Tag Archives: AR5

The increasing confidence seems to be very hard to explain

The UK parliamentary Inquiry of Energy and Climate Change Committee into “the science” of UNIPCC AR5 was a very interesting event. This in several ways. There was of course the fact that this time there was a alarmist panel and a skeptic panel that were questioned. On blogs and articles I heard a lot about the second session, the one with the skeptics. But I was more interested in the first session. It gave me a lot of ideas, so probably some later posts about things that caught my eye.

In previous post I spend some time on the question why there is now increased confidence in human influences and the answers were puzzling.

Why was I so interested in that little question? Earlier, I was confused by the statement of the IPCC that they had more confidence that humans caused the warming in the last 60 years. In the media I could not find a clarification about this, only a rehash of the statement. Now, in this UK parliamentary Inquiry, I came across an insider that seems to give some more background information. That insight could indeed fit the IPCC statement, but when looking into it, it also made it meaningless.

To recapitulate: the statement was that we can have more confidence that the human influence is towards warming because the higher confidence that the cooling influence is less than previously thought. Meaning the overall anthropogenic influence is warming. As I said before I can agree with that, but it doesn’t say nothing about the amount of anthropogenic warming. It could be more, true, but it could also be less or even the same.

This is what puzzled me: the inquiry was set up in order to inform policy makers about the science in the IPCC AR5 report. The answer given was to an interesting scientific problem, but was not aligned to MP’s, at least not without additional explanations. Policy makers don’t want to know that net anthropogenic influence is towards warming, they probably start from there already. What they want to know is how much that influence is.

Further in the inquiry there was another attempt to pose the exact same question. According to the transcript:

[Graham Stringer]
Going back to some of the questions that John asked previously, one of the things that puzzles me about the greater confidence that the IPCC now has in their analysis and predictions is the hiatus in increasing temperature was not predicted, and yet you have greater confidence as that particular prediction was wrong. I am a chemist, not a mathematician and so I am treading somewhere where I fear to tread, but my understanding of Bayesian analysis is that when you have high confidence of something and you miss your target, you should have less confidence, not more confidence. Why do you have more confidence?

[Professor Sir Brian Hoskins]
I was not part of it, so I will not say “we”, but let me say that the models were not in the position to predict the hiatus, because that would have been an initial value problem where they would have needed the observations over the whole depth of the ocean as well as more of the whole climate system to start the model off, like a weather forecast if you like. That information was not available and so they were just running and would not expect to predict that behaviour. There is no way that counters the model. It is the sort of variability one would hope that the long runs of the models would show and I suspect that the models do not quite have enough of that sort of variability in general, which means that climate can change faster in one spell and less fast in another spell. They are probably slightly too uniform in their behaviours. There was no attempt and, in fact, there would have been no ability to predict the hiatus at that time. That was a decadal prediction problem for which the initial data were not available, but perhaps I can hand over to you.

That is a diplomatic answer. “The models were not in the position” and “there would have been no ability to predict the hiatus at that time”, as if they are now. And “suspect that the models do not quite have enough of that sort of variability in general”, wouldn’t be easier to say they still don’t.

[Dr Stott]
Maybe it is just worth pointing out that back in the Second Assessment Report there was a statement when it was talking about warming rates to 2100. The actual annual to decadal changes would include considerable natural variability, and this is part of what we are seeing at the moment and part of what has been assessed. Of course, it has been relatively challenging to assess this, because we are talking about very up-to-date information and therefore, for the cut-off date that the IPCC had, which was 15 March last year, there has not been a huge amount of literature, but that literature was assessed. In fact, the assessment that was made in the Fifth Assessment Report has been borne out by further evidence we have had since.

In the assessment it talks about particularly the contribution from internal variability, including a possible redistribution of heat within the ocean, and we have seen some literature since then-we also discussed this in the series of notes that the Met Office produced as well-that has talked about the potential role of the Pacific decadal oscillation, all within a context where we have seen multiple indicators of climate change, including estimates of the increasing energy of the climate system, continuing to increase in the last 15 years. There is more work to do to further delve into exactly the details of that Pacific decadal oscillation and what that is doing, but there is clearly an implication behind that, which is that there is an expectation for warming rates to pick up in the future as the mode of that variability switches into the other mode.

[Professor Sir Brian Hoskins]
At a meeting in Downing Street in the late 1980s I said that we could easily have a decade that would be cooler than the previous one and that would not mean the whole global warming thing had gone away. It is not unexpected, given natural variability, to get a behaviour like this but there was no attempt to predict it.

Basically it is said to be natural variability. But that was not the question. Even when we believe that it is natural variability, doesn’t that add to less confidence if it is not understood? What does the standstill actually mean in terms of their understanding? They avoided talking about the standstill in global temperatures. They just assumed it was a natural process that can’t be ruled out. I agree that a period of non-warming can’t be ruled out in a warming world, but what does this mean for the theory? It was brought into the discussion by Peter Lilley and also by Robert Smith, but all the alarmists witnesses kept well away from answering the question.

To me this is the big question: if human influence on cooling is less and the total anthropogenic effect is towards warming and there is a standstill in temperatures while there is “unprecedented” amounts of CO2 spewed in the atmosphere and that CO2 being the main cause of global warming, what does this tells us about the theory? That there are unexplained forcings/feedbacks that are not taken into account yet. What if those unknown effects have a big influence on global temperature? To me, just as to Smith, Lilley and Stringer, this looks as there should be less confidence, because some elements in the system are not yet known, not yet quantified and they seem to have quite some influence.

Increased confidence in anthropogenic warming

morecertain

After the release of AR5 SPM end of September last year, I was really surprised that the IPCC declared even more confidence in the human influence on climate, despite indications of the contrary. I was puzzled about this contradiction. To my joy I found that in the UK parliamentary Inquiry of Energy and Climate Change Committee into “the science” of UNIPCC AR5 there was some discussion about it. Indeed, when looking at the video I noticed this gem of a question (my own transcript of the video):

[Sir Robert Smith MP]
Is it possible, in the time we got, to summarize why there was an increased confidence in the human influence?

That was the puzzling question that was buggering me for so long. Peter Stott of the Met Office took the word:

[Peter Stott]
We have more evidence across the climate system. And we have evidence in terms of the ocean warming in particular. We understand we have better observations now, we understand not only the changes, but also the variability in ocean temperatures. Of course, if we going to measure temperatures below the surface of the oceans we have to send buoys down to measure those temperatures and so on.

[Sir Robert Smith MP]
Isn’t that more confirming the outcome warming. How does it increase the confidence that the human activity was the cause?

[Peter Stott]
It is providing our, it is increasing our level of conf [interrupted] I am sorry, excuse me,

[Sir Robert Smith MP]
I want to pin down, that is more a symptom of the system warming, but does it increase the confidence that the human activity was the cause?

That was my opinion also. He was talking about warming in general, not about why he thinks there is now more confidence in human influence. Good that Smith noticed it quickly and intervened. Stott continued:

[Peter Stott]
What we are seeking to do is to quantify the overall role of human activity. I mean, our null hypothesis starting point is explaining by internal natural processes and we can rule that out with high confidence levels. Then it comes to the question of the so called attribution question of how we quantify the human role, and that is what we discussed, the starting point could be that it is dominated by natural factors and human factor is rather small. So this is why we have increased our confidence in saying that extremely likely, more than half the observed warming, therefor the dominant cause is human influence. And to do that you need to bring all the evidence to bearing including importantly the ocean temperatures. Previously in AR5 there was a lot of variability, apparently, in the observational record, which we now understood was an artifact of different biases of those measurements. So, that is one example on which we have improved our confidence. But it is not an in terms of ocean temperatures but also in other factors such as the retreating ice and snow. We developed our understanding of the water cycle. We developed an understanding of extremes. When you look into our chapter, chapter 10, you will see we have a table in the back there of 33 rows where we itemized all the evidence, both the evidence, well, we have multiple lines of evidence, and also were we also have remaining uncertainties. And we have a greater wealth of evidence now than we had. With better understanding. With more observations. And also with the improved models as well.

He had a real problem keeping with the question and explained the warming as such. Even after the intervention of Smith the answer stayed really vague and in some places even circular. How disappointing for a lead author and the “Attribution Expert” of the Met Office. What I take from it is “we have more data now”. Indeed, 6 years of data extra. For the ocean temperatures this means 9 years of data in stead of 3. Still not very impressive. This is even less than the standstill of global temperature data and in climate terms insignificant. Not really impressive.

Myles Allen came to the rescue to clarify the matter and came in a real juicy discussion with Peter Lilley and Robert Smith.

[Myles Allen]
Just briefly to come back to the earlier conversation. I recall one of the big arguments in AR4 was over whether human influence via aerosols could be masking the human influences due to greenhouse gases. That was one of the big uncertainties. Now the uncertainties came down, because we are more confident that the aerosol forcing is not at the really high end that was considered possible back in AR4. And that increases our confidence in the size of the total anthropogenic warming over the past 50 years. Because our confidence about anthropogenic cooling [interrupted]

[Peter Lilley MP]
It means it is less?

[Myles Allen]
No, because aerosols cause cooling, because less aerosols and the total anthropogenic warming [interrupted]

[Peter Lilley MP]
There has been less cooling

[Myles Allen]
The total anthropogenic warming … if you are saying you are more confident about the anthropogenic cooling is less, then obviously the total anthropogenic warming is, … then your lower band of total anthropogenic warming goes up. We are more confident that humans are not cooling the climate, therefor we can have greater confidence that the overall effect, including all the warming factors, is towards warming. That’s just one of the factors that play in.

[Peter Lilley MP]
Let’s look at the real world, you know how much warming there has been, it is not how much warming there is in the models. There has been, let’s say 0.7 degrees warming. If that is not the result of 1 degree warming through CO2 and 0.3 degrees cooling through aerosols, which for example we know there is cooling through aerosols, than the amount of warming through CO2 must be quite subtle.

[Myles Allen]
The statement is about the total human induced warming. We have greater confidence in what the total human induced warming is than what we been able to give in AR4, That is an example of an increased confidence, resulting [interrupted]

[Peter Lilley MP]
Increased confidence and a smaller amount.

[Myles Allen]
Actually, no, I don’t believe that is a correct representation of the conclusions. But the key thing is that by ruling out something that might be compensating for the warming effect, by assigning a lower probability that what might be compensating for the the warming effect of CO2, you can increase your confidence that the overall human influence on climate is towards warming.

[Sir Robert Smith MP]
You are talking about the net effect of human activity.

[Myles Allen]
Yes, and that is what the statement is

[Sir Robert Smith MP]
But Mr. Lilley was making the statement that if there is less human input to cooling, then the human input to warming

Very good persistent questions of both MP’s. But they got this nonsensical answer, not even close to a clarification of what they have been asking:

[Myles Allen]
This uncertainty in aerosols is reflected in our uncertainties, specifically the warming attributed due to greenhouse gases. Which is highly relevant to quantifying uncertainty in the future. And as I emphasized before, it is that uncertainty which contributes substantially to the range of uncertainty in future projections. It is this balance between aerosol induced cooling and greenhouse induced warming. But this is specifically focusing on how much warming of the past 50 years is attributed to overall human influence. And that is the situation where our reduced estimates actually help increase our confidence in the total human influence.

Then the chairman interrupted the discussion and put the conversation on the rails of the consensus (well, due to this “higher confidence”). Saved by the bell.

Initially I couldn’t really grasp what he was talking about. But suddenly it dawned on me. It is really simple actually. It took me a while, but I think both Robert Smith and Peter Lilley saw it on the spot and were pushing for the answer.

This is how I think it makes perfectly sense. In the IPCC universe there are two main elements in the temperatures: natural variability and anthropogenic influences. The latter consist of aerosols (which have a cooling effect) and greenhouse gases (which have a warming effect). They assume that the greenhouse effect will be increasing, because our emissions also increase. Now the statement was not about the amount of anthropogenic warming, but about the proportion of warming in the anthropogenic influences.

Big difference.

When being more certain that anthropogenic aerosols have a lower cooling effect, this means that the net anthropogenic forces are now mainly in the warming area. In a way this is indeed an “improvement” over AR4 were the influences of aerosols was less certain. This could explain the change in terminology in AR5 (95% certainty about total human influences) to AR4 (90% certainty about human influences by greenhouse gases alone). This throws another light over the triumphal message that there was more confidence of human attribution in AR5 than in AR4. Did they really wanted to communicate such a meaningless message?

It is not hard to understand that Smith and the other MP’s understood it differently when asking this question. As policy makers they want to know about the certainty of the amount of warming caused by humans, while the scientists were talking about certainty about the proportion of warming within the anthropogenic influences.

It is a waste of time to debate … we do not have the answers either

When searching for information on how the AR5 was portrayed in the media, I came to this remarkable interview with Jean-Pascale van Ypersele (vice chair of the IPCC) in MO* magazine.

Again the usual statement that “the 5th IPCC report confirms with even more certainty that the warming is persisting and that man is responsible”. No word of course about the lowered equilibrium climate sensitivity, the larger sensitivity range, the inability to give a best estimate, the low confidence in trend and attribution of droughts and hurricanes, the lowered confidence in climate models and so on. Also no mention that this assessment is a opinion, not a measured value. In the light of this it is a bit odd certainty goes from 90% to 95%.

The thing that surprised me was that van Ypersele didn’t hide the fact that politics is almost as important to him than science (his father and grandfather were politicians). Which is probably an advantage having in the IPCC process (the IPCC is a political organization that uses science for its purposes).

That is how I saw the rest of the article, a combination of science and politics. About the AR Summary for Policymakers (translated from Dutch):

For five days long they discussed almost day and night about the Summary for Policymakers. Climate scientists simply do not rush it. And once the evidence is obvious, they also want politicians to take responsibility.

The summary for policymakers is a combination of science and politics. Strangely he says that “once the evidence is obvious”. As if the evidence is that clear. It relies heavily on climate models, which received quite some criticism in the last year. There is a growing discrepancy between the output of models and the observations. That would also be a reason why the certainty should have been lowered in stead of increased.

The reporter suggested that the report was conservative because the numbers are old. The answer (translated from Dutch):

With all the filtering it underwent, it can only be conservative. But the final findings have been accepted by all countries, including Russia and Saudi Arabia. That’s very important, the report is very solid. If we don’t have controversy anymore over this data, it is a very important base. Therefor, the UN Climate Panel decided not to enter into debate with climate skeptics about science anymore. The debate should now be about the solutions.

This strikes me as a pure political statement, not a scientific one. Scientifically, it shouldn’t even matter if a finding is accepted by all countries. The fact that these countries agree, doesn’t mean this is a solid finding, in fact, it only means that the selected members of those countries agree on this text, not that it is scientifically correct. Big difference.

No controversy anymore? Sure, if there is no debate anymore with skeptics about the science, then there obviously is no controversy anymore and the debate could be continued to find the solutions. Which is the pure political part in the process.

This gives it a sense of urgency. Quick, quick, we have to do something. Never mind the failing models, never mind the observations that raise questions that could not even be answered, the lowered sensitivity,…

Besides, losing the debate because of the naughty uncertainty could mean losing credibility.

Acting upon the advice of the UN Climate Panel he refused a debate (translated from Dutch)

That’s why I did not accept the invitation of the RTBF program Mise au Point to debate with fellow chemist at UCL, István Markó. It is a waste of time to debate someone like that before a public that does not think about it in a scientific way. I don’t avoid the debate on scientific questions, but then it must be conducted in a scientific context, in meetings with scientists. If you have a science that is so firmly established, examined by hundreds of scientists, and if the result is accepted by all governments of the world, no, I have no more time to this yet to deal with a climate non-expert who cast doubt in the debate.

That really convenient. Back in 2011 he once debated István Markó and it didn’t fare well. After the discussion the majority of the audience said not believing that humans are responsible for global warming, while at the beginning it was the reverse.

He doesn’t want to discuss about it anymore, except with like minded people. That’s not a debate anymore, but a cozy get-together. It gives me the impression that here speaks a man that doesn’t want to prove his case and just want to press his political ideas without resistance. This is not a scientist speaking, but a politician.

Closing the gap or influencing others to jump off the cliff?

closethegap

A couple days ahead of the IPCC AR5 report release I found this video in which 20 climate scientists were interviewed about the “gap between science and public opinion”.

It starts rather innocent telling that public perception of climate change (I believe they use in the meaning of “global warming”) is hard to perceive. The reason given is the flow of the seasons which makes it difficult to experience climate change conscientiously. I could agree with that. After this the admission that climate science is really difficult field. It is complex and spans over different disciplines.

Then “climate communication” comes up and things become more slippery.

We need to find a way to take the science that we produce and make it digestible to the general public. And then we will see people generally asking government and really forcing the government go out and do something.

At first it seems plausible. There is nothing wrong with making science digestible to the general public. But the intention of this is not to let the public gain knowledge, but to put pressure on politicians. Is that what climate science is about? And it continues:

Recently we started to realize that the problem with us taking action now is not with we perhaps don’t have enough research, but it is constantly a fight to make the research that we do have gets heard properly outside of the scientific circles. And it is a fight that we are not winning at the moment.

A fight they are not winning? Really? They got the mainstream media in their pocket. Climate scientists only have to make a press conference and whatever information they want to give will be printed in the mainstream media. Some journalists will even crank up the volume. And now he is telling us they are losing the fight?

What’s that all about fighting a battle? Since when does science need to fight a battle about the perception of the public? But it get even weirder:

The most important is to communicate the science, accurately. The climate science community is being doing it for 50 years. In the 1960s the first science came out spelling the warnings of ongoing greenhouse gases emissions. President Carter commissioned a report on the dangers of ongoing carbon emissions. And he was told at the time this is a problem we need to urgently move away from fossil fuels in terms of a source of electricity and for power and transport.

As far as I know Carter was worried about energy independence, not greenhouse gases. It was the time of the oil crisis and prices of oil skyrocketed. The frightening word back then was OPEC. By the way, it was at the end of the ice age scare and warming by greenhouse gases would be more a desired thing than something that urgently need as solution. It was the beginning of climate science and back then climate scientists (at that time probably only Lamb and Bryson) acknowledged both greenhouse gases and cooling. But, at least in the media, their bet was on cooling, not on warming.

The most interesting statement was this:

They say the reason why we should believe the climate scientists is there is a consensus because of the globe warming because of humans etcetera. I think people then think that somehow the science works by consensus or that someone is deciding this is the way is going to be and then everyone says That’s cool, let’s do that. The truth is the complete opposite. The scientists are so argumentative, so hate to be wrong and so like to argue with their peers. It is like a constant battle amongst everyone. Then there is one viewpoint that is coming out of this constant battling. Then this viewpoint is extremely, extremely valid. And we know it gonna be very close to the truth. So if you understood it more how argumentative scientists were in the field they might have more respect for these mayor viewpoints that coming out of their research.

That’s a strange reasoning. “No, (climate) science doesn’t work by consensus, but climate scientists are so argumentative, so when many of them agree that is really, really valid”. That’s like a consensus in disguise. It still means the debate is over because there is … a consensus. “They just looooove the debate”. Therefor they exclude other scientists that think differently. By calling them deniers, which is the case even in this video. Words like “Denier of climate science” and “anti science” start flying around. Calling people who have another viewpoint “denier” or worse, is not exactly a good example of being “argumentative”. Or are they only “argumentative” with the like-minded?

The science is clouded by these messages from the anti science community. So people left with a confused message and they decide, well, who to believe? That guy in a suit or that guy in a suit. It has been a very difficult communication message

Science is not particular well suited to communicating all this stuff and it is unfortunate because in the last 10-15 years there has been such concerted campaign to distort what the scientific community actually found.

My heart is going weak, those poor little climate scientists. They really think that the science or the debate is hindered by people who have another opinion. Just a couple years ago I had the same misconception. I thought “progress” in climate understanding was hindered by “deniers”. But I didn’t have a clue what the arguments of those deniers were. I wasn’t aware the skeptics’ arguments were only brought by the climate scientists themselves and a lot of things went lost in translation.

Then of course the road we will have to go:

I think that it is clear that it is not hard to do, that it is not changing the economy, that it is not impacting all of your life. I think that would be a start.

That seems unbelievable. We have to wean off fossil fuel energy (and probably other reliable energy sources) and replace it with intermittent and expensive energy sources. Hey, that’s going to be really easy! 😉

Then the “misconception” that climate scientists have diverse political views and that politics doesn’t drive the science. Well, looking at this clip gives me another impression. Sure, scientists have different political views, as have most other fields in our society, but that is not the issue. At issue is not that these scientists have political views, but that they want to influence the public towards one view. Confirmed in depth by the scientists in this clip. When they themselves know the subject is a very complex matter, yet pretending to have the answers and dismissing other viewpoints in the process.

Low confidence in trend and attribution of droughts and hurricanes, doh!

In climate change communication droughts and hurricanes are two constants. We hear there is an increase in droughts, of course due to global warming. Also, more hurricanes are the fingerprint of a warming world. It looks strange because nor droughts nor hurricanes are on the increase, but it has been told so many time that a lot of people believe it.

Therefor it was a surprise to see the extreme weather table in SPM AR5 (table SPM.1 on page 23) and even more when comparing it with the similar table in SPM AR4 (table SPM.2 on page 8).

This is the comparison table:

  Likelihood of future trends
Phenomenon and direction of trend Likelihood that trend occurred in late 20th century (typically post-1960) Likelihood of a human contribution to observed trend Early 21st century Late 21st century
  AR4 → AR5 AR4 → AR5 AR4 → AR5 AR4 → AR5
Area affected by droughts increases Likely in many regions since 1970 → Low confidence on global scale/Likely changes in some regions More likely than not → Low confidence Likely → Low confidence Likely → Likely (medium confidence ) on a regional to global scale
Intense tropical cyclone activity increases Likely in many regions since 1970 → Low confidence in long term (centennial) changes. Virtually certain in North Atlantic since 1970 More likely than not → Low confidence Likely → Low confidence Likely → More likely than not in the Western North Pacific and North Atlantic
Droughts

Likelihood of the trend occurred since late 20th century went from “likely in many regions since 1970” to “Low confidence on global scale/likely changes in some regions”. Rather odd that they had to adjust their assessment for a trend that supposedly was there, “clear” to see. Some areas were likely to change, but globally there is not much confidence.

The same with human attribution: this went from “More likely” in AR4 to “Low confidence” in AR5. Nice to hear.

Likelihood of future trend went from “Likely” to “Low confidence” in the early 21th century and “Likely” on a regional to global scale. After hearing so many horror stories about droughts this seems serious backpedaling.

Tropical cyclone activity

Like droughts the trend occurred since the late 20th century went from “likely in many regions since 1970” to “Low confidence on a global scale”, but “Virtually certain in North Atlantic”.

The likelihood of human attribution went from “More likely than not” in AR4 to “Low confidence” in AR5.

Also the future likelihood short term went from “Likely” to “Low confidence” and on long term the confidence is assigned to “More likely than not”. Not that much confidence anymore.

Well, low confidence of human attribution and short term increases in droughts and hurricanes. That’s good news! Until now we were bombarded with loads of doom stories. Now those seems to to be based on nothing much. Pity this was not in the press release. Wouldn’t it be great when the public got to know such things? 😉

95% certainty or 95% opinion?

On September 27, 2013 the IPCC declared that the “human influence on the climate system is clear” in the AR5 Summary for Policymakers. In the current report they declared more than 95% certainty (extremely likely) that greenhouse gases account for more than half of the increased global average surface temperatures, while they declared only 90% (very likely) for something similar in the previous report.

For us, the great unwashed, this seems really impressive. We assume it is a accurate picture of reality and the science made quite some progress in the last six years and hey, 95% certainty isn’t that more than enough for action?

It made me wonder what this “certainty” exactly means and how it is determined. This is what the AR Summary for Policymakers says about it (page 2):

The degree of certainty in key findings in this assessment is based on the author teams’ evaluations of underlying scientific understanding and is expressed as a qualitative level of confidence (from very low to very high) and, when possible, probabilistically with a quantified likelihood (from exceptionally unlikely to virtually certain).

That seems more like the subjective interpretation of a certain group of scientists (that probably are selected for having these opinions in the first place).

But look at reality. The “pause” was not foreseen and unexplained. We were told many times (also in this report) that human produced CO2 drives temperatures. If this really is the case then a more than a decade long standstill in temperature increase, together with a record amount of CO2 in the atmosphere, must tell us something: there is at least one forcing or feedback with at least the (combined) strength of CO2 that is not accounted for.

That alone should have humbled the scientists and adjust their certainty DOWNwards in stead of upwards. Self declaring more certainty in the face of a glaring gap in their understanding of the climate system doesn’t make much sense.

95% certainty or 90% certainty, what does it even matter? It is both as impossible to evaluate anyway.

Climate sensitivity? What climate sensitivity?

After looking at the attribution certainty differences between AR4 and AR5, this time the differences of the equilibrium climate sensitivity. This is how AR5 defines it:

The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration.

Basically, the warming we would expect for a doubling of CO2, in most cases determined as doubling of pre-industrial concentration of 280 ppm to 560 ppm (when we continue emissions like this, we will maybe get there, dependent on how much fossil fuels stay available). This is how they were assessed in both reports:

AR4 (SPM p. 12)
Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.

AR5 (SPM p. 11)
Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence).

That’s a step back in sensitivity and no mention anymore of the best estimate (3 °C in AR4). Maybe it is still too high. The pause tells us that something doesn’t add up: the climate sensitivity could be even smaller than presented and/or there are other forces or feedbacks that are not represented in the models. Therefor the big surprise about the declaration of “even more certainty” in AR5, despite the known discrepancy between the observations and the outcome of the climate models.

It is strange that the certainty range is getting larger. The lower limit is 1.5 °C, but the higher limit stays at 4.5 °C and a best estimate can not be given anymore. Really? After 6 years the settled science is even less clear about sensitivity?

Nevertheless, if climate sensitivity could be lower than expected, then less warming could be expected. That’s good news. But no mention of the lowered limit in the press release. To be fair, it is mentioned in the Summary for the Policymakers, also that it is lower than the value in AR4, but because the media doesn’t read the Summary report, it is not communicated to the public. It almost didn’t get in the Summary report. According to the minutes of the IPCC plenary session:

Quantification of Climate System Responses: On equilibrium climate sensitivity, several delegations, including Australia, the Netherlands and others, noted that the message that the lower limit of the assessed “likely” range of climate sensitivity is less than the 2°C in the AR4 can be confusing to policy makers and suggested noting it is the same as in previous assessments. The CLAs explained that comparison to each of the previous IPCC assessments would be difficult, and new language was developed adding that the upper limit of the assessed range is the same as in AR4.

Some representatives in the plenary session found the lower sensitivity to be “confusing” and suggested to note it is the same as previous assessment (it is not). I can imagine vividly why this lower value would be confusing for policymakers.

Finally, when looking at how this lower sensitivity limit is communicated to the public…Nothing…Nada…Niente… only skeptical sites/blogs seem to report on it.