Wednesday, September 19, 2007

It could be when, not how much

Simon Donner has a nice post up describing the complexities governing sea ice extent, how other factors besides temperatures are important. Some time ago, Eli pointed out that not only was sea ice declining, but the time of the minimum was moving substantially later in September. With all the emphasis on sea ice extent (guilty, guilty, guilty) it is useful to return to that point
and take a look at how the Arctic is doing today
The later the minimum, the later the freeze up, the later the freeze up, the less time to grow new ice, and the easier to reach a minimum next year. Eli may up his bet with Stoat. 60 carrots??

UPDATE: Via the Scotsman comes the news from the NSIDC that sea ice reached its minimum in 2007 between the 16th and 20th of September. This leaves Eli in a bit of a quandry. If the day of the minimum went beyond the 20th, the Rabett was going to pile on his bet with Wm Connolley. OTOH, if it came earlier, he was going to gracefully retire to the carrot patch. The current strategy is to wait for a few days and see how steep or shallow the rebuilding is compared to future years. Advice o mice?

Overview of current sea ice conditions
Sea ice extent now stands at 4.18 million square kilometers (1.61 million square miles). This represents an increase of 50,000 square kilometers (19,000 square miles) compared to the value of 4.13 million square kilometers (1.59 million square miles) five-day running mean extent, observed on September 16, which appears to be the 2007 minimum.

39 comments:

Anonymous said...

I don't think anyone seriously disputes that Arctic ice is shrinking. The dispute is:

1) what does it mean - is it unprecedented, is it on a catastrophic positive feedback loop?

2) why is Antarctic ice expanding?

In short, are we dealing with a temporary (in terms of centuries) regional phenomenon, that will reverse given time, or are we dealing with a global and runaway situation?

The big question surely about this must be the MWP? We know the Vikings farmed during it. But we also know it cooled and ice expanded after that. So, if it could happen then, and we have no explanation of either why it warmed or why it cooled, why cannot the same thing be driving current warming, and then drive future cooling?

Belette said...

Are you offering £60?

Anonymous said...

Hey Rabett can you draw 2006 on that same graph. Just so you can confirm your logic.

JohnS

Anonymous said...

Anon 11:12 "why is Antarctic ice expanding? "

Oh, is it?

How can you be so sure when GRACE showed just the opposite? -- ice mass loss.

Perhaps you depend on something more accurate than GRACE for your definitive conclusion?

Anonymous said...

Anon 11;12 asked "- is it unprecedented"

Whether it has ever occurred in the past is not the relevant issue here.

What is relevant is whether it is occurring now (it is) and whether the current ice loss is due to human influence (the vast majority of scientists would also answer yes to the latter).

" is it on a catastrophic positive feedback loop?"

"catastrophic" is a loaded word (meant to cover your retreat?), but it's pretty clear that what is going on in the arctic does involve positive feedbacks.

The decrease in albedo with ice loss is a positive feedback causing temperature to go up which in turn melts more ice. Not imaginary, but very real.

Such positive feedback is one of the primary reasons why temperatures have been increasing more in the arctic than they have been on average worldwide.


What is most important in the case of sea ice is that the "melt season" in the arctic has been increasing* and the refreeze season shrinking.

According to NASA
"The length of the melt season inferred from surface temperature weekly data has been increasing by 9, 12, 12, and 17 days per decade in sea ice covered areas, Greenland, Eurasia (>60o lat), and North America (>60o lat), respectively. Longer melt periods would mean reduced growth season, thinner sea ice and less extensive sea ice cover in the summer. Credit: NASA"

J. said...

Anon at 6:01 am, you're confusing Antarctic sea ice (not measured by GRACE) w/ land ice.

The earlier anon (11:12 pm) is a bit misleading in talking about Antarctic [sea] ice "expanding". Yes it is at an "all-time" (ie, since 1979) high, but there's no statistically significant trend, as far as I'm aware. The southern hemisphere ice anomaly plot just looks like noise centered around a basically stationary mean.

In contrast, the northern hemisphere anomaly plot really does show a persistent downward trend.

Southern Hemisphere:
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.south.jpg

Northern Hemisphere:
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.jpg

Anonymous said...

From the Anon 7:24 post:

Anon 11;12 asked "- is it unprecedented"

Whether it has ever occurred in the past is not the relevant issue here.

What is relevant is whether it is occurring now (it is) and whether the current ice loss is due to human influence (the vast majority of scientists would also answer yes to the latter)."


If you're comparing today to the 1979-2000 line, then this statement is untrue - you're showing relevance ("is it happening now", yes it is: COMPARED TO THE LINE.)

It would be even more relevant to show the same kind of charts based on a 1930 to 1978 line.

Also, showing a 21-year average line, and comparing it to lines that are 5 TO 7 YEARS LATER?

It might mean more to break that chart down and show individual years. If the theory is true, then each successive year's line would dig a little deeper.

P.S. Did 2006 set new records, or did it skip fom 2005 to 2007?

Anonymous said...

J. said "you're confusing Antarctic sea ice (not measured by GRACE) w/ land ice."


"The first-ever gravity survey of the entire Antarctic ice sheet, conducted using data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (Grace), concludes the ice sheet's mass has decreased significantly from 2002 to 2005."

Which part of "entire Antarctic ice sheet" do you not understand?

Anonymous said...

"It would be even more relevant to show the same kind of charts based on a 1930 to 1978 line."

Not true. The last 3 decades are the most relevant with regard to human induced warming. That's what IPCC says, that's what NAS says.

Your saying otherwise does not change that.

J. said...

Anon 9:04, ice extent in September 2006 was slightly higher than Sept. 2005, so not a record but still well below the mean.

Anon 9:51 wrote "Which part of 'entire Antarctic ice sheet' do you not understand?"

Sea ice and land ice are two different systems. In the Antarctic, mass of ice on land is decreasing. Sea ice extent is more or less randomly fluctuating from year to year around a constant mean. This month's ice extent happens to be a record high, but I don't think anyone believes that it's particularly significant.

EliRabett said...

J is correct on the facts, but wrong on the blogs. Lots of noise about the Antarctic sea ice extent being a max.

Anonymous said...

Anonymous 9:57 AM said...

"It would be even more relevant to show the same kind of charts based on a 1930 to 1978 line."

Not true. The last 3 decades are the most relevant with regard to human induced warming. That's what IPCC says, that's what NAS says.

Your saying otherwise does not change that.


O.K, assume that the reason the ice has decreased in the last 3 decades is the result of human induced warming.

One way to prove that would be to compare it to the 30-year period BEFORE 1977.

If the average levels before then has less of a downward slope, that means you're correct.

If there is any upward slope, you're wrong.

If any year PRIOR TO 1977 had lower averages, you're wrong.

cce said...

This chart shows sea ice extent for the various seasons going back to 1900. Obviously, the further you go back the less certain the data since you have to rely entirely on observational data. However, it's unlikely that the Arctic sea ice extent has (recently) been as low as it is now.

http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seasonal.extent.1900-2007.jpg

Anonymous said...

The anon who posted that whether Arctic ice is at an historical low is unimportant:

The reason it seems important to some of us, is that we require evidence that the present retreat is in some way beyond the norms which are to be expected from a climate.

It is not to be expected, nor would it be a reasonable goal of policy, that Arctic ice should have exactly the same extent for centuries. So how much variation is to be expected?

Put in another way, We want to know how many standard deviations away from the mean the present shrinkage is, in terms of centuries.

This would be an automatic question in any other area. For instance, I see a fluctuation in monthly sales figures. My first question is, is this significant, or do sales by month normally vary up or down by this amount? I see that a certain treatment seems to be less effective this year than it was last year, for what we think is the same variant of the virus. Is it significantly less effective? Or is this within normal variation?

Personal abuse will not get to the root of this one. It is basically a question about the facts. If you know the answer (I do not) just please give it. In terms of standard deviations, please. That is how some of us are used to thinking about these things.

Anonymous said...

> I see a fluctuation in
> monthly sales figures.

Remember way back when "he could sell iceboxes to Eskimos" was only said about outstanding salesmen?

Anonymous said...

The last three decades are the most significant?

Cannot be, can it? No matter who says it. We only know they are the most significant IF they are significantly different from all other periods of three decades. If the climate naturally varies this much, who cares what it did in the last 30 years? It will go up and then down. So what? Its like summer and winter.

stevesadlov said...

Any mention of Sigma concepts, and it's bound to result in all sorts of snarling from the usual suspects.

Now, let me offer a different way. If I were an advocate of taking radical action in response to GHG emissions, and, if I were truly confident that the data said it was a problem, I'd have no qualms at all about adopting a Sigma methodology to prove my point. It was actually, if these two aforementioned things were true, help strengthen my argument.

If, on the other hand, I lacked the data needed to say "the sea ice extent is no longer within the 6/5/4/3/whatever the appropriate Sigma value is Sigma limits" I would be doing everything I could to get such data. I would be the worst enemy of those using weak methodologies to collect data and to process it. I would be doing the equivalent of an amicus brief.

Go as you will, and take this or leave it.

Anonymous said...

to cce:

Thanks for the chart. As you said, "Obviously, the further you go back the less certain the data since you have to rely entirely on observational data. However, it's unlikely that the Arctic sea ice extent has (recently) been as low as it is now."

And the chart does not show decreased levels like we have now.

But the chart you gave does have a couple of things on it:

1. The annual (average). There is a drop, but that appears to start in 1955, not within the last 30 years.

2. The chart that you gave breaks the year into 4 parts. The chart rabbit shows only gives 5 months (warmest months). Once again, it would be useful to see the whole peak-to-peak on a chart like rabbit shows (ie, is there any change in sea-ice MAXIMUM?)

If the sea ice max is falling at the same rate as sea ice min, then I'd say you have a case.

guthrie said...

How typical of the denialists to reject any idea that we might actually have a pretty good understanding of what has been affecting the climate in the past 30 and more years, despite the reams of evidence showing that we do.

Anonymous said...

"Sea ice and land ice are two different systems."

You are assuming that the "sea ice" does not go all the way to the bottom, which it does with much of the WAIS. Sea ice is not always "floating".

According to Wikipedia

"The WAIS is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves."

In other words, the part of WAIS that lies on the ocean floor is technically "sea ice".

But much of that volume of ice actually rests on the ocean floor, meaning that changes in the ice sheet can be detected by GRACE and can also have an impact on sea level.

http://en.wikipedia.org/wiki/West_Antarctic_Ice_Sheet

"In January 2006, in a UK government-commissioned report, the head of the British Antarctic Survey, Chris Rapley, warned that this huge west Antarctic ice sheet may be starting to disintegrate, an event that could raise sea levels by at least 5 m"

And what has been happened recently to several of the giant ice shelves that are floating? They have been collapsing, that's what.

Steve Bloom said...

I'm pretty sure that what defines sea ice as such is that it forms in the sea. The ice sheets and ice shelves are glacial in origin.

Anonymous said...

Steve,

The argument above started when J claimed that "Anon at 6:01 am, you're confusing Antarctic sea ice (not measured by GRACE) w/ land ice."

J is the one who is actually confused because much of the "sea ice" (also termed "marine ice") making up WAIS actually rests on the sea floor and changes in this can therefore be measured with GRACE.

Anonymous said...

"guthrie said...

How typical of the denialists to reject any idea that we might actually have a pretty good understanding of what has been affecting the climate in the past 30 and more years, despite the reams of evidence showing that we do."


And how typical of the alarmists to avoid the questions. I came on here, and asked a civil question. If you know the answer, say so.

So far, one of you says that anything past 30 years wasn't relevant. Now you say that you have the data I'm requesting for 30 years and more.

So the question was, Is it possible, among all the "reams of evidence", to see a chart like rabbit has, for the same period, with the full "peak-to-peak" of the full year.

I'd still like to be pointed to the data that shows the sea ice maximums plotted.

stevesadlov said...

What are the variation characteristics of areal extent over the Holocene, and what are the resulting sigma levels, warning limits and spec limits, as it were? "Anomalous" would mean, outside warning limits. "Unprecedented" would mean, spec limit exceeded (but even that might be argued, if the "process" was not innately in control, and there were past similar excursions. For the sake of this discussion, I am assuming that the process could be deemed "in control" over the Holocene.)

Dano said...

Sadlov non-sequiurs:
Any mention of Sigma concepts, and it's bound to result in all sorts of snarling from the usual suspects.

Looks like Hansen is yanking the denialist chains:

NASA Researchers Find Snowmelt in Antarctica Creeping Inland

09.20.07

On the world's coldest continent of Antarctica, the landscape is so vast and varied that only satellites can fully capture the extent of changes in the snow melting across its valleys, mountains, glaciers and ice shelves. In a new NASA study, researchers using 20 years of data from space-based sensors have confirmed that Antarctic snow is melting farther inland from the coast over time, melting at higher altitudes than ever and increasingly melting on Antarctica's largest ice shelf.


Poor, poor lil' chimps.

Best,

D

Anonymous said...

From what I read on NSIDC "Sea Ice" is frozen sea water, i.e. not glacial in origin. I'm not sure exactly how the WAIS is computed, but I don't think that it is, nor should be, considered sea ice, regardless how far out and down in the sea it extends.
I would expect that the freezing of sea water requires cold temperatures, prone to slow glacial ice, whereas warm temperatures would promote faster galcial ice flowing and calving. So I don't think that it would add clarity to include the WAIS in sea ice, although I'd have to verify how it's actually computed.

cce said...

Anon,

I would say that the drop in annual figures actually started in the '70s. There was a transition in methodology in the '50s that is responsible for the abrupt change. A bit of documentation is given here: http://arctic.atmos.uiuc.edu/SEAICE/

The sea ice extent seemed to take the early 20th Century warming in stride, but it has been significantly reduced by warming since the '70s.

Also, these are absolute figures, so the sea ice maximum would be whatever year/season had the largest numbers (albeit only seasonal resolution). On this chart, the record would be spring 1949, but given the uncertainty you can't say this with any assurance.

Despite the shaky data, it's safe to say that current conditions are unusual for over 100 years or so, and certainly during the "little ice age" immediately prior. So, probably 500 years, at the very least.

EliRabett said...

Given that 30 years is about when the first polar observing satellites went up, how accurate does anyone think data from earlier than that would be.

You might want it, but that does not mean it either exists or is very reliable.

Anonymous said...

guthrie, there are no such animals as 'denialists'. There are some people who want to see evidence for assertions, or want to see this evidence stated in the usual ways in which evidence in other fields is stated.

Calling such people 'denialists', as if that was a state, is a little like calling people 'right wing deviationists' or 'capitalist roaders'. It makes no contribution to the facts, which is what we are interested in.

What you seem to be doing with this term is to try to imply that such requests are in bad faith. The implication is that the facts are all in and known, and that only bad faith can lead to any questions. Well no, this is science. Questions are perfectly legitimate. All you have to do is answer them.

It is not, you notice, at all like smoking, where the statistics of the connection and the details of the studies were readily available and in the public domain. Here, one has to use the FOI to get at the data studies are based on, and the statistics, when available, turn out to contain howlers. And in the present case one's impression is that people simply do not have the data on Arctic ice. You are not able to show that there is anything significant going on there, over and above what is to be expected over a few hundred years, from the normal fluctuations in the environment of the planet.

Or maybe you are? If so, just produce it. What some of us want to see is a simple statement with evidence: the current melting or shrinkage is X standard deviations away from the 1,000 year Arctic ice mean.

I don't think you have it.

John Mashey said...

One interesting way to look at the daa is to look at the anomalies by month, which shows the downward trends in every month,with faster downward rates in melt season:
http://nsidc.org/cgi-bin/wist/wist.pl?annot=1&legend=1&scale=75&tab_cols=2&tab_rows=12&wcf=seaice_extent_trends&submit=Refresh&hemis0=N&img0=trnd&hemis1=N&img1=plot&mo0=01&year0=2007&mo1=01&year1=2007&mo2=02&year2=2007&mo3=03&year3=2007&mo4=04&year4=2007&mo5=05&year5=2007&mo6=06&year6=2007&mo7=07&year7=2007&mo8=08&year8=2007&mo9=09&year9=2006&mo10=10&year10=2006&mo11=11&year11=2006&.cgifields=no_panel

Another interesting way of looking at the data is to graph quarterly and half-yearly changes:
Au(p)->Wi
Wi->Sp
Sp->Su
Su->Au

Wi->Su
Su(p)->Wi

Here is the graph of the quarterly extent data, and the data itself:
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seasonal.extent.1900-2007.jpg

http://arctic.atmos.uiuc.edu/SEAICE/timeseries.1870-2007

Download into Excel, use Data>Text to Columns to split into 6 columns. create 6 new columns that compute the quarterly changes, and the Winter<->Summer changes. Create a scatterplot of the years, plus those 6 columns.

Before 1950, nothing much was going on, i.e., the graphs jiggle, but are essentially flat.

Since 1979:
1) The Winter->Spring melt has gradually deepened.
2) The Spring->Summer melt has gradually deepened, until this year, when it fell off.

3) The Previous Summer -> Winter has slowly increased (of course, since the downtrend is faster during melt).
The last 10 years:
7.5,6.9,6.7,7.1,7.0,7.4,7.0,6.7,6.6,6.4.
mean=6.9, stddev=.33

4) Suppose the current number ends up around 5.8, and we take the maximum re-freeze from above (7.5), that gives a guess of 13.3 for next Winter. So, I guess 13.0-13.3. To get back to 14.0, we'd need 8.2, about 4 sigmas higher than the mean of 6.9.
But of course, this year's melt looks so weird that I suppose the refreeze could be really weird too. Over the last 10 years, and again, guessing that 2007: -5.8 (not -5.6) the meltoff mean = -7.2, stddev=.48, so that this year's melt is about 3 sigmas out.

I'm sure this is way too simplistic, but it's least a back-of-the envelope guess.

Dano said...

Anon @ 11:18 PM:

Good joke. Funny ha-ha:

The only thing that will convince the denialists is...IS: knowing X std devs away from a 1,000 year mean.

I think this is Standard Denialist Tactic #16: raise the bar really, like, high. I am pre-coffee so I could have the tactic number wrong.

Best,

D

J. said...

Dear Anon at 2:37 / 3:29: With all due respect, you are mistaken. Repeatedly, and increasingly emphatically, mistaken.

"Sea ice" refers to a relatively thin layer (a few meters or less) of floating ice formed by freezing of seawater. In the Northern Hemisphere, some of this is multi-year ice, but in the SH it almost all melts and re-freezes every year.

This seasonal change in phase (solid / liquid) doesn't involve any change in mass, so as far as I know GRACE isn't useful for measuring it.

Land ice and the floating ice shelves are glacial features formed from precipitation on the Antarctic continent. In East Antarctica virtually all the ice sheet is grounded above sea level. In West Antarctica, the ice sheet is grounded below sea level, but it's still not "sea ice". The EA and WA ice sheets gain mass by precipitation and lose mass by melting and by discharge of ice into the ocean. GRACE can be used to measure changes in mass of the EAIS and the WAIS.

The ice shelves (Ross, Filchner-Ronne, etc.) are floating, but they're still glacial in origin, not formed from freezing seawater, and they're much thicker than sea ice (100s of meters).

Again, in Antarctica, the land ice component of the cryosphere is losing mass overall, which is a driver of sea level change. Sea ice doesn't affect sea level; it is primarily of interest because of its effects on albedo, on the formation of Antarctic Bottom Water (which plays a role in the thermohaline circulation) and on ecosystems.

In the Antarctic, sea ice extent recently reached a 28-year maximum, but as I said earlier that doesn't seem to be statistically significant ... because it's just oscillating around a more or less stationary mean.

I don't read a lot of denialist blogs, so I'll take Eli's word for whether or not the blogs are trying to imply some significance to it. But the bottom line is simple -- Arctic sea ice is at a record minimum and there is an obvious downward trend; Antarctic sea is is at a record maximum but seems to be just pseudo-random noise.

Cheers,
J.

stevesadlov said...

Dano why do you resist quality assurance? It's not rasing the bar any higher than what is the norm in most fields of science and engineering. I do this for a living, I'm not some trailer park vidiot. But go ahead, believe and spew what you will. I'm glad you don't work for me. (By the way, did you know that I am in the "green products" business? Just thought I'd share that little, as you might put it, "fact-y!")

guthrie said...

My advice is don't read the Hootsmon, it's been going downhill since the Barclay brothers got therir sweaty mitts on it and installed Andrew Neil, an ex- london broadsheet guy with ambitions but no real ability, who drove away the decent journalists. Their science reporting is extremely variable, some articles not even making sense, and one or two being of reasonable quality.

Plus myself and some others have to squash the denialists on a regular basis.

guthrie said...

anonymous 11:18- no, your a denialist- it's a perfectly good label for someone who exhibits the behaviour of many of the anonymous people who post on here. (Of course since you are anonymous and don't even have a tag we can't tie you in with other posts.)

The evidence for the assertions has already been given. Regarding the winter ice extents, presumably you are incapable of doing your own leg work:

http://nsidc.org/news/press/2007_seaiceminimum/20070810_faq.html

http://nsidc.org/news/press/20070403_winterrecovery.html

hmm, I see that the latter url says:

"NSIDC scientist Walt Meier said, "This year's low wintertime extent is another milestone in a strong downward trend. We're still seeing near-record lows and higher-than-normal temperatures. We expect the downward trend to continue in future years.""

You see, all these anonymous peopel swarming about asking questions aren't actually talking about facts, because whenever they run into any they don't like, the "turbo goalpost mover 2000" fires up.

S2 said...

NSIDC also have "A word of caution on calling the minimum", something the press seem to have overlooked.

Echoing Guthrie, I wouldn't recommend basing a decision on the size of your stake on what you read in the Scotsman.

Besides, you're identifiable. :)

Steve Bloom said...

If as is implied by William's comment the bet is in pounds, over the course of the next year exchange rates should serve to double or triple the size of the bet (fron Eli's POV).

Eli, also bear in mind that what William is doing here is betting on the results of his most recent paper (which basically concludes that holding other influences equal a loss of sea ice will lack persistence). This means that you should either a) stand pat or b) go all in. I leave the details of the analysis to you.

A serious comment: I haven't checked, but if it's the case that cloud cover has remained on the low side, it seem reasonable to expect that this summer's much larger area of exposed water will start "skinning over" quickly as soon as insolation drops by a certain amount. If so, the slightly early reversal of extent may be misleading. An examination of the pattern of extent recovery may inform about this.

J. said...

I have to make a correction. Earlier in this comment thread, I wrote:

The earlier anon (11:12 pm) is a bit misleading in talking about Antarctic [sea] ice "expanding". Yes it is at an "all-time" (ie, since 1979) high, but there's no statistically significant trend, as far as I'm aware. The southern hemisphere ice anomaly plot just looks like noise centered around a basically stationary mean.

In fact, if you look at the monthly ice extent data from NSIDC, I believe there is a very small, but statistically significant, upward trend in the SH ice anomaly, over the entire period 1978-2007. The trend is miniscule in comparison to the NH trend -- and it appears to have flattened out since 2000 anyway -- but if you just fit a linear trend to the whole data set, it is a statistically significant upward trend.

I commented about this in more depth in the "Polar Ice" thread over at Tamino's place: http://tamino.wordpress.com/2007/08/29/polar-ice/. The comment hasn't shown up yet, but it should soon.

Anonymous said...

What is happening with sea ice in the antarctic is all very interesting, but that is not what is most important.

What is most meaningful is what is happening to the ice sheet overall -- ie, "net ice mass balance" -- and as Scientist Tom James (Natural resources Canada) remarked (5/31/2007) "At present, however, it is uncertain if the net mass balance of Antarctica is positive or negative."

"The stability of the marine-based West Antarctic ice sheet, which contains about 5 m of global sea-level rise, has been called into question. Recent studies suggest an acceleration of glacial flow in a key drainage basin (Pine Island Glacier). At present, however, it is uncertain if the net mass balance of Antarctica is positive or negative."

http://ess.nrcan.gc.ca/ercc-rrcc/proj5/theme1/act3_e.php