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Arctic sea ice melt - natural or man-made?

Posted on 9 June 2008 by John Cook

Arctic sea ice has declined steadily since the 1970s. However, the 2007 summer saw a dramatic drop in sea ice extent, smashing the previous record minimum set in 2005 by 20%. This has been widely cited as proof of global warming. However, a popular mantra by climatologists is not to read too much into short term fluctuations - climate change is more concerned with long term trends. So how much of Arctic melt is due to natural variability and how much was a result of global warming?

The long term trend in Arctic sea ice

Global warming affects Arctic sea ice in various ways. Warming air temperatures have been observed over the past 3 decades by drifting buoys and radiometer satellites (Rigor 2000, Comiso 2003). Downward longwave radiation has increased, as expected when air temperature, water vapor and cloudiness increases (Francis 2006). More ocean heat is being transported into Arctic waters (Shimada 2006).

As sea ice melts, positive feedbacks enhance the rate of sea ice loss. Positive ice-albedo feedback has become a dominant factor since the mid-to-late 1990s (Perovich 2007). Older perennial ice is thicker and more likely to survive the summer melt season. It reflects more sunlight and transmits less solar radiation to the ocean. Satellite measurements have found over the past 3 decades, the amount of perennial sea ice has been steadily declining (Nghiem 2007). Consequently, the mean thickness of ice over the Arctic Ocean has thinned from 2.6 meters in March 1987 to 2.0 meters in 2007 (Stroeve 2008).

 

Global warming has a clearly observed, long term effect on Arctic sea ice. In fact, although climate models predict that Arctic sea ice will decline in response to greenhouse gas increases, the current pace of retreat at the end of the melt season is exceeding the models’ forecasts by around a factor of 3 (Stroeve 2007).

 


Figure 1: September Arctic Sea Ice Extent (thin, light blue) with long term trend (thick, dark blue). Sea ice extent is defined as the surface area enclosed by the sea ice edge (where sea ice concentration falls below 15%).

What caused the dramatic ice loss in 2007?

The sudden drop in sea ice extent in 2007 exceeded most expectations. The summer sea ice extent was 40% below 1980's levels and 20% below the previous record minimum set in 2005. The major factor in the 2007 melt was anomalous weather conditions.

An anticyclonic pattern formed in early June 2007 over the central Arctic Ocean, persisting for 3 months (Gascard 2008). This was coupled with low pressures over central and western Siberia. Persistent southerly winds between the high and low pressure centers gave rise to warmer air temperatures north of Siberia that promoted melt. The wind also transported ice away from the Siberian coast.

In addition, skies under the anticyclone were predominantly clear. The reduced cloudiness meant more than usual sunlight reached the sea ice, fostering strong sea ice melt (Kay 2008).

Both the wind patterns and reduced cloudliness were anomalies but not unprecedented. Similar patterns occurred in 1987 and 1977. However, past occurances didn't have the same dramatic effect as in 2007. The reason for the severe ice loss in 2007 was because the ice pack had suffered two decades of thinning and area reduction, making the sea ice more vulnerable to current weather conditions (Nghiem 2007).

Conclusion

Recent discussion about ocean cycles have focused on how internal variability can slow down global warming. The 2007 Arctic melt is a sobering example of the impact when internal variability enhances the long term global warming trend.

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Comments 501 to 529 out of 529:

  1. Re: Marc Morano I read articles on http://www.climatedebatedaily.com/ I don't look at the author's names, nor do I care. I read the latest posts bu BOTH sides (left and right columns) to see what the argument is. If you say this guy has an agenda, fine. Hansen has an agenda. Gore has an agenda. If I look for who has an agenda I will have nothing left to read. And yes Pat, good luck on the midterms. So far you have demonstrated an open mind but stick to the "consensus" views for the tests. Your teachers tend to be less open minded.
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  2. My teachers have not demonstrated any such lack of open-mindedness, but both they and I balance that with reasonable skepticism. Past tense, because I'm actually not currently enrolled in formal education (sorry for the wrong impression). Not that you should just trust people per se, but an agenda driven by awareness of a problem is not quite the same as an agenda that drives one to find a problem.
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  3. Sorry, I spoke from my college experience. Two "by the book" professors lowered my cum so I only graduated cum laude by giving me Cs. At least one of them apologized and admitted his error. The other one was an idiot that could not teach, most of the class flunked. I agree with you that it depends on the agenda. What do you think is the Green agenda is by trying to paralyze the country by destroying the energy giants?
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  4. sigh...:| There is an understandable and even justifiable amount of anger regarding Exxon's influences, but the policies being pursued by the mainstream green movement are not for the purpose of retribution; at the core, they are to correct an externality and stimulate innovation. There are some disagreements among fellow environmentalists/innovators/liberals/etc (and not aligned by group per se) regarding whether to cap-and-trade or tax-and-credit (or cap-and-dividend (Hansen), or tax-and cut other taxes, or cap-and compensate (for climate damages), etc.) - my concern is that, while I support some measures to cushion the economic shocks (ease into a tax rate (start low)), maybe aid for the poor and for some others who will be initially hit harder because of greater dependence on coal, oil, gas - but especially coal, - it concerns me that this may be seen as some permanent part of the solution. Ultimately, the tax/cap and trade portions of policy are supposed to work through market mechanisms and allocation of this or that to specific industries, regions, etc, will only muddy the price signals (if it is more efficient for industries to move to the sun belt to get cheaper energy, then maybe they should move there, and not be subsidized for staying elsewhere). This may be less of a problem for helping the poor (individuals and countries) specifically (increasing returns? - it might make them more energy efficient - but so long as the aid is in money back or some equivalent and not in a reduction in emissions taxes. That's not to say that it doesn't make sense to allow developing countries some grace period before being expected to join policies as full equals. See my referenced comments at 196 at http://www.skepticalscience.com/volcanoes-and-global-warming.htm ).
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  5. You should get back to reality and get away from the conspiracy theories, Quietman. There aren't any green helicopters out there to watch you.
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  6. Philippe Conspiracies are covert. The green socialst threat is quite visible. This red/green indoctrination is NOT environmentalism, it is overt anti-capitalism, plain and simple that actually started in the late 1960s. The ones that yell the loudest are the absolute worst true polluters. The home of the green movement is in california, the dirtiest state in the U.S. and home of the biggest hypocrites. But CO2 is NOT a pollutant. Cap and Trade, that's just a money maker for crooked politicians. So where is your conspiracy?
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  7. "Making Carbon Markets Work (extended version) Limiting climate change without damaging the world economy depends on stronger and smarter market signals to regulate carbon dioxide" David G. Victor, Danny Cullenward http://www.sciam.com/article.cfm?id=making-carbon-markets-wor "Climate Change Is Happening, Effects Will Be Severe, Now What Will It Cost to Fix It? Could it be true that staving off the severe effects posed by climate change won't impose ruinous costs? The IPCC thinks so" David Biello http://www.sciam.com/article.cfm?id=climate-change-happening-effects-severe-what-cost-fix-it "A Solar Grand Plan By 2050 solar power could end U.S. dependence on foreign oil and slash greenhouse gas emissions" Ken Zweibel, James Mason, Vasilis Fthenakis http://www.sciam.com/article.cfm?id=a-solar-grand-plan “Engineering Silicon Solar Cells to Make Photovoltaic Power Affordable” Steven Ashley http://www.sciam.com/article.cfm?id=engineering-silicon-solar-cells My comments 187, 200, 225, 229, 236, 254, 317, 322, 338,...(a few more in the pipeline) at: http://www.realclimate.org/index.php/archives/2009/03/olympian-efforts-to-control-pollution/langswitch_lang/tk And 324, 334-336, 338, 368, 387 and 388 (skip PART II if you want to), 393, 398 (and comments surrounding that by others), 412 (just entered, may not appear and might not be >412) at: http://www.realclimate.org/index.php/archives/2009/03/advice-for-a-young-climate-blogger/langswitch_lang/tk
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  8. "The green socialist threat". Wow, I wish I had that line when I was a kid and my mom made me do the dishes. I would'nt have had to wash another dish. Of course, my sisters would have done the same thing, and then mom would have decided that since no one else was washing the dishes, she wouldn't either. And my dad? Please! By this time however, we'd all be dead from food poisoning, so problem solved!
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  9. "The green socialist threat". Wow, I wish I had that line when I was a kid and my mom made me do the dishes. I would'nt have had to wash another dish. Of course, my sisters would have done the same thing, and then mom would have decided that since no one else was washing the dishes, she wouldn't either. And my dad? Please! By this time however, we'd all be dead from food poisoning, so problem solved!
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  10. Quietman says, "CO-2 is not a pollutant". I'm dissapointed. I would think that someone as obviously intelligent and educated as you are (no sarcasm) would'nt have to resort to the most simplistic and ignorent "arguements" against the MMGW concensis curculating around. No one is argueing that point. Like no one is argueing about whether plants need CO-2 to live. It's nothing but a manufactured arguement. You can't argue the actual facts of this subject, so you have to make up something to maintain the illusion of a debate. Also, you might want to ask Jim Lovell what he thinks of that claim, concidering that he's had actual experience dealing with CO-2 'pollution.
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  11. Same with your 'tectonic plate' arguement. You just made that up so you could keep "debating".
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  12. Quietman you're misunderestimating the skill of the conspirators. They're really really smart. Therefore, they know that the best way to destroy capitalism is to have capitalism destroy itself, by letting it go unchecked. The "green socialist threat" is nothing but a decoy. The real conspirators have been sitting in Wall Street and K Street (and the SEC?), encouraging always more deregulation, less regulation and weak enforcement of what little exists. Their plan is working like a (swiss) watch. It has already produced socialized banking. Not only the helicopters are black, after all, but they're also coporate...
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  13. Rossby Wave Wrap up again: So, if there is (diabatic) warming in one region, relative to pressure coordinates, this will displace isentropes downard in that region. The changes in Rossby wave propagation will then come from two approximately seperable effects: 1. for the given variation of f (approximately unchanging (for these purposes) over time periods less than millions of years) and wind distributions and vertical stability distributions in pressure coordinates, the wind and stability (and variations - horizontally and, if the regional heating varied in height except in certain ways, vertically) will change in isentropic coordinates due to the shifting of isentropic coordinates. 2. The vertical wind shear in pressure will tend to change in response to changing horizontal temperature gradient, and a vertical variation in warming tends to change vertical stability. ---- It occurs to me that diabatic fluctations in temperature offer another way for some wave activity to leak through barriers to propagation - by transfering air mass up and down through isentropic surfaces - of course, that can also change IPV. But if one uses material surfaces...
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  14. Lee I don't understand 508/509 so I'll ignore it. 510 I do understand. I worked in an emissions testing lab for 10 years before going out in the field. According to the Federal Register the goal is to limit CO, NOx and HC by converting it to CO2 and water vapor as the end product of combustion. Why would we intentionally produce pollution? The answer is that it is not pollution and is a very weak GHG. May I suggest a quick read of this A sensitive subject Nature Reports Climate Change Published online: 30 April 2009 | doi:10.1038/climate.2009.41
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  15. Quietman - CO, and I suspect, HC, will oxydize to CO2 anyway; but CO and HC are themselves undesirable in the air (above some limits). The reasons that these and NOx are called 'pollution' is not mainly from any direct radiative forcing but from more direct biological effects (NOx -> acid rain -> not good for plants, statues, etc.). On a per atom C basis, CO and HC may be worse for the environment than CO2 (but when those C atoms are fossil C emissions, they ultimately add CO2 to the atmosphere anyway), but this does not in any way imply that massive anthropogenic CO2 emissions do not have serious consequences. "According to the Federal Register the goal is to limit CO, NOx and HC by converting it to CO2 and water vapor as the end product of combustion. Why would we intentionally produce pollution?" 1. So what if the policy at one time ignores global warming risk? Once upon a time, there were few restrictions on smoking and ciggarette advertising - why would people not restrict an unhealthy product? Well, it is unhealthy, and (libertarian objections aside) now we have restrictions. 2. There are many substances or other things (heat, sound) that are present naturally at some level, but can be harmful if found in significantly higher concentrations. Some things (so far as I know, Hg, some artificial substances, etc.) are not beneficial to most or all organisms at any concentration, but there is some (generally very low) background level that can be tolerated. Some substances (Pb, Se) are benificial or even necessary for some or most organisms in small or trace amounts, but will generally be harmful in higher quantities. UV radiation can work as a natural disinfectant, produce hyroxyl radicals in the atmosphere that help cleanse the atmosphere of some substances (though in the short term it can produce photochemical smog when other pollutants are present), and it helps our bodies make vitamin D (though some of us do not need this because of modern food processing); but UV radiation can also cause cancer and fade colors, etc. Generally people obviously enjoy and benifit from light and sound, some level of heat, etc, but there can be too much (or too little). There are such things as light, sound, and heat pollution (they can have environmental effects as well). Early in earth's history, HCN and H2S may have been very important to life, while free O2 was toxic, but for most ecosystems today, the reverse is the case. Rain is naturally a litte acidic, but acid rain is problematic. Some places may naturally have different geochemical and geophysical conditions that only a small subset of organisms are adapted to - they may thrive there, but this doesn't imply such conditions should be artificially replicated in many other places. Life has and will adapt to changing conditions, including changing CO2 levels and their climatic effects, but evolution can only go so far so fast; large sudden changes into relatively unfamiliar conditions (as measured by time since last occurence) are costly; either such an increase in CO2 or such a decrease (unless immediately following such an increase - and even then...) can/will be problematic; I don't know what such a decrease would be called, but in the context of such an increase, additions of CO2 from outside a balanced cycle can be considered pollution.
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  16. "very weak GHG" A milligram is not much mass. A trillion milligrams = a billion g = a million kg = a thousand metric tons. Massive.
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  17. Patrick Considering that was a pro-AGW article in the "call for actions" for the greens columns, you don't seem very receptive.
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  18. Re 517 - I haven't gotten around to reading it yet; not sure what you mean...
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  19. I've now read it. I still don't know what you mean...
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  20. Patrick Re: "A trillion milligrams = a billion g = a million kg = a thousand metric tons" A lot for you, nothing to the Earth. Everything is relative. The surface is 70-75% water. Water controls the climate it just so many ways (but it's the sun that gives the oceans most of it's heat potential). Re: "I still don't know what you mean... " The science isn't settled. Re: "1. So what if the policy at one time ignores global warming risk? Once upon a time, there were few restrictions on smoking and ciggarette advertising - why would people not restrict an unhealthy product? Well, it is unhealthy, and (libertarian objections aside) now we have restrictions." Because it's not a pollutant, it's perfectly safe mixed into the air we breathe and required by plants, a part of nature's carbon cycle. We have known about GHGs and CO2 as a GHG for over 100 years so you don't seem to realize that real scientists know it's not a problem. Never has been and never will. It's O2 that can be a bugger.
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  21. "A lot for you, nothing to the Earth. Everything is relative. " It was an analogy, Quietman. There are actually over 700 billion tons of C in the atmosphere in the form of CO2. Anthropogenic emissions could easily double atmospheric CO2. On a per molecule basis, CH4 is certainly a stronger greenhouse gas than CO2, and CO2 may be stronger than H2O vapor, but there's a lot more H2O in the atmosphere than CO2 (although there are variations in concentration such that H2O has less effect than it would if it were well-mixed), and much more CO2 than CH4. Etc. There is much less of any of these than there are N2 and O2, but N2 and O2 have little effect on LW radiation. Of course, the per molecule strength of a gas depends on overlap with other gases and with itself - these things depend on the concentration; for example, the radiative forcing of CO2 is roughly logarithically proportional to CO2 amount within a range of values. ------ "The science isn't settled." Depends on which part of the science. I never claimed that ever last piece of the puzzle had been fit together. What we know: Human activity has caused a dramatic and rapid increase in atmospheric CO2 and is still adding to that, pushing the atmospheric CO2 level to well above levels seen for hundreds of thousands of years. Human activity has also added other greenhouse gases. The mechanism of radiative forcing of climate is settled. Changes in greenhouse gas levels have made important contributions to climate change in the past, both as externally forced changes (volcanic activity and other geologic emissions, tectonically/mineralogically forced changes chemical weathering, changes in the C cycle due to biological evolution) and as feedbacks (orbitally-forced glaciations/deglaciations and monsoon changes, changes in solar TSI over 100s of millions of years, biological evolution, other climate changes). Water vapor and snow and ice albedo are important positive feedbacks. The best estimate of climate sensitivity from physics, paleoclimate, and the historical record is about 3 deg C per doubling of CO2 (or its radiative equivalent, adjusting for efficacy of different forcing agents), give or take roughly 1 deg C or so. Climate sensitivity from the paleoclimatic record can be problematic because climate sensivity could vary as a function of climate itself (and perhaps geography/geology and biological evolution); the climate sensitivity determined by greenhouse gas changes between preindustrial time and the last ice age may be larger than the climate sensitivity now because there is less ice sheet area available to melt/disintegrate and the snow and ice is more confined higher latitudes; however, Hansen's calculation of a 3 (+/- 1 ?) deg C for the radiative equivalent of a doubling of CO2 is actually calculated from the radiative forcing of greenhouse gases, ice sheets, aerosols, and land albedo changes from vegetation, and thus the climate sensitivity to greenhouse gas forcing and other anthropogenic forcings could be greater than that, as any ice sheet and vegetation responses would be feedbacks in AGW context. (Off hand I am not sure if sea ice albedo was included in Hansen's forcings; I think it was not. I inferred that nnow cover, along with water vapor and clouds, were treated as feedbacks. Snow cover would have extended to lower latitudes in the ice ages, but some higher latitude snow cover would be replaced by ice sheets, and the total area of seasonal snow might actually have been less (??), although it would have been at lower latitudes...)); however, there is evidence for a relationship even farther back in time than the last several ice ages, going back hundreds of millions of years; the Earth has been both warmer and cooler in the past. A persuasive case that there is some strong negative feedback missed by climate models is lacking. There are also both largely settled and unsettled aspects to what climate change means on the regional level. Greenhouse effect-driven warming will tend to warm the surface and troposphere but cool the upper atmosphere; solar forcing tends to warm both or cool both; volcanic aerosols can cause warming in the stratosphere while cooling the surface and troposphere. Any surface and tropospheric warming will tend to be enhanced in the mid-to-upper troposphere at lower latitudes (due to a moist adiabatic lapse rate feedback) and at higher latitudes near the surface (due to snow and ice albedo feedbacks and perhaps alsob because of the relatively larger vertical static stability in the air)(except, at least at first, in the Southern hemisphere due to the Antarctic Ice Sheet's stability, cold upwelling water driven by winds, and the dominance of water and lack of land in southern midlatitudes). Sea level will rise from thermal expansion of water and from melting glaciers and land ice (and not just until 2100); it will not be perfectly the same everywhere because regional sea level variations are caused by wind and temperature variations. Generally, a greater portion of precipitation will come in shorter time intervals at any given location. There is some expectation that midlatitude storm tracks will shift poleward, with drying trends on their subtropical flanks. Depending on how much more moisture the soil can hold in the spring from winter melt, there may be significant drying in midlatitude continents in summer in particular if extra precipitation in the winter is lost to runoff. There are significant costs to adapting to large, rapid, sustained changes into relatively unfamiliar conditions; if taken far enough, such change leads to mass extinctions. Ecosystems are stressed by such changes. The economy is an ecosystem and it depends in part on the larger natural ecosystems. Human society is obviously capable of rapid evolution, but this can still involve much pain, and that can be amplified by an evolved expecation of modern comforts, as well as population growth and politics (somewhat unlike in the stone age, it is not a simple matter to pack up one's belongings and just migrate; even if you are poor enough to carry all you own, borders and property rights get in the way). Buildings and infrastruture have been designed for conditions and will need adjustments. Agricultural productivity will decrease in the tropics; it may increase at first at higher latitudes, but only up to a point; tropical conditions are not kind to some valued food crops; the growing season cannot get any longer than a full year and growing season quality is important. Some crops are photoperiod sensitive. Breeding new crop varieties takes time and effort. There is great concern about fresh water availability as glaciers melt (regularity is important, not just total amount). Warming could increase risk and spread of some tropical diseases. Loss of biodiversity is a cost; biodiversity is a resource for new crops, drugs, etc. There is a great concentration of people near sea level. Uncertainty in climate change could itself be a cost because uncertainty hinders planning (there will always be uncertainty in the future climate, human effects or not, but presumably the total uncertainty is greater when there remains natural fluctuation and uncertainty in climate response to anthropogenic forcing). Even if adaptation were easier than mitigation (although it is not an either-or issue but a question of how much of each), it makes sense that the benificiaries of climate-change causing activities should pay the costs of climate change (the market response to that imposed price signal would tend to favor mitigating economic pathways - efficiency and clean energy, etc.). Even with various levels of uncertainty, there are actions for which an actionable level of inteligence will have been met. Even decades ago, it made sense to invest in clean energy and energy efficiency technology, at least to have it available to deploy with sufficient pace, as an insurance policy for the risk that global warming would be a problem. We have reason now to actually impose a price signal on emissions (or some other policy) to shift the economy towards greater demand of and investment in clean energy and energy efficiency as well as other emissions-reducing pathways. I do agree that there is room for debate about how large that price signal should be, because the science is not FULLY settled (it is likely it never will be FULLY settled).
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  22. Re: "We have reason now to actually impose a price signal on emissions (or some other policy) to shift the economy towards greater demand of and investment in clean energy and energy efficiency as well as other emissions-reducing pathways. I do agree that there is room for debate about how large that price signal should be," And there is where the problem resides. In countries where the civilians are disarmed it may work but it will not worked in truly free countries. This is why the gun grabbers have been trying to impose gun laws that allow them to disarm as many of the populace they can. And they already know that our military will not forcibly disarm Americans (they asked the troops if they would be willing to exactly this about 10 years and they plain out refused). This is a built-in check to keep our government from imposing laws against the will of the people and subvert the constitution. So they have to tread softly on such issues and try to get the populace to actually believe that this is for our good and not making headway. If they force the issue it will get very ugly very quick. This is why this is a political hot potato and needs to be approached with unassailable science that everyone can accept, and they can not do that.
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  23. "This is why this is a political hot potato and needs to be approached with unassailable science that everyone can accept, and they can not do that. " There is no such thing - unassailable science that everyone can accept. In recent weeks I've witnessed a case of a person who cannot accept simple arithmetic. We need good science. We already have a lot. We shouldn't stop trying to get more, but we needn't wait to use what we have in appropriate ways. However good the science is, there will be people who cannot accept it. I don't really care anymore what they think - I hope those of us living in reality are a strong enough majority, and those who stubbornly bury their heads in the sand while feigning self-righteous protest - they will just have to live with the decisions the rest of us make. Should I be so optimistic as to expect a room of adults to agree that the sun is a star?
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  24. Patrick " 523. Should I be so optimistic as to expect a room of adults to agree that the sun is a star?" Maybe Not, LOL. Some think the Earth was created 6,000 years or so, and rely on prophets of Doom and Death. The wackos expect the Earth to end in 2012 and Hansen says were are "Toast". With wackos like this what can we expect.
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  25. You do realize Hansen does not actually mean that we are slices of bread that have been exposed to heat emanating from nichrome wires, don't you?
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  26. NOTE I'm now using @ as a substitute partial derivative symbol for ease of typing and readability. Correction/clarification: For a reference frame that rotates with the planet (or star, etc.) u = x component of velocity v = y component of velocity For flow patterns along surfaces (in x,y,z coordinates, surfaces of constant z; otherwise, for x,y,p coordinates, surfaces of constant p (isobaric surfaces), or for x,y,q coordinates, where q is potential temperature, using surfaces of constant q (isentropic surfaces), and where 'vorticity' refers to the vertical component of the vorticity vector, the vorticity is equal to the sum (two horizontal dimensions) of variations over horizontal distances of the horizontal component of flow perpendicular to those direction; RV (relative vorticity) = @v/@x - @u/@y, f = planetary vorticity = the coriolis parameter = the vertical component of vorticity associated with planetary rotation, AV = absolute vorticity = RV + f where horizontal distances x and y are the horizontal components of distances along the surfaces within which the vorticity is being determined; when x and y are rotated so that the x direction is locally aligned with the flow, @v/@x becomes the orbital (curvature) vorticity (the vorticity associated with curvature of streamlines), and - @u/@y becomes the shear vorticity (the vorticity associated with changing the wind speed or direction across streamlines) For a pattern of flow that is like rigid solid-body rotation, @v/@x = - @u/@y = 1/2 * vorticity = shear vorticity = orbital (curvature) vorticity = angular velocity of rotation = angular frequency The circulation* (*not to be confused with the more general usage of the term) along a closed perimeter (the integral over length along the perimeter of the component of velocity locally parallel to the contour, taken to be positive in the counterclockwise direction) is equal to the integral of vorticity over the area enclosed. Circulation = line integral of (component of velocity parallel to the line in the counterclockwise direction) area-average vorticity = circulation/(area enclosed) If the perimeter follows a closed loop streamline (streamlines are parallel to the wind velocity), then the average of the counterclockwise speed over the length of the streamline is equal to the area-average of vorticity within the streamline multiplied by the area and divided by the length of the streamline. Circular streamlines will be centered on a circularly-symmetric vorticity field (which maintains symmetry out to infinitiy) if the flow is nondivergent (streamlines can only be assigned to a nondivergent component of the flow). For a circular streamline of radius r: Circulation = 2*pi*r * (circumference-average speed) = pi*r^2 * (area-average vorticity) (average speed) = r/2 * (average vorticity) Note that for two different closed-loops that completely envelope a region of nonzero vorticity which is surrounded by a zero vorticity field, the circulation about each loop is the same and hence the average speed (component along the loop) will vary among the loops in inverse proportion to loop length. Hence, the average speed about such loops which are circular is inversely proportional to the radii. If such circular loops are centered about a circularly-symmetricy vorticity field which goes to zero within a radius smaller than the loop radii (and remains at zero to infinity), then the loops are streamlines and the average speed on each loop is the speed at all points on each loop. Outside a circularly-symmetric region of nonzero vorticity, the orbital (curvature) vorticity and shear vorticity cancel each other. (There is a planetary circulation for planetary vorticiy, relative circulation ('circulation' by default) for RV, etc.) Linearly-superpositional vorticity fields and circulation values are associated with linearly-superpositional wind fields. Thus, if the total wind velocity = V1 + V2 ..., then the total RV = RV1 + RV2 ..., etc. Hence, any circularly-symmetric vorticity field has a set of concentric circular streamlines that are contours of that vorticity field's streamfunction (whether or not there are other nonzero vorticity fields at any location); the streamfunctions of multiple vorticity fields can be summed to find the total streamfunction. A particular component of a vorticity field can be called a vorticity anomaly, and a vorticity anomaly has a flow-pattern anomaly (streamfunction anomaly). Within a circular homogeneous RV anomaly, the anomaly flow pattern acts like rigid-body rotation. True that: 1. a point or homogeneous circular RV anomaly is mathematically associated with a wind anomaly which forms circular streamlines centered on the RV anomaly with wind speed being proportional to 1/r outside the RV anomaly, where r is the distance from the center (such a relationship has curvature/orbital vorticity same sign as the RV anomaly which is equal and opposite to the shear vorticity). AND for a homogenous RV anomaly along a line or constatn-wideth band, the velocity anomaly outside the line or band does not vary with distance from the line or band and is everywhere parallel to the line/band. HOWEVER (and this is where I may have made a mistake earlier): For a barotropic PV anomaly (with flat bottom topography) that is either a point-anomaly or has a 'hard edge' (as opposed to a sinusoidal or sufficiently-gradually tapered distribution (?), etc.), assuming geostrophic balance, the velocity field will generally have to decay in magnitude more rapidly with horizontal distance. This is because, for example, a cyclonic PV anomaly must have a low pressure anomaly associated with it. The pressure can't have a sharp jump within infinite geostrophic winds; thus, the pressure must decline gradually going towards the PV anomaly, even in the region of zero PV anomaly; this requires a RV anomaly of opposite sign to surround the PV anomaly (there will still generally be an RV anomaly of the same sign within the PV anomaly, and the circulation along closed loops (closure at infinity for infinite band/line PV anomaly) will still be cyclonic at any distance from the PV anomaly, but will decrease in strength outward from the PV anomaly due to the oppositely-signed RV anomaly required by geostrophic balance and a continuous pressure distribution. In the case of a baroclinic PV anomaly that is either a point or otherwise 'hard-edged' or not-sufficiently tapered, with limited extent vertically and horizontally, similar arguments apply. Along the isentropic surfaces that intersect the PV anomaly, for a cyclonic anomaly in geostrophic balance, the spacing between isentropes must decrease approaching the anomaly even outside the anomaly, so that there is a positive stability anomaly, which requires an anticyclonic RV anomaly outside a cyclonic PV anomaly in order for the PV anomaly to be zero outside the nonzero PV anomaly. Above and below a cyclonic PV anomaly in geostrophic balance, the isentrope spacing anomaly must be negative, requiring a cyclonic RV anomaly, as previously described. Qualitative arguments suggest a pattern with something like a conical shape with upper and lower cones that have vertices at/within the PV anomaly; outside the region of nonzero PV anomaly, for a cyclonic PV anomaly: within the cones, the stability anomaly is negative and the RV anomaly is cyclonic (as is generally the RV anomaly within the PV anomaly), while outside the cones, a ring of anticyclonic RV anomaly surrounds the PV anomaly along with a positive stability anomaly that generally extends through the PV anomaly. The RV anomaly doesn't cause anticyclonic circulation about the cyclonic PV anomaly but to some degree it spatially restricts the strength of the cyclonic circulation.
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  27. The RV anomaly doesn't cause anticyclonic circulation (in the totality of the flow anomaly of the PV anomaly) about the cyclonic PV anomaly but to some degree it spatially restricts the strength of the cyclonic circulation.
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  28. The northern hemisphere sea ice extent graph for this winter/spring is a bit remarkable. For most of the winter the ice was tracking the 2006-2007 extent line quite closely. But instead of plateauing and starting to drop in late Feb. or early March, it just kept going: It will be interesting to see what NSIDC has to say about this in their next monthly update, which should be out soon.
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  29. Ned, i'm no expert so i'll only place my bet: it has something to do with the Arctic Oscillation which became about neutral in March. We'll soon have the response.
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