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Uncertainty, noise and the art of model-data comparison

Filed under: — gavin @ 11 January 2008 - (Español) (Chinese (simplified))

Gavin Schmidt and Stefan Rahmstorf

John Tierney and Roger Pielke Jr. have recently discussed attempts to validate (or falsify) IPCC projections of global temperature change over the period 2000-2007. Others have attempted to show that last year’s numbers imply that ‘Global Warming has stopped’ or that it is ‘taking a break’ (Uli Kulke, Die Welt)). However, as most of our readers will realise, these comparisons are flawed since they basically compare long term climate change to short term weather variability.

This becomes immediately clear when looking at the following graph:

The red line is the annual global-mean GISTEMP temperature record (though any other data set would do just as well), while the blue lines are 8-year trend lines – one for each 8-year period of data in the graph. What it shows is exactly what anyone should expect: the trends over such short periods are variable; sometimes small, sometimes large, sometimes negative – depending on which year you start with. The mean of all the 8 year trends is close to the long term trend (0.19ºC/decade), but the standard deviation is almost as large (0.17ºC/decade), implying that a trend would have to be either >0.5ºC/decade or much more negative (< -0.2ºC/decade) for it to obviously fall outside the distribution. Thus comparing short trends has very little power to distinguish between alternate expectations.

So, it should be clear that short term comparisons are misguided, but the reasons why, and what should be done instead, are worth exploring.

The first point to make (and indeed the first point we always make) is that the climate system has enormous amounts of variability on day-to-day, month-to-month, year-to-year and decade-to-decade periods. Much of this variability (once you account for the diurnal cycle and the seasons) is apparently chaotic and unrelated to any external factor – it is the weather. Some aspects of weather are predictable – the location of mid-latitude storms a few days in advance, the progression of an El Niño event a few months in advance etc, but predictability quickly evaporates due to the extreme sensitivity of the weather to the unavoidable uncertainty in the initial conditions. So for most intents and purposes, the weather component can be thought of as random.

If you are interested in the forced component of the climate – and many people are – then you need to assess the size of an expected forced signal relative to the unforced weather ‘noise’. Without this, the significance of any observed change is impossible to determine. The signal to noise ratio is actually very sensitive to exactly what climate record (or ‘metric’) you are looking at, and so whether a signal can be clearly seen will vary enormously across different aspects of the climate.

An obvious example is looking at the temperature anomaly in a single temperature station. The standard deviation in New York City for a monthly mean anomaly is around 2.5ºC, for the annual mean it is around 0.6ºC, while for the global mean anomaly it is around 0.2ºC. So the longer the averaging time-period and the wider the spatial average, the smaller the weather noise and the greater chance to detect any particular signal.

In the real world, there are other sources of uncertainty which add to the ‘noise’ part of this discussion. First of all there is the uncertainty that any particular climate metric is actually representing what it claims to be. This can be due to sparse sampling or it can relate to the procedure by which the raw data is put together. It can either be random or systematic and there are a couple of good examples of this in the various surface or near-surface temperature records.

Sampling biases are easy to see in the difference between the GISTEMP surface temperature data product (which extrapolates over the Arctic region) and the HADCRUT3v product which assumes that Arctic temperature anomalies don’t extend past the land. These are both defendable choices, but when calculating global mean anomalies in a situation where the Arctic is warming up rapidly, there is an obvious offset between the two records (and indeed GISTEMP has been trending higher). However, the long term trends are very similar.

A more systematic bias is seen in the differences between the RSS and UAH versions of the MSU-LT (lower troposphere) satellite temperature record. Both groups are nominally trying to estimate the same thing from the same data, but because of assumptions and methods used in tying together the different satellites involved, there can be large differences in trends. Given that we only have two examples of this metric, the true systematic uncertainty is clearly larger than the simply the difference between them.

What we are really after is how to evaluate our understanding of what’s driving climate change as encapsulated in models of the climate system. Those models though can be as simple as an extrapolated trend, or as complex as a state-of-the-art GCM. Whatever the source of an estimate of what ‘should’ be happening, there are three issues that need to be addressed:

  • Firstly, are the drivers changing as we expected? It’s all very well to predict that a pedestrian will likely be knocked over if they step into the path of a truck, but the prediction can only be validated if they actually step off the curb! In the climate case, we need to know how well we estimated forcings (greenhouse gases, volcanic effects, aerosols, solar etc.) in the projections.
  • Secondly, what is the uncertainty in that prediction given a particular forcing? For instance, how often is our poor pedestrian saved because the truck manages to swerve out of the way? For temperature changes this is equivalent to the uncertainty in the long-term projected trends. This uncertainty depends on climate sensitivity, the length of time and the size of the unforced variability.
  • Thirdly, we need to compare like with like and be careful about what questions are really being asked. This has become easier with the archive of model simulations for the 20th Century (but more about this in a future post).

It’s worthwhile expanding on the third point since it is often the one that trips people up. In model projections, it is now standard practice to do a number of different simulations that have different initial conditions in order to span the range of possible weather states. Any individual simulation will have the same forced climate change, but will have a different realisation of the unforced noise. By averaging over the runs, the noise (which is uncorrelated from one run to another) averages out, and what is left is an estimate of the forced signal and its uncertainty. This is somewhat analogous to the averaging of all the short trends in the figure above, and as there, you can often get a very good estimate of the forced change (or long term mean).

Problems can occur though if the estimate of the forced change is compared directly to the real trend in order to see if they are consistent. You need to remember that the real world consists of both a (potentially) forced trend but also a random weather component. This was an issue with the recent Douglass et al paper, where they claimed the observations were outside the mean model tropospheric trend and its uncertainty. They confused the uncertainty in how well we can estimate the forced signal (the mean of the all the models) with the distribution of trends+noise.

This might seem confusing, but an dice-throwing analogy might be useful. If you have a bunch of normal dice (‘models’) then the mean point value is 3.5 with a standard deviation of ~1.7. Thus, the mean over 100 throws will have a distribution of 3.5 +/- 0.17 which means you’ll get a pretty good estimate. To assess whether another dice is loaded it is not enough to just compare one throw of that dice. For instance, if you threw a 5, that is significantly outside the expected value derived from the 100 previous throws, but it is clearly within the expected distribution.

Bringing it back to climate models, there can be strong agreement that 0.2ºC/dec is the expected value for the current forced trend, but comparing the actual trend simply to that number plus or minus the uncertainty in its value is incorrect. This is what is implicitly being done in the figure on Tierney’s post.

If that isn’t the right way to do it, what is a better way? Well, if you start to take longer trends, then the uncertainty in the trend estimate approaches the uncertainty in the expected trend, at which point it becomes meaningful to compare them since the ‘weather’ component has been averaged out. In the global surface temperature record, that happens for trends longer than about 15 years, but for smaller areas with higher noise levels (like Antarctica), the time period can be many decades.

Are people going back to the earliest projections and assessing how good they are? Yes. We’ve done so here for Hansen’s 1988 projections, Stefan and colleagues did it for CO2, temperature and sea level projections from IPCC TAR (Rahmstorf et al, 2007), and IPCC themselves did so in Fig 1.1 of AR4 Chapter 1. Each of these analyses show that the longer term temperature trends are indeed what is expected. Sea level rise, on the other hand, appears to be under-estimated by the models for reasons that are as yet unclear.

Finally, this subject appears to have been raised from the expectation that some short term weather event over the next few years will definitively prove that either anthropogenic global warming is a problem or it isn’t. As the above discussion should have made clear this is not the right question to ask. Instead, the question should be, are there analyses that will be made over the next few years that will improve the evaluation of climate models? There the answer is likely to be yes. There will be better estimates of long term trends in precipitation, cloudiness, winds, storm intensity, ice thickness, glacial retreat, ocean warming etc. We have expectations of what those trends should be, but in many cases the ‘noise’ is still too large for those metrics to be a useful constraint. As time goes on, the noise in ever-longer trends diminishes, and what gets revealed then will determine how well we understand what’s happening.

Update: We are pleased to see such large interest in our post. Several readers asked for additional graphs. Here they are:

UK Met Office data (instead of GISS data) with 8-year trend lines
GISS data with 7-year trend lines (instead of 8-year).
GISS data with 15-year trend lines

These graphs illustrate that the 8-year trends in the UK Met Office data are of course just as noisy as in the GISS data; that 7-year trend lines are of course even noisier than 8-year trend lines; and that things start to stabilise (trends getting statistically robust) when 15-year averaging is used. This illustrates the key point we were trying to make: looking at only 8 years of data is looking primarily at the “noise” of interannual variability rather than at the forced long-term trend. This makes as much sense as analysing the temperature observations from 10-17 April to check whether it really gets warmer during spring.

And here is an update of the comparison of global temperature data with the IPCC TAR projections (Rahmstorf et al., Science 2007) with the 2007 values added in (for caption see that paper). With both data sets the observed long-term trends are still running in the upper half of the range that IPCC projected.


624 Responses to “Uncertainty, noise and the art of model-data comparison”

  1. 451
    Fred Staples says:

    It is very hard to see 3 degrees centigrade per CO2 doubling, Barton ( 391 )

    If we start from the satellite records from 1978, the overall regression line to date produces an increase of 0.41 degrees centigrade over 29 years. But that line starts from the temperature trough, down from the previous peak 35 years earlier.
    From that peak in the forties, Hansen quotes the fall in temperature as 0.2 degrees centigrade.

    Over 60 plus years, therefore, we have observed a temperature increase of a net 0.21 degrees centigrade. Ignoring the recovery from the little ice age to the forties, and using the Arrhenius version of the logn “saturation laws”, we have experienced 44% of the maximum temperature effect we can expect from doubling CO2.
    The full effect, due in the twenty-first century when CO2 reaches 560ppm, will be less than half of one degree.

    If you want to attribute the Ice Age recovery to CO2 increases, and go back more than 100 years, we have about 0.6 degrees overall for 44% of the maximum effect.
    That gives a temperature increase of 1.4 degrees for CO2 doubling, which is often quoted.

  2. 452

    Re #440 Barton,

    You are calculating the partial pressure using the volume (ppm) but you should be using density. The partia pressure depends on moles of gas which is the volume divided by the moleculatr weight. See http://library.thinkquest.org/12596/dalton.html

    HTH, Alastair.

  3. 453
    Tracy P. Hamilton says:

    Number 440 (Barton) “The volume fraction of CO2 is 384 ppm. Using a carbon dioxide
    molecular weight of 44.0096 AMUs and a dry air figure of 28.9644, I get a
    mass fraction for CO2 of 0.000583. That means there’s 2.99e15
    kg of CO2 floating around up there.

    Now, Dalton’s law of partial pressures indicates the CO2
    partial pressure should be .000384 * 101325 or 38.9 pascals. But when I
    put 2.99e15 into the equation above for atmospheric pressure, assuming
    only the carbon dioxide were present, I get 57.5 Pa. Why don’t the two
    answers match? I assume there’s some simple answer I’m missing here.”

    Try 384 ppm by weight, rather than volume.

  4. 454
    Hank Roberts says:

    Fred, rate of change. We affect addition; removal is by natural processes that have various rates, none of them relatively fast.

    If you trickle water into a bucket with holes in it, the level rises slowly. If you pour water into the same bucket, the level rises faster. If you use a firehose, you may break the bucket.

    Plus feedbacks — like melting permafrost –increase the fill rate further.

    Current rate of CO2 increase is some 100x past rapid events. The 3 degrees number is for where temperature levels off some centuries after we stop adding extra carbon to the atmosphere. Say “when” ….

  5. 455
    Ray Ladbury says:

    Rod, a quantum mechanical harmonic oscillator represents a bound state, and so the energy levels must be quantized. There will be multiple excited states (all multiples of the same fundamental frequency), but one (the 1st excited, I believe) particular state is relevant as the one that absorbs IR radiation. The higher states don’t absorb as strongly and are not in the IR.

  6. 456
    Chris Colose says:

    450

    actually when planck said that energy is quantized, that allowed him to compute the total irradiance of a blackbody for a given T, and is in fact important for determining Stefan-boltzmann constant (k). Not sure where this is an issue as far as problems with ray’s argument

    451

    You are ignoring feedbacks (1.2 K comes from deltaT(2x CO2) without feedbacks), and you are getting strange results for the temperature change, which is around 0.6 C over the last 3 decades, and just about 0.8 C over the last century. Moreover, we still have some warming in the pipeline even remaining at 380 ppmv like conditions

  7. 457
    Fred Staples says:

    The Physics in this post, particularly from Ray, is very quantum mechanical – “a molecule is either excited or it is not” (445).
    I have always believed quantum mechanics to be essentially a high energy phenomenon where classical physics breaks down. Incoming solar radiation will excite N2 and O2 to higher electron orbital energy levels, and they will re-radiate as you describe when they return to their original states. That is an effect inside the electron shell of which the classical physicists knew nothing.
    Does that happen at infra-red energies over a 30 degree temperature range? I doubt it. When Hug repeated the Angstrom experiment recently he saw a steady rise in temperature to a limit, without quantised discontinuities.

    At these energy levels we are on the wave side of the wave-particle duality, and your image of photons transferring from ghg molecule to ghg molecule is not convincing.

    The atmosphere is subjected to electro-magnetic radiation from the surface, which all matter will absorb to a greater or lesser extent, (stand in front of a fire), through molecular vibration and motion. Matter will conduct the heat, and radiate (by slowing down) at its surfaces, all the while observing the laws of entropy (at equilibrium, all the matter will come to the same temperature).

    If the ghg molecules absorb radiation through their vibration bands (manifested as peaks in their absorption spectra) will they not transfer kinetic energy to N2 and O2 and raise the air temperature by increasing the collision rate? Do we believe that N2/O2 molecules are mere bystanders, unaffected by their excited ghg neighbours?

  8. 458
    Rod B says:

    Ray (455), thanks. This makes sense with one exception: Since the first absorbing state would be exactly the same in all CO2 molecules, what causes the distinct line absorption spectra? A clarifying question: how do the higher vibration energy get filled? Only from collision?? By more IR absorption after level 1 is filled (and before it relaxes)??

    Chris (456) Planck’s quantum energy levels, ala Planck’s Constant are (almost) a totally different phenomena than the quantum (“discrete” might be a better word) molecular energy states of rotation, vibration, and electronic. [Molecular translation/kinetic energy tends more to Planck’s quanta.]

    Fred (457) I’ll let Ray address the first two paragraphs (if he wants…) as I’m still trying to absorb it (no pun intended).

    Your last paragraph is correct. A CO2 that has picked up energy by absorbing IR into a vibration band will relax that energy either by re-emission or via collision and transfer to N2 or O2 (most likely) molecular translation energy (=kinetic = heat), the latter seeming to prevail at higher atmospheric concentrations — low altitudes.

  9. 459
    Hank Roberts says:

    Um, Fred, it’s quantums all the way.
    There’s no duality.

  10. 460
    Ray Ladbury says:

    Fred, unfortunately, you cannot pick and choose where the laws of physics are quantum and where they are classical. If a system is in a bound state, its energies are quantized. Because a solid is composed of a lattice of atoms, it will have many different vibrational modes and will absorb much of the radiation incident on it. Gasses are quite different. A diatomic atom (undistorted by collision, etc.) has no modes that can be excited, while triatomic and above will absorbe only certain frequencies. When those frequencies are in the energy range of the blackbody spectrum of the planet, the gasses are greenhouse gasses. Collisions, and other interactions can affect the vibrational spectra–collisional broadening is one of the factors giving rise to the wings of the absorption lines. However, this does not change the basic nature of the phenomenon.
    Again, Fred, learning a little physics would pay dividends. It must be very difficult to argue against what you don’t understand.

  11. 461
    Ray Ladbury says:

    Rod, The line corresponds to single wavelength (actually a narrow range), frequency or energy. Since the energy levels of a harmonic oscillator are quantized–> hw(n+1/2), you get absorption and radiation at hw/2, 3hw/2, 5h2/w… The ground state is hw/2–that’s the lowest state of the molecule (i.e. it’s always vibrating). The transition that absorbs IR is (I think) the one from 3hw/2 to hw/2. There are no energy states between, and to populate the ones above, you’d need higher energy photons. Keep in mind that the coupling between different states varies–that is you have different probabilities of absorbing a photon for the different states. The 15 micron line for CO2 is a pretty strong line.

    A couple of references may help refresh your memory:

    http://www.wag.caltech.edu/home/jang/genchem/infrared.htm

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

  12. 462
    Steve Reynolds says:

    Alastair McDonald> Since strong El Ninos seem to happen roughly every sixteen years, then we can expect the next one within six years. The result could be disruption to global agriculture with a possibility of famine.

    Are you willing to take a bet that the next El Nino does not cause a statistically significant increase in famine?

  13. 463
    tharanga says:

    I have a silly question.

    I’ve read here that ENSO events affect the global mean temperature (as in 1998). Yet I understand ENSO to be an internal/local phenomenon, where heat is simply being redistributed. If this is so, shouldn’t the global mean temp be unaffected? Is it that the total heat content of the system (oceans + atmosphere + land) is unchanged by El Nino, but the mean surface temperature can still change?

    [Response: Not silly at all. Actually, there is a bit of a conversation above that addresses some aspects of that. I think most scientists would say that a) there is a lot of energy being redistributed during a El Nino, b) there is a clear connection between El Nino and global mean temperature, but also c) that because of cloud changes and surface temperature increases, they would be surprised if the net change at the top of the atmosphere was zero. It’s not known (to me at least) what the net change should be, but it’s certainly conceivable that the planet loses heat in such circumstances (all else being equal). – gavin]

    [Response: There is a lot of heat stored beneath the tropical Pacific ocean surface in the tropics. During an El Nino event, the trade winds weaken or dissappear, the the tradewind-driven upwelling of cold sub-thermocline (that’s the boundary between warm near surface and cold deeper waters) water which normally keeps surface temperatures low, ceases. Consequently, the surface warms up. Note that there need not be any net change in combined ocean/atmosphere heat content in this process, its just a redistribution that does indeed tend to elevate ocean surface tempertures over a large part of the tropical Pacific. That increased surface warmth is also available to be advected poleward by air currents. As Gavin notes, it is not inconcievable that these changes do lead to a net change in the planetary heat balance–one of the reasons we need as many remote sensed and in situ tropical ocean and atmosphere observations as we can get! -mike]

  14. 464
    Paul Klemencic says:

    Roger Pielke, in #429 You wrote: “Chris (No. 423)- How to respond on blogs when one’s work is misrepresented always requires choices (e.g., let the misrepresentation stand, or perhaps look like a complainer). For example, in your comment you obviously have me confused with someone else, as we’ve never turned our blog comments off. So if you have accurate, substantive critiques, please do share them, and I’ll be happy to engage. But do get your facts right first.”

    Chris isn’t the only one having difficulties posting on Prometheus. I have just tried again, to post a follow-up comment on your January 9th blog post, and no luck. I can log-in, but not post. I just gave up after awhile, and moved on.

    And the inaccurate information presented in the NY Times article, and sourced from you, lives on. John Tierney still has not printed a correction to the statistically fallacious chart printed there, even though in the comments posted, the chart was roundly discredited.

  15. 465
    Chris Colose says:

    # 463

    El Nino does involve “mining” some heat out of the ocean to put into the atmosphere, and that redistribution does tweak the global mean temperature; it may also affect albedo a bit but I never looked into that. There is not a perfect offset between the places that cool and the places that warm, and so there can be a small net tropical average warming.

    In general, the ocean and atmosphere are constantly exchanging heat back and forth on time scales characteristic of the mixed layer (so the planet does not need to be in radiative balance on very small timescales, and El nino does appear to tweak global temp when you look), and the changes in atmospheric circulation can affect clouds and WV, which could affect albedo or shortwave/longwave.

  16. 466
    Tilo Reber says:

    Chris,
    Since we have moved from 280 ppm CO2 to 390 ppm CO2 we are nearly 40% of the way to a doubling. The temperature effect of that 40% has been .6C. Why would you propose that we are going to get 2.4C more out of the remaining 60%. Considering that the forcing effect of CO2 is logarithmic, meaning the later part of that doubling contributes less forcing than the earlier part, and considering that the solar influence more likely contributed at least some warming over that time period, I simply cannot see how you get 3C per doubling.

    [Response: It’s hard to tease climate sensitivity out of the warming record so far, but Tilo does the arithmetic wrong and left out some important physics. First, the logarithmic effect. To use round numbers, 4*log2(390/280) = 1.91 W/m**2, and 4*log2(560/390) = 2.09 W/m*2, so you get about the same forcing again for the rest of the way to doubling. Then, Tilo’s ignored the fact that part of the radiative forcing to date is canceled by aerosols (which don’t grow so much in the future), and also that the ocean has delayed the warming so we haven’t seen the full equilibrium warming yet. –raypierre]

  17. 467
    Joseph O'Sullivan says:

    Roger Pielke Jr has very accurately wrote that there is pervasive tribalism in the global warming debates. He could get much further in his posts if he remembered this before he wrote his posts. ;)

    He could take a lesson from Bryan S who at first took a knee-jerk reaction to the ocean heat content and AGW issue, but now on this thread is taking the time to do the science and ask thoughtful questions. Bryan S pushes his position but does in in an informed and if I might say humble way.

    Its not what you say, its how you say it. If Roger Pielke Jr posed his questions in a more careful way, it could lead to a more instructive and enlightening discussion. He does ask interesting questions that could lead to informative discussion if he were to pose his questions better.

    I recognize his style, its very similar to the way law school classes are taught. The socratic method is the normal method in that arena, but it will be misunderstood by people, like scientists, who have been trained in a completely different way.

  18. 468

    raypierre wrote in the inline to 466:

    … and also that the ocean has delayed the warming so we haven’t seen the full equilibrium warming yet.

    Higher levels of carbon dioxide have made the atmosphere more opaque to thermal radiation, leading to a radiative imbalance at the top of the atmosphere where the rate at which energy in the form of sunlight enters the atmosphere has remained roughly constant (since about 1952, actually), but the rate at which energy in the form of infrared thermal radiation leaves the atmosphere has dropped. As long as this situation exists, the climate system will absorb more energy until it warms up enough to radiate sufficient thermal radiation and restore the balance between incoming and outgoing radiation.

    However, the oceans have a higher thermal inertia than land, meaning that they must absorb more energy in order to reach their equilibrium temperature. But in the process of warming up, the partial pressure of water vapor coming off the surface of the oceans will increase, leading to more atmospheric water vapor in a process known as “water vapor feedback.” Since water vapor is also a greenhouse gas, this will amplify the greenhouse effect of carbon dioxide — and the full equilibrium response will take a fair number of decades. Incidentally, the greater thermal inertia of the oceans is what explains why land warms more quickly than ocean, and even why the Southern Hemisphere has warmed more slowly than the Northern Hemisphere: the Southern Hemisphere has less landmass, more ocean.

  19. 469
    oyvinds says:

    Re #439

    Yes, I claim that fig 10.26 give a wrong impression w.r.t sulphur emissions.
    This is partly due to the fact that 10.26 is only valid up to 1990 for SO2. As seen from the different colours, the number for 2000 is a scenario, not an estimate. It also underestimates the reduction in Eastern Europe between 1985 and 1990. Updated numbers are given in section 2.4.4.1. The estimate are now a reduction between 15 and 30 % from 1980 to 2000. Anyhow, even with no SO2 reductions in this period, one should expect an increase in warming since concentrations of CO2 has continued to increase.

    An interesting phenomenon to study is that radiative forcing efficiency of aerosols vary strongly from region to region. The direct radiative forcing (“cooling”) is probably larger over south-east Asia than over Europe given the same concentration of aerosols.

  20. 470

    Fred Staples (#457) wrote:

    The Physics in this post, particularly from Ray, is very quantum mechanical – “a molecule is either excited or it is not” (445).

    That it is, Fred.

    That’s why you have the lines and bands in the spectra of greenhouse gases. With the HiTran database, they have recorded well over a million spectral lines, their intensities and locations. The result of precision lab experiments.

    Minus the collisions which bend the molecules, you can derive the spectra of the molecules from the first principles of quantum mechanics. Otherwise you would need the quantum mechanics and a great deal of computer power to achieve the somewhat better level of accuracy you can get in the labs. Doable, but expensive.

    Fred Staples (#457) wrote:

    I have always believed quantum mechanics to be essentially a high energy phenomenon…

    Well, fortunately physics isn’t a matter of someone’s opinion or beliefs. The stuff is repeatable and well-documented. It is obseved with a great deal of accuracy at the surface (the spectra of backradiation from the atmosphere) and from satellites, including satellites that are able to view the atmosphere in over 2000 channels.

    Here are some videos:

    Multimedia Animations
    http://airs.jpl.nasa.gov/Multimedia/Animations/

    … and here is a still you might like:

    NASA AIRS Mid-Tropospheric (8km) Carbon Dioxide
    http://www-airs.jpl.nasa.gov/Products/CarbonDioxide/

    It shows carbon dioxide at 8 km. You will notice the plums rising off the heavily population East and West coast of the United States.

    What is being measured is the infrared radiation being absorbed and then reemitted by carbon dioxide. The thicker the carbon dioxide, the more opaque the atmosphere becomes to the infrared radiation in that channel. So in essence, you are seeing the enhanced greenhouse effect in action when you look at that photo. And we are able to do the same thing with water vapor and methane — a few of the videos deal with those.

  21. 471
    Fair weather cyclist says:

    Re #430

    Note that euballing the charts shows that there is a trend for El Ninos to get stronger and La Nnas to get weaker.

    That does seem to be the case on first inspection (and maybe closer inspection too). It would seem a reasonable conjecture that this be due to background AGW superimposing itself on the natural ENSO variation. However, I don’t think that one can simultaneously hold that view (or even the possibility that it might be correct) and ascribe short term deviations from the AGW warming trend to the magnitude of a nino/nina. This is an example of the potential for circularity I mentioned in an earlier post. I’d be grateful for any comments as always.
    on the natural

  22. 472
    Pekka J. Kostamo says:

    Trying to understand the ENSO impact on global temperature.

    La Nina phase has a high temperature pool on the Asian side. This means high evaporation rates. A part of the excess water vapor returns east in the Walker circulation, but another part moves south or north to the middle latitudes. In the La Nina case it soon encounters land masses in Asia, Indonesia and Australia, causing heavy precipitation there.

    During the El Nino phase the process is similar starting from the hot pool close to American coasts, except that there is no land masses nearby. I presume that the water vapour then is mixed and distributed more widely before it is precipitated as rain, further away.

    The difference would mean that in the El Nino case the water vapor stays a longer time in the atmosphere, trapping more IR in the process.

    Gavin stated somewhere that the relative humidity is considered rather constant (in case of a warming atmosphere). Regional short term variations certainly exist, but could this radiation mechanism cause the apparent connection between ENSO and the global temperature? In addition to the latent heat transport, of course.

  23. 473
    Nick Gotts says:

    Re #462 #

    Steve Reynolds Says>

    Alastair McDonald>>
    Since strong El Ninos seem to happen roughly every sixteen years, then we can expect the next one within six years. The result could be disruption to global agriculture with a possibility of famine.

    >Are you willing to take a bet that the next El Nino does not cause a statistically significant increase in famine?

    That is rather a tasteless offer: if Alastair accepts it, he puts himself in the position of standing to win money through an increase in the number of people dying of hunger.

  24. 474

    Tracy — the 384 ppm figure is by volume, not by mass. The mass figure would be 583 ppm. And the paradox still stands. I’m assuming the physical pressure equation isn’t appropriate for one constituent of a mixed atmosphere, but I don’t know why. Does anybody here know? Preferably a professional.

  25. 475
    Alan K says:

    #218, 222, 223 (an age ago it seems..)

    RESPONSE FROM HADLEY CENTRE:

    “Dear Alan, Thank you for your email. Your comment #221 in the RealClimate tread was in response to #218.This particular thread was concerned with the observations of temperature rise and not with the Hadley Centre climate model. The HCclimate model of course has polar amplification just as every other climate model does. The point was the interpolation of existing observational data over the polar regions. If you look at the raw observations that GISS uses you can see how little data they are basing an interpolation on. Regardless of what they consider the correct spatial length scale for observations,the Arctic sees large regional changes in temperature, which are being glossed over with a large correlation length. The Had/CRU treatment of the observations simply states that the error is greater due to lacking data, [edit]. There are no EXTRA observations that GISS has access to, that Had/CRU does not. Thus there is no reason to believe GISS’ observations vs Had/CRU observations of recent global temperature rise when the errors are taken into account. Kind Regards, ”

    [Response: Can you forward me who sent this to you? It is true and freely acknowledged that everyone is working from the same data, and that the differences between the products is a measure of the uncertainty in those products. But the different approaches are all reasonable ways of dealing with the data. – gavin]

  26. 476
    Chris Colose says:

    466 Tilo and raypierre

    I don’t think the aerosols play a huge role in this discussion because it would be better to say “Aerosols have offset everything but CO2″ (e.g. methane, N2O, trop. ozone). In fact, the RF for CO2 and the total net RF is roughly the same. Right now, we’re simply talking about the RF of 2x CO2; if it were all GHG’s, or all things that would be a new story. In that sense, if aerosols stay the same or decline, and the other GHG’s rise, their effects will show up a bit more. I would guess you can roughly estimate the future temperature change using just CO2 + feedbacks, though I do not know how you can calculate the forcing from water vapor feedback, just that it goes up 7%/K from Clausius-Clapeyron. This and ice-albedo haven’t shown their ugly face yet. Dr. Pierrehumbert’s comment about the warming “still in the pipeline” still applies, which is probably about half a degree to a degree, so if you use 5.35 ln (Cf/Ci) you need to realize that is an equilibrium concept, and we are not yet at 380-ppm like conditions.

    Actually, for those who say that the real sensitivity is ~1.2 C per 2x CO2, I would say that at equilibrium we have already hit that.

  27. 477
    Douglas Wise says:

    re #447. Thanks, Hank, for correcting my loose terminology in stating that CO2 molecules reflected photons (as opposed to re-emitting them) back to the surface. However, you triggered the following thought: SO2, a triatomic molecule and therefore having a dipole moment, is presumably able to absorb and emit photons of an appropriate wavelength. In the case of SO2, this must be in the visible rather than infra red range. I have read that SO2 contributes to global dimming through an albedo effect (by reflecting solar radiation out of the atmosphere). Should one more correctly state that SO2 absorbs solar radiation and re-emits some of it out of the atmosphere? If not, why not?

  28. 478
    Hank Roberts says:

    Douglas, “presumably” is not your friend. If you got

    > SO2, this must be in the visible

    from a source other than your own logic, would you tell us what source you’re relying on and why you consider it trustworthy? If from your own logic, I’d suggest checking the math or asking someone more competent than me for help.

    It’s important to check what you believe, even if you are sure it was true yesterday.

    Google is your friend. Google Scholar can be too.

    Try this search for starters.

    http://www.google.com/search?q=sulfur+dioxide+spectrum+absorbtion

  29. 479
    Hank Roberts says:

    Douglas, a bit more for fun:
    This (a lab notebook, nothing fancy, but interesting tidbits I think relevant to your question)

    http://www.thecatalyst.org/experiments/Finholt/Finholt2.html

    That led to this (it’s a web archive, you may find a current version if you look around at NIST)

    http://web.archive.org/web/20010516030227/webbook.nist.gov/

  30. 480
    Ron Taylor says:

    This is OT – sorry. Stu Ostro of the Weather Channel has been posting some very interesting studies of the relationship between global warming and weather (for which he has been hammered by trolls). He has especially remarked on the appearance of persistent, intense high pressure ridges that are contributing to extreme weather events, and other atmospheric features that are appearing in the wrong places or at the wrong times. The latest is here:

    http://climate.weather.com/blog/9_14587.html

  31. 481
    Ray Ladbury says:

    Douglas, That’s not quite what happens. Unlike CO2, SO2 is pretty reactive, so it doesn’t stay a gas, but rather condenses into tiny particles. The particles actually reflect light like a tiny mirror. So it’s not molecular spectra that are relevant but rather the effects of the SO2 aerosols.

  32. 482

    Re 477 Where Douglas asks “Should one more correctly state that SO2 absorbs solar radiation and re-emits some of it out of the atmosphere? If not, why not?”

    No, SO2 is extremely hygroscopic and combines with the water vapour in the air to form sulphate aerosols which reflect (scatter) solar radiation. So CO2 absorbs and re-emits thermal radiation. SO2 reflects solar radiation.

    SO2 would act as a greenhouse gas in the 8 micrometre band if water vapour wasn’t present, as that is where its vibrational frequency is.

  33. 483

    Re 462 where Steve Reynolds asks:

    “Are you willing to take a bet that the next El Nino does not cause a statistically significant increase in famine?”

    If I am correct there is a strong possibility of social disorder. In that case it is unlikely that I would be able to collect my bet. Would you really hand over $1000 dollars to me rather than use it to by food for your wife and kids?

    It is very easy to bury ones head in the sand and say that disasters never happen. But the Boxing Day tsunami happemed, the Katrina hurricane happened, and the First World War happened. Are we really more civilised and less greedy than those people who cheered when war was declared on Germany, and who were unable to prevent a repeat in 1939?

  34. 484
    Hank Roberts says:

    Oh, one more for Douglas, I missed the obvious error!
    You asked about sulfur dioxide. Look up _sulfate_ to understand how aerosols reflect. Different chemical. These change over time in the atmosphere.

    E.g.
    http://www.google.com/search?q=aerosol+sulfate

  35. 485
    B Buckner says:

    Timothy #468
    Everything you say here is straightforward and uncontroversial. It takes more energy to heat the water than the land or air. The ocean surface has shown a smaller increase in temperature than the air over land. OK. What I don’t understand is the lag, or decades required for equilibrium. The oceans are heated directly by solar radiation, and are warmer by a few degrees C than the atmosphere near the surface. The atmosphere is not heating the ocean, although a warmer CO2 laden atmosphere will slow down the cooling of the ocean (effecting its energy budget) and result in a warmer ocean. But why the lag? It takes more energy to heat the ocean but why does it take more time?

  36. 486

    #475, There is other ways to confirm NASA GISS trend. It is utterly wrong to suggest that there has not been significant warming in the Arctic, especially since 2005, all leading to a massive ice melt of 2007, and by the looks of it 2008. Vertical sun disk diameters particularly exploded in size in 2005. Atmospheric refraction is directly proportional to the density (coldness) of the atmosphere as whole (its a little more complicated by the sum effect of multiple density layers). Looking through 5 to 20 atmospheres by using the sun as a fixed sphere of reference is a powerful analysis
    of a huge swath of the atmosphere. High Arctic oblate sun results confirm NASA GISS, totally reject Hadley if the met office shows no significant warming in the Arctic, which I find unbelievable if they do.

  37. 487
    Martin Vermeer says:

    Re #440 (Barton): Thank you for a very good question! You made me think real deep about something that also for me was unexpected.

    Indeed the partial pressure of CO2 (at sea level) and the weight of the CO2 fraction of the atmosphere are not equal — 38.9 Pa vs. 57.5 Pa. Nor should they be.

    Consider it this way: if you could magically remove all the oxygen, nitrogen, argon etc. and only be left with CO2, the resulting atmosphere would have a scale height of only (29/44)*8 km instead of the current 8 km. In other words, the CO2-only atmosphere would sag down, most molecules moving downward, and density at sea level would increase 44/29 times. Only then would pressure at sea level become equal to the weight of this atmosphere.

    In the current situation, where turbulence etc. keeps the atmosphere well mixed, CO2 is spread out more in the vertical direction than in the above, hypothetical CO2-only atmosphere (or equivalently, in a stagnant atmosphere where each molecular species finds it own scale height, on a very long diffusion time scale). It is the N2, O2, Ar etc. molecules that (on average) are exerting an effective uplift on the CO2 molecules. This uplift equals the difference 57.5 – 38.9 Pa that you found.

    Does this make sense?

  38. 488
  39. 489
    Hank Roberts says:

    Mr. Buckner, what’s your source for the statements you make above, including

    > The oceans … are warmer by a few degrees C
    > than the atmosphere near the surface. The
    > atmosphere is not heating the ocean …

    This is true in some locations and probably true if you consider the overall global average, but that’s not how temperature moves.

    What you state is certainly not true where upwelling from deep water replaces warm water with cold, for example.

    This may help:
    http://www.atmos.washington.edu/2004Q4/101/lect13_overheads.pdf
    http://eesc.columbia.edu/courses/ees/climate/lectures/o_atm.html

  40. 490
    henry says:

    Re #475:

    Recently on Open Mind, Tamino took the GISS and Had/CRU curves and merged them in the same chart. Other than the .1 degree zero offset (due to the use of different reference periods), the curves follow nicely.

    This seems to show that GISS’ extrapolation of the Arctic sites doesn’t add to the global temp.

  41. 491
    Rod B says:

    Ray (461), just to be certain before I get to far in what looks like great info: I assume hw is h-bar X omega. Correct?

  42. 492

    B Buckner (#485) wrote:

    Timothy #468
    Everything you say here is straightforward and uncontroversial.

    Sometimes it helps, I figure, to explicitly state what may be well-understood with a simple allusion by the regulars but, if stated with too much brevity may be misunderstood by the newcomers.

    B Buckner (#485) wrote:

    It takes more energy to heat the water than the land or air. The ocean surface has shown a smaller increase in temperature than the air over land. OK. What I don’t understand is the lag, or decades required for equilibrium. The oceans are heated directly by solar radiation, …

    Correct, but they are also absorbing backradiation from the atmosphere which increases over time as the result of more water vapor — acting as a greenhouse gas.

    B Buckner (#485) wrote:

    … and are warmer by a few degrees C than the atmosphere near the surface. The atmosphere is not heating the ocean, although a warmer CO2 laden atmosphere will slow down the cooling of the ocean (effecting its energy budget) and result in a warmer ocean….

    The same applies to water vapor. And as the water vapor content of the atmosphere increases, the atmosphere becomes more opaque to infrared radiation.

    B Buckner (#485) wrote:

    But why the lag? It takes more energy to heat the ocean but why does it take more time?

    In part because it is receiving solar radiation at the same rate, and to raise the temperature to the same degree will require more energy — and this can only come from the difference between incoming and outgoing radiation. In part because the atmosphere is becoming more opaque over time due to increased levels of water vapor as the ocean warms — further raising the equilibrium temperatures that the oceans must achieve if there is to be a balance between incoming and outgoing radiation at the top of the atmosphere.

    *

    However, another factor is that the energy which the ocean receives becomes distributed as the result of ocean circulation, and since it becomes distributed, with a portion of it being removed from the surface layers, the ocean surface will have to receive more energy in order to warm to the same degree. With a simple “block” model of the ocean that involves no ocean circulation, the majority of the rise in temperature will take place within only a few years. Consequently you will get a smaller climate sensitivity with the slab model.

    But when we take into account the fact that the ocean is not a solid slab and that it takes time for the additional heat content to become distributed (what one might call the “equilibrium distribution”), we find that the majority of the rise in temperature takes decades. For this reason, those who are intent on claiming that climate sensitivity is low seem to have an almost fatal attraction for slab oceans.

    Just this year I believe we have seen a couple of papers come through which used slab oceans, and at least one was peer-reviewed. I remember that with the latter of the two papers there was a fair amount of press from many of those who think that General Climate Models with forty layers of atmosphere and twenty layers of ocean are too simple to capture the behavior of the climate system. They thought that the study using the single layer slab model of the ocean had conclusively shown that we were overestimating climate sensitivity. Funny how that works. And underestimating climate sensitivity as they do may very well prove to be a fatal mistake — for much of the world’s population — given the floods, drought and famine implied by the higher, more realistic climate sensitivity.

    *

    Anyway, I should have mentioned the bit about ocean circulation earlier, but I was trying to keep things short, and not being a climatologist, I didn’t recall it until seeing your post. Thank you — the bit about ocean circulation needed to be included since (if I remember correctly) this is the biggest reason for the lag.

  43. 493

    Re #488 where # Barton Paul Levenson Says: “I have no idea.”

    If you are going to calculate using mass then you have to use moles not kilograms. In other words you have to divide the mass of gas by its molecular weight.

    Surely it does not need a “professional” to point out that it is moles of ideal gases which have the same volume 22.4 litres IIRC.

    Cheers, Alastair.

  44. 494
    B Buckner says:

    Timothy – thanks for taking the time to prepare a thorough and thoughtful response. It was helpful to me.

    Hank – I was referring to global average temperatures. Using 20th century averages, the average land surface temperature is 8.5 C and the average sea surface temperature is 16.1 C.

    http://www.ncdc.noaa.gov/oa/climate/research/anomalies/anomalies.html#means

  45. 495
    Ray Ladbury says:

    Rod, Yes–best I could do with an English keyboard.

  46. 496

    Global Temperature Trends: NASA GISS vs. Hadley Cru

    henry (#490) wrote:

    Recently on Open Mind, Tamino took the GISS and Had/CRU curves and merged them in the same chart. Other than the .1 degree zero offset (due to the use of different reference periods), the curves follow nicely.

    This seems to show that GISS’ extrapolation of the Arctic sites doesn’t add to the global temp.

    I found it.

    From the essay:

    I’ve plotted the two data sets using different y-axes, because they’re on a different scale. That’s because NASA GISS computes anomalies compared to the 1951-1980 baseline while HadCRU uses the 1961-1990 baseline.

    Clearly there’s a strong warming trend in both data sets, with GISTEMP data rising at 0.018 +/- 0.003 deg.C/yr, HadCRU at 0.019 +/- 0.003 deg.C/yr. We can also see some some sizeable ups and downs, like the cooling for a few years around 1992 caused by the Mt. Pinatubo explosion, and the strong warming in 1998 caused by el Nino.

    Garbage is Forever
    August 31, 2007
    http://tamino.wordpress.com/2007/08/31/garbage-is-forever

  47. 497

    B Buckner (#494) wrote:

    Timothy – thanks for taking the time to prepare a thorough and thoughtful response. It was helpful to me.

    Not a problem. Writing longer pieces helps me learn and forces me to think through the issues.

    Incidentally, I might not do a one-to-one comparison, though, between sea and land surface temperature. Most of the land is at higher latitudes and there is plenty of ocean around the tropical middle. Oceans at the same latitude may still be warmer, but probably not by as much as the global ocean vs. global land suggests.

  48. 498
    Arch Stanton says:

    B Buckner (485), at the risk of being as presumptuous as to add to what Timothy (492) has said; I will point out that seasonal warming rarely extends much more than a meter on the land (less than half of the earth’s surface). Because part of the ocean is constantly overturning much of the ocean’s warmed upper layer is constantly forced down to warm areas much deeper than one meter (indeed hundreds if not thousands of meters) below the “earth’s” surface. It will take hundreds of years for this previously “warmed water” to resurface and when it does so it will present the “delayed” effect you described.

    Besides the overturning factor described you must realize that the surface of the ocean is subject to mixing that has little comparable equivilent on the land. Local turbulence mixes the upper few meters of the ocean. This is driven by local winds, waves, fish, passing boats, tides, toddlers in inner tubes, etc. The closest terrestrial equivalents to these factors are earthworms, gophers and human earth moving machinery including toddlers in sandboxes. Because water is more fluid than soil, this “cooling effect” is more dominant on water than it is terrestrially.

    I hope this helps

  49. 499
    Arch Stanton says:

    B Buckner (484), at the risk of being as presumptuous as to add to what Timothy (498) has said; I will point out that seasonal warming rarely extends much more than a meter on the land (less than half of the earth’s surface). Because part of the ocean is constantly overturning much of the ocean’s warmed upper layer is constantly forced down to warm areas much deeper than one meter (indeed hundreds if not thousands of meters) below the “earth’s” surface. It will take hundreds of years for this previously “warmed water” to resurface and when it does so it will present the “delayed” effect you described.

    Besides the overturning factor described you must realize that the surface of the ocean is subject to mixing that has little comparable equivilent on the land. Local turbulence mixes the upper few meters of the ocean. This is driven by local winds, waves, fish, passing boats, tides, toddlers in inner tubes, etc. The closest terrestrial equivalents to these factors are earthworms, gophers and human earth moving machinery including toddlers in sandboxes. Because water is more fluid than soil, this “cooling effect” is more dominant on water than it is terrestrially.

    I hope this helps

  50. 500
    Hank Roberts says:

    > global average temperatures

    Without knowing the waves, wind, currents, and much else, the global average number isn’t going to be useful as a way to decide how heat’s transferred. Sunlight penetrates a good ways into the ocean. Measurements have been done to come up with numbers useful for modeling, worth looking these up I think.

    One example, rather old, from one set of cruises:
    http://www.agu.org/pubs/crossref/1996/95JC03205.shtml

    “Accounting for all factors, the net surface heat transfer to the ocean was 17.9 ± 10 W m−2….”

    Just an example, mind, not meant to be a global average number.

    Joan Baez said in a radio interview a few years ago, “when faced with a choice between a hypothetical situation and a real one, always take the real one.”

    Perhaps she learned that from her physicist father.


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