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Explain the Medieval Warming Period Edit
The romans grew grapes in the UK, so it must have been warm then. -- Andrew Dodds, summarizing Plimer at Open Mind From the distribution of the vines, olives, citrus and grain crops in Europe, UK and Greenland, calculate the temperature in the Roman and Medieval Warmings and the required atmospheric CO2 content at sea level to drive such warmings. What are the errors in your calculation? Reconcile your calculations with at least five atmospheric CO2 proxies. Show all calculations and justify all assumptions.
* Relevance: 0 - poor. Basic logical fallacy. The existence of prior warm periods that may have been caused by different effects (such as solar changes, orbital variation, continental configuration etc.) does not imply that the human-caused increase in CO2 is not causing warming now.
* Scientific Content: D - phenology (the distribution and timing of species) can potentially be useful for tracking climate changes, but it is just one of many different types of proxy information available, and has its own regional, temporal, and seasonal limitations. Even more problematic, it is well known that the patterns of surface temperature variability during the "MWP" - more accurately, the Medieval Climate Anomaly (MCA) - and LIA periods were spatially quite heterogeneous, and a record at one or two locations generally tells us very little if anything about global patterns. Even a cursory examination of the relevant recent literature (for instance, Osborn and Briffa, 2006) reveals that the pattern of warmth during the Medieval era was far regional in nature, and does not approach the truly global scale of warmth evident in recent decades.
* Sources: Greater extent of vineyards today in England than in medieval times. Ice core records. Incoherence of the Medieval warm period. -- Gavin Schmidt, grading Plimer at RealClimate
It is rather strange why Plimer requests agriculture as a proxy for paleotemperature (and what's stranger is what this has to do with anything). The answer about the error is simply that it will be 'large' and will not be indicative of global temperatures, even if temperature is the dominant climatic variable picked up by changes in vines and crops. What's more interesting is the statements about Carbon Dioxide. Plimer plays an interesting trick here and assumes that these climate changes were in fact caused by Carbon Dioxide (which they weren't). The question about CO2 content at sea level is a hypothetical question (how much CO2 would it take to cause the climate change?), but then he asks Monbiot to reconcile this with observations (I'm not sure why he needs five, I'd just use ice core records here as a starting choice.) But what if CO2 didn't cause those warmings? In that case, Monbiot could answer the hypothetical first question, but there would be nothing to answer for the second part of the question since the justification itself would be expected to be void. It's like if someone asked me how much the globe would cool if we removed all the CO2 from the atmosphere (which is an interesting question) but then asking me to back that up with proxy data, as if this actually happened before. Even to answer the first question about 'CO2 content' would include some caveats since it's the *change* in CO2 that matters for driving warming, not the absolute content, and it's the fractional increase (not the absolute increase) that matters and so you'd need to know the baseline value of CO2. This is not difficult to get from proxies (and not much different from pre-industrial values) but the question as stated makes little sense. Furthermore, CO2 is well-mixed in the atmosphere and so there's no reason to confine the question to sea-level, especially since CO2 in the upper atmosphere matters significantly for radiative transfer.-- Chris Colose, Climate Change 
The natural proxies including species distribution and farming conditions we have now have been observed with more precision than anything going on in the Medieval Warm Period, so the error for that period from natural proxies has to be far higher than for similar proxies now -- Marion Delgado, commenting at Climate Change• Kola Peninsula temperature during the medieval warming period. doi:10.1016/j.palaeo.2004.02.018 This calculates multi-proxy using tree cover, seasonal tree growth, and lake beds. However, much of this is evidenced by warmer summer temperatures, not warmer temperatures in general. The link between solar activity and temperatures before 1900 is in good agreement, but breaks down after 1900 doi:10.1016/j.asr.2004.01.020 That the MWP is not global in nature can be seen from surveys of other areas during the same period http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm06&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm06%2Ffm06&maxhits=200&=%22PP43A-1228%22. That the MWP is natural is indicated by survey data, for example: Climate and Human Influences on Global Biomass Burning Over Past Millennia Marlon, J
The medieval warming period is explained relatively well, according to the proxies of solar activity, by solar forcing, but present warming is not. The MWP is in fact evidence that previous warming periods could be explained by solar forcing, but present activity is not. More over, this good explanation of previous warming periods would indicate that other effects that are mentioned for long term changes in temperature, as opposed to short term variance, are not important, otherwise would would see less correlation between solar forcing and temperature proxies. An example of short term variance that differs from global temperatures from local signals can be seen in Box's Survey of Greenland instrumental temperature records: 1873-2001, which shows that Greenland has been cooling recently, even when there is indisputable evidence of global increase in surface temperatures. However, it also shows how short these changes are, which would tend to rule out long term cyclical changes based on vulcanism.
Also, the above data, and other surveys from other areas correlate decreased humidity with natural increase in temperature during the MWP, for example Effects of Past Climate Changes on Ecosystem Biogeochemical Cycles in Rocky Mountain Forests and Lakes Shuman, B et al This answers the assertion that water vapor is a driving natural green house gas. Stirling Newberry 02:13, September 15, 2009 (UTC)
Rule out volcanos I Edit
All the CO2 comes from Volcanoes.-- Andrew DoddsTabulate the CO2 exhalation rates over the last 15,000 years from (i) terrestrial and submarine volcanism (including maars, gas vents, geysers and springs) and calc-silicate mineral formation, and (ii) CH4 oxidation to CO2 derived from CH4 exhalation by terrestrial and submarine volcanism, natural hydrocarbon leakage from sediments and sedimentary rocks, methane hydrates, soils, microbiological decay of plant material, arthropods, ruminants and terrestrial methanogenic bacteria to a depth of 4 km. From these data, what is the C12, C13 and C14 content of atmospheric CO2 each thousand years over the last 15,000 years and what are the resultant atmospheric CO2 residence times? All assumptions need to be documented and justified.
* Relevance: 0 - pure misdirection.
* Scientific Content: F - We know what CO2 and CH4 levels have been over the last 15,000 years and they oscillated within about 10 ppmv (CO2) and 100 ppbv (CH4) of their Holocene values since the start of the current era - until the industrial period (around 1750) since when CO2 has increased by 35%, and methane concentrations have more than doubled. In each case the values being measured today are way higher than anything measured in 800,000 years of ice core records, and likely higher than anything since the Pliocene (~3 million years ago). The idea that bacterial methane production at 4km in the Earth's crust has anything to with this is laughable.
* Sources: The IPCC FAQ is all that is required. Do volcanoes produce more CO2 than human activity? Not even close -- Gavin Schmidt
The whole point here is to sound smart. There's nothing here remotely relevant to what Monbiot wanted to debate. One can certainly pull up tree ring or ice core data to get C13/C12 ratios as a time-series, or (probably more relevant) CO2 concentration in the atmosphere whose fluctuations will tell you something useful about net emission/uptake and whether the carbon cycle has been perturbed, as well as residency time. We don't know emissions from every individual source and there's also not much point in throwing the 'From these data' connection since certain proxies record CO2 content in the atmosphere to pretty high accuracy without knowing where it came from. -- Chris Colose 
Rule out clouds Edit
There is uncertainty over the effects of clouds, therefore there is no CO2-greenhouse effect. -- Andrew DoddsFrom first principles, calculate the effects on atmospheric temperature at sea level by changes in cloudiness of 0.5%, 1% and 2% at 0%, 20%, 40%, 60% and 80% humidity. What changes in cloudiness would have been necessary to drive the Roman Warming, Dark Ages, Medieval Warming and Little Ice Age? Show all calculations and justify all assumptions.
* Relevance: 3 - clouds certainly have an effect on climate and understanding their variability is the subject of much research.
* Scientific Content: F - The question makes no sense. Clouds at 0% humidity? Is humidity supposed to be globally uniform? And where should these cloud changes occur? The change for low-level clouds will be of the opposite sign to changes in high level clouds, and changes in the Arctic will give different answers than changes in the tropics. It should go without saying that Plimer is mistakenly assuming that he has accurate information for global temperatures over 2000 years.
* Sources: Cloud Feedbacks in the Climate System.-- Gavin Schmidt
Changes in cloudiness at 0% humidity indeed!!
Obviously some of these situations are unphysical and so preclude any realistic calculation and cloud change is generally thought to be a feedback. It also depends not only on cloud amount but distribution of cloud type, as changing high clouds and low clouds (or for example one could decrease the area coverage but increase the cloud top altitude) would have different, even competing effects.
One crude estimate presented by Dennis Hartmann is his book 'Global Physical Climatology' is that the fractional area of cloud cover is about 50% and has a net -20 W/m2 impact on the energy balance, and thus the partial derivative of the net radiative energy input at the TOA with respect to the total fractional area of clouds would imply that a 10% change in cloud fraction would either offset or double the RF for a doubling of carbon dioxide.-- Chris Colose 
Um. I don't think "The Dark Ages" is a climate epoch, and I don't think the term is really in vogue anymore amongst historians. Unless Plimer is suggesting that the Dark Ages were dark because it was extremely cloudy. MarkG commenting at Deltoid 
He's trying to raise the "Iris effect." There is no need to engage in these calculations when empirical measurements show that the iris effect is a positive, not negative, feedback loop. That is, during naturally occurring warming periods, the atmosphere doesn't have water vapor as a driving green house gas, but instead the air gets drier. During periods of green house gas based warming, the atmosphere gets wetter, and clouds hold in more radiation than they reflect. Lin, B., B. Wielicki, L. Chambers, Y. Hu, and K.-M. Xu (2002). "The Iris Hypothesis: A Negative or Positive Cloud Feedback?". J. Clim. 15 (1): 3-7. doi:10.1175/1520-0442(2002)015<000
The only supporters of the iris effect as a negative feedback are in the fossil fuel industry, and even their most special pleading offers no substantial negative feedback. It's tobacco science. So yes, clouds are ruled out from a solar forcing model, because under solar forcing the atmosphere is drier in the Holocene. Stirling Newberry 03:51, September 15, 2009 (UTC)
Rule out volcanos II Edit
All the CO2 comes from Volcanoes.Truly bizarre attack on the isotope evidence proving otherwise -- Andrew DoddsCalculate the changes in atmospheric C12 and C13 content of CO2 and CH4 from crack-seal deformation. What is the influence of this source of gases on atmospheric CO2 residence time since 1850? Validate assumptions and show all calculations.
* Relevance: 0 - completely irrelevant.
* Scientific Content: F - for those that don't know 'crack-seal deformation' is a geologic process that causes the veins of crystals/minerals etc. in many rock types. (see here). Its relevance to atmospheric concentrations and isotopic composition is absolutely zero. It has no influence on atmospheric residence time - whether since 1850 or at any time in the past.
* Sources: Discussions of the actual carbon cycle and the real influences upon it.-- Gavin Schmidt
Get around CO2 follows T I - explain faint sun paradox Edit
There was more CO2 in the air in previous ice ages [when the sun was fainter], therefore CO2 does not drive temperature. -- Andrew DoddsFrom CO2 proxies, carbonate rock and mineral volumes and stable isotopes, calculate the CO2 forcing of temperature in the Huronian, Neoproterozoic, Ordovician, Permo-Carboniferous and Jurassic ice ages. Why is the "faint Sun paradox" inapplicable to the Phanerozoic ice ages in the light of your calculations? All assumptions must be validated and calculations and sources of information must be shown.
* Relevance: 0 (again). The acknowledged climate changes in the past caused by natural events in no way implies that human effects are negligible today. Does the existence of forest fires caused by lightning imply that arson can never happen?
* Scientific Content: C - There is a lot of interesting science related to deep time, but any discussion of such changes must be prefaced with the acknowledgment that our knowledge of greenhouse gases, temperatures or any other potential forcing or response is very limited resume writingcompared to what we know about climate today or even in the last ice age. Given that we don't know precisely what CO2 levels were (let alone CH4, N2O, ozone, aerosols, ice sheet configurations, vegetation distribution etc.), the attributions of climate change at this distance is speculative at best.
* Sources: The faint young sun paradox.-- Gavin Schmidt
This is the same argument used by Young Earth Creationists, and in fact the question is the same tactic used by YECs to question evolution: use an area of research within science to attack science, because it isn't as settled as scripture is. The most recent and interesting explanation is http://www.ku.dk/english/news/ global warming from COS and the stronger solar wind sheltering the earth from cooling effects of cosmic rays http://adsabs.harvard.edu/full/2002ASPC..269...85G. These effects, given even a wide range of uncertainty, account for the temperature difference, and as importantly, the temperature variability of the periods he's asking about.
So the answer is "Carbon Dioxide wasn't the forcing gas, and while the sun put out less heat, it put out more solar wind, and thus the earth cooled less from cosmic rays." Stirling Newberry 04:06, September 15, 2009 (UTC)
Get around CO2 follows T II Edit
The increase in CO2 comes from the oceans heating, not the other way around. -- Andrew DoddsFrom ocean current velocity, palaeotemperature and atmosphere measurements of ice cores and stable and radiogenic isotopes of seawater, atmospheric CO2 and fluid inclusions in ice and using atmospheric CO2 residence times of 4, 12, 50 and 400 years, numerically demonstrate that the modern increase in atmospheric CO2 could not derive from the Medieval Warming.
* Relevance:1 - There are amplifying feedbacks between climate and CO2 which are most evident in the long ice cores from Antarctica, but this argument is trivial to dismiss without any recourse to ocean current velocities etc.
* Scientific Content: D - You can calculate the change in CO2 per deg C global warming over long (multi-centennial) timescales from the ice age data - it's roughly 100ppmv/5°C = 20 ppmv/°C. The increase in atmospheric CO2 in the last 200 years is now about 110ppmv, implying that any natural driver would have need to have been more than 5°C natural warming in recent centuries. This would have been noticed by someone.
* Sources: None required. -- Gavin Schmidt
Basically in an overcomplicated way, Plimer is asking to show that the modern rise in CO2 is not a feedback from medieval warming. This is probably some offshoot of the whole 'CO2 lags temperature' line. One issue here is that the perturbation lifetime of CO2 is different than the 'lag time' which occurs as a response to warming. Even then no one expects an abrupt decadal scale rise of CO2 centuries after the event, which isn't even what occurred in the glacial-interglacial cycles, which were considerably larger in magnitude and spatial extent then the MWP. The rate of change is today of CO2 is orders of magnitude larger, and the absolute concentration is also much higher than the whole ice core record show and even much longer. We also know that today's CO2 increase is anthropogenic because CO2 is going into the ocean (ocean acidification, duh) not going out and isotopic signatures of light and heavy carbon changes. -- Chris Colose 
Yes, Plimer is really arguing that recent CO2 increases were caused by Medieval Warming. Isotopes measurements show that this increase mainly came from burning fossil fuels, but my link goes to RealClimate so Plimer will dismiss it with one of his usual ad hominem attacks. -- Tim Lambert Deltoid 
Rule out aerosols I - space dust Edit
There is some uncertainty over aerosols, therefore there is no CO2-greenhouse effect. -- Andrew DoddsCalculate the changes in the atmospheric transmissivity of radiant energy over the last 2,000 years derived from a variable ingress of stellar, meteoritic and cometary dust, terrestrial dust, terrestrial volcanic aerosols and industrial aerosols. How can your calculations show whether atmospheric temperature changes are related to aerosols? All assumptions must be justified and calculations and sources of information must be shown.
* Relevance: 4 - aerosols are an important climate forcing, and their history through time (even in the 20th Century) are quite uncertain.
* Scientific Content: C - Calculating the impacts of aerosols is quite hard, first because we don't have great records for their distribution through time and space, and secondly there are uncertainties in how the mix with each other and how they interact with clouds. Forcing estimates for the human-caused changes in aerosols over the 20th Century therefore have quite large uncertainties associated with them and are a principle reason why attempts to constrain climate sensitivity from the recent record along have not been very successful. Volcanic effects are however quite well characterised, and actually provide one of the many lines of evidence for why GCM simulations are reasonable since they get the right magnitude and character of the volcanic effects on climate. However, there is no evidence whatsoever for large changes in interstellar dust changes in recent millennia and trying to pin recent warming on that is really grasping at straws.
* Sources: Climate sensitivity and aerosol forcings.-- Gavin Schmidt
Use geology jargon to prove you understand climate Edit
I'm pretty sure that MORB circulation systems don't metamorphose rocks to GS facies, the pressures are far too low; Pilmer has his geology wrong in this meaningless question. -- Andrew DoddsCalculate 10 Ma time flitches using W/R ratios of 10, 100 and 500 for the heat addition to the oceans, oceanic pH changes and CO2 additions to bottom waters by alteration of sea floor rocks to greenschist and amphibolite facies assemblages, the cooling of new submarine volcanic rocks (including MORBs) and the heat, CO2 and CH4 additions from springs and gas vents since the opening of the Atlantic Ocean. From your calculations, relate the heat balance to global climate over these 10 Ma flitches. What are the errors in your calculations? Show all calculations and discuss the validity of any assumptions made.
* Relevance: 0 - again more misdirection. The throwing around of irrelevant geologic terms and undefined jargon is simply done in order to appear more knowledgeable than your interlocutor. The argument appears to that climate is changing due to tectonically slow changes the direct heat input from ocean sea floor spreading. This is absurd.
* Scientific Content: F.
* Sources: Definition of 'flitch'. -- Gavin Schmidt
Prove sea level rise is not due to retreating glaciers Edit
I think he is trying to claim that sea level rise is due to the last deglaciation. -- Andrew DoddsCalculate the rate of isostatic sinking of the Pacific Ocean floor resulting from post LGM loading by water, the rate of compensatory land level rise, the rate of gravitationally-induced sea level rise and sea level changes from morphological changes to the ocean floor. Numerically reconcile your answer with the post LGM sea level rise, oceanic thermal expansion and coral atoll drilling in the South Pacific Ocean. What are the relative proportions of sea level change derived from your calculations?
* Relevance: 2 - pretty much irrelevant.
* Scientific Content: C - isostatic issues in sea level are important on long time scales, and there is still an effect today from the deglaciation 15000 years ago. It contributes a decrease of about 0.3 mm/yr to the global sea level rise, compared to 3 mm/yr total (i.e. about 10%). If the idea was to imply that current sea level rise is simply the response to the deglaciation, then it was completely misleading.
* Sources: Reconciliation of the sea level rise, thermal expansion and ice melt. -- Gavin Schmidt
Get around CO2 follows T III Edit
The increase in CO2 comes from the oceans heating, not the other way around. -- Andrew DoddsFrom atmospheric CO2 measurements, stable isotopes, radiogenic Kr and hemispheric transport of volcanic aerosols, calculate the rate of mixing of CO2 between the hemispheres of planet Earth and reconcile this mixing with CO2 solubility, CO2 chemical kinetic data, CO2 stable and cosmogenic isotopes, the natural sequestration rates of CO2 from the atmosphere into plankton, oceans, carbonate sediments and cements, hydrothermal alteration, soils, bacteria and plants for each continent and ocean. All assumptions must be justified and calculations and sources of information must be shown. Calculations may need to be corrected for differences in 12CO2, 13CO2 and 14CO2 kinetic adsorption and/or molecular variations in oceanic dissolution rates.
* Relevance: 5 - the carbon cycle is actually a key issue.
* Scientific Content: A - understanding the carbon cycle given multiple constraints on the carbon fluxes (including some of the issues raised in the question) is important in showing that the ~35% rise in CO2 since ~1750 is in fact anthropogenic. This has been shown numerous times to be consistent with the known human emissions, increases in oceans and terrestrial carbon, the decrease in 14C content of the atmosphere, the decrease in 13C content in the atmosphere, the decrease in O2 in the atmosphere.
* Sources: Read the FAQ. -- Gavin Schmidt
Give a short course in historical climate modeling Edit
Models are all unreliable and wrong. And if you don't know absolutely everything about climate change, then you don't know anything. -- Andrew DoddsCalculate from first principles the variability of climate, the warming and cooling rates and global sea level changes from the Boelling to the present and compare and contrast the variability, maximum warming and maximum sea level change rates over this time period to that from 1850 to the present. Using your calculations, how can natural and human-induced changes be differentiated? All assumptions must be justified and calculations and sources of information must be shown.
* Relevance: 4 - detection and attribution of climate change is an important issue.
* Scientific Content: B - First principles calculations of climate variability are most closely approximated by GCMs and multiple modelling groups have done various Holocene simulations. Attribution of any climate changes requires model simulations with and without each particular forcing and for the Holocene, this involves changes in the orbit, greenhouse gases, solar, meltwater regimes, ice sheet change, aerosols etc. and a comparison of the signature of the responses with patterns observed in the real world. However, comparable data to 20th Century sea levels or temperature changes are not available going back to the beginning of the Holocene.
* Sources: Attribution of mid-Holocene hydrologic changes to orbital forcing. Attribution of patterns of cooling at 8.2 kya to drainage of Lake Agassiz. Attribution of pre-industrial variability over the last millennia to solar and volcanic forcing . -- Gavin Schmidt
Rule out aerosols II Edit
Is really a rehash of 'If CO2 is responsible for climate change now, it must ALWAYS be responsible for EVERY climate change'. -- Andrew DoddsCalculate the volume of particulate and sulphurous aerosols and CO2 and CH4 coeval with the last three major mass extinctions of life. Use the figures derived from these calculations to numerically demonstrate the effects of terrestrial, deep submarine, hot spot and mid ocean ridge volcanism on planktonic and terrestrial life on Earth. What are the errors in your calculations?
* Relevance: 1 - irrelevant. Has nothing to do with current causes of species extinction nor sources of CO2.
* Scientific Content: D - insufficient data exist to infer atmospheric composition, nor the sources of any hypothesised fluxes. We think that it is likely that mass extinctions are probably bad for 'planktonic and terrestrial life on Earth' with very little error.
* Sources: This is a good intro to the P/T extinction event which is fascinating even if mostly irrelevant to today. -- Gavin Schmidt
Rule out water vapor Edit
Water vapour is the only important GHG -- Andrew DoddsFrom the annual average burning of hydrocarbons, lignite, bituminous coal and natural and coal gas, smelting, production of cement, cropping, irrigation and deforestation, use the 25?m, 7?m and 2.5?m wavelengths to calculate the effect that gaseous, liquid and solid H2O have on atmospheric temperature at sea level and at 5 km altitude at latitudes of 20°, 40°, 60° and 80°. How does the effect of H2O compare with the effect of CO2 derived from the same sources? All assumptions must be justified and calculations and sources of information must be shown.
* Relevance: 3 - radiative transfer is a key issue.
* Scientific Content: F - the question as it stands makes no sense. How can using fossil fuel emissions of CO2 allow you to calculate the impact of total H2O? And why only three wavelengths? You would need the whole atmosphere distribution of water (in all three phases and which doesn't exist outside a model) in order to calculate the radiative fluxes, and a full GCM to calculate all the other fluxes that influence the temperature. If Plimer is actually alluding to the impact of the direct injection of water vapour into the atmosphere from the combustion of hydrocarbons, then this makes even less sense since the perturbation time for water vapour is measured in days (rather than decades to centuries for CO2) and the relative importance of anthropogenic fluxes is much much less.
* Sources: Importance of water vapour and clouds compared to CO2 for the total greenhouse effect (roughly, 50%, 25% and 20% once overlaps are apportioned). Complete irrelevance of anthropogenic addition of H2O. Calculation of radiative forcing for anthropogenic CO2. -- Gavin Schmidt
At least one question (#13) is just stupid - I think he's just trying to sound impressive.
There are numbers of little peaks between 1 and 10 microns for water vapor - which peaks near 2.5 and 7 microns does he want? Liquid water has peaks at 3 and 6 microns, so 'use the 25?m, 7?m and 2.5?m wavelengths to calculate the effect that gaseous, liquid and solid H2O have on atmospheric temperature' just doesn't make sense. Ice has a broad band around 2 microns.
There's a CO2 absorption band at 4 microns which doesn't exist for water vapor, so if you try to examine 'How does the effect of H2O compare with the effect of CO2 derived from the same sources?' you'll get something completely meaningless.
In any event the answer is pretty simple - all this water vapor will get precipitated out pretty quickly so it won't have much of an effect.-- Pessce commenting at Greenfyre 
I laughed very hard at this last one. It's all a complete mess and meshes together many different problems. There's absolutely no relation between what we burn for example and the absoprtion properties of water vapor or CO2 (and by the way, for CO2 you want to look near 15 microns and the unsaturated bands on either side). There's also essentially no effect from water vapor from those sources since it precipitates out quickly and is thus not a climate forcing. This is of course in the gas phase, I'm not sure what information he wants about liquid or solid phases. All togther this is jumbled up nonsense and shows that Plimer is intentionally trying to mislead others. -- Chris Colose 
So now this waste of electrons (referring to a water vapor trumps C02 claim earlier than Plimer's) is reduced to the claim that the optical depth of the atmosphere is held constant at 1.87 by compensating variations in the water vapor column density in the atmosphere. Well, at least for the Earth's atmosphere right now that is the value, but is it fixed and if it is what is the mechanism?
We know that the atmospheric concentration of CO2 is rising which would increase the optical depth. The only way that the optical depth could then decrease is for the water vapor column density to decrease. Measurements show that the surface is warming. Since warmer temperatures will result in higher water vapor pressure, the only way that total water vapor would decrease would be for the excess water to rain out quickly. Although there is some data on water vapor column density from way back when, it is not of the highest quality, and the further back you go, the worse it gets. Is there another way to get at this?
Why yes young fella, if you had the right satellite instruments you could look at the water vapor concentration as a function of temperature on the surface over a relatively small area. And guess what we have the right satellite instrument, the Atmospheric Infrared Sounder (AIRS) and Andrew Dessler and friends have used data from this instrument in a paper currently appearing in the Journal of Geophysical Research, and as you would expect if you were not Ferenc Miskolczi, the column density of water vapor increases as the surface temperature below increases. -- Eli Rabett Rabett Run 
One of the interesting things about the questions is how Plimer acts as if he has the answers in the back of the book, and that he is allowed to grade the world. The tone is more than just bellicose and bombastic, it's also poorly written and thought out. For example, here he could have just said "from all human generated sources." The list isn't exhaustive, and invites intellectually dishonest dodges if aggregates, which are measurable, are used on his replies. Next off many of the questions imply models which are not referenced, which do not have their assumptions enumerated, which don't have their data sourced, and which do not proceed from first principles. As many here have pointed out, many of these questions are non sequiturs, or are straw men. For example the faint sun paradox question.
On this one he makes the engine battery mistake. Water vapor stores heat, but it does not drive heat. No model currently being used has WV as a driving green house gas. It isn't even clear how a water vapor model would drive global warming, since, as Eli pointed out, water would precipitate out. Now water vapor can be a contributing factor to global climate change, but it's effects are more significant in changing local temperatures and changes in the larger hydrologic cycle, see for example the citations on the MWP. This is important to us, since average temperature is a great deal less important to human civilization than what happens to specific areas, and climate variability. It would, in fact, be easier to deal with a 2C warmer planet, than a planet that should radically larger shifts in annual temperature, or which had radically different distribution of rain fall and temperature.
These kinds of highly specific questions are only valid when enough variables have been controlled to make the calculation decisive between competing explanations. Absent this kind of specificity, these questions, taken as a whole, ask for extreme specificity, without once proving that the specificity would falsify the theory that recent changes in climate and global temperature are driven by human release of GHG, specifically Carbon Dioxide with a smaller contribution by other by products of human activity such as methane etc. In many cases the questions are contradictory: for example the emphasis on water vapor as a green house gas that drives natural warming, and the focus on the MWP, which, in fact, indicates that natural warming cycles, at least locally, are correlated with drier, not wetter, conditions. Stirling Newberry 05:56, September 15, 2009 (UTC)
- ↑ Monbiot vs Plimer Open Mind
- ↑ Medieval warmth and English wine RealClimate
- ↑ Carbon Dioxide Information Analysis Center Historical CO2 Records from the Law Dome DE08, DE08-2, and DSS Ice Cores
- ↑ Medieval Climate Not So Hot Science Daily
- ↑ Plimer's homework assignment RealClimate
- ↑ 6.0 6.1 6.2 6.3 6.4 Ian Plimer's questions to George Monbiot Climate Change
- ↑ Coby Beck Volcanos emit more CO2 A Few Things Ill-considered
- ↑ Do you believe Ian Plimer? Open Mind
- ↑ It's volcanoes (or lack thereof) Skeptical Science
- ↑ Cloud Feedbacks in the Climate System: A Critical Review AMS Journal of Climate
- ↑ 11.0 11.1 Plimer fails to answer Monbiot Deltoid
- ↑ Climate sensitivity and aerosol forcings RealClimate
- ↑ Flitch Wikipedia
- ↑ Improved estimates of upper-ocean warming and multi-decadal sea-level rise Nature
- ↑ Evaluation of PMIP coupled ocean-atmosphere simulations of the Mid-Holocene Developments in Paleoenvironmental Research
- ↑ A.N.Legrande et al. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proc. Natl. Acad. Sci.
- ↑ the IPCC IPCC Ch8, p680+
- ↑ Water vapor: feedback or forcing? RealClimate
- ↑ Climate scientists hide water vapor A Few Things Ill-considered
- ↑ The C02 problem in 6 easy steps RealClimate
- ↑ Monbiot is wrong about Plimer's questions Greenfyre
- ↑ The first principle is that you must not fool yourself Rabett Run