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Climate Hustle

How climate skeptics misunderstand past climate change

Posted on 20 August 2010 by James Wight

This post is the Basic version (written by James Wight) of the skeptic argument "Climate's changed before".

A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. To understand this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.

There are a number of different forces which can influence the Earth’s climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.

It is obviously true that past climate change was caused by natural forcings. However, to argue that this means we can’t cause climate change is like arguing that humans can’t start bushfires because in the past they’ve happened naturally. Greenhouse gas increases have caused climate change many times in Earth’s history, and we are now adding greenhouse gases to the atmosphere at a increasingly rapid rate.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, for instance, we can work out the degree of past temperature change, the level of solar activity, and the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods and timescales in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat. We can even quantify this: when you include positive feedbacks, a doubling of CO2 causes a warming of around 3°C.

What does that mean for today? Rising greenhouse gas levels are an external forcing, which has caused climate changes many times in Earth's history. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the greenhouse warming. So past climate change doesn't tell us that humans can't influence climate; on the contrary, it tells us that climate is highly sensitive to the greenhouse warming we're now causing.

Note: we're currently going through the process of writing plain English versions of all the rebuttals to skeptic arguments. It's a big task but many hands make light work. If you're interested in helping with this effort, please contact me.

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Comments 1 to 49:

  1. Claims like "Our climate is highly sensitive to changes in heat. We can even quantify this: when you include positive feedbacks, a doubling of CO2 causes a warming of around 3°C." have always amazed me. How can you call this 'highly sensitive'? A DOUBLING of the independent variable is causing only about a 1% change in the dependent variable.

    That does not sound like "highly sensitive" to me. Neither does it to the skeptics.
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  2. Good post James. Another lesson ( from past changes is that there is no magic negative feedback mechanism that magically kicks in to stop global warming. This wishful thinking (eg that plants are suddenly going to grow faster and gobble up CO2) is completely negated by the dramatic nature of past changes,
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  3. MattJ: The doubling of a small number is still a small number. It is actually pretty amazing that just a few hundred ppm of a trace, largely inert, gas could cause such a change to the entire planet.
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  4. MattJ @1 commits a common "Straw Man" fallacy in his reply, in presuming that the term "highly sensitive" must necessarily describe a climate response more extreme than what is actually observed.
    Another error is assuming there should be a linear 1:1 relationship between climate driver and climate response.

    That said, one of the persistent complaints by AGW skeptics is accusations of hyperbole and exaggeration in describing the impacts of climate change. This is the origin of the odious term "Alarmist", which is commonly applied to anyone who accepts AGW as proved, or likely.

    Accusations of "Alarmism" are, themselves, mostly a Straw Man. Most scientific papers on climate change are quite careful and measured in their conclusions. Even climate studies that predict more severe long-term impacts are based on a particular set of assumptions, and should not be presumed to be intended to frighten little children.

    At the same time, if the goal is to reach the broadest possible audience, and to use the most non-inflammatory language, perhaps some further explanation should be added, or perhaps even the word "highly" dropped.
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  5. If 3dC is 1%, then MattJ's range of potential temperatures is 300Cdegrees. That's unrealistic.

    The ice-core record gives the reality range - 10Cdegrees.

    Doubling CO2 with a plus 3dC consequence would be a dependent variable increase of about 20% (factor in the lower rate of increase with the higher levels of CO2).
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  6. Yes, but the actual correlation between greenhouse gas levels and climate change in the past is not that good. This suggests variations in levels of greenhouse gases are not that infleuntial in affecting temperature, but rather follow other causes (such as c02 released when oceans warm due to lower solubility).

    Eg. C02 is known to follow rising ocean temperatures at the end of ice ages (soemthing Al Gore conveniently left out).

    Currently the correlation between c02 and T since 1850 is about 22%, and falling.

    One has to introduce a bit of gymnastics in the geologial record to explain why greenhouse gas levels don't often corralate with T, such as lower solar output in the Ordovician, and even in recent times such as increase in aerosols ~1940-1970, which all sounds a bit convenient.
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    Response: "One has to introduce a bit of gymnastics in the geologial record to explain why greenhouse gas levels don't often corralate with T"

    Gymnastics? I would characterise it as taking into account all the forcings that drive climate. If the sun was cooler in the past, this obviously needs to be taken into account when calculating the planet's net radiative forcing. Similarly for the mid-century cooling, when you factor in solar dimming which is a directly observed phenomena and couple that with increasing temperatures at night even during the period of mid-century cooling, there is no inconsistency with greenhouse warming effect.

    Many of the misunderstandings about climate would disappear if everyone were to take into account the full picture, not just cherry pick the bits and pieces that lead to a misleading conclusion.
  7. "Our climate is highly sensitive to changes in heat."

    Can you quantify when the positive feedback stops since it has always stopped in the past? Are there models that max out temperature at some point? What specifically causes positive feedback to stop?
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  8. Good point, owl905. The comparison should be with the natural variation of global temperatures, rather than the absolute temperature in Kelvin.

    On that basis, a 20% shift is pretty significant. Especially as we're talking about a 20% shift upwards on top of an already relatively high interglacial temperature...
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  9. Yes, Eric, models do project maximum temperatures. If you don't know that, may I recommend Spencer Weart? Look for the section on GCMs, follow the references.
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  10. @thingadonta - did you miss the part of the article where he said that external forcings in the past included a number of different elements, including solar irradiation, volcanic eruptions (& aerosols), and greenhouse gases?
    Obviously that list should include orbital variations, and potentially meteor impacts (although it'd have to be a big one to trigger a really long-term climate shift).
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  11. Ogemaniac, CO2 is not inert with respect to IR, which is the only aspect that matters for the topic under discussion.

    The doubling of the number of CO2 molecules means drastically increasing the effect of CO2. Since the pre-doubling effect already was large, the increased effect will be large. 2 x 1 = only 2, but 2 x 1,000 = 2,000.
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  12. Thingadonta stops being serious when he mentions Al Gore. Stick with the science, Thingadonta.
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  13. Bern, just to nit pick, orbital variations are about variation primarily to distribution of solar radiation. The effect is dependent on land/sea distribution so going back in time, the changing plate configuration is also a factor in climate. Meteor impacts I would say are mostly aerosol in effect?
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  14. From Weart, Annan, Hargreaves 2006 uses a probabilistic argument to estimate sensitivity from two observable variables, forcing and temperature. Using both negative and positive forcings and corresponding temperature changes, they calculate ranges of sensitivities and most likely values. No discussion of what physically causes those limits.

    Weart also mentions Lindzen's theory of self-regulating climate. Then he dismisses it by saying "...climate experts (aside from Lindzen and a few followers) were now nearly certain that serious global warming was underway". Apparently Weart believes (1) "serious" global warming is a reality rather than just a model result and (2) the "seriousness" of global warming means that Lindzen's theory of self-regulation is wrong.

    Can't find "Climate Models and Their Evaluation" at where Weart says it is. I did find this
    but haven't finished reading it.
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  15. Eric, there are a lot of ways to assess climate sensitivity. The commonest is direct calculation from model output (which comes out at around 3) but ECS may be higher has AR4 models dont track carbon feedbacks. This is ONLY about physical causes.

    IPCC WG1 has a section on estimating sensitivity from past response to forcing with many papers. Annan and Hargreaves just one of many different approaches. Necessarily, uncertainties are high. Annan at least uses an approach to limit the upper end.
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  16. Eric - positive (and negative) feedbacks with an absolute value < 1.0 damp out. Feedbacks > 1.0 absolute value are un-physical; they would increase infinitely, requiring an infinite amount of energy to do so.

    Here's a post on feedbacks, also here, which I wrote a while back - these may give you some idea on how these work, and how they taper out after a fixed amplification/dampening of the initial forcing.
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  17. scaddenp: yep, orbital forcings affect distribution of solar radiation - but they don't change the total amount. And, yes, land/ocean distribution would also be a significant factor, even if not (directly) a forcing. They'd still generate feedback forcings, though (e.g. albedo changes to due cloud / ice / vegetation cover).
    Meteor impacts certainly would kick up a lot of (short-lived) aerosols, but can also result in enormous greenhouse gas kicks, depending on what type of rock is hit, and how much organic matter burns / decomposes as a result of the short-term effects. This may provide a nudge that pushes the global climate from a relatively stable state into a transition state.
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  18. Probably the most important issue in the ongoing climate debate is that of "Feedbacks".

    James Wright buys into the (majority) view that the feedbacks are positive.

    Right now the range of feedback estimates is so wide that the models are worthless when it comes to prediction or even explaining past climate. For the moment, one scientist's guess is as good as another's.
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  19. I'm not so sure GC, note how the cooling effects from major volcanic eruptions show up in the model runs?.
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  20. KR, those are fascinating posts. Do you have any physical explanation of how the feedback mechanism always knows to add 0.5 of the anomaly? For example, how does the tropical feedback limit itself to 0.5 of a worldwide anomaly if the tropics expand due to warming? Or are tropical, subtropical, temperate, desert, etc all the same feedback amount? All based on the anomaly and not the absolute temperature?

    Is this only long term feedback that ignores short term fluctuations? (e.g. worldwide average albedo changes, short term solar fluctuations, etc) see UAH for example:
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  21. scaddenp, do you have a link to ECS/AR4 (I don't know what those are)? Also do you know of a specific physical mechanism that limits the feedback in the models? Is that physical mechanism weather? Doug, what I was looking for is a model output that stops some time in the future at some temperature. The model outputs I have looked at all continue upwards indefinitely. If the models never have an upper limit to temperature, it's hard to take them seriously.
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  22. Third paragraph, last sentence, 4th word from the end should be "an" rather than "a".

    Eric, positive feedbacks 'end' when the factors driving them do... which obviously means that it varies by feedback.

    For instance, increased atmospheric temperatures allow the atmosphere to hold more water vapor... thus, as CO2 (or anything else) drives up the atmospheric temperature the amount of water vapor in the air increases. Water vapor is itself a very powerful greenhouse gas... which means that the increased water vapor causes MORE warming. In short, a positive feedback. This feedback would thus continue until the underlying cause of temperature rise (in this case rising CO2 levels) ended OR the planet ran out of surface water... which we'd pretty much have to be TRYING to kill ourselves off to achieve.

    One of the other major positive feedbacks is ice albedo... as the temperature rises ice melts, which exposes more dark land and water, which raises temperatures more. Again... the positive feedback continues until the external warming factor (rising CO2) ends or the planet runs out of ice... which couldn't happen for a long time and then only if we burned all available fossil fuels.

    Thus, the reason models show feedbacks 'continuing upwards indefinitely' is that the planet isn't going to run out of water or ice any time soon. The limiting factor is really fossil fuels. When we run out of or stop using those then positive feedbacks will continue playing out for a few more decades and then temperatures would level off. However, if you assume we go after deepwater oil (check), tar sands (check), oil shale (not yet, though Bush tried), methane clathrates (Russia is starting to), and other 'unconventional sources' we could theoretically be burning fossil fuels for another two hundred years or more. With just conventional sources we'd run out completely by around 2100.
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  23. I suppose “highly sensitive” is a somewhat subjective way of putting it. I’ll consider rewording that part.

    gallopingcamel, estimates of climate sensitivity from paleoclimate studies have roughly the same range as estimates based on models. This is discussed elsewhere on Skeptical Science – see here and here.

    Eric, you’ll find the IPCC chapter “Climate Models and Their Evaluation” here.
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  24. It is a crucial point to make - positive feedbacks are required to reconstruct past climatic change, and as mentioned that leads to sensitivities similar to those calculated for future warming. Otherwise, you cannot account for glacial-interglacial climate shifts. Also important is that a positive feedback does not equal a runaway positive feedback - if the 'gain' is less than 1, then the feedback is necessarily self-limiting. e.g CO2 rise of X leads to a temperature rise of Y and a feeback of 0.5*X, leading to a temperature rise of Y', feedback of 0.25*X, rise Y'', feedback 0.125X etc etc in a geometric series, which is self-limiting. Our best estimate is that the doubling CO2 leads to a temperature rise of ~1C with the feedbacks summed adding another ~2C.

    In the short term, warming will cost us a lot in albedo, as we have lots of sea ice and snowfileds that can be melted quickly, and so that feedback will operate substantially at first, but eventually that feedback will slow down as there simply won't be as much snowfields/sea ice per unit temperature rise to drive the feedback. The large ice sheets also have the albedo feedback, but changes in their areas will likely take rather longer to become apparent (still a bad thing as like a heavy runaway train their changes are harder to stop).
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  25. OK if CO2 is such a strong positive feedback, as claimed, how does the earth dramatically cool when it is about 250? It has before. In fact before we slipped into our last ice age CO2 hung on rather stubbornly as temperatures fell dramatically. Seems it isn't quite the force some may think. Let's put it logically. If CO2 is the big dog we should be warmer now than we were at any time in glacial history. We are not even close. What gives?
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  26. One thing most folks don't realize is that we are heading to a place most of us are not going to be comfortable with without any help from CO2. If we don't head back into an ice age we will keep melting. there is no static perfection here. It goes one way or the other and it usually goes till things are quite a bit different than now.

    The coast of FL will go under water again (along with many other places). It is just part of the cycle we have been in for close to 1,000,000 years. Plants and animals will extend their ranges N. It is simply the way it is. Then the ice will come again. That happens when sea currents get so screwed up that warm waters do not make it far enough N anymore. The N hemisphere really controls all of this.

    Weather is not constant. Get used to it.
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    Moderator Response: The rapidity of the current warming does not give us enough time to "get used to it." See "We’re heading into an ice age."
  27. CBDunkerson, it would still be nice to see a model output that didn't have temperature rising indefinitely using capped CO2. The argument that CO2 will increase forever is probably discussed in another thread which I will have to look up.

    skywatcher, running out of ice with high albedo to melt is great example of a physically limited positive feedback. Unfortunately it is negligible since, according to the albedo page here, cloud albedo dominates.

    But I would like to see that type of physical limit demonstrated for water vapor. It won't come from GCMs which lack the resolution to model small scale convection and that impact on latent heat transfer (negative), clouds (positive and negative), subsidence (negative), or lack of convection (positive). The amount of positive water feedback depends on the distribution of water vapor, not the amount. So formulas using amounts of increase in water vapor based on warming from CO2 won't work. The answer lies in how the weather changes in a world slightly warmed by CO2. I have read theories here that the amount of extreme climate (concentrated heat and rainfall) is increasing. Climate extremes are negative feedbacks but need to be quantified (and I'm not convinced that they exist).

    Thanks for the link James. First, in the lapse rate discussion, low latitudes show negative feedback (as I implied above). In the middle and higher latitudes, the feedback is positive. What they don't mention is that the coverage of negative feedback will expand during global warming.

    Second, in the cloud discussion they mention increases in storm intensity and poleward shifts in storm tracks and other negative feedbacks without seeming to recognize them. Otherwise that section is just muddled and inconclusive. It is actually very simple to use climate models to determine the sign and strength of feedbacks. Concentrated convection is warming, diffuse is cooling. High clouds are warming, low are cooling, mid are ambiguous. Tropical cyclones cool (mainly from subsidence surrounding them). None of this is difficult to understand.

    One interesting question is how much current measurements match up to the positive and negative feedbacks. A much harder problem is determining the overall evenness of water vapor in world warmed by CO2. Uneven water vapor is cooling, a negative feedback. Evenly distributed water vapor, especially at high altitudes is a positive feedback. Models will not help much until they figure out better ways to integrate small scale weather models into the coarser climate models or computer power increases enough to model small scale weather in climate models. That should not take more than 10 or 20 years IMO.
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  28. Cruzn @ 25 - the ice core data seem to disagree with your assertion.

    See also CO2 lags temperature
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    Moderator Response: Cruzn, in addition to that link, see CO2 is not the only driver of climate.
  29. Eric, you seem to have a rather confused understanding of climate, such as your claims regarding water vapor
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  30. Dappledwater, thanks for link to the other thread. The discussion was all gummed up unfortunately by long tangents into anthropogenic water vapor forcing (in a word: negligible). Feedback works locally on the entire quantity. The climate in any location cannot possibly know or care that the CO2 caused a small amount of global warming, it simply reacts to whatever local warming or cooling is caused by whatever anthropogenic and natural factors are in that location. CO2 warming is almost always going to be the least of those factors. Where it is not (e.g. hot and cold deserts), the feedback will probably be positive (added WV) but limited by lack of water and those being a small portion of the earth.
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  31. Cruzn246 #25: "OK if CO2 is such a strong positive feedback, as claimed, how does the earth dramatically cool when it is about 250?"

    At present CO2 is a human induced climate driver rather than any sort of significant feedback. In order for CO2 to become a major feedback effect you'd need to see something like extreme heat causing massive wildfires without resulting increased plant growth elsewhere... or rising temperatures causing the oceans to expel excess carbon... in short, heat causing a rise in CO2 which causes more rise in heat.

    I suppose it could be argued that we have something like that now with extra fossil fuels being burned to run air conditioners more due to increased temperatures... but for the most part the increase in CO2 is due to human industry in general rather than being a feedback effect as you describe.

    As to how the earth cools when CO2 decreases... that seems so obvious that the mere existence of the question indicates some sort of profound misunderstanding of the situation. CO2 and other greenhouse gases 'trap' heat within the climate system. If the quantity goes down less heat is trapped and it gets colder.
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  32. Eric, your arguments contain so many false assumptions that I can't even follow what you are trying to say half the time. To start with... water vapor feedback definitely DOES depend on the amount. It is just like any other 'greenhouse gas'. The greater the quantity in the atmosphere, the more it will act to prevent heat escape to space and warm the climate.

    No, atmospheric water vapor is not maintained at a constant level worldwide... but as global temperature increases the global total amount of water vapor does as well. Higher total water vapor = greater greenhouse warming.

    BTW, you also said that 'clouds dominate albedo'... most studies show that to be the case SO FAR. However, there has been very little change in albedo due to ice melt to date. Only a tiny percentage of the planet's surface which had ice now does not. As time goes by and the large areas of ice on Greenland, Antarctica, the Arctic ocean, et cetera disappear the albedo effects of ice loss will become much greater. Clouds also trap heat in the same way that greenhouse gases do... creating a great deal of debate over whether their NET feedback effect is positive or negative. What we CAN say with a fair degree of confidence is that the net effect is SMALL.
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  33. CBDunkerson, warming from WV forcing depends on the amount of WV (in general, but see below). WV feedback from other warming depends on the amount of that other warming. Hopefully we agree on those two things. Where I disagree is that "as global temperature increases the global total amount of water vapor does as well". You are trying to make a global case for a local effect which is not necessarily true and will become false at some level of warming.

    My question from the beginning of the thread is "what is the physical process that limits the feedback when WV increases warming and warming increases WV". I stated it poorly as "when does positive feedback stop". The actual question is when does weather start to limit the positive feedback from WV. It already does in the tropics (in fact increasing warmth has negative WV feedback). It does not seem to be limited outside the tropics, so there is positive feedback there (except where there is no available water). So how is that feedback limited? When will we start to reach the limits (i.e. will we reach them soon or will it warm a lot more first)?

    The most basic error in your statement is that the distribution of water vapor (caused by weather) determines the amount of warming from water vapor, not the "total amount of water vapor". This is easy to prove. Suppose all the water vapor in the world was moved to one hemisphere (case 1). Suppose that amount was doubled. Now do the same doubling without first moving the water vapor (case 2). In both cases the "total amount of water vapor" went from X to 2X. But in case 1 the world would not warm with the doubling, and probably cool because of large increases in precipitation in an overall wet world. In case 2, the world would warm by some amount certainly greater than any warming in case 1.
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  34. Eric - if global temperatures rise, and relative humidity remains constant, the total amount of water in the atmosphere will rise, as the peak absolute humidity, the amount of water the air can hold (driven by the vapor pressure of water at atmospheric pressure and temperature) rises. The relative humidity is the percentage of possible absolute humidity at stated conditions.

    So - unless you know of some mechanism that will globally reduce relative humidity as temperature rises, the total WV (kg water/kg air) will rise in lockstep with temperature.
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  35. I witnessed a mechanism that locally reduced RH as the temperature rose today and my weather forecast tonight is for RH to be lower than last night. Repeat that globally and there's the global mechanism. No doubt that my mechanism could work the other way or it could keep global RH constant. But the only way to prove that it stays constant is to show that weather doesn't change in a warming world. But there are threads here claiming that climate extremes such as excessive rainfall have increased globally. That's certainly one way to reduce RH globally if it is true.
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  36. Excessive rainfall requires excessive evaporation - rainfall doesn't occur without sufficient water vapor present.

    In regards to cloud cover - there appears to be a slight inverse relationship to temperature over the last 60 years (sorry, can't currently locate where that was discussed on skepticalscience), but all of the analysis I've seen on it indicates that cloud cover feedback (negative or positive) is fairly minor.
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  37. Eric #33: "Where I disagree is that "as global temperature increases the global total amount of water vapor does as well". You are trying to make a global case for a local effect which is not necessarily true and will become false at some level of warming."

    I'm still not following you. In what way is global warming a "local effect"? If temperatures increase globally then water vapor increases globally... you say you dispute this seemingly obvious fact, but provide no explanation WHY.

    Also: "what is the physical process that limits the feedback when WV increases warming and warming increases WV"

    Are you asking why positive water vapor feedback does not cause 'run away' warming all by itself? That was already explained to you in the '0.5 increase' bit. Simple MATH shows why the positive feedback effect is limited. Again, suppose that a global average anomaly of +1 C from CO2 or some other forcing causes a global average water vapor increase sufficient to introduce another 0.5 C of warming from the water vapor... in short, an additional 50%. Well, then 50% of that 0.5 extra is 0.25 and 50% of that is 0.125 and so forth. If you add these fractions up you will see that the result gets closer and closer to 1 C... +0.99999 repeating to infinity. Thus, the limit you are looking for is MATHEMATICS. So long as the water vapor feedback for 1 C of external warming is itself less than 1 C there will always be a finite limit to the feedback warming.

    As to when weather starts to limit water vapor feedback... NEVER. Yes, rainfall takes water out of the atmosphere... and evaporation puts it back in. In a warmer world both effects are increased, because the total amount of water the atmosphere can hold increases with temperature.

    Your claim that concentrating water vapor in one hemisphere would somehow cause net global cooling has no basis in science and no explanation that I can see in your post. As you describe it the northern hemisphere, magically devoid of water vapor, would cool drastically... but the southern hemisphere, now holding double the previous global total, would heat up far more than the northern had cooled. Net effect... average global temperatures would increase.
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  38. CB, for the first question let me give a simpler example. Take an area with high temperature and humidity (e.g. 80 and 75%) and drop tiny grains of dry ice uniformly over it. The moisture will precipitate out and the temperature will drop. Apply forcing to warm the area back to 85. At that point it will be warm and dry perhaps 30-40% RH. Or warm to 85 without precipitating and the RH will be approx 65%.

    The only difference in the two scenarios is weather, but one has much lower RH and the rest of the water is in the river (where it flows away without evaporating). There is no physical law of any sort that favors one scenario over the other, or any law that requires balancing scenarios (more evaporation to balance more condensation). Weather simply doesn't balance. So as the world warms, there is no reason that the RH would stay constant unless the weather stayed precisely the same on a globally averaged basis.

    The problem with the simple math is that there is no physical reality to base it on. There is no global physical mechanism that results in a particular percentage increase in WV for an increase in temperature. It mostly depends on weather as in the above scenarios. Even though there will be more evaporation, it is unevenly distributed and unevenly condensed and precipitated giving an unknowable relationship between the global average temperature and total amount of water in the atmosphere. In the extreme case I merely need to add more dry ice and more forcing to keep the temperature higher with lower RH.
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  39. Eric, no. None of that makes ANY sense.

    Dry ice: Presumably here you are talking about cloud seeding... though you didn't mention any clouds to be seeded (water vapor is not synonymous with clouds). If you drop dry ice into clouds the chances of precipitation increase. That precipitation would then cause a temporary local cooling effect. No 'extra forcing' is then required to raise the temperature again... water vapor will naturally return to the atmosphere in very short order provided there is a water source nearby (such as the recently fallen rain or the surrounding air for instance). So no... the air does not remain dry after rainfall.

    All of which is completely irrelevant because you are talking about a short term localized process rather than long term global averages. So one city gets rained on and cools slightly during part of one day.... this is MEANINGLESS on a global scale.

    Look at it this way. New York City and Miami are both right on the Atlantic ocean. So why is the absolute humidity of Miami on average alot higher than that of New York? Because Miami is closer to the equator and thus on average warmer... which allows more water vapor into the air on more days. If the entire planet gets warmer then that same effect takes place worldwide. How is this not obvious?

    The connection between temperature increase and atmospheric water vapor which you wrongly state does not exist is known as the Clausius-Clapeyron relation... which has been around since the 1830s.
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  40. CB, Clausius-Clapeyron describes an ideal local effect and can't be used to determine a water vapor number in real-world circumstances (advection, limited water supply, etc). But in looking that up I see where you and KR are coming from, as described here (Held and Soden 2000), including Moller's fixed RH assumption.

    I was trying to show above that a C-C relation can't be applied in nonideal circumstances (i.e. anyplace but flat water with no wind, and certainly not in what I described above). Is your argument then that C-C is applicable globally because C-C will apply to the average situation? Perhaps, but that average situation will be quite complex to describe, perhaps it could be a weighted average of several or many typical circumstances.

    The best answer I have is already above which is that weather will change as the global average temperature rises. On some threads here it is claimed that it already has changed to an increase in extremes. The extremes in precipitation obviously remove water vapor from the air (that should be obvious?) but the amount will depend on how much those extremes have increased on average. The extremes in temperature which may not be CO2 correspond with extremely low RH. Again if those increase globally, then RH decreases by that amount globally.
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  41. Eric - extreme precipitation requires extreme evaporation to provide the WV needed. It ain't gonna rain without sufficient water in the air. So an increase in extreme storms indicates an increase in evaporative generation of water vapor, not a total decrease in relative humidity.

    That doesn't mean that there might not be changes in RH with temperature - just that you cannot assume drier conditions based on more extreme storms, quite the contrary.
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  42. KR at 14:15 PM, whilst extreme precipitation indicates that a large amount of rain is concentrated on a small area, it doesn't necessarily mean that a large amount of moisture was evaporated from a similarly small area.
    Certainly in tropical storms that is what drives the storm with the moisture being picked up and then dumped rather quickly over a relatively small area.

    However at other times it all depends on the circulation patterns with small amounts of moisture being picked up traveling over a vast distance until the system happens to collide with another system that creates the conditions that triggers the rain event. This is what often happens when moisture that is picked up in the Indian Ocean under the normal course of events is transported across Australia to the SE corner where it is released. However there are also times when the remnants of tropical storms that also originated in the Indian Ocean to the NW of Australia also cross to the SE corner where it is released.
    Whether the rain is released over a wide area or dumped over a smaller area is really dependent on what conditions the rain bearing system collides with.
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  43. johnd - what goes out (storms) must come in (latent heat via evaporation). First law of thermodynamics...
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  44. Eric, as I have explained before, and KR and johnd both just did again, the REASON there is more precipitation in a warmer world is that there is more water vapor in the air to precipitate out... because more heat causes more evaporation which puts that water vapor into the air.

    As you seem to insist on adhering to beliefs which are clearly false without making any effort to examine the proof to the contrary presented to you I don't see much point in continuing this. You should head to Miami and enjoy the ultra low humidity your logic indicates exists there.
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  45. CB, that's hardly a fair comment (no effort), I looked up C-C and found the reason you quoted it and KR assumes constant RH. I believe that Soden and Held are oversimplifying by using C-C globally. I don't disagree that a warmer world has more water vapor in the air, my question at beginning of the thread was when does the positive feedback stop. Your answer was two-fold: never (22) and use the math (37) which I answered in 38.

    It may interest you and others to know that there were number of days here in the DC area this summer where our humidity was higher than Miami including total precipitable water. Also his year we had many days that were warmer than Miami where it never got above 95 (we were above 100 several times). Here's our plot: and theirs:
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  46. Perhaps you may like to illustrate how this blog, and climate science in general, doesn't seem to understand past climate change.

    This site suggests that warming starts in the SH, is amplified by greenhouse gases and other positive feedbacks, and greenhouse gases transport this to the NH.

    1) Richard Alley (at ~38m) suggests the "physically realistic" models show reduced summer sun causes ice sheets to grow in Canada eventually leading to a globally average reduction of ~5-6 °C, yet this page contains Vostok ice core data suggesting temperature falls before CO2 does. If the change starts in the NH, how does CO2 still lag temperature (during these cooling periods) in the SH if CO2 has such a significant effect, particularly in transporting heat from one hemisphere to the other?

    2) This page suggests charts provided by Ned (located here), which don't appear to bear any relevance to climate science, are a good example of how positive feedbacks don't lead to a runaway climate. Looking at the Vostok data here:

    a. How do you explain the almost linear temperature rise in the three warming periods at ~125,000, ~240,000, and ~325,000 years BP? If positive feedbacks don't lead to a runaway climate they need to illustrate asymptotic behaviour, which the empirical evidence doesn't support.

    b. In the most recent warming (~20,000 years BP), where in the EPICA Dome C data (which is what I look at, but it's seems to also be visible in the Vostok data) there's warming for a few thousand years, then cooling for about 1,500 years, then warming again for about 1,000 years. If positive feedbacks are so strong, how do you explain such (relatively) rapid changes in gradient (as they're significant, rather than being almost zero)?

    c. How do you explain the significant (~10-15 %) drop in CO2 around ~325,000 years BP with no corresponding change in temperature?

    In response to Dappledwater (Comment #28) and the Moderator Response, thank you for illustrating what seems to me as the biggest issue with this ice core data, the correlation between temperature and numerous variables that apparently cause the change.

    Hansen et al. 2008 Figures S18 and S19 illustrate the strong correlation between temperature and CO2, CH4, and surface albedo. Page 2 attributes the change in temperature between the LGM and pre-industrial Holocene to a radiative forcing of ~6.5 W/m^2 with ~54 % surface albedo, ~35 % CO2, ~6 % CH4, and ~5 % N2O. We also know that warmer climates lead to less snow and ice as well as cause vegetation changes, which are changes in surface albedo, and warmer climates also lead to greater concentrations of CO2 due to out-gassing. In other words, temperature has some influence over at least two of the supposed forcings.

    My reasoning is along these lines. If the data showed perfect correlation between these variables, then the only possible explanation is that changes in temperature caused all the other changes and they had no influence on temperature (as climate science suggests there are external forcings
    of temperature such as solar variability, orbital effects, etc.). In reality there's strong correlation (particularly between the LGM and pre-industrial Holocene) suggesting that change in one variable (independent) is the significant cause of change in the other variables (dependent), since external forcings only directly influence temperature it's unlikely to be a common cause for all variables. It also suggests that changes in any of the dependent variables would have little if any influence on the independent variable. There's a couple of things that suggest temperature would be the independent variable:

    1) Temperature is directly influenced by external forcings.
    2) The relationship describing the correlation doesn't correspond to the forcing (e.g. the trendline for CO2 appears to be a quadratic which is contrary to the logarithmic relationship provided by climate science).

    I'm not worried about the lag between temperature and CO2, how do you explain the strong correlation between all these variables if there are multiple drivers of climate which would tend to break the correlation?
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  47. CO2 concentrations in the upper atmosphere increase with an increase in temperature and not the other way around.
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    Moderator Response: This argument is addressed on the page "CO2 lags temperature".
  48. Carbon Di-oxide Claims - "it was a bum rap - it was the Di-hydrogen oxide wot did it"

    The tropics are the place where the solar irradiance is strongest - can't argue with that one. The majority of the earth in the tropics is ocean. Approximately 2400 j/gram is required to evaporate good old H20. As vapour it has a specific heat of about 2.1 j/gram - insignificant but still double that of CO2.

    It evaporates and is carried by convection to the upper atmosphere and to the cooler regions of the earth where it releases this energy. I think we can't argue with this as it rained here not long ago.

    Consequently water vapour makes up approximately 2 % of the atmosphere.

    CO2 comprises less than 0.04% of the atmosphere and has a specific heat of less than 1 j/gram at ambient atmospheric temperatures.

    So, every gram of water vapour rising from most of the surface of the hottest parts of the globe carries with it enormous amounts of energy.

    Convection in both the atmosphere and the oceans swamp the radiative effects - the earth is simply not hot enough for this to not be true. This is not saying there is no radiative effect from the heated surface of the earth simply that the warm air and ocean currents move much more energy than is radiated at the surface.

    Doesn't the process of water vapour convection seem a much more powerful way of transferring energy than radiation forcing by a gas which is some 60 to 70 times less abundant and which has a thermal capacity some 2400 times less ?

    Note, this does not say there is no atmospheric effect keeping the earth warm - I didn't say greenhouse deliberately as that is associated with CO2 as a driver.

    And none of this implies that the earth is at its potential blackbody temperature but it does imply that there may be a possible exaggeration of the radiation imbalance.

    Radiation is a relatively poor method of transmitting energy in the atmosphere.
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  49. Rosco - see water is the most important greenhouse gas

    And you think scientists havent done the maths then think again (or read the IPCC report and relevant linked papers).
    See also Schmidt et al for detailed attribution.
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