Jai: And if the curve continues exponential growth through 2060 (12 doublings) it'll be many times that. If it continues until 2100, the Earth will collapse into a black hole made of water * My point is that melting the entire Greenland ice sheet would cause about 7 metres of sea level rise. Unconstrained extrapolation of exponentials is insanity. * Well, maybe not. I wonder how long it would take?

... and when it turns out to be one of those weather systems that has little or no effect on the melt, or even apparently increases extent, it's the most spectacular own goal in the history of ever. Not to mention the fact that there are only half a dozen or so idiots worth mentioning, so the list runs out in a couple of years and you have to start reusing names.

For me, about 5 years of experience following Arctic sea ice. I don't get deeply into the mathematics or do any modelling myself, so I count as a 1 in your system. However, I think you're missing something here, which is that not all mathematicians, models and arguments are equal. I am a professional scientist, my main field being molecular biology, and the reason I don't do the modelling myself is because I know the limits of my ability. Regrettably, I have seen several extensive "modelling" posts on this blog and others by those with no such self-awareness, proposing quite unphysical models. The Dunning-Kruger effect applies to fake-skeptics and alarmists alike.

Wayne: Thanks for clarifying that you weren't talking about net melting - that makes more sense now. I have no idea how well the buoys detect the active thaw/refreeze zone at the base of the ice. P-maker: You're a loony. Buckets and spades? Even leaving aside the sheer implausibility of the scale of it (work out how much you can reasonably expect a man to shovel in a day, what area that equates to, and how many men you'd need to cover the ablation zone), it's a totally wrong-headed endeavour. What will you do with the snow you scoop up? You can't just move it to a different part of Greenland, you know. That wouldn't achieve anything. You'd have to take it somewhere else, God knows how many gazillion tons of it. And then refrigerate it forever, using God knows how much energy to do so, because if you just let it melt, that itself constitutes a significant mass loss off the sheet. And what of the heat generated by the men on the ice sheet, and the machines required to cart the snow away? "Just sweep up the dirty snow" my sweet aunt Fanny. Honestly, I'd expect more sense from any seven year old.

To make the same point another way: the mean and the median of the crowd-sourced estimate are both currently around 2.9, which means that Hydrologist is less of an outlier than the folks predicting zero.

No need to throw out data, it's irrelevant given how many estimates are already in. Let's say he's 4 million above the consensus (which he isn't). With ~100 guesses already in, this one wild guess will only skew the average up by 0.04 M km^2, which is completely irrelevant. If you look at the graph, it's pretty symmetrical: the tail at ~5M balances the tail at zero. Both are equally unrealistic.

Hi Wayne - could you let us know how you are observing this underside melt, and where you're based? This melt doesn't seem to be visible to any of the sonar buoys across the Arctic. http://imb.crrel.usace.army.mil/newdata.htm All of these show that the ice is is still thickening slowly, including 2013A which is quite far south within the Canadian Archipelago. The only exception is 2012L, which is in ~3.4 metre thick ice and so further thickening by congelation is not really expected.

~4 million. Predicting this far out is a crap-shoot though, I'm just going for somewhere between 2012 and 2007 on the grounds that we have an accelerating downward trend, but two record years in a row seems unlikely.

we can pull out the properties of the exact pixel corresponding to the buoy. No you can't. You can pull out the properties of maybe the 100mx100m area immediately around the buoy, most likely substantially less. This is an insignificant fraction of the corresponding pixel.

Just to whet your appetite, buoy 2012L is not reporting a single centimeter of ice gain or loss this whole time. It's stayed 3.35m thick. If this holds for a lot of other buoys, I'm planning to hit the trash button on a whole lot of model papers, products and posts. Why would you do that? It's well known that once you get past about 2 metres thick, ice doesn't thicken much more by accretion. Instead it thickens by ridging, over-riding and slabbing. If any of that happened to the ice the buoy was embedded in, it would destroy the buoy! It's therefore not surprising that none of the buoys are able to observe it. The models operate at a much coarser resolution: each pixel represents the average thickness across many square miles of ice, and thus averages together the ridges and the areas between the ridges.

Aircraft contrail.

Lodger: That's not how the averaging works. It's not that they get a readout once each day and then average two successive days together. Rather, they take all the swaths from the last 48 hours, combine that into one picture and then calculate the extent. Some parts near the Pole will have been scanned dozens of times during that period, others will have been scanned only once - I don't think one day's data is quite enough to image the the whole Arctic, and even if scanned twice some parts will have been obscured by cloud in one or other of the scans. Moreover, the pattern of exactly which areas have been oversampled / undersampled on each day will vary from day to day. Trying to extract a "single day's worth" of data from the final product is absolutely futile endeavour, and would be meaningless even if you could do so.

Final pedant point: there is no "release of energy from the fragmentation of the meteor". Fragmenting a rock takes energy, as you'll appreciate if you've ever hit one with a hammer. The energy source is the stored kinetic energy of the rock, which gets dumped into the upper atmosphere as heat energy and compression.

Putting it another way, "sonic booms" are (the sound caused by) a particular subset of "shockwaves". Your definition restricts it simply to being the bow waves from objects moving faster than sound. I'm using it to mean any shockwaves caused by objects moving faster than sound, including those caused by frictional/compressive heating of the atmosphere. Wikipedia doesn't help: either of those definitions is technically valid. Either way, it's a bit of a sterile debate. Changing the words doesn't alter the physics, which we all agree on anyway!

I think we're at major league cross purposes here. I'm not denying that the object fragmented explosively, causing a shockwave. The point is that this is still simply a conversion of the kinetic energy of a moving object into sound/pressure waves, i.e. the same process involved in generating a "normal" sonic boom. There was no extra input of energy (e.g. chemical or nuclear energy in a bomb, electrical energy in a thunderclap), just the kinetic energy of the incoming rock.

the shockwave wasn't due to sonic boom This is meaningless. A sonic boom is a shockwave, and vice versa. It simply means that something is travelling faster than the speed of sound, and so the compression waves all stack up on top of each other. In the case of an explosion, the thing travelling faster than sound is the expanding hot gases from the explosive reaction. In the case of an aircraft's sonic boom, it's the bow waves from the aircraft's movement. In the case of the meteor, the movement is sufficiently fast to heat the gas surrounding the bolide, so you get both processes going on.

You have to think of it in terms of ice cohorts. 2007 was a record summer melt, which means that a record amount of new 1st-year ice formed during the winter of 2007-2008. This ice became 2nd-year ice in winter of 2008-2009, 3rd-year ice the following year, and so on. Naively therefore one would expect a substantial increase in 5th year ice in December 2011. However, this didn't come to pass. There are two reasons for this. Firstly 2008 was also a huge melt, so this cohort did particularly poorly in its first year. Secondly, this cohort also got slaughtered in summer 2010, and failed to mature from 3rd year to 4th year ice. In contrast, the ice that formed during winter of 2008/2009 was followed by the comparatively weak 2009 summer melt: i.e. proportionally more of this cohort survived its first year, and has subsequently matured. It shows as a "bulge" of 2nd year ice in December 2009, 3rd year ice in December 2010, 4th year ice in December 2011 and now it has just tipped into the 5+ bracket. Steven Goddard has been obsessively following this and touting it as a "recovery" of multi-year ice depite the fact that it's the only cohort of ice to show any improvement. All other brackets are down dramatically. What I'd love to see would be a series of graphs showing the absolute area of each ice cohort from formation onwards.

Sorry Al, but that plot of PIOMAS vs troposphere temperature is just linear regression in disguise, since the temperature itself also has a trend. You might just as well plot PIOMAS against my house price and discover that ice volume will hit zero when the house price hits £350k (or whatever).

In fact, tell you what, why don't you tell us as soon as you have something that you can drop from 5 feet onto a frozen water butt, and have it drill through and measure the thickness. Come on, time's a-wasting!

I propose herewith to invent cheap mass produced air droppable floating ice probes, which drill automatically a sensor stick through the ice and keep track and transmit ice & snow thickness, salinity, temperature, radiation balance. Then spray the ice sheet with thousands of them. Wow! What an amazing idea! I wonder why nobody's thought of it before! Please tell us as soon as you have a working prototype!

P-maker: They are clearly open water, i.e. polynyas. They are far too dark to be melt pools. Melt pools over sea ice look greyish or blueish in visible wavelengths - you can see a margin of ponded sea ice all round the edge of the glacier tongue, which looks completely different to the polynyas. It's grossly insulting to label work from peer-reviwed, pulished scientists as a "fairy tale" based on nothing more than your own boundless ignorance and gut feelings. The quality of commentary on this blog really has taken a catastrophic downturn this year. I hope it clears up by next year or I'll just write it off like I do for YouTube and other such sinkholes.

L1 libration point is unstable, so you can't use ordinary chaff - the radiation pressure from absorbing/reflecting light would push it out of the L1 orbit. You need to use a transparent sheet that refracts light in all directions ~equally. That cuts down the direct transmission (i.e. you shade the Earth) while keeping the radiation pressure to a manageable level (i.e. the shade stays put). Even then, you still need computer-controlled guidance to keep it in orbit. You're not going to be able to manufacture something that specific from whatever randomly happens to be the composition of the asteroid you land on. It basically boils down to the fact that trying to get something to stay at the L1 libration point is equivalent to balancing a pencil on the pointy end.

How about dredging the sea bottom? How energy intensive is that? I have no idea, but I can tell you it takes a lot more energy to dredge up a cubic metre of gloop from the sea floor than it does to siphon off a cubic metre of water from the sea surface. And you still have the same problem of where to put it. Filling Death Valley with seawater or with sea-bottom mud really doesn't make a lot of difference, from that perspective.

http://www.pnas.org/content/103/46/17184.full.pdf+html Requires us to put 20 million tons of stuff into orbit, which in turn requires us to cut the cost of doing so by three orders of magnitude. Oh, and the L1 libration point is unstable, so you can't just use chaff, you have to use spacecraft with onboard computer controls and adjustable elements so they can move themselves around by radiation pressure. 16 trillion of them.

Oh come on people, scale! Saying the ice will damp down waves in the surrounded area is like saying there are no waves in the Mediterranean because of the surrounding land. The size scale for wind/wave dynamics is smaller than a single pixel on satellite maps.