Walt sent me this essay unsolicited, and I think it is very useful for establishing some baseline techniques. There’s more useful information on techniques here than in the entire Catlin Arctic Survey website. UPDATE, a response has been posted at the end of the article. – Anthony
Dr. Walt Meier
There have been several recent posts on sea ice thickness, particularly in regards to the Catlin expedition. I don’t have any direct connection to Catlin and in my research focus, I don’t anticipate using the Catlin data. I’m not responding to defend them or their methods. Thus, I can’t address details of their operation. However, from reading the posts and comments it seems like some basics on how sea ice thickness is estimated might be of interest.
Sea ice floats in the ocean. Because sea ice is a lower density than unfrozen water, it floats and a portion (~10-15% depending on density) rises above the water line, while most of the ice (~85-90%) is below the surface. The part of the ice cover above the water line is called the “freeboard”; the portion below is called the “draft”. The sum of the freeboard and the draft is the total ice thickness. There may or may not be snow on top of the ice which can add to the “snow+ice freeboard” and the “snow+ice thickness”.
A variety of techniques have been developed to obtain information about sea ice thickness. Most of these methods don’t actually directly measure thickness but estimate thickness from a related measurement. Here are some examples:
Upward Looking Sonar: Mounted on a submarine or on the ocean floor, these instruments measure the return of sound waves bouncing off the bottom of the sea ice. They measure the sea ice draft from underneath the ice. From this draft measurement, the thickness can be derived with knowledge of the ice and water density and the snow cover.
Altimeter: Compared to sonar, altimeter measure the opposite side of the ice. They measure the freeboard from above the ice, from which the total thickness is derived. The NASA ICESat is a laser altimeter, which actually measures the snow+ice freeboard, so knowing something about the snow cover is particularly important (http://www.nasa.gov/images/content/324868main_kwokfig2_full.jpg). Radar altimeters are also often used (including the European Cryosat-2 scheduled to be launched later this year); these penetrate through the snow and thus measure the ice freeboard. ICESat can take a lot of measurements over a large region of the Arctic, but there are limitations, which are discussed below. Altimeters can also be flown on airborne platforms.
Ground radar: This carried on or near the surface and sends out a radar pulse that echoes off the ice-water boundary. Thus it is an estimate of the total ice+snow thickness.
Drill holes: This is the simplest way to obtain ice thickness and it is the only direct measurement of ice thickness – drill a hole and stick measuring tape through it and you have the thickness (whether it is in units of meters, feet, or smoots [http://en.wikipedia.org/wiki/Smoot]). A variant of drill holes are the ice mass balance buoys that Steven Goddard wrote about – drill a hole and put in instrumentation to estimate thickness automatically over time.
There are errors associated with any estimate, but the errors tend to be higher the farther one is away from a direct measurement. For example, for ICESat, you need to know very precisely: (1) the altitude of the satellite above the surface, (2) the ocean surface topography [sea level isn’t constant], (3) the density of the ice and water, and (4) the density and height of the snow cover. All four of these are challenges, though by far the biggest one is #4. There just isn’t a lot of information about snow. ICESat has already provided valuable information about sea ice thickness over large regions of the Arctic and more results will be forthcoming. However, the goal is to continue to improve these estimates to make them even more useful.
This is where surface measurements, radar and drill holes are particularly valuable because they provide “ground truth” – of both ice and snow thickness. The problem with these ground measurements is that it is difficult to obtain a large number of them over a broad area. And this is particularly important for sea ice thickness, which can vary considerably over short distances. This is a limitation of the ice mass balance buoys. There are only a few within the entire Arctic and they measure thickness on a single floe. Even in the immediate vicinity, ice thickness could be quite different than that being measured by the buoy. Thus, while the buoys provide an excellent measurement of thickness at a point through the seasons, they do not provide good information on the large-scale spatial distribution of ice thickness.
Ideally, we’d send a few thousand people out to the Arctic and drill thousands of holes and get good sampling of thickness, but this is just not possible. Even putting out more than a few autonomous buoys are impractical because of the cost of the buoys and the fact that they only last a few years (the ice melts and the buoys are lost, though people are looking about buoys that can float and could potentially be recovered and recycled).
This is where the Catlin expedition can be particularly valuable. To have a group out on the ice taking direct measurements of thickness across a relatively large region (compared to most field expeditions) of the Arctic is something that has only rarely, if ever, been done before. It is unfortunate that the radar may not have worked as well as hoped, but that is the nature of field work, especially in harsh polar environments – things almost never go according to plan. The radar would essentially provide a continuous transect of thickness estimates over several hundred kilometers. However, the drill hole measurements taken regularly over the route will still likely be valuable.
It is also unfortunate that they are not likely to get as much data from multiyear ice as hoped because that is of greater scientific interest, but any ground truth estimates can help improve data from satellites like ICESat is useful. Their planned route looked like it would’ve taken them over ice of varied ages, but the older ice moved out of the area over the winter and, as Steven Goddard showed comparing their position with the ice age data on NSIDC’s web page, they started squarely in first-year ice. Generally, logistics for an expedition need to be planned several months in advance, long before anyone can know how and where precisely the ice will move. Like many scientific expeditions, it seems like they won’t get as much data as hoped, but ground data from the ice is so rare that every little bit helps.
As a final note, since it seems the measuring tape used by Catlin is of great interest, I’ll end with a bit of information on that. Basically, it is simply a measuring tape, but with a collapsible metal flange at the end of the tape. The weight pulls the tape down through the hole to the bottom of the ice. Then you pull the tape taught and the flange opens and catches on the bottom of the ice. You make your measurement, then pull hard on the tape and the flange collapses and you can pull it up through the drill hole. Since such tapes with flanges are relatively specialized, there aren’t many places to get one. One place is Kovacs Ice Drilling Equipment
http://www.kovacsicedrillingequipment.com/ice_thickness_gauge.html
NSIDC has a gauge from Kovacs and it has units of meters and feet, on opposite sides of the tape. I would guess that the Catlin tape is similar, but I don’t want to jump to conclusions.
Response to Dr. Meier by Steven Goddard.
First, I want to thank Dr. Meier for his candid explanation of how Catlin landed on first year ice, and how ice is measured. As always, he has treated our concerns seriously and that is very much appreciated.
Dr. Meier said that the ice “can vary considerably over short distances” and the Catlin web site has said “the team systematically seeks out flatter ice.” That implies to me that there is a geographical bias to the data which makes the entire data set suspect. (That might be analogous to having a temperature set where a disproportionate percentage of the thermometers were located in Urban Heat Islands.) If I were traveling across the Arctic pulling a 100Kg sledge in -40 degree weather, I would certainly seek out the flattest ice, as they have done.
The Catlin team has reported “Snow thickness, measured by the team during the first 2 weeks of March, shows an average snow depth of around 11 centimeters. Since then the average has risen to around 16cm.” Four to six inches of snow hardly sounds like a serious problem in estimating ice thickness in metres. They also said “March snow depths in this area should be 32‐34 cm on multi‐year ice.” If snow thickness is less than expected, does that imply that the satellites may be slightly underestimating the thickness of the ice?
If the multi-year ice shifted over a period of several months ahead of the expedition launch, why was the Catlin team seemingly surprised upon their arrival to find first-year ice? NSIDC knew it was first year ice in February. This reminds me of Lewis Pugh’s attempt to kayak to the North Pole, at a time when NSIDC maps showed the route blocked by 600 miles of ice.
It sounds like the new European satellite Cryostat-2 will provide the desired ice thickness data, without any geographical bias or concern about snow thickness. Speaking as a former amateur explorer, I certainly appreciate and admire the adventurous nature and grit of the Catlin team. However, I don’t see that there is a lot of scientific value to their ice measurement efforts – particularly given their stated disposition towards arriving at a seemingly pre-determined result.
bill (19:55:04) :
“Someone has to design turbines for example. Other have to construct the design, and still more have to erect them. The grid will need upgrading = more jobs.
Renewables will not replace all conventional power stations . But ever kW generated by wind equates to a bit under kW of fossil fuel equivalent that can be passed onto future generations.”
Every dollar spent on wind/solar that produces less energy than the same dollar spent on fossil/nuclear/hydro is some fraction of a dollar that cannot be spent on other goods and services in society.
You end up with the same amount of energy to use but miss out on the other goods and services including the ability to pay for research that will increase energy availability from ever cleaner and more efficient sources.
I’m still hoping that the Bussard/Nebel Polywell fusion reactor will work and produce nett power. It is looking good so far. If it does, all talk of energy shortages will be over. If it doesn’t fission or fission/fusion hybrid with fuel recycling and thorium will do for a couple of thousand years although I doubt the human race will be using it for that long before something better comes along.
With enough cheap and abundant energy you can MAKE “fossil fuels” from CO2 and water.
Here’s a link to the Bussard device
http://nextbigfuture.com/2009/04/inertial-electrostatic-bussard-fusion.html
And this one will make you think about the nuclear “waste” problem.
http://blog.the-thinking-man.com/nuclear-waste-does-not-exist
bill (19:55:04) :
That is a well known economic fallacy. It goes something like this:
A vandal throws a rock through a store window. This creates jobs! Because the glazier will get extra work replacing the window, and he will then have extra money to pay for a new pair of shoes, so the shoemaker will have more income… and so on.
But here’s what really happens:
Because the store owner must pay for the damage, he cannot buy the new shoes his kid needs, so the shoemaker loses business, and can’t paint his shop, so the painter loses income… etc.
When the government forces taxpayers to pay for unnecessary job creation, the same thing happens. The country becomes poorer.
Every new “green” job is the equivalent of throwing a rock through a store window. Businesses and taxpayers are made poorer as a result of not allowing them to direct their money into the most productive uses. Every green job created is created by money confiscated from taxpayers. There is no free lunch.
“Smokey,
Every green job created is created by money confiscated from taxpayers. There is no free lunch.”
I’ve never seen this fallacy so simply and so utterly destroyed. I was watching the Green Channel (sorry) and saw some of the new green workers. I think they were in Chicago. They even had cheerleaders and signs and everything! It was explained that these green workers were cleaning up around the lakefront. Also, they were spreading potted plants on the roof of a three story building! Of course, the plants were first raised in lifts of hundreds of plants to the roof by a crane. Wow they made a green roof!!! What tremendous strides we have made, and soon these gangs, uh I mean groups of our highly educated high school grads will be cleaning lakefronts and spreading out plants all over this great country of ours!!
I have a lump in my throat…
Thanks, Smokey,
Mike
OT: some posters suggest that eco-schemes will rebuild the economy by providing thousands of hi-tech jobs.
This is just a variation on the Broken Windows Fallacy
( http://en.wikipedia.org/wiki/Broken_windows_fallacy )
In this fallacy, a child breaks a window – this provides work and pay for the glazier – who then buys bread off the baker and shoes off the cobbler, who each buy …..
Maybe if we all decided to walk round in lead divers’ boots: it would provide jobs for the boot makers, jobs for floor repairs, jobs for physiotherapists, ….
A brick through a window creates work but does not provide a benefit – therefore not the same as building renewable energy sources benefit = kWh.
Have thorium reactors produced commercial power yet? India has one I believe.
Bill,
if the “renewable energy source” is less efficient and more costly than fossil fuels over it’s expected lifetime, then it IS breaking a window.
If you have to put a gun to my head in order to get money from me for a green project, I can almost assuredly guarantee that such is NOT efficient, and thus represents a wast of resources that decreases economic growth.
I.e. it’s a “broken window”, Dude.
The last few posts are funny. I was going to comment that bill would claim that building inefficient renewable power plants wasn’t breaking windows and there he comes in right on schedule.
As for thorium reactors not producing commercial power yet, that is because there was an incentive to use uranium and that path was chosen. There don’t appear to be any fundamental reasons why thorium reactors won’t produce commercial power.
I would also argue that renewables (other than hydro) have yet to produce any commercial (unsubsidised) power to the grid and are not likely to in the foreseeable future.
The only “commercial” power they produce is if you are off grid and need to rely on them and you’ll find the price per Kw-hr is very high and your personal economic circumstances are poorer as a result..
It’s a “broken window” , Bill.
Speaking of renewables. While watching the AGW channel, er Weather Channel, they were talking about wind turbines. And I learned that not only do they not produce if the wind doesn’t blow (duh), but have to be shut down if the wind blows too hard. If I recall correctly they have to be shut down if winds reach 50 mph. Not very practical if you ask me.