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I read that differently than you, Roger:

The way I see it, these uncertainties were left out of TAR because they thought they were known to be small, but they were left out of AR4 because they are no longer confident that they are small.

So the reason that they were left out has changed. and the take-home message is that sometimes more research gives you larger preceived error bars, not smaller ones.

So the basic explanation that dynamic effects have moved from the "knowns" to the "known unknowns" catagory seems to be consistent with both your quotes adn the realclimate explanation.

LL- Thanks much .. but I don't think that the TAR supports this interpretation, it states:

"We emphasise that estimates of the relevant factors are highly uncertain"

Just a layman here, I'm not sure I understood correctly. It seems from your quotes that:

Neither number has the dynamical changes.
TAR has mass balance numbers included.
AR4 doesn't have mass balance numbers included.

So it is left to someone to quantify the effect of these mass balance numbers on the upper bound. (not dynamical changes.)
What would the number have been in TAR if they were left out, and what would it have been in AR4 if current state of the art was taken into account?

I hope I'm clarifying and not adding to the confusion. :)

Roger, you have not got the story right here. There are a number of reasons why the two sets of numbers are different. Two are purely methodological - the change to 2090-2099 from 2100 for the headline changes, and the reduction to 90% from 95% in the uncertainties. Both of these reduce the headline number but not the substantive change. The physical change is how the ice sheet mass balance is treated. Looking purely at the thermal expansion numbers, the two reports best estimates are within 10% of each other for each scenario.

The uncertainty for the ice sheet change has increased on one side only - people now think there is a greater chance of more rapid melt (due mainly to dynamical effects) than they thought before. Thus the impact of the ice sheet change has not been reported in the same way. The partial inclusion of ice sheet effects in TAR is thus removed, but the potential magnitude of these changes in AR4 is larger. This cannot be described as a reduction in the top range.

However, this is clearly a complicated and confused issue, and so we will try and give a more quantitative analysis in a couple of days.

Minor note, if there is a signed comment on RC, it is not 'RC' talking, it is simply the person that signed it. 'group' posts are discussed among ourselves and can be assigned to the group.

Thanks Gavin, much appreciated. A few replies:

1. The AR4 SPM says:

"For each scenario, the midpoint of the range in Table SPM-3 is within 10% of the TAR
model average for 2090-2099. The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions."

OK, I'll bite -- when it says "within 10%" is that above or below the TAR model average? Someone obviously knows the answer to this, and I suppose we'll see in May. But I cannot imagine why the IPCC leaves this completely ambiguous -- can you report the actual numbers for us? I am assuming that the AR4 values are below the TAR.

The AR4 SPM suggests that the narrower range is due to "improved information about uncertainties" and doesn't indicate what the quantitative effects of changing the significance level. There is the footnote which says, "The TAR would have had similar ranges to those in Table SPM-2 if it had treated the uncertainties in the same way." Perhaps this is an allusion to the changing confidence level? Why not just tell us the numbers? Why the unclear presentation?

If the IPCC believes that the uncertainty ranges are actually larger than they reported, then why in the world didn't they report them as being larger? It makes no sense to report quantitative numbers indicating a 90% confidence range and then explain that they don't actually mean that.

So yes, I am sure that many people would welcome some clarification. Until then, given what a muddle this is, I do not think it is at all fair to criticize anyone who reads the AR4 SPM and concludes that there has in fact been a decrease in the top range projection of sea level rise. That is absolutely what it looks like to me, but I will await your further quantitative explanation and we'll be happy to prominently post that quantitative information once provided.

On the attribution to RC, we'll try to keep that straight in the future ... Thanks!

Gavin- A further confusion -- the use of the 90% confidence interval isn't exactly clear as none of the sea level rise ranges appear within the square brackets said to denote the 90% range. Thanks!

Roger, re your concluding comment and question:

"Thus, I conclude that the top end estimate has indeed come down from the TAR to the AR4, and those making this observation are accurately representing the AR4. Why didn't the IPCC just say so?"

It couldn't be, could it, that the IPCC team writing the SPM for AR4 didn't want to have any statements where the level of alarm in TAR is being reduced?

Hi Roger. You tell us that RC concludes that IPCC hasn't lowered the top of the SLR range. I'm still unsure about the details, but we actually say "Note that some media have been comparing apples with pears here: they claimed IPCC has reduced its upper sea level limit from 88 to 59 cm, but the former number from the TAR did include this ice dynamics uncertainty, while the latter from the AR4 does not, precisely because this issue is now considered more uncertain and possibly more serious than before." which asserts that some of the media discussion is wrong (and of course some, from eg the WSJ, has been nonsense) but doesn't say what the real answer is. As far as I can tell, IPCC says the range has decreased, so they are probably right.

Roger,

I can see your reference to the TAR which says that ice sheet dynamics were not included for the WAIS, but there is no mention of Greenland. Are we to assume that it is the dynamics of the Greenland glaciers that cause the difference between the TAR and 4AR estimates? In the absence of statements in the TAR saying that Greenland's glaciers were similarly treated to the WAIS I am inclined to believe Gavin. The section of the SPM which discusses this makes no mention of this being a post 21st Century problem, the implication is that this dynamic instability is connected to the 2100 sea level rise estimates. It says that the 0.1m - 0.2m uncertainty could be added on directly to the upper bounds of the estimates.

Thanks William -- a nice clear statement like "As far as I can tell, IPCC says the range has decreased, so they are probably right" would have been useful in the RC discussion.

Paul- I don't think we can answer this precisely until we see the actual numbers used in the IPCC.

Thank!

Roger,

An observation on a minor note. Gavin says:

"if there is a signed comment on RC, it is not 'RC' talking, it is simply the person that signed it. 'group' posts are discussed among ourselves and can be assigned to the group."

But William says:

"You tell us that RC concludes that IPCC hasn't lowered the top of the SLR range. I'm still unsure about the details, but we actually say..."

So I remain confused whether RealClimate speaks for their individual poster or for their group or if comments about RealClimate posts, as the ones seen here, can be addressed by an individual or the group.

Let's face it. The entertaining hair-splitting about how many millimeters global sea level will rise in the next 100 years is another indication that climate alarmists are concerned about the new mood within the IPCC. Despite lobbying, leaks and antechambering, the SPM has thrown out the more extreme scenarios regarding sea level and temperature rise, polar ice melting, hurricane activity, etc. This has come as a huge disappointment to many interested parties. After all, the extreme scenarios have been carefully developed and advanced by the disaster lobby since the notorious Met Office conference (Avoiding Dangerous Climate Change) exactly 2 years ago.
http://meteo.lcd.lu/globalwarming/Peiser/Peiser_Exeter2005_report.html

One only has to compare the SPM with the Stern Review to appreciate just how much the IPCC has softened its assessment and estimates of core issues. No wonder many doom merchants are fuming about the new moderate temper. Others are simply denying that any moderation has actually occurred.

To make matters worse, every single government has now signed on to the IPCC consensus. Some bloggers are seething about President Bush’s conversion as this newfangled consensus deprives campaigners of a natural target in the “science wars.” Now, that no government is disagreeing with the basic science, campaigners are forced to engage in the much more complex issues of climate policy and economic analysis.

Nonetheless, I don’t expect that the prophets of doom will surrender that easily and accept the IPCC consensus. In fact, I expect the stream of disaster predictions, catastrophe scenarios and hyped media alarmism to go on as usual – in the hope that - never mind - at least the next IPCC report will be more alarmist!

As a green campaigner recently stressed: “one of the big stories around this Friday's release by the Intergovernmental Panel on Climate Change (IPCC) is the conservative edge to the final product ... That means public awareness needs to be raised about two things: the missing pieces, and future IPCC reports that include models of disintegrating ice sheets…”

Guess what? I've just arrived at home - and what do I find in the post? Another New Scientist shocker: Their front page headline reads as if it's right out of a conspiracy fruit cake:

"GOOD BYE COOL WORLD: WHY OUR FUTURE WILL BE HOTTER THAN WE'VE BEEN TOLD"

So what's the IPCC secret?

Here is there revelation:

"But here's what they didn't tell us: If the official verdict on climate change seems bad enough, the real story looks far worse...."

http://environment.newscientist.com/channel/earth/mg19325903.800-climate-change-what-the-ipcc-didnt-tell-us.html;jsessionid=HIJLBDEJNAHA

No wonder the editorial is headlined: "Consensus is not enough"

Another non scientist, (but not ignorant!). Reading the IPCC summary for policymakers, (yes, I did read through it, turgid and poorly written though it was), I was struck by the table SPM0 on page 5 of the on-line pdf offering.
The numbers simply do not add up, either on the suggested contributions or the associated errors. As a minor,but maybe significant aspect, this table, and others is centered in format. Shouldn't a table which is meant to be added be aligned on the decimal points? It may be that this is why the addition is wrong!

Post script. The 18 page pdf file that I have just checked on at http://ipcc-wg1.ucar.edu/wg1/docs/WG1AR4_SPM_Approved_05Feb.pdf

differs markedly from the one earlier released through Junkscience which, apart from anything else, was 21 pages long and had things in a different order. Even the table numbers are different, the one I referred to before is now tablespm1, the numbers have been changed, but the addition is still odd in the errors column. This does not bode well for the final report, particularly when you realise that the man made part of the total co2 load is only about 1/6th anyway.

A related comment about the "Summary for Policy Makers". My thought throughout reading the SPM was "a policy maker is supposed to read this!!?". I think the report would more accurately be called "Summary of briefing for climate advisors to give to Policy Makers".

Tony,
The issue you talk about wrt table spm0 has been addressed here

http://atoc.colorado.edu/~seand/headinacloud/index.php/2007/02/02/first-thoughts-on-ipcc-spm-did-i-find-an-error/

Long story short: It looks like a units conversion error.

I posted a corrected version here:

http://www.gpc.edu/~dthieme/Weather/IPCC_AR4.htm

I actually think that Table SPM-0 made more intuitive sense in meters per century, but apparently the British per mm/yr.

On another thread Ian Castles raises a point worth thinking about and following up on:

"Questions such as whether or not the Panel reduced the upper end of its range for sea level rise between the Third and Fourth Assessment Reports are of minor importance compared with the key issue of whether or not the higher emissions projections used in both Assessments are plausible."

Benny:
I've never seen New Scientist to cover any Earth Science-related issue with any level of competency. However, I haven't read it for years, so in theory they may have lifted their game.

Off topic question. How was one chosen to be a member of IPCC? was there an effort made to include varying viewpoints?

New Scientist isn't a science magazine. I got a reply years back to a complaint about content in which the person answering me made exactly that statement. He wrote further: New Scientist is an 'Entertainment' magazine.

They said it, I believe it.

Roger

Here is another good example why climate alarmists and green campaign journalists are losing credibility fast:

Remember the extensive lobbying and media campaign against the IPCC's handling of the sea-level issue in the run-up to the AR4 release?

"Early and changeable drafts of their upcoming authoritative report on climate change foresee smaller sea level rises than were projected in 2001 in the last report. Many top U.S. scientists reject these rosier numbers.

Those calculations don't include the recent, and dramatic, melt-off of big ice sheets in two crucial locations: They "don't take into account the gorillas — Greenland and Antarctica," said Ohio State University earth sciences professor Lonnie Thompson, a polar ice specialist. "I think there are unpleasant surprises as we move into the 21st century."

Michael MacCracken, who until 2001 coordinated the official U.S. government reviews of the international climate report on global warming, has fired off a letter of protest over the omission...."
http://www.iht.com/articles/ap/2007/01/28/america/NA-FEA-GEN-US-Climate-Report.php

A few days ago, Science published a new paper that shows that this kind of gorilla-hysteria is misplaced. The paper reports that two of the largest Greenland glaciers have suddenly slowed. At one of them observations revealed “average thinning over the glacier during the summer of 2006 declined to near zero, with some apparent thickening in areas on the main trunk.”
http://www.sciencemag.org/cgi/content/abstract/1138478v1

I commend science writer John Tierney for concluding that the current moderation on this issue is more than appropriate:

"But for now, with the glaciers moving in fits and starts, it’s wise not to make any sweeping predictions based on a few measurements. Although the Intergovernmental Panel on Climate Change was criticized for not incorporating the recent scary data from Greenland into its long-range projections, these new results seem to vindicate its caution...."
http://tierneylab.blogs.nytimes.com/2007/02/08/greenlands-glaciers-take-a-breather/

As yet another climate scare has fallen flat, it is instructive but not surprising that alarmists and their groupies in the green media are simply ignoring an inconvenient truth.