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Comments 129501 to 129550:

129501. Patrick 027 at 15:25 PM on 31 October 2008
        Arctic sea ice melt - natural or man-made?
        Vorticity: Clarifications: 1. the derivation of vorticity from the wind field, as described above, is based on the wind at an instant in time. Streamlines are also based on the wind at an instant in time; streamlines are everywhere parallel to the wind velocity, and the wind speed is inversely proportional to the spacing of streamlines in two-dimensional flow** (**for non-divergent winds: divergence = du/dx + dv/dy, non-diverence wind implies du/dx + dv/dy = 0; a two dimensional wind field can always be decomposed into an irrotational portion with zero vorticity, and a non-divergent portion which must have any vorticity that exists in the total wind; streamlines, which are contours of the streamfunction, can be defined for the non-divergent component of the wind.) So for example, pure orbital/curvature vorticity corresponds to streamlines that, in the direction locally perpendicular to themselves, are equally-spaced. The streamlines curve; if the vorticity is constant, the streamlines must form arcs that are parts of concentric circles ... No, wait, is there another way to do that? Can constant orbital vorticity correspond to something other than solid body rotation? For a moment I had an idea***... have to think about that - anyway, it's not important for the rest of this. For pure shear vorticity, the streamlines are straight; they're spacing varies along a direction locally perpendicular to themselves. This is at an instant in time, so if the streamlines are changing in time, the motion of the air parcels can vary - they could have trajectories that are straight even where streamlines curve and vice versa, or the curvature could be in the opposite direction. So I was inaccurate when I wrote "Where there is only shear vorticity, the wind is not changing direction." Pure shear vorticity, an absence of curvature vorticity, requires the wind is not changing direction along a streamline at an instant in time, which is a relationship among different air parcels. It is not required for each individual air parcel to not be changing its own direction of motion. 2. I have been so far discussing vorticity as a scalar quantity. It can be treated as such for flow in two dimensions. In general, though, vorticity is a vector. The component of that vector which I've been focussed on is that which is perpendicular to the two dimensions of the flow I've been describing; for horizontal flow, it is the vertical component. If the horizontal flow varies with height, it can/will have horizontal components of vorticity as well, but for introductory purposes, assume the flow is identical at each level so that the only component of vorticity is the vertical component (in the z direction for x,y,z coordinates). --------- So I left off describing an irrotational (zero-vorticity) wind field of concentric circle streamlines centered on some region which contains voriticity. Now for Stoke's theorem: Take an enclosed area. Take the wind velocity at all points along the perimeter of that area. At each point, take the component of velocity that is parallel to the perimeter at that same point. If that component points such that continuing in that direction is counterclockwise around the area, count it as positive, otherwise count it as negative. Now integrate this value along the length of the closed perimeter, stopping where you started (a complete revolution). This is the **circulation** around the enclosed area (it has units of wind speed times length, such as square meters per second). (obviously a somewhat different usage of the word than in the context of 'general circulation of the atmosphere or ocean' or 'the fluid is circulating'...) It turns out that the circulation around such a closed path is equal to the area-integrated vorticity contained within it (this can be proven mathematically). Or in other words, the circulation around an area, divided by that area, equals the area average of vorticity in that area. In three dimensions, that area is on a surface. That surface can be any surface whose edge is on the same path (so if the path is in a single plane, the surface need not be on that plane - it could be a curved surface). In this more general case, it is the component of vorticity locally perpendicular to the surface that must be integrated over the area of the surface to find the circulation, or be averaged over that area to get the circulation be unit area. (PS in general, the vorticity as a vector is equal to the 'curl of the wind vector' which is written mathematically as the gradient operator *cross* the wind vector (somewhat like a vector cross product, except the first 'vector' is an operator; the divergence is written like a vector dot product with the wind vector, but again with that operator as the first 'vector').) Now back to two-dimensional flow. Consider the wind field where the streamlines form concentric circles around a center point, and starting at a distance R from the center, the wind is inversely proportional to the distance from the center. Since the circumference is proportional to the distance from the center (the radius of the circular streamline), the product of the wind speed and the circumference of the streamline it is on is constant over a range of distances. Since the wind velocity is parallel to the streamlines, this means the circulation around each streamline is the same. Which means that the circulation of two different concentric streamlines are the same, which thus means the circulation around the area of an annulus between two such streamlines is zero (the circulation around an area with a hole in it is equal to the circulation oround the outer boundary minus the circulation around the inner boundary, in other words, it is the circulation of the larger area included the hole, minus the circulation of the hole). This means the average vorticity within the annulus is zero. This is true except within the circle of radius R at the center of this structure. Notice that maintaining the same wind field outside of that central circle which must contain all the vorticity, I can redifine the wind field between radius R and radius R2 from the center to again be inversely proportional to the radius, with the same wind speed at R, and then all the vorticity is concentrated into a smaller circle of radius R2. The circulation around this smaller circle must be the same as that around the larger circle. Thus the wind field outside this central circle doesn't depend on the size of the cirle, only on the circulation of the circle and thus the area-integral of vorticity within it. A point vortex can be defined, which has infinite vorticity at a single point, but only some finite circulation, and this would correspond to a wind field with concentric circular streamlines, with wind speed inversely proportional to radius, and the wind speed at a reference radius R0 being proportional to the circulation 'possessed' by the point vortex. Will have to continue later; but what is coming is an illustration that the wind field can be reconstructed by it's vorticity distribution. (At the same instant in time, anyway, although under some conditions vorticity is conserved following the motion, so that ... etc.)
129502. Patrick 027 at 13:01 PM on 31 October 2008
        Arctic sea ice melt - natural or man-made?
        "This is getting interesting." Great! "Where are you getting this information from?" physics, math, college courses, and textbooks, the last including: "Introduction to Dynamic Meteorology - Third Edition" by James R. Holton "Introduction to Geophysical Fluid Dynamics" by Benoit Cushman-Roisin If you wanted to get these, of course you'd want the most recent editions; the third edition of Holton does have an error in Chapter 8 in section 8.2.1 (for a while I couldn't figure out how the math was being done and then I figured out why! But I think the ultimate conclusions may be correct anyway (perhaps the math was originally done correctly and then a few steps were copied wrongly)). As for Cushman-Roisin - a great dynamics book, but be aware the brief description of global warming is not good. -------------- Before going on, a few clarifications: 1. the constant basic-state density with height approximation: The description of gravity waves I had in mind was derived mathematically from equations using a constant-density approximation. In at least some ways this can be a good approximation because and so long as the individual air parcels themselves do not rise or fall so much with height, but obviously a vertically-propagating or perhaps even an evanescent wave will propagate through a greater depth of the atmosphere, and of course the atmospheric pressure and density decrease to a first approximation exponentially with height. Again, I haven't gone through the math entirely for myself, but from Ch 12 of Holton, it seems there's a general tendency, if not exactly than approximately, for vertically-propagating waves of various kinds - (including planetary (kind of Rossby) waves, equatorial Kelvin waves, equatorial Rossby-gravity and gravity waves and (are there vertically-propating equatorial waves that are purely Rossby?)) - to increase amplitude in proportion to the square root of the inverse of density (the inverse of density also known as specific volume (volume per unit mass)). However, I don't think this means the energy or momentum flux is increasing with height, at least not just from that alone. 2. Just to be completely clear, I was refering to the group velocity and phase velocity earlier relative to the flow of air. Thus they generally have horizontal components even if the waves are stationary relative to the surface; the wind moves through the waves, hence the waves move through the air. (ps for waves in one dimension, phase velocity = frequency times wavelength: c = f*l group velocity = change in frequency per unit change in wavelength: cg = df/dl the angular frequency w = 2*pi*f, and the wave number k = 2*pi/l, so: c = w/k cg = dw/dk In multiple dimensions, c and cg can be vectors and k can also be a vector (the wave number vector, or I think it's okay just to say wave vector). In this case, cg = the gradient of w in k-space (which means, the x component of cg is equal to the rate of change of w per unit change in the x-component of k. Note that the gradient may vary over k-space, which makes cg a function of the k vector). One has to be careful using c (phase velocity - I'm going to use that term here though I'm not 100% sure if that's technically the correct term) as a vector (which is to be perpendicular to planes or lines of constant phase (troughs, crests, etc.)- the different components of c are not equal to the phase speeds in those different dimensions. The reason for this: imagine a diagonal line on the x-y plane moving perpendicular to itself at speed c. The phase speed in the x-direction, cx, is the speed of the x-intercept. If the line is nearly parallel to x, cx can approach infinity for a finite value of c. However, the inverses of c, cx, and cy add like vectors - as if 1/cx and 1/cy were the vector components of 1/c. Note that the cx (the phase speed in the x-direction) = w/(x-component of k). (see appendix A of Cushman-Roisin, "Wave Kinematics") Without going into the precise mathematical derivation based on the relationship between group velocity and other wave properties for vertically propagating gravity waves, it can be seen from the geometry that the horizontal phase speed (the inverse of the horizontal component of the vector in the phase velocity direction with magnitude 1/c) must be equal to the horizontal component of the group velocity (I think the horizontal and vertical components of group velocity do actually add as vectors - No, wait...????????) ... well, what I was going to say was that if the wind speed slows down with height, then (if I am correct here**) the group velocity not only becomes closer to horizontal (as the phase velocity is increasely farther from horizontal), the vertical component must become smaller, which means the propagation of energy and momentum slows down. This would then explain more clearly why, per unit vertical distance, all else being equal, wave damping would increase, so that the wave would dissipate faster per unit distance, so that per unit volume, the momentum and energy transferred from the wave to the background (basic or mean) state would increase. One has to divide by density to get the effect per unit mass, of course (momentum = velocity times mass). *NOTE* that this happens where the frequency of the wave relative to the air is reduced - where the wind velocity relative to the horizontal velocity of phase propagation is reduced (I stated that more generally because some waves may not be stationary relative to the surface, depending on what causes them, changes in the wind, etc.). PS the geometrical relationship between phase plane orientation, group velocity and phase velocity - what is parallel or perpendicular to what - is common to more than just vertically-propagating gravity waves. It applies to some other kinds of vertically-propagating waves, including equatorial Kelvin and Rossby-gravity waves. In the case that the wind relative to the horizontal position of the waves goes to zero, the group velocity, as I understand it, must go to zero. This means there is a convergence of wave energy and momentum - if the group velocity is still upward at some level beneath this critical level. Even if the waves have reached a steady state beneath this level, wave energy and momentum must continuously accumulate at such a critical level. This would explain the quote from Holton earlier: " Holton p.284: "Amplitude enhancement leading to wave breaking and turbulent mixing can occur if there is a 'critical level' where the mean flow goes to zero," - 'critical level' in the original is italicized instead of in single quotes " I would expect that this is another way that the momentum of the wave is transferred to the air at that level as opposed to just propagating through it and moving on. 3. Pressure perturbations and temperature perturbations: What allows gravity waves to exist is that, within a stably-stratified fluid (lapse rate less than the adiabatic lapse rate, so that potential temperature increases with height (or potential density decreases with height, where potential density is the density the material would have if brought adiabatically to some reference pressure) is that when air is displaced vertically so that the pressure changes, the temperature changes more than the temperature of the surrounding air, so that lifted air is more dense and will tend to sink, and air pushed down will be less dense than surrounding air at its new level and will tend to rise. The vertical accelerations do require pressure perturbations that are not in hydrostatic equilibrium (in other words, there must be some small imbalance between gravitational force and the vertical pressure gradient force). However, there are also pressure perturbations that are in hydrostatic equilibrium with the temperature perturbations. The temperature perturbations are in part due to the pressure perturbations but also to the vertical displacements that cause changes in pressure following the air as it moves. Thus, if we designate the vertical maxima in trajectories as crests and the vertical minima as troughs, the troughs tend to be warm and the crests tend to be cold. This means that, relative to the background (basic) state, the 'hydostatic pressure' decreases more with height through crests and less with height through troughs. When the phases are tilted, the hydrostatically-balanced portions of the pressure perturbations are thus high pressure under crests and above troughs and low pressure above crests and beneath troughs, with pressure extremes 90 degrees (1/4 wavelength) out of phase with the vertical displacements. These pressure perturbations push the actual temperature perturbations a little higher than otherwise, which in turn pushes the pressure perturbations a little higher than otherwise, but probably not much for small-amplitude waves. On the other hand, non-tilting phases require that the hydrostatically-balanced portion of the pressure perturbation is high pressure in the crests and low pressure in the troughs, and decreasing with height even for constant vertical displacement amplitude; but of coures the amplitude decreases with height, and it works out if both decrease exponentially with height (at least before taking into account the density variation with height of the basic (background) state, but I think the picture is still qualitatively similar). In this case, the temperature perturbations are reduced from what they would be because the pressure perturbations are opposite the pressure variations following the air motion. As long as the amplitude is small enough, however, everything should be the same sign as so far described (I think. It's possible to imagine the opposite scenario... well I'd have to think about that - I haven't done the math yet**). The non-hydrostatic portion of the pressure perturbation is necessary to balance the vertical accelerations. It will thus be high pressure beneath troughs and low pressure above troughs, to accelerate the air upward from the troughs. And the opposite for crests, where the air's upward motion slows and reverses: a downward acceleration. Notice that for tilted (vertically-propagating) waves, this is 180 deg (1/2 wavelength) out of phase of the hydrostatically-balanced pressure perturbation (before the readjustment to the adjustement to temperature). There is also horizontal acceleration because, subtracting the basic state wind from the total, the waves involve cycling slantwise motion parallel to crests and troughs. So... Now I have to do more to figure it out, but the diagram on p.202 of Holton shows high pressure over the trough and under the crest, with cold near or at the crest and warm near or at the trough, and the pressure and temperature perturbations 90 degrees (1/4 wavelength) out-of phase. As for the evanescent waves, ... I need to figure more about before going farther with that...** 4. Form drag and layers of air: It is important when considering momentum transfer by form drag to define layers of air by material surfaces (or in cross section, material lines). A material surface is either parallel to motion or moves with the air; trajectories never cross material surfaces. Streamlines can cross material surfaces so long as the material surfaces move along them. In the case of the layers of air considered here, the material surfaces are displaced vertically along with the air as it moves, so a layer of air is ridged. For vertically propagating waves, the pressure perturbations are such that, on the upper material surface, there are higher pressures on one set of slopes and lower pressures on the other set, so that there is a net force acting on the air layer in the horizontal. However, if the amplitudes of both the the 'pressure wave' and the vertical displacements are the same at the bottom of the layer, then the forces acting from below on the bottom material surface exactly oppose that from above. The difference in forces acting on the bottom and top of material surfaces that would be due to variation in the wave 'strength' with height would result in a net sideways force on the layer of air. If this is not the case with amplification with height of the wave displacements due just to decreasing basic-state density, then that could be because the pressure perturbations simultaneously decrease with height (due just to the density decreasing with height, again) in sufficient proportion (?). This concept of form drag based on forces on material lines is also useful with vertically-propagating equatorial Kelvin and Rossby-gravity waves (among others, I'd think). So there are a few things there I'm not sure about but overall I think/hope that still helps. Now back to vorticity and Rossby waves... to be continued...
129503. chris at 09:11 AM on 31 October 2008
        Human CO2 is a tiny % of CO2 emissions
        "More or less in balance" isn't "a cop out". There's a pretty good understanding of the short term and medium term carbon cycle that dominates the carbon flux between the atmosphere and biosphere, and on longer periods, the atmosphere and terrestrial environment. So to answer your first question: ["How much out of balance does it have to be before you consider it not in equilibrium?"] If atmospheric CO2 levels haven't varied much more than about 20 ppm (maybe 30 ppm according to some plant stomatal index analyses) around 280 ppm for the last 10,000 years before the 20th century, one can conclude that the system has been more or less in balance. It's not "a cop out" to state the obvious. The flux of carbon into the atmosphere has been reasonably closely balanced by the flux out of the atmosphere for vast periods of time before the 20th century. And if one considers the 10 million years before the 20th century, the atmospheric CO2 seems to have been pretty much near equilibrium. So if one considers only the interglacial periods, the atmospheric CO2 was below or around 300 ppm during this entire period according to the proxy record: e.g. Pearson, PN and Palmer, MR (2000) "Atmospheric carbon dioxide concentrations over the past 60 million years" Nature 406, 695-699. M. Pagani et al. (2005) "Marked Decline in Atmospheric Carbon Dioxide Concentrations During the Paleogene", Science 309, 600 – 603. T. K. Lowenstein and R. V. Demicco (2006) "Elevated Eocene Atmospheric CO2 and Its Subsequent Decline" Science 313, 1928. R. M. DeConto et al (2008) "Thresholds for Cenozoic bipolar glaciation" Nature 455, 652-656 Note that it's worth distinguishing the interglacial and glacial periods here, since the shift of atmospheric CO2 down to around 170-180 ppm during glacials is similarly part of the short term carbon cycle that relates to the distribution of carbon between the terrestrial biosphere, oceans and atmosphere. In this case it's the temperature-dependent element of the cycle and its response to very slow insolation variation (Milankovitch cycles). So we can talk about being "near equilibrium" or "more or less in balance" in quite explicit terms: (i) On the timescale of 1000-10,000 years, the relatively fixed amount of ACCESSIBLE carbon distributing between the atmosphere, oceans and biosphere has maintained an atmospheric CO2 concentration that has undergone relatively little variation (the overall variations during 1000's of years of the order of the changes now occurring in about a decade). (ii) on the timescale of 10 million years the longer term carbon cycle involving the sedimentation of carbon as carbonates in the deep oceans and the slow release of carbon from ocean plate subduction and volcanic activity has also been more or less in balance. The atmospheric CO2 record of the last 10 million years suppoorts that conclusion. (iii) On top of the equilibrium carbon distributions of the carbon cycle on the millions of years timescale, insolation variations (Milankovitch cycles) cause very slow requilibration of CO2 between the atmosphere and ocean/terrestrial environments. Now something quite different is happening. A massive store of excess carbon inaccessible to the carbon cycle for many 10's of millions of years is being rapidly reintroduced into the system in an extraordinarily short time period. Not surprisingly the atmospheric CO2 concentration is rising very rapidly indeed. The atmospheric CO2 concentration is out of equilibrium (there's a large nett flux into the atmosphere from previously long-sequestered sources), and the atmospheric CO2 concentration is being driven up towards some new equilibrium concentration. And the above also address your second question: ["How does all that CO2 locked up as carbonate sediment compare to the oil/gas/coal deposits?"] That's not quite a relevant question. Considering carbonate sediments and their formation, the long term paleoCO2 record of the last 10 million years or so indicates that carbonate sedimentation has been pretty much in balance with the return of CO2 from subducted carbonate back through volcanoes into the atmosphere. ...where the "out of balance" element has arisen is the awesomely rapid oxidation and return to the atmosphere of massive stores of carbon previously sequestered out of the short and medium carbon cycles for 10's and 100's of millions of years. Note that dynamic systems CAN be in equilibrium. In general they fluctuate around equilibrium states. Of course one can raise semantic issues about the extent to which a particular fluctuation constitutes a departure from equilibrium. But it's quite easy to be explicit and define exactly what one means by the particular equilibrium in question.
129504. chris at 08:13 AM on 31 October 2008
        It's Urban Heat Island effect
        O.K. so you agree that the colours denoting changes in temperature are entirely appropriate in the light of the completely general use of the colours asssociated with the visible part of the EM spectrum to denote temperature and temperature change (e.g. in thermal imaging of body temperature as in my link in post 5). That's good. But you're going to carry on maintaining that there's some sort of a disconnect between the satellite photo of the Earth at night (which is a pretty good identifier of urbanization and its density) and the surface temperature anomaly. It should be obvious that if one were to take the satellite picture of the Earth each night and average this for a year, that it would look pretty much the same as the snapshot. Or do you consider that averaged over a whole year there would magically appear lights from massive connurbations in the Arctic and Alaska, the vast Northern territories of Canada and Serbia, the empty regions of Australia, North and Central Africa and so on...? ...I think not.
129505. Mizimi at 06:44 AM on 31 October 2008
        It's Urban Heat Island effect
        Chris: thermal imaging does not produce colours..these are added by the software interpreting the input data as selected by the user. I accept we 'naturally' take white/orange/red to be 'hot' and blue/green to be 'cold' but how cool is a methane flame? Our bodies detect IR quite well but not UV, yet more people get sunburnt than fireburnt. My point about the two images above is that the temp anomaly is a compilation of a years data and thus the UHI's would be obscured. We regularly fly thermal imaging 'sorties' over our airfields to assess which buildings are inadequately insulated ( as well as to give the pilots some practise), but we do it at night to improve the image contrast. UHI effect will also vary according to season. So if these images are supposed to show that UHI effect is so minimal that they do not affect global temp, then, in my opinion, they fail.
129506. Mizimi at 05:46 AM on 31 October 2008
        It hasn't warmed since 1998
        PS: that should be 2108 of course!!
129507. Mizimi at 05:28 AM on 31 October 2008
        It's the sun
        QM: Yes, the plastic is modified polythene which is translucent. Part of the effect is due to albedo (colour) and part due to low incident angle reflection (shiny surface). Both effects diminish with age, (darkening of the colour and accumulation of dust) but the plastic only lasts about 3 years and then is replaced. Much of the plastic is recycled ( even the plastic string used for baling and tying plants!!) and turned into garden and playground equipment. Some is even re-inforced with steel bar and used as I beams for lightweight construction. True greenhouses hold temps up because they minimise losses thro' convection/windage rather than trapping heat by preventing re-transmission of IR ( although you can buy IR glass which acts just like a GG...a bit expensive tho'). Translucent fibreglass will do the same but I do not know the light transmission characteristics of this material...you might end up cutting the light frequencies the plants need. Why tempered glass? Ordinary 2 or 3mm plain window glass is perfectly OK and the cheapest. Don't forget to have vents to control the temp in summer....you don't want to cook the plants!
129508. Quietman at 16:49 PM on 30 October 2008
        Arctic sea ice melt - natural or man-made?
        Patrick This is getting interesting. Where are you getting this information from?
129509. Quietman at 16:44 PM on 30 October 2008
        It's the sun
        Mizimi That is interesting. I am planning to build a greenhouse to counteract the cooling conditions here. Is the increased albedo because of the plastic? I was planning on using tempered glass and translucent fibreglass.
129510. Patrick 027 at 15:04 PM on 30 October 2008
        Arctic sea ice melt - natural or man-made?
        Rossby waves: First, notes on vorticity. Vorticity = dv/dx - du/dy ; that is, the variation in the meridional wind component (v = Dy/Dt) going from west to east, MINUS the variation in the zonal wind component (u = Dx/Dt) going from south to north. Or in any coordinates (s,n) where facing in the direction of positive n, s points to the left, then the vorticity is the rate of change in the n direction of the s-component of velocity over n MINUS the rate of change in the s direction of the n-component of velocity; voriticity = d(Dn/Dt)/ds - d(Ds/Dt)/dn. (where Dq/Dt for any q is the velocity in the q direction; D/Dt is the langrangian or material derivative, which means it is the time derivative following the motion of the air; hence Dq/Dt is the rate of change of location along q following the air's motion.) Vorticity is the sum of two components: shear vorticity, and orbital or curvature vorticity. If there were only orbital/curvature vorticity, then the motion is simply rotation about a point. At each point at which this is the case, du/dx = - dv/dy, and d(Dn/Dt)/ds = - d(Ds/Dt)/dn, for any orientation of (s,n) axes. Over the space in which the vorticity is constant, the air would be rotating as if parts of the same rigid object, and there would be no deformation (if the air were tagged with shapes, the shapes would be rotated but remain the same size and shape). If only shear vorticity is present, then for a given location it will be possible to find some orientation of (s,n) such that one of d(Dn/Dt)/ds or d(Ds/Dt)/dn is zero. Suppose it is the first which is zero; in that case vorticity = shear vorticity = d(Ds/Dt)/dn. Where there is only shear vorticity, the wind is not changing direction. It is possible to have shear vorticity and orbital/curvature vorticity of opposite signs, in which case, if of equal magnitude, the total vorticity would be zero. One such case would be a wind field in which the streamlines form concentric circles, but outside of the central point or a central circle, the wind speed is inversely proportional to the distance from the center. Within the central circle, there would have to be some vorticity, or if there is only vorticity at the central point, that would have to be infinite vorticity (but just at one point, so that the vorticity integrated over area (for now, call that C) would be finite). To be continued...
129511. Patrick 027 at 11:22 AM on 30 October 2008
        Arctic sea ice melt - natural or man-made?
        ... well, now I'm not quite sure about the lack of form drag (the phase of pressure relative to displacement) with non-vertically propagating gravity waves, but anyway, moving on: If the forcing is at lower frequency than the buoyancy frequency, then: Vertical propagation occurs. surfaces of constant phase (crests and troughs) tilt with height. An interesting thing about these kinds of waves is that the group velocity is at right angles to the wave vector (which is perpendicular to the crests and troughs). The wave vector is in the direction of phase propagation. The group velocity is parallel to the crests and troughs. Relative to the air, for gravity waves emanating from the surface (such as from wind blowing over ridges), the crests and troughs move downward at an angle but build upward (along themselves) at an angle (at the group velocity), so that in steady state conditions, the wind blows through a stationary tilted crest and wave pattern. The pressure perturbation and vertical displacements are positioned so that there is form drag - there is higher pressure on the windward sides of the ridges and lower pressure on the lee sides. Thus there is a net force on the ridges, which means the air is losing momentum to the ridges. However, as each layer of air loses momentum to the layer below by the same process, it gains momentum from the layer above. If the gravity wave propagates upward without dissipation, there is no net loss of momentum. Ultimately the momentum transfered to the solid Earth from the air by the form drag is then taken from the air at levels where the gravity wave is dissipating (or otherwise ceasing to propagate as just described?). When the winds vary in time, the formation of gravity waves will change and I expect those changes to propagate at the group velocity. The winds and static stability can and will change with height, which will affect gravity wave propagation. Where the wind is slower, the frequency of the waves is reduced relative to the air following its motion - my understanding is that this (perhaps just because of the period of motion, or perhaps also because the tilts change so the group velocity goes farther away from the vertical?) allows for enhanced thermal and mechanical damping of the wave at such levels (per unit volume ?). Mecahnical damping would be by viscosity - including eddy-viscosity (the eddies in this case would be on smaller scales); concievably it might include something else**??. Thermal damping can occur because there are pressure perturbations in a gravity wave, which cause small adiabatic temperature variations, which then cause small variations in radiative (photons) cooling rates, which is not an adiabatic process and will reduce the gravity wave amplitude. In such gravity waves, the perturbation velocity and motion is parallel to the constant phase surfaces (crests and troughs, etc.) and oriented so that the horizontal projection is parallel to the mean wind. Inertio-gravity waves are gravity waves in which the fluid parcel oscillations are slow enough (slow wind, very very very broad ridges, low static stability) for the coriolis effect to become significant - so that the perturbation trajectories form ellipses rather than a line segment (following the air with the mean wind). As this happens, the coriolis effect becomes part of the restoring force. I haven't gone thoroughly through the math but from what I've read ("Introduction to Dynamic Meteorology - Third Edition" by James R. Holton - see chapters 7 and 9 in particular for gravity waves) vertically propagating inertio-gravity waves must have frequencies (following the motion of air parcels) between the buoyancy frequency (generally much much more rapid, and in that limit, crests and troughs approaching vertical) and the inertial oscillation frequency (proportional to the coriolis effect, and in that limit, crests and troughs approaching horizontal). I'm not sure what happens when the frequency is less than the inertial oscillation frequency - I suppose in that case the wave can't propagate. That might be why, in the context of inertial oscillations in the ocean excited by the wind, I've read that these can not propagate toward higher latitudes (but I was skimming that material, so don't take my word for it). Typically ridges don't have the profile of an endless sinusoidal wave with constant wavelength. Wind blowing over irregular topograph, or a single ridge, may excite a spectrum of gravity waves; depending on conditions, some may propagate vertically and some others may decay with height exponentially. (Of course, at high amplitudes, nonlinear effects, such as wave-wave interaction, may become a bigger factor). Sometimes conditions may allow vertical propagation but only up to some level, at which point the waves don't propagate further. I expect there'd be evanescent waves above that level (because the amplitude can't discontinuosly jump to zero - the same condition that requires evanescent electromagnetic waves beneath a reflecting surface). The gravity waves may reflect from that level. Repeated reflection between the surface and the upper level can generate trapped lee waves (Holton, p.284). Reflection may play some role in downslope windstorms but nonlinear processes are important in that phenomenon (Holton, p.284-285; also try looking up 'Froude number', 'hydraulic jump'). Without going into all details, Holton p.284: "Amplitude enhancement leading to wave breaking and turbulent mixing can occur if there is a 'critical level' where the mean flow goes to zero," - 'critical level' in the original is italicized instead of in single quotes (see also 'Scorer parameter'). Gravity waves with downward group velocity may occur presumably upon reflection from above - perhaps they could also occur from wind blowing underneath and relative to a disturbance in the air, though I haven't read of anything like that. Gravity waves can be excited by wind blowing over cumulus convection, and also may be produced by that convection itself (in that case, gravity waves may radiate away from the disturbances). 2. Rossby waves (to be continued)...
129512. Patrick 027 at 05:35 AM on 30 October 2008
        Arctic sea ice melt - natural or man-made?
        "Try to remember my background is engineering not climatology or theoretical physics. " I have a little bit of engineering and basic physics background but I'm not sure exactly what you mean about eddy currents in electricity. However, I really haven't begun to explain how these mechanical waves propagate. So to correct that, here are two important examples: 1. Gravity waves exited by wind blowing over sinusoidal ridges. Take the (arbitrarily-defined) layer of air closest to the surface. Without wave-breaking, the air moves up and down over ridges. It thus has to accelerate. So there will be pressure variations. Take the next layer of air - because the first layer is displaced, the next layer must be displaced, etc. As a function of the wavelength of the ridges and the wind speed, there must be some frequency of oscillation for the air as it blows through this set-up. If the air is stable (potential temperature increasing with height), air displaced vertically will tend to fall back to where it was, and oscillate about an equilibrium level - in the absence of forcing, this continues except for thermal (radiative - photons) and mechanical dissipation of the potential and kinetic energy involved. This natural frequency is called the buoyancy frequency or Brunt-Vaisalla (sp?) frequency. If the forcing of gravity waves is at a higher frequency, (unless I have this backwards), then the gravity waves produced by wind blowing over ridges decrease exponentially with height; there is no vertical propagation of the energy. There is no form drag - that is, the pressure perturbations associated with the gravity waves are aligned with vertical displacement maxima and minima so that there is no sideways forcing. The dissipation that would occur is by viscosity, which would occur in the absence of gravity waves (wind blowing across the surface tends to lose momentum to the surface (and hence the Earth), and wind at different levels at different speeds can exchange momentum via viscosity, though that is not generally a dominant factor in atmospheric motions away from the surface). On the other hand, if the forcing frequency (determined by the wind and the wavelength of the ridges) is less, than ... to be continued...
129513. Quietman at 16:16 PM on 29 October 2008
        Arctic sea ice melt - natural or man-made?
        Patrick So you are saying that eddy waves are similar to eddy currents in electricity rather than in aerodynamics? Try to remember my background is engineering not climatology or theoretical physics.
129514. Patrick 027 at 14:57 PM on 29 October 2008
        Arctic sea ice melt - natural or man-made?
        ... I think that - either when there is zero EP-flux or when the EP-flux divergence is zero?, then the wave is not dissipating (or amplifying or breaking) and so is not altering the mean state (it must be the first, because even without EP flux divergence, there is some alteration - but maybe it has to be reversed in time?). In such a case a wave can propagate by making only temporary changes (as electromagnetic waves may nudge the matter in a transparent material along the way (which affects how the wave propagates).
129515. Patrick 027 at 12:43 PM on 29 October 2008
        Arctic sea ice melt - natural or man-made?
        "Re: 276 I am just guessing but doesn't direct nonstop sunlight on the poles during a 6 month long day have a little to do with this?" YES! (And that will bear on any changes in solar forcing, but in and of itself is just part of the regular seasonal cycle which changes on timescales of several thousand years and longer...) "Re: 277 I have no clue as to what that means. Are you talking about wind or radiation? Eddys are circular currents caused by turbulence (in water or air) so I do not follow. " Not about electromagnetic radiation as in photons. These are mechanical waves. Waves are often thought of as sinusoidal in some way, but one can have a single wave pulse. There's phase velocity and group velocity - for dispersive waves, not the same thing. (Energy propagates witht the group velocity.) In order for waves to occur there must be a restoring force - gravity, pressure, elasticity, etc... In the context of geophysical fluid dynamics, I'm not sure exactly if there is a distinction between eddies and waves. Eddies can propogate. Rossby waves involve rotation. Cyclones that develope in midlatitudes are an aspect of baroclinic waves, which I think may be considered a kind of Rossby wave... It is true that in order for baroclinic instability to occur, there must be some vertical level, called a critical level (or steering level in this case, for obvious reasons) where the wind averaged across the baroclinic wave (a basic state wind) is equal to the velocity of the motion of the baroclinic wave (at least in the case where the average wind is not changing direction with height); however, above and below, the wave is propagating through the air. Even if we ignore vertical motion, the air at the center of a cyclone is not necessarily going to be at the center of the same cyclone in the near future; Even if a cyclonic circulation is strictly two-dimensional and axisymmetric, so that at any instant the streamlines (parallel to the wind vector at each location) are circles, propagation of this streamline pattern can be such that individual trajectories spiral into and out of the cyclone and in some cases may curve anticyclonically. ... Often an analysis of the atmosphere is made using zonal averages - these are averages over all longitudes - so that they might be graphed in two dimensions (if averaged over some specific time period or at some particular moment in time, etc.). Then there are zonal means, mean temperature, zonal (westerly) wind, and the mean meridional circulation (north-south and up-down). Motions that average to zero are attributed to eddies. Correlations of some parts of eddies with other parts of eddies can yield nonzero eddy fluxes - for example, in this perspective, the average eddy temperature deviation is zero, the average eddy meridional wind velocity is zero, but the average of the product of the two can be nonzero - thus there is a nonzero northward temperature flux by eddies. The average zonal velocity of eddies can also be zero, but a correlation between eddy zonal wind velocity and eddy meridional wind velocity can yield a nonzero northward eddy flux of zonal (westerly) momentum. The average of the square of the eddy wind speed will be nonzero, and hence so will the eddy kinetic energy. One way of analyzing how eddies affect the mean is by looking at the EP flux, which is a mathematical expression derived from distributions of eddy temperature and momentum fluxes, and is related to the eddy potential vorticity flux. In this perspective, everything 'eddy' would include some of such things as (internal) gravity waves, (internal) intertia-gravity waves, Rossby waves (baroclinic waves, planetary waves, etc.), equatorial Rossby waves, Rossby-gravity waves, and Kelvin waves; thus these can all have eddy fluxes. I'm a bit vague on much of this, but the gist of what I've gotten is: These waves can propagate through the atmosphere in some ways; depending on the type and at least sometimes the wavelength of the wave, there can be a index of refraction assigned to parts of the atmosphere which is a function of the wind field, the coriolis effect (varies with latitude), stability, and/or quantities derived from those things - vorticity, potential vorticity, etc. Waves may propagate horizontally only or they may also propagate vertically. Relevant to either direction, there can be critical levels. There may be regions where the wave can not propogate in an oscillatory manner - upon reaching a boundary it may be felt on the other side as an evanescent wave, one which decays exponentially away from that boundary - if another boundary is reached where it can again propagate, perhaps some of it will have tunneled through the barier (perhaps analogous to electron tunneling, considering the quantum-mechanical wave nature of electrons; also perhaps analogous to the evanescent portion of an electromagnetic wave which exists on the opposite side of a reflecting surface). Some gravity waves are actually evanescent waves - for example, a gravity wave produced by wind blowing over a ridge may decay with height and have vertical phase planes vertical with - zero group velocity? - whereas otherwise a gravity wave produced by wind blowing over a ridge will propagate vertically (the energy will propagate with the group velocity, and this carries momentum). Waves may be concentrated by variations in the index of refraction. They may reflect. They may over-reflect - I'm not sure but maybe that's analogous to the stimulated emission of radiation. Generally, disturbances may radiate waves. Waves may grow, and thus must be taking something from the background state they inhabit. They may dissipate, and in doing so they may deposit momentum back into a background state. Waves can also break (like waves crashing on a beach). I started going into this because wave-mean interactions are important in the global circulation; wave propagation also is important in stratospheric and mesospheric motions; etc...
129516. Mizimi at 05:49 AM on 29 October 2008
        Temp record is unreliable
        Thanks Chris; somehow I missed the link. This thread is about temperature records and how reliable /accurate/representative are they. CO2 levels are assumed to vary only slightly due to effective atmospheric mixing, but this is very different from temperature which has much greater variation. Given the paucity of temperature recording stations I cannot accept that the data used for models is sufficiently representative of the global condition, and thus the resultant of the model is questionable. Even satellite records are questionable as recently demonstrated by the modification needed to the attitude correction algorithm.
129517. chris at 05:46 AM on 29 October 2008
        Global warming stopped in 1998, 1995, 2002, 2007, 2010, ????
        Re #24: That's incorrect. The overall trend of the last 5 million years has been a mildly cooling one. Re #21 There's no such thing as "normal" temperature in relation to the Earth. The Earth is on a journey through time, and it's properties (atmosphere, temperature, biosphere, geology and so on) evolve according to a whole range of intrinsic and extrinsic factors. For human kind and the current biosphere, "normal" only really has a meaning in relation to evolutionary adaptedness. The biosphere in its current state is adapted (i) to the relatively cool period of the last several million years, and (ii) to a world with rather more continuous and connected environments that has, until the recent past, allowed migration as a fundamental means of adapting to climate change.
129518. chris at 05:31 AM on 29 October 2008
        Global warming stopped in 1998, 1995, 2002, 2007, 2010, ????
        Re #23 The notion that one can change reality or somehow diminish real world implications with semantics is a dismal notion...it's politics, not science. The world is warming..the evidence indicates that the massive enhancement of greenhouse gases is a dominant causal factor....our understanding of the climate system and its response to enhanced greenhouse effect indicates that we're very likely to get a considerable amount of additional warming. That's "global warming"...and it's already causing "climate change"... As for the "shift in emphasis" from "Global Warming" to "Climate Change", much of that "shift" has come from the sectors of the political spectrum, especially in the US, that has had such a degrading effect on the entire US sociopolitic during the last several decades: So, for example, it was Frank Luntz, the Republican party strategist, that urged Republican candidates in a memo some years ago, to use the phrase "climate change" rather than "global warming", because (in his words): "Climate change is a lot less frightening than global warming". At that time, Luntz's aim was to misrepresent the science and to play the "uncertainty" game ("there's no proof that cigarette smoke causes cancer"..."there's no proof that aspirin enhances the liklihood of Reyes syndrome in children".."there's no proof that CFC's denude high altitude ozone concentrations" etc. etc. ad nauseum). Happily, like an awful lot of people that combine politics with at least a semblance of honesty, Luntz has shifted his viewpoint, such that he said in an interview a couple of years ago: "It's now 2006...I think that most people would conclude that there is global warming taking place and the behaviour of humans is affecting the climate..." The take home messages are, first, that the natural world sadly doesn't bow to ones' political pursuasions (see King Canute's political advisors!), second, that on the supposed use of semantics to politicise/downplay real world consequences, one should be a little more careful in assessing where the politicizations are coming from.... ....and third, if one considers that it is appropriate to misrepresent and deliberately misunderstand the science in pursuit of political agendas, one might consider who is actually benefitting from one's contrived misrepresentation...one might discover at some future time that one was being treated as a chump to service someone else's agenda!
129519. Mizimi at 05:31 AM on 29 October 2008
        It's the sun
        FYI: University of Almeria (Spain): A study by Pablo Campra (published in the Journal of Geophysical Research)on the effect of greenhouses in western Almeria province reports an 0.3C/decade drop in temp over the last 25 yrs....roughly the same as the rise in temp for the rest of the world during that time. Western Almeria has over 30,000 hectares of plastic covered greenhouses supplying produce to European supermarkets. The plastic sheeting increases the local albedo, reflecting sunlight back into space. The plants grown also act as a carbon 'sink' absorbing around 10 tonnes of carbon/hectare...an annual equivalent of 300,000 tonnes of carbon.
129520. Mizimi at 05:18 AM on 29 October 2008
        What does CO2 lagging temperature mean?
        Chris: You have listed catastrophic events which cannot be predicted and occur infrequently; yes, they had an enormous effect ( from which life recovered) but in the scheme of things were relatively transient. As such they should be factored out of any attempt to model climate.
129521. chris at 21:05 PM on 28 October 2008
        CO2 measurements are suspect
        Re #9 The ocean isn't really "covered by a few ships". The oceans have a scattering of data stations in isolated islands (see map in the World Data Centre For Greenhouse Gases in John Cook's top article). It's pretty hard to see what your difficulty is. If we can measure CO2 in the atmosphere from a whole slew of data stations in isolated positions around the world situated away from urban centres, and these give rather similar atmospheric CO2 measures (yearly averaged), then we can be pretty confident that we are obtaining accurate and valid measures of the atmospheric CO2 concentration, particularly if we have extended time series that allows us to determine year on year variations in the level from individual sites. That's rather consistent with what we understand about the nature of atmospheric gases that are highly diffusive, and so are pretty well mixed on the annual basis. Of course it's important to monitor yearly averages if we wish to determine the year on year variation in atmospheric CO2 levels, since there are significant intraannual (cyclic) variations, especially in relation to the yearly cycle of plant growth and decay that is dominated by the N. hemisphere seasonal growing/decay cycle. And we do know what the CO2 levels west of the Brazilian rainforest are. We have data from Huancayo in Peru from various periods in the 1980's. These are within a few ppm of the global average from the ocean surface stations (or the Mauna Loa observatory). We have data from Easter Island that lies to the west of the Brazilian rainforest. Likewise these data are within a few ppm of the rest of the globally averaged data. I expect you can find more data from sites west of the Brazilian rainforest if you try (it really depends how interested you are in finding out this stuff). We do know what the atmospheric CO2 levels are in the Sahara. We have extensive data from Assekrem in Algeria in the N. Sahara, for example. The data are rather close to the atmospheric CO2 levels measured from the globally averaged data (or the Mauna Loa data). In other words wherever we look, we find a rather consistent set of atmospheric CO2 concentrations throughout the world, so long as these are measured in isolated sites unperturbed by major sources of atmospheric CO2.
129522. Quietman at 14:54 PM on 28 October 2008
        Volcanoes emit more CO2 than humans
        Patrick Re: 108 Maybe not global, I was thinking on a much more localized scale (as in the cause of El Nino) that has wide effects (as in El Nino).
129523. Quietman at 14:51 PM on 28 October 2008
        Arctic sea ice melt - natural or man-made?
        Patrick Re: 276 I am just guessing but doesn't direct nonstop sunlight on the poles during a 6 month long day have a little to do with this? Re: 277 I have no clue as to what that means. Are you talking about wind or radiation? Eddys are circular currents caused by turbulence (in water or air) so I do not follow.
129524. chris at 08:50 AM on 28 October 2008
        Temp record is unreliable
        Re #34 No, not paleoproxies. That's clear from the data I linked to: http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf (see page 3) The atmospheric CO2 record is the directly measured atmospheric CO2 either in the atmosphere (from the many sites around the world and the continuous record from Manua Loa since 1959), and trapped in bubbles in ice cores extending back many 100's of thousands of years, but at a high resolution extending back 1000 years: e.g. D. M. Etheridge et al (1996) "Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn" J. Geophys Res. 101, 4115 -4128. and later extended to 2000 years: CM Meure et al (2006) "Law Dome CO2, CH4 and N2O ice core records extended to 2000 years BP" Geophys Res. Lett. 33 Art. # L14810
129525. Dan Pangburn at 07:07 AM on 28 October 2008
        CO2 lags temperature
        At post 43 above “for the last glaciation” appears to have been mistakenly interpreted as the transition from interglacial to glacial. The words were intended to be understood as ‘during the glacial period’ which excludes the interglacials and transitions to avoid these murkier periods and also to avoid significant influence of Milankovitch cycles. It may have been less ambiguous to have said “during the last glacial period” because what is meant is the period from about 115,000 ybp to about 20,000 ybp (and previous glacial periods). Similarly in post 45, ‘glaciations’ is intended to mean ‘during the glacial period’. Of course the planet is warmer because of ‘greenhouse gases’ than it would be without the effect. Most people that are knowledgeable on climate understand that positive feedbacks occur with carbon dioxide and water vapor and should understand that the climate responds to NET feedback which is the combined effect of all feedbacks, both positive and negative whether known or not. Much less well understood is that there has to be substantial negative feedback because the trends in the temperature record prove that the NET feedback can not be significantly positive. Without significant net positive feedback, the GCMs do not predict significant global warming. The lag of atmospheric carbon dioxide level to changes in global average temperature in paleo data is readily explained by the change with temperature in solubility of carbon dioxide in water.
129526. chris at 23:39 PM on 27 October 2008
        Water vapor is the most powerful greenhouse gas
        Re #9 Yes water vapour amplifies the warming. One needs to be a bit more explicit in relation to the question of whether it amplifies the source of the warming (it does under some circumstances). So in general it's more explicit to state that water vapour amplifies the effect of the source of warming to which raised water vapour concentrations is a response. So yes, raised water vapour amplifies the warming. It "makes it larger" (it "exaggerates" or "increases" the warming). As your definitions indicate "amplification" is an appropriate word; there's nothing emotive about it! It doesn't have a cooling effect. And as you also indicate the GMT is somewhat higher than it would be otherwise be...
129527. Mizimi at 07:01 AM on 27 October 2008
        CO2 measurements are suspect
        Most of the stations are located in the N hemisphere, very few in the southern, and the ocean is covered by a few ships. I would be a lot more comfortable with the idea that CO2 is rapidly homogenised if we had some hard data from the areas not currently monitored, especially since most of them are not industrialised. For example: we might well find that the CO2 levels west of the Brazilian rainforest are higher than 'average' or that Saharan levels are markedly below. The point is we don't know and we should.
129528. Mizimi at 06:41 AM on 27 October 2008
        Water vapor is the most powerful greenhouse gas
        Amplify: lit. to increase or make bigger. 1. To make larger or more powerful; increase. 2. To add to, as by illustrations; make complete. 3. To exaggerate. So by what means does any GG ( water vapour inc.) amplify the source of warming? It doesn't. It moderates the rate at which heat is lost which means the GMT is somewhat higher than it would otherwise be.
129529. Mizimi at 06:00 AM on 27 October 2008
        It hasn't warmed since 1998
        Dan: I just did a simple linear calc 0.18/decade for 10 decades = + 1.8c rise. Totally wrong I know but it was to illustrate a point that even if the 30 year trend continues in a linear fashion ( which it can't as the real effect is logarithmic) we would only see GMT rise to 15/16C by 2008.
129530. Mizimi at 05:51 AM on 27 October 2008
        Temp record is unreliable
        Chris: "If one examines the high resolution atmospheric CO2 record over the last 1000 years,"...... What highres records are we talking about please? Paleoproxies?
129531. Patrick 027 at 14:09 PM on 26 October 2008
        Arctic sea ice melt - natural or man-made?
        Characteristics of the atmosphere can be divided into a mean state (as in zonal mean - averaged over all longitudes) and eddies. Eddy winds blow north and south, east and west; average of eddy wind velocity would be zero (unless...?). Eddy thermal anomalies are warm and cold; average is zero. But correlation can exist so that the average eddy heat and momentum fluxes are nonzero. These eddies are waves. They can propagate. They can be 'emitted' (generated, grow), can be reflected, can be absorbed, depending on the type of wave and conditions of the atmosphere.
129532. bit_pattern at 14:02 PM on 26 October 2008
        Is Pacific Decadal Oscillation the Smoking Gun?
        Thank you!
129533. Patrick 027 at 13:58 PM on 26 October 2008
        Arctic sea ice melt - natural or man-made?
        Basic info on global circulation patterns: Troposphere: latitudinal variations in solar heating drive thermally direct Hadley cells - altered and enhanced by the role of water vapor: Widespread sinking over subtropics; adiabatic warming, radiative cooling. Rising in cumulus convection (hot towers) in the ITCZ - latent heating, adiabatic cooling. Seasonal Variation in forcing: ITCZ migrates north and south; the most intense Hadley cell is from the ITCZ to the winter subtropics. Seasonal land-sea contrasts: monsoons, enhanced by water vapor (latent heating). Also, Walker Circulation. Hurricanes, cumulus convection, land-sea breezes and mountain-valley breezes. -- Baroclinic instability in midlatitudes: Eddies carry heat polewards, concentrate zonal (westerly) momentum from north and south, transfer zonal momentum downwards to surface. Drives a thermally-indirect Ferrel Cell. -- Stratosphere and Mesosphere: Radiative equilibrium would be a temperature maximum around stratopause, Latitinal variation is - in lower stratosphere, warmest on the summer-side of the equator, but still cooler at summer pole; going up, latitude of greatest temperatures shifts all the way to summer polar region (happens while still in stratosphere). Winter polar region very cold. Circulations driven by propagation of mechanical energy upward from troposphere drive stratospheric and mesospheric motions that alter the temperature distribution: Quasi-stationary planetary waves produced in the troposphere can (under certain conditions) propagate upward - this can only happen with westerly winds with certain ranges of speeds - this happens in winter; not in summer. In winter, planetary waves propagate upward and dissipate in the stratosphere, which drives poleward motion; BREWER-DOBSON circulation in the stratosphere is upward over tropics, poleward into the winter hemisphere, and downward at higher latitudes. This warms the mid and high-latitude lower stratosphere and cools the tropical tropopause. Sometimes this happens in bursts called 'sudden stratospheric warmings'. But on average the winter polar stratosphere is still colder than the winter midlatitude stratosphere. Some gravity waves produced in the troposphere can propagate up to the mesosphere where they are dissipated, driving motion that is from the summer hemisphere to the winter hemisphere, with upward motion over the summer high latitudes and downward motion over the winter high latitudes. This cools the summer upper mesosphere and warms the winter mesosphere. Thus on average: At tropopause, coldest over tropics (the tropopause is highest over the tropics). In the lower stratosphere, summer polar region is warmer, tropics are colder, midlatitude winter is a bit warmer again, but the polar winter is colder. Higher in the stratosphere, there is a general decline in temperature from summer pole to winter pole. This continues somewhat into the mesosphere, except starting in the lower mesosphere winter high latitudes, the temperature gradient reverses; going up this condition spreads across the tropics and all the way to the summer pole, so that in the upper mesosphere and mesopause region, the summer pole is cold and the winter pole is warmer.
129534. Patrick 027 at 13:23 PM on 26 October 2008
        Volcanoes emit more CO2 than humans
        That last comment was about "'Dead' planets might be livable after all". hope to get back to AO discussion within a few days...
129535. Patrick 027 at 13:20 PM on 26 October 2008
        Volcanoes emit more CO2 than humans
        That was interesting. One important point is that the heat would take time to build up from such a process. In spite of all the Earth's internal heat, it counts little for regional or global scale climate (directly), because the heat flux is very small. For internal heat to make a difference to surface temperatures on a large scale, the heat flux has to be significant compared to the heating by radiation from the planet's star. Assuming a rocky crust as on Earth, the thermal gradient must then be that much greater, which means perhaps a thin crust on a molten mantle.
129536. Quietman at 11:40 AM on 26 October 2008
        It's the sun
        It is becoming more apparent that we picked either a very bad time or a very good time to screw with mother nature because the earth itself is very active as well as the sun. This is not a coincidence.
129537. Quietman at 11:34 AM on 26 October 2008
        Volcanoes emit more CO2 than humans
        Patrick I found a little more background data: Evidence Mounts For Arctic Oscillation's Impact On Northern Climate: ScienceDaily (Dec. 20, 1999)- A growing body of evidence indicates that a climate phenomenon called the Arctic Oscillation has wide-ranging effects in the Northern Hemisphere and operates differently from other known climate cycles. Arctic Oscillation Has Moderated Northern Winters Of 1980s And '90s: ScienceDaily (July 10, 2001) - The Arctic Oscillation has been linked to wide-ranging climate effects in the Northern Hemisphere, but new evidence shows that in recent decades it has been the key in preventing freezing temperatures from extending as far south as they had previously. Synchronized Chaos: Mechanisms For Major Climate Shifts: ScienceDaily (Aug. 2, 2007) — In the mid-1970s, a climate shift cooled sea surface temperatures in the central Pacific Ocean and warmed the coast of western North America, bringing long-range changes to the northern hemisphere. It seems that someone has been ignoring this data for quite a few years now. I wonder why.
129538. chris at 09:57 AM on 26 October 2008
        CO2 measurements are suspect
        Re #7, Well yes, that's rather the point. If one wants to obtain reliable global estimates of atmospheric CO2 concentrations, it makes sense to sample the atmosphere in isolated locations far from major sources of CO2 production. So one expects to see a bit of variability of atmospheric CO2 in measurements made in industrialised countries especially in the Northern hemisphere, and of course there is the yearly plant growth/decay cycle dominated again by the N. hemisphere. However if one examines the yearly average of atmospheric CO2 in isolated locations (there are dozens of these), the variability is low. These locations give a good measure of the global CO2 in the well-mixed atmosphere averaged on a yearly basis. Obviously local measures of CO2 concentrations can be somewhat higher, especially in or near cities (where they can be locally very much higher). There's masses of data that indicate that rather obvious consequence of measuring near human sources of CO2 (industrial/transport/heating etc.). Clearly if one wishes to assess the extent to which global atmospheric CO2 concentrations are changing in time, one asesses the global average on the time scale of good atmospheric mixing (e.g. annually) at the wealth of sites in isolated locations far from CO2 sources... ..it ain't rocket science!
129539. chris at 09:35 AM on 26 October 2008
        Is Pacific Decadal Oscillation the Smoking Gun?
        Re #42: The infrared electromagnetic (EM) radiation reaching the earth's surface is transformed to thermal energy which is re-irradiated eventually as longwave infrared (IR), having a lower energy than the incident IR. This longwave IR has energies that overlap with those of the vibrational transitions of certain atmospheric gases. These are molecules with asymmetric bond vibrations; i.e. CO2, H2O, CH4 and others (symmetric diatomic molecules that dominate the atmospheric composition - O2 and N2, don't absorb this longwave IR). So electronic transitions aren't excited, nor are bonds broken. However the absorbed longwave IR is either re-emitted by the greenhouse gas molecules, or else the gases transfer their thermal energy to other molecules directly by collision (thermal energy is essentially the same as heat). This has the effect of suppressing the escape of IR into space, and thus warming the atmosphere. In other words, the longwave IR emitted by the earth's surface radiates essentially "upwards" into towards space; however the "trapped" IR is re-emitted in all directions, and so the return of thermal energy to space is suppressed. I suspect that's what the sentence is summarising...
129540. Mizimi at 08:45 AM on 26 October 2008
        CO2 measurements are suspect
        Well, the World Data Centre for GG's shows more than 1% differences...eg... Syowa Station * Japan NOAA/GMD 13CO2 2007 379ppm Hegyhatsal * Hungary HMS CO2 2007 405ppm Minamitorishima * Japan JMA 2008 380ppm Puszcza Borecka/Diabla Gora * Poland 2008 398ppm A small sample, there are others. 7% differential....5% differential; a bit difficult to accept the idea that there are no significant global variations in CO2 levels. Especially when no-one has bothered to measure the rather large areas mentioned in #1
129541. Mizimi at 08:15 AM on 26 October 2008
        Human CO2 is a tiny % of CO2 emissions
        Depends what you define as 'short', 'medium' or 'long'. Yes, atmospheric CO2 levels have risen in the last 50 years or so....is this short or medium? Climate-wise I suggest it is very short. Paleoproxy data shows atmospheric CO2 rising and falling by very much greater levels over longer periods of time. The system is clearly never in equilibrium. 'More or less in balance' is a cop out. How much out of balance does it have to be before you consider it not in equilibrium? How does all that CO2 locked up as carbonate sediment compare to the oil/gas/coal deposits? And that form of sequestration is still going on. Human population is expected to grow from 6 to 9 billion by 2100...which equals (roughly) 540 million tons of carbon locked up in people for say, 60 years? And yes, people die, but the release of carbon back to the environment is not immediate. No dynamic system can be in equilibrium...
129542. bit_pattern at 08:08 AM on 26 October 2008
        Is Pacific Decadal Oscillation the Smoking Gun?
        I have a question - my grasp on physics is pretty basic and I was using this article to make a point on an internet forum and this was the response I got. Can you possibly give a brief explanation? "Greenhouse gases absorb outgoing longwave radiation" I am not sure what this is trying to say. The absorption of energy must culminate in some sort of effect. Either the energy excites e-, causing it to jump from a lower energy level then falling back again; emitting light during the drop. Or the energy absorbed breaks bonds. Or the energy is reflected. Do you know what this is trying to say?
129543. Dan Pangburn at 02:39 AM on 26 October 2008
        It hasn't warmed since 1998
        

        It is unclear how the calculation at 15 was made. NOAA data is available at LINK and Hadley data at http://www.cru.uea.ac.uk/cru/data/temperature/hadcrut3gl.txt . These both show a trend of about +0.1 degree per decade. Although short term trends can be misleading, like the 22 year run up from 1976 to 1998, the dramatic drop of global average temperature in 2008 may be indicative of a change in character of the climate. The current UAH satellite numerical data (these data consist of the differences of lower atmospheric temperature from the 1979 thru 1998 average) is at http://vortex.nsstc.uah.edu/data/msu/t2lt/uahncdc.lt . According to these data, the AVERAGE GLOBAL TEMPERATURE for the first 9 months of 2008 is LOWER than the average from 2000 thru 2007 by an amount equal to 43.1% of the total linearized increase (NOAA data) during the 20th century. Since 2000, the CARBON DIOXIDE LEVEL HAS INCREASED by 14.4% of the total increase since the start of the Industrial Revolution.

129544. Quietman at 15:03 PM on 25 October 2008
        It's the sun
        pps Sorry, I put it in the "CO2 measurements are suspect" thread since I could not find a more pertinant thread for it.
129545. Quietman at 05:05 AM on 25 October 2008
        It's the sun
        chris Re: Otherwise it's not obvious - Greenhouse gases yes, to some large extent. But there is still more to it in my view. PS I put a link to yet another GHG in the volcano thread that might interest you.
129546. chris at 03:40 AM on 25 October 2008
        CO2 measurements are suspect
        Nice pudding Quietman, but it doesn't really go with the main course. This thread (and my posts) is about the accuracy of global CO2 readings and the mixing of the atmosphere on the annual timescale. The fact that efforts are being made to measure the concentrations of atmospheric NF3 is a seperate issue and not related at all to the accuracy of atmospheric CO2 measurements. Note that NF3 concentrations are extraordinarily low (I calculate around 42,500,000 times lower that those of atmospheric CO2 based on the info in your link)...no doubt it hasn't been easy to measure these...or perhaps no one has bothered up to now...
129547. Quietman at 11:45 AM on 24 October 2008
        CO2 measurements are suspect
        chris May I offer you a little pudding perhaps.
129548. chris at 08:43 AM on 24 October 2008
        There's no empirical evidence
        Re #11 A skeptic would easily recognise that Beck's analysis is nonsense! see, for example, post #172 here: http://www.skepticalscience.com/solar-activity-sunspots-global-warming.htm remember that this is a "skepticalscience" site...we should make at least a little effort to be skeptical!
129549. chris at 08:38 AM on 24 October 2008
        It's the sun
        Re #179 Well yes, the large scale global warming of the last 30-odd years hasn't had a significant solar component. If anything the solar contribution has been a slight cooling one during the last several decades. Even those that push for solar contributions such as the cosmic ray flux concede that the solar contribution has been negligible at best. So solar contributions to warming in recent decades just isn't a viable proposition. The evidence is flat against it. Otherwise it's not obvious what else can have contributed significantly to warming other than the very well characterized massive enhancement of the Earth's greenhouse effect.
129550. chris at 08:28 AM on 24 October 2008
        It warmed before 1940 when CO2 was low
        Re: #2 (and #1) ..and yet there is a massive amount of evidence that the warming of the last 30 years is not "natural". The warming has followed the truely massive increases in CO2 emissions especially since the 1960's. Atmospheric CO2 concentrations rose rather slowly throughout the early 20th century. They were approaching 300 ppm in 1900 and reached 320 ppm in 1962. Since then we've raced up to 386 ppm. That's the likely source of the large scale global warming of the last 30-odd years. So to suggest that "our contribution, of various sources, not just CO2 is negligible" (whatever that might mean!) just doesn't accord with the real world evidence. I'd like to know what these "arguments presented against CO2" that "have some merit" actually are...can we have a list please?

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