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The significance of past climate change

Posted on 21 April 2010 by John Cook

A common skeptic argument is that climate has changed naturally in the past therefore humans aren't causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. When I try to explain why to people, I usually get blank, confused stares. I gave a presentation to a roomful of engineers this week and after explaining the significance of past climate change, complete with slides of climate sensitivity PDFs and examples of positive feedbacks, the result was a long, silent pause. I asked if anyone understood what I'd just talked about. A few asked some follow-up questions which made it clear they didn't. So I'm reworking my whole explanation of past climate change in an attempt to make it as clear and simple as possible. Comments, particularly on anything confusing or unclear, are welcome!

In the past, climate has changed, sometimes very dramatically. This has gone on long before SUVs and coal fired power plants. If climate can change on its own, couldn't current global warming be natural as well? To answer 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.

This can happen in a number of ways. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, all the particles suspended in the atmosphere reflect sunlight and the planet cools. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, we can work out past temperature change, the level of solar activity plus 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 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.

What does that mean for today? Rising CO2 levels are an external forcing. 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 CO2 warming. So past climate change doesn't tell us that humans can't influence climate. On the contrary, the past tells us that climate is highly sensitive to the CO2 warming we're now causing.

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Comments 51 to 87 out of 87:

  1. #30 unrecovered "Another aspect, I have found difficult for engineers and many other scientists to grasp is: How a system where all feedbacks sum up to a net "positive" does not constitute a run-away system. This is not and easy thing to explain and typically requires too much time in a short talk. But may constitute a stumbling block for engineers in particular. " Perhaps the key point to get across is that the feedbacks are positive, but they also finite or decline with the increase. Methane runs out when the sources are drained, CO2's impact is logarithmic with concentration, when the ice sheets are melting away the albedo change tapers off. And some feedbacks such as Milankovitch Cycles actually cycle between positive & negative.
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  2. So if someone asked about CO2 being much higher in the past (noone did bring that up at the talk), I would say that the sun has been getting steadily brighter over Earth's history. I go a bit further - but as I'm a layman I would like to check with you. I put it that the configuration of the biosphere was also very different, notably a big difference in where land masses are situated. Albedo has a strong influence, so if there is more or less land on our near the poles, this can strongly impact the radiation budget. Directly comparing the biosphere now with that of millions and millions of years ago, in terms of CO2 influence, is a bit of an apples to oranges thing. Is this a fair comment or have I been getting it wrong? Also, would different ocean/atmosphere heat transport dynamics in the distant past have an impact on the total radiation budget, or does it even out for the global total? (Hope that made sense)
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  3. I have a pet hypothesis that is in the same vein as Barry's. Another, layman speculation here; so, keep that in mind. I somewhat think that there are plateaus in global temperatures, and the within a plateau, a certain amount of change in forcings has less effect that near the edge of one. I'm imagining a few mechanisms for this, but I'll just try to explain just one. Let's consider the Greenland ice sheet and the albedo of the surface area it occupies. Let's say there is an ice sheet, and the mean temperature over it is -6 C. (Yeah, I know it'll be warmer in the south and colder in the north, on average, and the actual degrees are just for illustrating a point. Humor me; I like to keep things simple.) About 90% of the energy received is reflected. Let's say you have a change in some forcing that raises this area's mean temperature by 4 C to -2 C. The ice sheet remains, and the change in climate is more or less linear (or probably a log) of the forcing. Let's say there is some additional forcing that, by itself, would push the mean temp to +2 C, and the ice cap melts. The albedo changes, the temperature raises some additional factor, and whatever relationship between forcing and effect that existed when the temp was between -6 and -2 C is not the same relationship that you will find between -2 and +2(+x). Same effect on the way down, in reverse. So, if it likely to be the case that there is not a consistent relationship between the amount of forcing and the amount of climate change, it is going to be difficult to anticipate where the current change in forcing will take us. Will we stay within this plateau, or will we be leapfrogged to some other? I believe Hansen has written along these lines better than I can. The point is, engineers are used to thinking in terms of coefficients that relate x to y, but they aren't always prepared to deal with coefficients that are not constant. If some engineer isn't prepared to deal with a variable coefficient, and they detect that it wasn't the same in scenario A as it was under scenario B, you'll loose them. When they learn that the coefficient isn't constant, but don't have a grasp of the larger forces in the equation, they can also come to some erroneous conclusions. In my own experience, I remember aerospace students, when they learned that the coefficient of drag of a sphere decreased with an increase in velocity, jumped to the erroneous conclusion that the drag itself reduced. No, the drag (subsonic) is still proportional to the square of the velocity, and the square of the velocity goes up a lot faster than the coefficient goes down. If there are questions about runaway feedbacks, you can always point out that the energy radiated by a body is proportional to the fourth power of the absolute temperature, and no other part of the equation has that large an exponent. Engineers will get that. I get the same feeling as I did with the decreased drag arguments when I hear arguments that the negative feedbacks, like increased cloud cover, will prevent warming. a) What, water molecules are going to anticipate a warming and leap up to form clouds to prevent it? No, they evaporate more than they had been because they are warmer than they had been. It's not like they weren't evaporating and forming clouds before at some equilibrium level. Clouds might mitigate a warming, but they can not prevent it. Nevermind that water vapor is itself a GHG. b) The changes in the past are a strong counterpoint to the idea that the climate is inherently stabilized by negative feedback mechanisms. Maybe that is my second point. It may be too much of a stretch to jump in saying that changes in the past support that we are changing climate now; it might be better to say that changes in the past establish that a change in forcings can very well change the climate. From there, those that are with you up to that point will naturally start to ask how much forcing are we doing now as compared to some other forcing at some other time. I haven't seen your talk; so, I'll just hazard a guess that some were still eating the first course when you yanked it and presented desert. Well, it's late, I've let myself get long winded, and I'm a choirboy preaching to the preacher. Good night and good luck.
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  4. #39 Momerath "Being engineers I’m sure at least one asked to see a graph of atmospheric CO2 concentration verses temperature for our planet. Since such a graph does not exist ..." That bugged me too, about a year ago. No amount of googling could find such a graph. So I downloaded the Vostok ice core data, graphed it over the last 100 kYears. You can see the results here: http://home.exetel.com.au/chemware/Paleoclimate.htm I also grabbed some sea-level data by Bard et al., and interpolated that to construct a similar graph for sea levels. Both plots are remarkably linear. They also have alarmingly large slopes.
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  5. Chemware - is this the kind of thing you're looking for? http://residualanalysis.blogspot.com/2008/06/anthropogenic-global-warming-is.html http://residualanalysis.blogspot.com/2009/12/statistical-proof-of-anthropogenic.html http://moregrumbinescience.blogspot.com/2009/03/does-co2-correlate-with-temperature.html http://bp3.blogger.com/__6PO0G1BcJM/SHjWm0gCguI/AAAAAAAAACk/BsbMNnwz0Go/s1600-h/figure2b.JPG
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  6. John If you are talking to Engineers, admitedly a very specific audience, try following the energy trail. Radiation Budget, Joules of heat in the ocean etc. Get them onside by doing the 'laymen may focus on temps but we know its about the energy trail'.
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  7. There's the ice age CO2/temp graph at wiki, too, a bit down the page on the right, though it's only a straightforward comparison. http://en.wikipedia.org/wiki/Ice_age#Origin_of_ice_age_theory
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  8. "The past geological record indicates that changes on a global scale are invariably very slow. We're talking tens of thousands, to hundreds of thousands and even millions of years in the vast majority of cases." "The fact that virtually *every* natural climate change event in the past (with the exception of those caused by extremely massive natural disasters) has occurred at a rate of between 1/2 to 1/10th [???] of the rate of recent change is simply further proof that nature is not the cause of the most recent climate change event." ... This is a representation of knowledge - science, with a few years. At least. Repeatedly (here) I proposed-presented, a graph: http://www.pnas.org/content/103/28/10536/F4.large.jpg. Why? 1st L. G. Thompson is a definitely not a skeptic. 2nd Certainly his work only for Africa but (as one of the few) dates back to 2003. Please look at the changes in the eighteenth century. It is difficult to explain only the increase in volcanic activity in the LIA. LIA began, moreover, a rapid and permanent change of temperature in about a few years of at least circa 0.8 deg C. Chris - why? After all, there were no significant changes in ice surface ... Certainly the current change is unprecedented, but certainly no more violent than the old change! For example, according to v. Storch once looked like this: http://www.korthweb.de/PhZT/Temperatur_Intcal2.gif Moreover, the current temperature change fits neatly into the sequence of Bond Events (not to be confused with D-O). CO2 is not needed to explain the very existence of the current changes in temperature ...
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  9. John, according to my reading, pre-Quaternary global temperatures were frequently around 22 degrees C, around 6-8 degrees warmer than at any other point since. The only exception was a very brief period at the end of the Ordovician Era and again at the end of the Carboniferous Era-where temperatures fell to those closer to modern averages. Both climate change events were preceded by significant drops in atmospheric CO2 (from 7,000ppm to 4,000ppm between the Cambrian & Ordovician Eras & from 4,000 down to less than 1,000 over the course of the Devonian & Carboniferous Eras). What's important, though, is that even though the sun was around 10% cooler than the modern era, the planet's temperature was significantly warmer than today-& the only real explanation is the very high levels of CO2 in the atmosphere-CO2 that we're now re-releasing back into our atmosphere. Yet still some people would have us believe that this massive release can have *no* impact on our global climate.
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  10. Arkadiusz Semczyszak-1st Bond events really would only explain climate change in the Northern Hemisphere. 2nd, almost *every* Bond Event in the Holocene has resulted in *cooling* of the Northern Hemisphere, not warming (like the Iron Age Cold Epoch of 900BC-300BC or the 4.2 KYr event thought responsible for the collapse of the Egyptian Old Kingdom). 3rd, these cooling events have been relatively slow compared to recent warming. So one wonders how you think GLOBAL WARMING is in anyway consistent with Bond Events, when past Bond events-like the 30-year decline in solar activity-suggest that we should be moving into a modest global *cooling* event, if anything-not warming. The fact remains that the only thing consistent with the rapid rise in temperature the last 30-50 years is the equally large rise in CO2 & methane during that same time period.
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  11. Arkadiusz, another point. The little ice age didn't occur quickly. Reconstructions show a 0.8 degree fall in temperature over a period of around 600 years-a rate of -0.013 degrees per decade! By contrast, global warming since the 1970's has been at a rate of +0.16 degrees per decade-ten times faster! Also, the LIA is directly connected to both the Maunder & Dalton minimums-coupled with some volcanic activity, wheras recent warming has no solar connection.
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  12. Arkadiusz Semczyszak at 21:50 PM on 22 April, 2010 1. You are misrepresenting Lonnie Thompson's PNAS article, which shows that current warming is likely unprecedented in the areas studied for 5000 year. Since this is an open access article, anyone wishing to know understand this can read it here Note that this work is about mountain glaciers in S. America (Peru); the analyses presented are interpreted in line with similar data from mountain glacier cores elsewhere in the world. 2. You're misrepresenting the science on the LIA. It certainly wasn't "a rapid and permanent change of temperature in about a few years of at least circa 0.8 deg C". One can assess this by inspection of published data also in open access articles. See here, for example (Fig 3). On can only get "at least circa 0.8 deg C" if one goes from the "height" of the MWP to the "depths" of the LIA, and that took around 500 years (not "in about a few years"!). If one takes the more generally accepted temperature decrease to LIA (from around 1200-1600 AD), the temperature decrease was in the range 0.3-0.5 deg C. However one looks at this your interpretation is grossly incorrect. 3. Bond events. There's not much evidence that these are truly manifest within the Holocene. In any case since the last N. hemisphere warmish period was around 1000-1100 AD and the Bond events have a supposed 1500 year periodicity, we shouldn't actually be having a Bond event now at all. In any case you'll need to provide us with some evidence for Holocene Bond events. It's simply not possible to "explain the very existence of the current changes in temperature", without massiverly enhanced anthropogenic greenhouse gas levels. Making stuff up isn't going to aid our understanding!
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  13. As an engineer I thought it behoved me to read and comment, so here goes. . When the sun gets brighter - I baulked at this as my immediate reaction was that you are saying that the sun is emitting more energy. (With thought, I accept that your statement is correct because the sun gets brighter from our perspective because it's closer or the earth is differently tilted due to the Milankovic cycles). When volcanoes erupt - my immediate reaction was we've had recent cases (Pinatubo, Krakatoa) where volcanoes have erupted and only caused very short term climate change. (I'm just giving you my first reaction here) So I was in a doubtful mood when I came to the second last paragraph. . the second last paragraph seems to be the key and I didn't find it convincing (Ie on a quick read. Similar to hearing it as part of a talk). It gave me the impression that there are a jumble of variables here and how would you know what's causing what and which variables are feedback. (Again with contemplation I start to realise what you are saying and how it does make sense. By the way, how can ice cores tell us temperature change and solar activity?) PS. I am not a skeptic on climate science. I live in a hot bed of activism on climate change and my daughter tells me you are giving a talk at UQ next month. I'll try and make it. PPS. I read a great comment in an IPCC article or related document about the greenhouse effect and saturation and the build up of pressure due to extra CO2 and the temperature of Venus being 400+ due to it being 98% CO2. I can't find it right now but I'd be really interested in your take on this topic as I didn't understand the build up of pressure bit and how it affects termperature.
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  14. #21 chris at 00:05 AM on 22 April, 2010 they [Dansgaard-Oeschger events] are not representative of global warming/cooling events Think again. PNAS online Ice-core evidence of abrupt climate changes Richard B. Alley "Ice-core records show that climate changes in the past have been large, rapid, and synchronous over broad areas extending into low latitudes, with less variability over historical times." It is not "global warming" anyway, it is "climate change". And D-O events brought hell of an abrupt climate change for sure. With such an instability, current agriculture would be impossible, billions would perish of mass starvation. Even if such events can not happen in the present warm regime, it is well known at least 7 years worth of food storage is needed. Still, world economy is run on a "just on time" philosophy with less than a year food backup. Grim.
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  15. BC #63, The energy emitted by the sun is not constant. There are, and have been over the ages, real differences in its energy output that have nothing to do with any orientation or orbital phase of the earth. Sorry, but I'm going to use you as a case in point that engineers often expect things to be constant that are not.
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  16. When the sun gets brighter - I baulked at this as my immediate reaction was that you are saying that the sun is emitting more energy. (With thought, I accept that your statement is correct because the sun gets brighter from our perspective because it's closer or the earth is differently tilted due to the Milankovic cycles) As I understand it, our sun has been getting hotter over the aeons. It's not a cyclical process.
    Even during its current life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising.
    http://en.wikipedia.org/wiki/Sun
    In the long term, the greatest changes in the Solar System will come from changes in the Sun itself as it ages. As the Sun burns through its supply of hydrogen fuel, it gets hotter and burns the remaining fuel even faster. As a result, the Sun is growing brighter at a rate of ten percent every 1.1 billion years.
    http://en.wikipedia.org/wiki/Formation_and_evolution_of_the_Solar_System
    The continuous fusion of hydrogen into helium will cause a build-up of helium in the core. The rate at which this process occurs depends on the initial mass of the star and ranges from millions to billions of years. Larger, hotter stars produce helium more rapidly than smaller, cooler ones. The accumulation of helium in the core causes a gradual increase in the rate of fusion and gravitational self-compression, as helium is denser than hydrogen. Higher temperatures must be attained to resist this increase in gravitational compression and to maintain a steady state. Eventually, the core exhausts its supply of hydrogen, and without the outward pressure generated by the fusion of hydrogen to counteract the force of gravity, it contracts until either electron degeneracy becomes sufficient to oppose gravity or the core becomes hot enough (around 100 megakelvins) for helium fusion to begin. Which of these happens first depends upon the star's mass.
    http://en.wikipedia.org/wiki/Stellar_evolution
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  17. Chemware, Barry, I believe Hansen et al cover this topic to some extent in "Target Atmospheric CO2: Where Should Humanity Aim?" http://arxiv.org/ftp/arxiv/papers/0804/0804.1126.pdf
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  18. Chris G and barry, I believe you misunderstood BC's intent. He was helping John by giving his knee-jerk reactions, to simulate an audience's immediate reactions upon seeing the slide presentation, to help John revise his slides to keep that audience engaged and on track.
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  19. @Berényi Péter at 22:56 PM on 22 April, 2010 #21 chris at 00:05 AM on 22 April, 2010 they [Dansgaard-Oeschger events] are not representative of global warming/cooling events Think again. PNAS online Ice-core evidence of abrupt climate changes Richard B. Alley I don't think your reference quite supports what you're intending. For example: Geographic Coverage. The ice-core record of abrupt climate changes is clearest in Greenland. No other record is available that spans the same time interval with equally high time resolution, complicating interpretations. It appears, however, that ice cores from the Canadian arctic islands, high mountains in South America, and Antarctica contain indications of the abrupt changes. Dating is secure for some of the Antarctic cores. The Canadian arctic cores show a sharp cold reversal during the deglaciation that is probably the Younger Dryas event (29). Ice cores from the high peaks of Huascaran and Sajama in the Andes also show a deglacial reversal in the ice isotopes that may be correlative with the Younger Dryas (2, 30). However, for various reasons, the exact timing and abruptness of the changes are difficult to ascertain in these records, and records of older abrupt changes are even less secure. In Antarctica the Byrd core from West Antarctica, and probably the Vostok and some other cores from East Antarctica, show events that are correlative to the larger millennial events of Greenland, including the Younger Dryas (6, 31). Byrd and Vostok also contain indications of events that may be correlative to nearly all of the Greenland events (31). However, the ice isotopes indicate an antiphase behavior, with Byrd warm during the major events when Greenland was cold; dating control is not good enough to determine the phase of the smaller events. The general impression of the Antarctic events is that they are smaller and less abrupt than those in Greenland, although fewer paleothermometers and other indicators have been brought to bear in Antarctica, reducing confidence somewhat. To further complicate the issue, the Taylor Dome core from a near-coastal site in East Antarctica appears to be in-phase with Greenland and out-of-phase with Byrd during the deglacial interval centered on the Younger Dryas (32). As reviewed in ref. 33, non-ice records from broadly distributed sites in the Northern Hemisphere indicate large, abrupt changes (near-)synchronous with those in Greenland, with generally cold, dry, and windy conditions occurring together although with some sites wet perhaps because of storm-track shifts (cf. ref. 28). Some Southern Hemisphere sites also exhibit the Greenland pattern during the deglaciation, although high-resolution (annually resolved) southern records are still lacking. However, southern sites near and downwind of the south Atlantic show an anti-Greenland pattern with millennial warming when Greenland cooled, superimposed on the slower orbital variations, which are broadly synchronous in both hemispheres. Sudden 'climate change' is probably regional. Global climate change is clearer in the long-term. The last sentence in the reference reads:
    Abrupt changes have been especially large when atmospheric carbon-dioxide concentration, insolation, and other important climatic variables were changing rapidly, with possible implications for general behavior of the climate system.
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  20. Tom, I meant no disrespect to BC, and I firmly believe that he gives an honest opinion. Above, I had presented the argument that there is a tendency in engineers, when you present information that only makes sense if something they have been assuming is a constant, is in fact a variable, they don't believe what you are telling them. BC, describing himself as an engineer, reported that he balked upon reading something that only made sense if the solar output was not constant, and then came to the incorrect conclusion that John was talking about changes related to earth's attitude or position. That follows very closely with the example I gave above.
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  21. I'm not clever enough to spot that kind of game unless I know the participants well enough. It occurred to me that Tom was being facetious. But it's better to assume straightforwardness if you're not sure when chatting amongst unfamiliars. Wasn't going to comment about that, but seeing as you mentioned it Chris... On another note, I find the reactiveness to skeptics a bit disappointing at times. Don't get me wrong, I've been as frustrated as hell quite often and been pretty abrupt, but I've lately seen well-meaning questioners mobbed with vitriol. I also hope that erstwhile obstinate interlocutors in these climate debates might eventually modify their approach, if not their doubts, if the door remains open. I ask myself "what's my purpose here" and try to operate as if the answer is meaningful. And I'm basically sick to death of the Punch and Judy. Seems to me it achieves the opposite of what is wanted (unless slap-downs are a turn-on for you). That's Nana's lecture for the year. Now back to our feature presentation.
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  22. Ther has not been a D.O. event for over 20000 years. Of all the known Bond events, only a small fraction had the potential to affect climate. They seem to lead to an opposite change in the other hemisphere, a phenomenon that has been dubbed the "bipolar see-saw." All in all, the resemblance with what's happening now is so poor as to be irrelevant. All that has already been discussed and referenced on this site before.
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  23. #55 Barry: very nice analysis and data, but it only covers the last 150 years. So the temperature is not in equilibrium as [CO2] is rapidly changing, and the whole climate system is far from equilibrium. That is, if we stopped emitting CO2 right now, the temperature would continue to climb for several decades-centuries. I was after the long-term, equilibrium temperatures, which I think are best obtained from paleo data, because the [CO2] is changing very slowly, and the climate is close to equilibrium. #57 Barry: Yes, this is the Vostok data, which I simply re-graphed to plot Temperature as a function of [CO2], over the last 100 kYears. #67 Chris: Yes, the same old Vostock data, combined (in Fig 1A) with Red Sea sea level data. But simple-minded experimentalist me has just taken the next step and said that if Temperature and Sea Level correlate with [CO2] (as Hansen's graphs clearly show), then let's just plot them against [CO2]. This is shown below. Re-plot of Vostok Ice Core data Plot of Bard's sea level data against Vostock CO2 data These are the sort of graphs that Engineers love: if [CO2] is X ppm, then I need to design for a Temperature of Y C, and a Sea Level of Z m. While one must always be extremely careful with extrapolation, the goodness of fit and the slope of both graphs is very worrying.
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  24. The argument: "Rising CO2 levels are an external forcing" would be easier to understand if you emphasize that the extra carbon is coming from sources of stocks of fossil carbon i.e. the carbon is coming in to the system (biospehere) from external sources (“carbon is coming into the system from under ground”). That is what is different in burning trees than burning fossils. In the first case you burn carbon that is already in the system (internal carbon). In the second case you burn carbon that is not in the system (external carbon).
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  25. Chemware at 09:53 AM on 23 April, 2010 I'm not sure how seriously you're taking your graphs, but they're not teriibly helpful in relation to the expected/anticipated response of the climate system (temperatures; sea levels) to raised CO2 levels. The first graph tells us something about the response of the carbon cycle to raised earth temperature, and more specifically the temperature-induced redistribution of CO2 from ocean and terrestrial stores into the atmosphere. Your data indicates that if Earth temperatures rise by 1 oC, then the equilibrium atmospheric [CO2] levels rise by around 20 ppm. Since the glacial to interglacial earth temperature rise is 5-6 oC globally averaged and the glacial to interglacial [CO2] increase is around 90 ppm (from 180 ppm glacial to around 270 ppm interglacial), we get 15-18 ppm of increased [CO2] from a 1 oC temperature rise through glacial cycles; pretty much in line with your plot. (It's not quite so simple, since one must also factor in the feedback warming from the raised [CO2]. A similar problem arises with your sea level plot. Sea levels respond to temperatures. One can relate these causally to [CO2] levels only under the circumstance that the dominant influence on temperature is [CO2] changes (as is largely the case now). That wasn't the case during glacial-interglacial cycles, so the second plot has the same flaw as the first...
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  26. might be useful to look at the reverse situation. If climate had NOT changed significantly in the past, change skeptics would then be arguing that man could not possibly have any effect on world climate - that climate is self correcting in response to various challenges. The only overall conclusion from the fact of past change is that future change is possible. As other posts have pointed out the skeptic argument above is really a non-sequitur. reversing the situation helps people understand that. Only then get into the details - discussions of time scales and the like.
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  27. Mikko, good point, I try to make point this out whenever I hear complaints about cows or local grassland burning. Chemware and Barry, Interesting graphs, but I think there are a couple of major flaws, one each per graph. On the temp graph, it's pretty well established that temperature response is logarithmic, and you have applied a linear best-fit. Hansen tends to be on the upper end of estimates for long-term equilibrium response, and I think he's estimating about 6 K for a doubling of CO2, under current planetary conditions. Aside from thermal expansion, there is a hard limit of how much sea levels can rise that is determined by how much ice there is to melt. Even if you accept that the earth will eventually be ice-free if a 450 or greater ppm level is maintained, there's only about 80 m of ice available. In contrast, you graph is off the chart at 450 ppm. http://pubs.usgs.gov/fs/fs2-00/
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  28. #73 chemware - if you were to plot the average global monthly surface temperatures anomolies from HADCRUT3 vs. Mauna Loa average monthly CO2 concentrations you would not get a very nice correlation. from 1958 to 1974 there was a flat response, followed by a linear reponse (and what appears to be good correlation) then back to a flat response in 1999 which continues today. it just shows there are other variables at play here.
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  29. Incremental CO2 in the atmosphere could just as easily act as a staging for additional IR radiation. Why not then consider this as providing as much negative feedback as positive feedback (basically resulting in a net canceling towards warming)?
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  30. Can someone explain to me why most of the observed global surface warming of recent decades is occurring mostly at night? Why, if CO2 is a warmer, is it that daytime temps have remained pretty much stable, but nighttime temps have risen? Wouldn't this indicate that increased cloudiness is a major factor? Thanks.
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  31. Unrecovered, here where you talk about understanding positive feedback, you have a good point. Many people seem to assume that positive feedback is unstable. One way to explain this is to note that positive feedback results in the system moving further from the starting point than the unmodified forcing would take it. That extra change causes more feedback, and so on, and so on. If the feedback is <1x the change, each successive feedback amount is smaller, and the sum of all the changes damps out. If the feedback is >1x the change, however, each iteration of feedback will be larger, and the system will run away (until some other form of feedback limits matters). Fortunately the vast majority of systems are stable - I suspect unstable ones just blow up when they arise.
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  32. RSVP, greenhouse gas radiation does not cancel out, because the gas acts as layers instead of a single slab. Each layer radiates in all directions. When a layer has another layer above it, that upper layer in turn intercepts the radiation that is headed toward space, then radiates it in all directions, including down. I'll not finish the explanation here, because you can read it on Spencer Weart's Simple Models of Climate page. Use your browser's Find function on that page to find the text "shorthand way," and read down from there.
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  33. Steve Berry at 04:57 AM on 24 April, 2010 Steve, the Earth's thermal energy comes almost exclusively from the sun (a tiny amount from radioactive decay within the Earth). Since greenhouse gases are largely transparent to solar radiation of the energies that directly impact the atmosphere and the Earth, enhanced greenhouse gases don't have much of an effect on incoming solar radiation, which impacts those regions of the Earth's atmosphere and surface that are facing the sun (i.e. during daylight hours). On the other hand radiative equilibrium (largely constant Earth surface temperature) is achieved by the incoming solar radiation being balanced by radiation returning to space. Some of this occurs by reflection [from the atmosphere, or clouds or highly reflective (high albedo) Earth surface, especially surface ice]. A large part of the radiation returning to space is that which is emitted from the Earth's surface. This is long wave infra red (LWIR), that is emitted by all bodies that have temperatures above zero Kelvins. It is this thermal radiation that is "trapped" by greenhouse gases and which warms the atmosphere. LWIR is an inherent property of all warm bodies, and occurs continuously, including during the night. So whereas daytime temperatures are dominated by the incoming solar radiation (depends on incident angle; i.e. seasonality) and reflective cloud cover, night time temperature variability is dominated by warmth trapping cloud cover and the efficiency of emission of LWIR back to space. All else being equal (i.e. solar irradiation, albedo and cloud cover), in a world with continuously rising greenhouse gas concentrations, we expect the night time temperatures to increase faster than day time temperatures, since LWIR is only one of several contributions to loss of thermal energy to space during the day, whereas it is a dominant factor in night time heat loss. This isn't the whole story. In a warming world we might expect to observe somewhat higher cloud cover at night, since enhanced atmospheric temperatures means more atmospheric water vapour with possible enhanced cloud formation as the atmosphere cools at night. On the other hand, man made increases in atmospheric greenhouse gases is accompanied by enhanced atmospheric aerosols, and this tends to decrease solar radiation at the Earth's surface, reducing the night/day warming difference...
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  34. "Using ice cores, we can work out past temperature change, the level of solar activity plus the amount of greenhouse gases and volcanic dust in the atmosphere." A bit of explanation on how all that information can be gleaned from ice cores might be helpful. Isn't this an important feedback... in the past, during a warming trend, increasing temperatures caused the oceans to release dissolved CO2, which caused further warming.
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  35. Good presentation but here is a much more straight-forward method when presenting to non-scientists. <<< Point #1 >>> The primary mover of Earth's overall climate is Milankovitch Cycles (the main one being changes in the orbital shape from circular to elliptical then back). http://en.wikipedia.org/wiki/Milankovitch_cycles When the orbit is circular like now, we experience interglacial periods which average 20,000 years in length. When elliptical, we experience glacial periods which average 100,000 years in length. This is verified by ice cores from Greenland (Century Station), Antarctica (Vostok Station), as well as deep sea cores from the Indian Ocean (Vema 28-238). <<< Point #2 >>> Effects from volcanoes, thermohaline circulation, atmospheric currents, storms systems, and feedbacks from greenhouse gases add or subtract from the Milankovitch cycles. <<< Point #3 >>> During the previous interglacials, warming always occurred first which then triggered the release of dissolved CO2 from the oceans. This caused the release of other greenhouse gases like methane and water vapor (to only name two) which the forces Earth's climate hotter. In our current interglacial which started 11,700 years ago, industrial humans released CO2 ahead of the main warming trend. So a run-away warming effect could be right around the corner. reference (see the second graphic): http://www.southwestclimatechange.org/climate/global/past-present <<< Point #4 >>> Heat doesn't always immediately increase temperature. By definition: one calorie of heat will raise the temperature of one cc of water by one degree Celsius. However 80 calories of heat are required to convert one cc of zero degree ice into zero degree water. Question: what happens when all the ice is melted? Answer: heat will begin to raise water temperature. http://en.wikipedia.org/wiki/Enthalpy_of_fusion 70% of the Earth's surface is water. When it is heated, atmospheric and ocean currents move this excess energy toward the poles. So why is the arctic melting much faster than the antarctic? Answer: The Arctic is mostly an ice structure at sea level where it is exposed to oceanic heat. The Antarctic is an ice covering over a continent with an average elevation of 2300 m (which is the largest of all contents). As everyone already knows, higher elevation means cooler temperature. But higher elevation also means less exposure to warm oceanic water. Most people already know that ice reflects 90% of incoming sunlight while water absorbs 90%. This albedo change (hysteresis) is another snap-action feedback. <<< Optional-Speculative Point 5 >>> Has Earth always been effected by glaciations this way? Scientists don't think so. Major changes to Earth's climate occurred after geological forces formed the Panama land bridge 3 million years ago (joining North America to South America). This blocked east-west ocean currents between the Pacific and Atlantic. Many believe that this triggered changes in Atlantic currents which caused warmer temperatures to be delivered to Europe. (London England is warmer than Labrador while Glasgow is warmer than Moscow). Since it now seems that ocean currents are more important than previously thought, we now need to wonder what will happen if climate change disrupts the thermohaline current or even the gulf stream. If this happened, global warming could actually cool Europe while melting Russia's Arctic (I'm sure Russians would think this a benefit)
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  36. Posted by John Cook on Wednesday, 21 April, 2010 at 19:56 PM Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes. You mean surface temperature, of course. However, surface temperature depends not only on the heat content of the climate system (atmosphere+hydrosphere+soil), but also on surface pressure (via lapse rate). If surface temperature is increased by 1%, surface temperature goes up by 0.5°C. Therefore while it is true a positive energy imbalance implies surface warming, it does not work in the reverse direction. You can have warming with no net heat energy gain, just by increasing pressure. It has no direct consequences on our present situation, for the volume of atmosphere is pretty stable on historic timescales. However, on geologic timescales the situation is radically different. In the late Carboniferous - early Permian atmospheric oxygen level is assumed to be much higher than today, perhaps as high as 35% per volume. If N2 partial pressure is held constant (at 0.754 atm), it implies an overall atmospheric pressure 24% higher than today. This pressure increase alone would make surface temperatures some 12°C higher. Of course according to the Standard Solar Model luminosity of the Sun was 2-3% lower 300 million years ago than it is today. But it would make surface temperatures only 2°C cooler at most. On the other hand we do know surface temperatures in that epoch were about the same as they are today (with slightly more CO2 in the atmosphere). We have 10°C unexplained lack of warming here (and a permanent polar ice cap). That's more than we asked for. The only way out is to assume surface pressure was also about the same as it is today. However, in this case N2 partial pressure must have been about 20% lower than its present value. As average residence time of N2 in the atmosphere is ~20 million years (due to biological nitrogen fixation) and there is about sixty times more Nitrogen in the crust than in air, it seems to be entirely possible, especially because sequestration of organic detritus was particularly intense in those times. However, if N2 partial pressure can vary on a multi-million year timescale, we should be extremely cautious in interpreting the paleoclimate record, for as far as I know, there is no reliable reconstruction in the literature of past atmospheric pressure changes so far. The upshot is that it is rather difficult (or should we say impossible?) to derive climate sensitivity from paleoclimate data while one of the major ingredients is indeterminate. Understanding the nitrogen cycle should be given high priority.
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  37. #86 Berényi Péter at 08:36 AM on 21 September, 2010 If surface temperature is increased by 1%, surface temperature goes up by 0.5°C. That's plain silly. Of course it should read "If surface pressure is increased by 1%, surface temperature goes up by 0.5°C." It is calculated this way: Density of air at ambient pressure is about 1.3 kg/m3. As mass of air above an 1 m2 surface is about 104 kg, a 1% increase implies an additional 100 kg/m2. To get to this value, the air column should be 77 m deeper. With a 6.5°C/km environmental lapse rate it makes the surface 0.5°C warmer. Now, here is this paper. Nature Geoscience 2, 891 - 896 (2009) Published online: 15 November 2009 doi:10.1038/ngeo692 Nitrogen-enhanced greenhouse warming on early Earth Colin Goldblatt, Mark W. Claire, Timothy M. Lenton, Adrian J. Matthews, Andrew J. Watson & Kevin J. Zahnle They hypothesize nitrogen partial pressure during the Archaean was twice as high as it is today and they say there would be 3-5°C warming for N2 doubling. It is much lower than the result given by the first-order approximation above and I just can't see where the huge difference comes from.
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