sumarizes:
More importantly for this discussion, it shows clearly that the limited number of thermometers is not an issue. Geographical bias, however, is an issue. For that reason all global temperature series prior to 1880 are suspect (ie, HadCRUTx, and for Land only, BEST), and the HadCRU series is not as good as that of either NCDC or GISS.
Zen , #9.
Thomas Stocker did indeed say what you quote, but in my recollection (I listened to the webcast live) he was referrring to the length of time over which observations have been taken, and not to the coverage or reliability of the measurements.
www.theguardian.com/environment/2013/sep/27/ipcc-world-dangerous-climate-change
@13 Doon9000
Well Said. I agree with almost everyingth you wrote. I guess I just think its a bad argument to discredit an opinion because its slightly outside someonse area of expertise. For example, James Hansen writes about the possibility of specieces exticntion in the next 50 years in "storms of my grandchildren". Do you think that his opinion should be dismissed becaouse James Hanses is not an ecologist?
ptbrown31, #24,
I once had a lump in a dangerous spot and went to my doc. He knew my fears and said "You are older than the age group for that type of cancer. I think it is just a cyst, but I will refer you to a consultant". He was right, I was relieved to find. His opinion/ gut feel was correct, but he did not 2nd guess the expert.
Hansen does write about species extinction in his book, but he refers to documented cases of mass extinctions in the past, and mentions the work of biologist E.O.Wilson on 20th century species extinction. While he might have given better references, he gives enough for anyone to follow up and find out how justified his opinions are on a topic outside his expertise.
I think anyone can give an opinion, as long as he or she points out where expert knowledge can be consulted. There must be grounds other than the "gut" of the speaker for a calibration of the opinion. The impression with Judy Curry is that she conceals where her own expertise begins and ends, and does not point to evidence or reference, thereby committing the Argument from Authority fallacy.
TonyW, the "all" is in the math lower in the article. One could say "more than half" just based on the summary box, but what does "more than half" mean? Well now we know. If Dana is being biased, it's only in choosing not to count those natural +0.1 to -0.1 ranges as positive values. It's very clear from the math that "more than half" means nowhere near 51%. And, of course, it's extremely clear from the existing literature that the human contribution is nowhere near 51%. I do think the IPCC could have been stronger in the box -- "nearly all" would have been representative (if still conservative).
Having downloaded & now looking at the report, I find figure SPM.5 especially interesting (I'd like to paste it here but dunno how:().
It is much better detailed than equiv. figure from AR4. The differences worth noting:
- the best estimate of negative aerosol forcings (direct + cloud formation) has been lowered from (-0.5;-0.7) to (-0.27;-0.55) respectively. I don't yet know what it means & be interested in hearing other opinions on that
- total antropo radiative forcings since 1750 increased from 1.6 to 2.29 (in 2011) ARF was only 1.25 in 1980. What does it mean? That AR4 was underestimating ARF a bit that 6y of CO2 increase caused the forcing numbers to grow that much?
[DB] Here is SPM.5:
One of blogs I was reading claimed that the IPCC could not agree on the level of climate sensitivity and this is backing away from the earlier reports. I can't find any evidence of that. Is this some kind of play on words or something?
@Stranger #29:
In response to your question, it's complete B.S.
Have your read Dana's OP?
Hi all,
Dana's position on Dr. Curry is, I'm afraid, spot on. As a fellow scientist in her field I have given up on her. People need to remember that Dr. Curry appears to be more concerned with soliciting attention and obfuscation nowadays than she is with making facually-based or scientifcally-credible or scientifically-defensible arguments.
Dr. William Connolley states the problem with bluntly and accurately:
"Judith Curry’s understanding of climate is not helped much by climate models
Or so she says. Personally, I find that my understanding of the deeper aspects of General Relativity [GR] isn’t helped by me not taking the time to concentrate on the maths. But at least I’m able to realise that’s a flaw in me, not GR.
Mind you, Curry’s comment does help explain why some of her papers are crap – if you write a paper in which “the model simulations … were the main source of data used in the analysis” and yet you don’t think the models help, you’re not really going to write anything sane."
This highlights Curry's double standard, logical fallacies and even hypocrisy when it comes to using climate model data.
Sadly, this is but just one of several problems when it comes to Dr. Curry's musings in the media and to journalists concerned with fabricating disinformation (see here and here)for a couple of examples. Curry did not even bother to check Rose's fallacious claims about the Arctic sea-ice extent, nor do I recall her calling him out on it when his egregious error was highlighted.
Again, Dana is spot on.
Shoyemore @25,
Exactly! People must not confuse a scientists speaking outside their area of expertise but basing their comments on experts' research in the field (as Hansen does), with someone like Curry making unsubstantiated and unsupported comments in areas outsider her area of specialization.
All: I have deleted Lei's most recent comments. They are all "off-topic slognaeering".
Concern trolling by Lei or anyone else will not be tolerated.
Lei: Please note that posting comments here at SkS is a privilege, not a right. This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.
Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.
Note: This is Warning #1. SkS Moderators adhere to a "three-strikes, you're out!" process.
[DB] Note that if behavior is egregious enough, strikes 1-3 may be combined into just strike 3.
Need many more temperature measuring contraptions in the oceans, so that interpolation requirement is minimal and fine-grained. As long as it`s possible (rational) for one of you sensible commenters to argue with me when I reference Balmaseda, Trenberth, Kallen (the graph from same) and state +137 ZettaJoules in last 10 years (which one of you did a few weeks back) and you say maybe still +5 to +6 per year into oceans there will be substantial doubt. Need that incontrovertible annual ocean heat increase, year by year as it happens, actually measured.
Me #35 I mean ...and I state +137 ZettaJoules in last 10 years....
TonyW @26 - it's very likely more than half, most likely 100%.
chriskoz @28 - somewhere in there the IPCC says that roughly half the increased anthro forcing estimate is due to the rise in GHGs over the past 6 years, and roughly half is due to the decreased aerosol negative forcing estimate (decreased meaning smaller in magnitude).
Stranger @29 - those people are probably referring to the fact that the IPCC AR5 only provides a range of equilibrium sensitivity estimates (1.5 to 4.5°C for 2xCO2) and not a best estimate. This is because while paleoclimate and model-based estimates agree (2 to 4.5°C, most likely 3°C), estimates based on using recent observational data in simple models (Otto, Lewis, etc.) arrive at somewhat lower estimates. So basically two methods agree, one disagrees, so they don't have a consensus best estimate anymore and instead just put the range.
Were I a betting man, I'd put my money on the former two methods. Not only is it 2 against 1, but the latter method has large uncertainties, discussed by Andrew Dessler here.
Thanks so much Dana. I've been an everyday lurker from almost day one on this site. I've had no science training beyond chemistry 101. The fact is I’ve learned so more about climate science then I ever wanted too, but you guys make it the most interesting.
TonyW @40 - see the quotes from the IPCC report in the Current Global Warming Caused by Greenhouse Gases, Not Nature section above.
TonyW - it would seem to be the logical conclusion to draw from looking at figure SPM-6 in the just released report.
TonyW - the figures are not inline with the text but at the bottom of the document. While I dont think Dana is making direct quotes, it is certainly the inference from the figure. Also when it says human is "most likely" in range in 0.5 to 1.3 and natural forcing and internal variability to be" -0.1 to 0.1", then it seems a perfectly reasonable inference to me.
I've noticed another detail: RPC3PD scenario is not there anymore: looks like it was replaced by RPC2.6. I'm not sure if those two scenarios are the same or if RPC2.6 has been slightly changed. But looking at Figure SPM.10 (the last one) it seems that the emissions are pacing about the same in all scenarios until 2030, then RPC2.6 starts slowing down in 2040 and stops abruptly in 2050 at cumulative ~750GtC, which is way below the suggested 1Tt alowance known from earlier reports.
RPC2.6 being the only acceptable scenario for those who care about the future of the planet, I'd like to have a feeling how realistic it is compared to the old RPC3PD. Are they realy the same? The emission halt within 2040-2050 seems to be too abrupt. And then, does AR5 still asume that humans start "scrubbing off" CO2 from the atmosphere around 2070? Or the Earth natural sinks (ocean + biosphere) would significantly outweigh the natural sources (permafrost + clathrate methane) helping to realise RPC2.6 scenario?
chriskoz @47 - RCP 3PD and RCP 2.6 are the same. PD is peak and decline, with the decline being from 3 W/m2 to 2.6.
I also think some assessments, while they may be well-meaning, fall into a trap of statistical methodology. In particular, surface temperature profiles from, say, HadCRUT4, express a single realization of surface temperature development on Earth. It's highly probable that even if the entire system were magically reinitialized to the state it was in 1980, with exactly the same GHG forcings and solar radiation vs time, it would track a different path of temperature. What we observe is but one realization.
Now, you can try to assess the so-called "internal variability" by looking at ensembles of temperature subsets, appropriately adjusted for serial dependency, but classical ("frequentist") techniques will never produce temperatures outside of the observed range. This is also true of climate models, e.g., 37 models from CMIP5.
What's needed is a Bayesian extension of both data sets, perhaps using predictive posteriors and weak priors, or priors initialized with paleoclimate results. (I'm thinking of Geisser and Eddy, 1979, "A predictive approach to model selection", http://dx.doi.org/10.1080/01621459.1979.10481632, or the paper on the same subject by Gelfand in the compendium, Gilks, et al, Markov Chain Monte Carlo in Practice.)
Facts are, and to use a very rough analogy, if you try to estimate the proportions of differing colored balls in an urn by sampling, knowing the possible colors but not knowing how many balls there are in the urn, a frequentist assignment will give you zero as the proportion for any colors you have not observed. A Bayesian assessment assigns some non-zero probability to colors which have not been observed, so after the data, they have some proportion, possibly small. Temperatures which have not been observed are not impossible and, to the degree to which models try to minimize error against all possible futures, they go for that, not a specific path.
There are also problems with trying to estimate magnitude of "internal variability" without taking such an approach, but the analysis is more involved there, and includes serious questions of identifiability without making unrealistic assumptions. More on that some other time.
I have not done the calculation so cannot assert, but I have done toy problems that are pretty analogous, and what happens is that there is a much greater overlap between the range of possible temperatures HadCRUT4 and others suggest and CMIP5 and others, and, so, assertions of incompatibility stand on less evidence than other kinds of analyses indicate.