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Climate Change: Past, Present, and Future

Posted on 30 September 2010 by John Cook

I love a simple, accessible graph that tells a clear story. A good example can be found in a new paper Climate Change: Past, Present, and Future (Chapman & Davis 2010). They plot past climate change over the past 1000 years together with what we can expect to experience over the next century. In a single figure, it tells us a number of stories which are fleshed out further in the paper.

The past 1000 years feature a number of temperature reconstructions (the thin coloured lines) using various proxies such as tree rings, corals, sediments, glacier length and boreholes. The black dotted line is the average temperature over the decade centered on 1 Jan 2000. Having a variety of independent proxy methods gives us confidence that current temperatures are warmer than any experienced over the past 1000 years.

The coloured areas represent future projections of global temperature. The yellow projection (C3) tells us what would happen if CO2 concentrations were held steady at year 2000 levels. In other words, what would happen if humanity had suddenly stopped emitting CO2 in the year 2000 (but it's okay, we'd still be allowed to breath). Even in this imaginary case, temperatures would still continue to increase due to the thermal inertia of the oceans.

The other coloured areas represent the IPCC scenarios. Scenario A2 has global population increasing to 15 billion, accompanied by a warming of 4°C by 2100. Scenario A1B has population peaking at 8.7 billion and a mix of fossil-fuel and renewable energy use. This gives us a warming of 2.5°C. Scenario B1 assumes the same population growth as A1B but also assumes more aggressive cuts in CO2 emissions, leading to a warming of less than 2°C.

It's a simple graph but quite useful to have past temperature history displayed with various future scenarios, providing the benefit of both hindsight and foresight. All the emission scenarios present global warming in the 21st century that dwarfs warming seen in the past millennium. Of course, it's also important to realise that this is the global average. Warming at high latitudes may be 3 times as much.

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Comments 1 to 50 out of 87:

  1. Of course this is too optimistic now, right?
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  2. Its a wonderful graph. But its still a theory that CO2 is the cause. Maybe its a coincidence that the temperature is rising everso slightly and the CO2 content of the atmosphere has risen everso slightly. Its just like I cannot prove to you God exists, but you cannot prove to me God does exist. Both theories God and CO2 are currently unprovable.

    It just needs for someone to put two and two together and come up with four.
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    Response: That CO2 is causing warming is confirmed by multiple, independent lines of evidence. We have both a shorter intermediate version of the evidence for an increased greenhouse effect and if you're hungry for more meat, a more detailed advanced version. On top of that, we have an extensive list of the human fingerprints on climate change that provide further corroboration. There is no shortage of directly observed, empirical evidence and it's all given to you on a silver platter here at Skeptical Science.
  3. "In other words, what would happen if humanity had suddenly stopped emitting CO2 in the year 2000."

    This should really read something like: "In other words, what would happen if humanity suddenly started emitting just enough CO2 to balance natural sinks and thus keep atmospheric concentrations at 2000 levels".
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  4. Miekol @2,

    "But its still a theory that CO2 is the cause"

    Did you mean to say "hypothesis"? Regardless, you are grossly over-simplifying things. A quick perusal of this site (which discusses and presents the relevant scientific literature on the subject) demonstrates that there is overwhelming evidence from independent sources which support the theory of AGW. It is no coincidence as some might wish to presume-- the evidence (from different scientific disciplines) is far too coherent for that.

    GHGs are not the only cause of climate variability of course, but recently radiative forcing from elevated GHGs has become a primary forcing mechanism and that is only going to increase as GHG levels continues to increase.

    If you want to debate what the equilibrium climate sensitivity is for doubling CO2, then that is still an open question. As of now, multiple, independent studies point to +3 C warming (globally) being most likely response to doubling of CO2.
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  5. You might even add that... "Average temperatures at high latitudes may be 3 times and much, AND individual summer high temperatures much more than even that."

    In other words, don't just go running north if you're looking to cool down.
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  6. miekol #2: "Maybe its a coincidence that the temperature is rising everso slightly and the CO2 content of the atmosphere has risen everso slightly."

    All reconstructions of past temperatures (even the skeptic ones) indicate that the recent temperature rise is the largest in the past few thousand years. In what way is that "ever so slightly"? The phrase is even LESS applicable to CO2, which has risen 40%... and is now at a level not seen for at least 800,000 years.

    ptbrown31 #3: Once atmospheric CO2 levels are elevated the natural processes which can reduce them take thousands of years to work. Thus, on the time scale of the graph the difference between 'human industry emitting just enough CO2 to keep the atmospheric level constant' and 'human industry emitting no CO2' would be insignificant.
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  7. I saw an interesting experiment/demonstration a while ago that rather neatly demonstrates to the dis-believers why CO2 causing warming is more than just a theory:
    Have a glass cylinder about e.g. a metre long and e.g. 10 cm diameter. At one end of it (outside the cylinder) put a candle or other heat source. At the other end (also outside the cylinder) put a thermal camera, pointing at the candle. On the camera's display, the candle should show up perfectly. Now fill the cylinder with CO2. You'll still be able to see the candle, but the IR camera will not.
    This proves that CO2 acts as blanket over the atmosphere - energy can still get to the surface as short wavelength light, but the longer wavelength IR radiation can't escape back out into space.
    I'm not sure where I saw it now. I've just checked on youtube and am unable to find it.
    Maybe someone who is better at writing and a little more scientifically literate than me can turn my summary into a simple demonstration? It seems to me that it is an approach that is not often tried: instead of using historical data (which people refuse to believe or say that it's just a coincidence), how about demonstrating the mechanics in a repeatable way?
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  8. Bamboozled @7,

    Was it this one?
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  9. Albatross... I've actually used that very same video to turn a few skeptics into believers.

    The other one that always seems to get skeptics thinking is the electric dragster video.
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  10. That is way cool Rob...I'm hoping to replace our Prius with an EV....
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  11. Miekol @2

    Your analogy to proving the existance of God is faulty. A God by definition is an unknowable phenomenon. We might postulate that one exists or not but the nature of the postulate is of the existence of a phenomena that has absolutely no physical interaction with the physical universe that we could observe, even less interactions that we could make predictions about EXPECTED physical phenomena.

    Greenhouse effect and AGW is very much a case of of a postulate that observable physical phenomena WILL occur.

    One core expected phenomena are changes in the emitted radiation spectrum for the planet from what otherwise would be the case due to the GH effect of these gases. And that this change to the spectrum will vary over time as the amounts of GH gases changes. Expected observable phenomena. What scientists would call a 'falsifiable' prediction. If we expect this phenomena and we don't see it, whoopsy we have a problem. If we see the phenomena then strong support for the postulate.

    And both expected phenomena are directly observed from satellite, high altitude aircraft and ground based observations, and have been for years. Not just theory. Direct observation

    And since that emission spectrumrepresents the energy flow from the planet, we are directly observing changes in the energy balance. We are directly observing a phenomena that is changing the energy balance of the planet.

    Take an analogy. If I put a pot of water onto the stove top and turn on the heat, and I have instruments that allow me to observe the heat flowing through the steel of the pot, I can make a reasonable conclusion that this heat flow will then continue into the water and heat it.

    By your reasoning if we observe the heat flowing through the steel and observe the water heating, we cannot assume that the heat from the steel is the cause. Something else might be heating the water. While there may be other sources of heat going into the water as well, we might be holding a flame to the top of the water at the same time, to assume that the heat flow through the steel will not have any effect on the water is nonsensical.

    Coming back to AGW. We know there is a radiation imbalance, we can see it. We know it is large enough to explain the observed warming. Other processes have been put forward that may contribute as well, but everything we know about them says they are not of anything like the same magnitude - see the recent post on Galactic Cosmic Rays at this site for a discussion of one of these areas.

    The only way we can say that we have a very strong understanding of why the phenomena should occur, we have observations of the phenomena occurring, but then say the consequence of the phenomena, heating of the planet may not be caused by the phenomena is if we assume that the Law of Conservation of Energy is wrong. If that is true Miekol, DUCK! The universe is about to self destruct.
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  12. If it's any consolation, U.S. debt next year will exceed its GDP, and many countries are sure to stop loaning us money so that we can continue to buy oil and gas. Peak Oil, just over the horizon, promises to be another damper. (Or is Peak Oil just another one of those myths?)

    In any event, a 15 billion world population would be impossible, because the land can barely support the population we currently have, even when using fossil fuel-based fertilizers. Gas production in many countries has also peaked, and fertilizers are becoming more difficult and expensive to come by. Without artificial fertilizers, world food production will drop sharply. So even the 8.7 billion population is questionable.

    And why are we still allowed to breathe, while holding CO2 production constant? Suppose the world has 6 billion people consuming 1000 Kilocalories per day. That equates to 6 trillion kilocalories per day. Modern food production and transportation processes consume about 7 kilocalories of fossil fuel per kilocalorie of food we consume. Thus modern food production and transportation would consume about 42 trillion kilocalories of fossil fuel per day. If we stopped modern food production and transportation, most of us would die, so I suppose we would not breathe or contribute CO2.
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  13. The current mainstream [ie, non-feral :-)]sceptical position at least as I understand it could be summed up as follows:

    1) CO2 has been rising
    2) Temperatures have been rising
    3) (1) very likely has made a substantial but not exclusive contribution to (2)

    But

    4) We're not as confident that temperature rise is unprecedented - ie, we have some doubts about the palaeoclimate proxy record when it is 'spliced' onto the modern record

    5) We don't want to overlook the role of other feedbacks which may be important whether as exacerbating or mitigating factors

    6)We're not as confident of catastrophic outcomes even if temperatures and CO2 rises more or less as projected

    7)Even if our reservations in (4),(5), and (6) prove to be correct, becoming much less dependent on fossil fuel and decarbonising our economies and our emissions is a very good idea anyway for lots of other reasons.

    8)However, we're much more likely succeed at (7) if we avoid a panicky response and scare the proverbial horses whilst triggering the laws of unintended consequences.
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  14. Dangerously reasonable, even heretical in some circles, Chris. (3) is dubiously relevant given that we seem to have introduced a durable secular trend in energy retention. (4) relies on past as prologue; the past did not include what's happening in the present so anachronisms are a questionable means of discounting our influence.

    Past those points, I'm not sure what you're describing is really skepticism about science as much as it is worry over human nature. We do have many examples of how our nature has led us to underestimate our impact on the various systems surrounding us, which we more or less depend on to continue thriving. The body of evidence we have available suggests we've made some serious mistakes in the past and should be forewarned about scaling up those errors.
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  15. #12: "why are we still allowed to breathe"
    Breathing was discussed here. I don't want to presume that there was a consensus, nor speak for one, but at least some folks said something to the effect of "Don't hold your breath."
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  16. #15, if breathing is allowed without eating food that has a carbon footprint, then I agree.
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  17. Doug @ 14

    Really, the so-called sceptical/AWG divide when you look at the output of mainstream contenders is akin to the two religious sects of Lilliputians who are divided between those who prefer cracking open their soft-boiled eggs from the little end, and those who prefer the big end.

    WUWT for example has of late put a fair bit of very conventional science into play much of which would pass without raising eyebrows if posted here and presented as coming from a 'warmist' source. WUWT posts are much more sophisticated and subtle than a lot of its readership.
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  18. #16:"is allowed"
    I don't quite understand what you're getting at here; where's the discussion of allowing this and prohibiting that?

    However, there are intelligent choices that can be made regarding food consumption.
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  19. #17: "WUWT posts are much more sophisticated and subtle than a lot of its readership."

    What kind of readership does Skeptical Science have? A bunch of people beating their chests, rehashing the same stuff over and over, trying to demonstrate who's most clever?

    I would venture that 99.94% of skeptics don't have a degree in earth sciences or related areas, and would have trouble following most of these discussions. I thought Skeptical Science was intended to educate those 99.94% of skeptics, but the ones I've interacted with stay about 30 seconds and delete the page. That's not going to help solve our worsening climate change problems.

    Skeptical Science should serve the congregation, not the choir.
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  20. #18: "there are intelligent choices"

    Cubans make intelligent choices; they use Oxen. Americans don't make intelligent choices; they use fossil fuel guzzling tractors and semi tractor-trailers barreling down the highway at 130 kilometers/hour. Their only thought is making a living.
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  21. Roger, "skeptics" of the type commonly found in the comments threads at WUWT are not generally amenable to any form of persuasion. Please see Roy Spencer's attempt to educate "skeptics" on the single most important feature of this "debate," paying attention to comments: Yes, Virginia, Cooler Objects Can Make Warmer Objects Even Warmer Still

    Following a descending slope of obduracy to the very bottom is not how people holding politicians accountable for effective policy are going to be engaged on this topic.
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  22. Doug @ 14
    '(4) relies on past as prologue; the past did not include what's happening in the present so anachronisms are a questionable means of discounting our influence.'

    However, both sides of the divide invoke the past as prologue. After all, we've just had a thread disputing Loehle's claims of being 'vindicated' following a recent study suggesting a robust MWP and LIA.

    It does matter then whether the LIA and MWP were not as hot as today - if they really proved as warm or warmer than today, then our debate around climate sensisivity changes significantly.

    However, becoming much less dependent on fossil fuel and decarbonising our economies and our emissions being a very good idea anyway for lots of other reasons, continues to apply.
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  23. Chris: ...becoming much less dependent on fossil fuel and decarbonising our economies and our emissions being a very good idea anyway...

    On that we certainly agree!
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  24. ptbrown31 #3
    "This should really read something like: "In other words, what would happen if humanity suddenly started emitting just enough CO2 to balance natural sinks and thus keep atmospheric concentrations at 2000 levels"."

    We are in the year 2010, (and 2011 is right around the corner).

    And an even smaller point...

    Posters (including myself) tend to use the word "humanity" quite a lot when in actuality they are referring to the overall negative impact humans are having on the natural environment. The word "civilization" is always available, although destroying the environment in a civilized manner doesnt ring that well either.
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  25. #20 Roger A. Wehage at 11:47 AM on 30 September, 2010
    Cubans make intelligent choices; they use Oxen. Americans don't make intelligent choices;

    Very true. In Cuba, the ox is mightier than the tractor.



    "Each tractor can do the work of five teams of oxen," Andalio said. "Work with tractors hasn't stopped, but it will only go as far as the economy allows," he added.

    "We use tractors when there are tractors, but there almost never are," said Alvarez, 59.

    "I am thankful for the revolution," the 52-year-old [Zenaida Leon] said. "But we don't get boots, tools, irrigation that works."


    High time for Americans to get rid of their boots; tools have already gone to China anyway. It's much healthier to walk barefoot, also makes your footprint look prettier, smaller and more natural.

    See the merits of going barefoot.

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  26. A cost benefit analysis incorporating the carbon footprint of oxen versus tractors in the context of parameters such as productivity of land and the quality of life of the farmer would be an interesting exercise. Any takers?
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  27. Bother. My comment disappeared (my own fault)

    Oxen? I think it's more productive to look at a better mix of animals suited to purpose. Chooks and pigs do a fantastic job of weeding, ploughing and fertilising all in one hit. I've only seen people talk about using this technique on horticultural sized enterprises. For agricultural purposes, perhaps chook and pig farmers could work a little like beekeepers, moving their charges from one area to another as needed. Goats are brilliant at clearing weed infested land, but finding enclosures that are both movable and secure enough would be an issue. Cows and sheep are good at turning crop residues into fertiliser, not very wonderful at clearing weeds or turning over the soil.

    Cuban farmers are in the unusual position of not being able to get boots, irrigation or any other useful items because of the collapse of the USSR taking away their sources while the trade embargo still continues. (Their position is not directly relevant to us, or even to farmers in other countries whose access to equipment is restricted for other reasons.)

    That has led to a great system of fruit and veg raising within Havana itself. Smallish plots using compost rather than the unobtainable chemical fertilisers are doing a fair job - and a lot healthier too.

    Oxen should have a medium role - I don't see chooks being an asset to seeding or sprouting crops, more of a menace. But we really do have to start thinking along the lines of how we farm grain crops or raise veg and fruit without FF based fertiliser. The old-fashioned mixed farm had some advantages. All we need to do is to see how these advantages can be modified for broadacre grain crops. Or even to rethink the whole industrial farming approach.
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  28. chriscanaris@27, "Any takers?"

    The The Power of Community: How Cuba Survived Peak Oil video produced by Community Solutions may be a good start. They're already into Plan C.
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  29. chriscanaris wrote : "It does matter then whether the LIA and MWP were not as hot as today - if they really proved as warm or warmer than today, then our debate around climate sensisivity changes significantly."


    Sticking to the MWP, it being globally warmer would in itself not contradict AGW but the so-called skeptics think it would - most of them seem comfortable with the sort of thinking that reckons that natural forest fires in the past disprove present man-made forest fires.
    If it was proved to be globally warmer (albeit at different locations over a period of hundreds of years), the so-called skeptics would declare the final (final) nail in the coffin of AGW and would then be able to internally rationalise-away any inconvenient facts in the real world (extreme weather, etc.), while looking for other science to 'audit' and misunderstand.

    Meanwhile, as you mention, the debate around climate sensitivity would increase in the real world (as would the effects from that sensitivity) and the science would carry on developing, hopefully without interference - the self-declared 'auditors' having declared victory and moved on.
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  30. OT, but the BLOG REVIEW OF SCIENTIFIC COHERENCE thread seems to have disappeared.
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    Response: Steve asked me to take it down once the blog-review process had done its job. The URL still exists but won't load for most users. The final transcript of the podcast will be posted here next Wednesday - with luck, an audio podcast will also be available to listen to.
  31. adelady@29 "rethink the whole industrial farming approach"

    Peak oil and gas will eventually do that for us, but it will be countries like Cuba who leap ahead of us, because of their experience. Food production must go more local, because the costs of processing and transportation will become prohibitive.

    I've been thinking that the power of the sun should be harnessed to till the land. A lot of, but clearly not enough, research has gone into Batteries for Electrical Energy Storage in Transportation (BEEST). Why not use this research to produce small walk-behind or ride electric-powered tractors with detachable instruments and drop-in battery packs charged by banks of solar cells or the grid? There could be smaller units for individual gardens and larger units for community gardens.

    The advantage of BEES (Batteries for Electrical Energy Storage) is that they don't eat much compared to animals of labor, saving more of Earth's scarce resources for the burgeoning human population.
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  32. Roger@33: bad link. The corrected link is Batteries for Electrical Energy Storage in Transportation (BEEST).
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  33. This is a great graph. Is there a link to it, other than on this site? The primary paper requires registration. Also, when presenting to skeptics the first thing they would say if I linked to this site is--"well of course, what do you expect from this site"
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  34. Roger A. Wehage, I strongly disagree with the notion that reversion to some kind of romanticized agricultural utopia is the way forward. Worse yet, the more traction that argument gets, the less likely it is that we'll actually do anything to address the risks of climate change.

    Insofar as people pay attention to them, your comments about Cuba here are a gift to the "skeptics" and a setback for those who are trying to do something constructive in the way of preventing or limiting climate change.

    Given a choice between (a) destroying the quality of life offered by our modern industrial civilization, or (b) burning all the oil, gas, coal, and tar sands on the planet to sustain that quality of life, people will opt for the latter.

    Our only hope is to offer a third option: evolution of our technological and social infrastructure in a direction that offers a high quality of life (and not just to that fraction of the human race that is fortunate enough to live in "developed" countries) but at a much lower level of GHG emissions.

    I know that "skeptics" like to claim that the only choices are burning fossil fuels or de-industrializing. I absolutely refuse to accept that claim.
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  35. Roger, the electrical vehicles are terrific.

    But without FF based fertilisers, we're going to have to find other fertilisers. We could finish up with animals being valued more for their dung than for their meat or milk. And we'd be mad to waste any kind of fuel on removing or disposing of crop residues when animals can convert it back into nutritious, water retaining soil.

    Home gardeners will be a lot better off with a few chooks fertilising their veg patch when they don't even have the option of getting a bag of stuff from the garden centre. Even a couple of guinea pigs in one of those rolling cages can mow a smallish lawn and keep it going with their little contributions. And home gardeners or other smallholdings will need lawns or other sources of green stuff to feed chooks or bulk up compost heaps.
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  36. Ned "I know that "skeptics" like to claim that the only choices are burning fossil fuels or de-industrializing. I absolutely refuse to accept that claim."

    This is my number one pet hate. Why on earth would we "de-industrialise" when we're faced with so many opportunities to make clever, advanced technological things and make bulk money doing so? I resign myself to silly statements about properties of CO2 and the like, but this is the number one anger maker.

    Especially since the argument is supposed to be about 'successful' economic activity. Good business people should be able to take opportunities that present themselves and find the best way to make money from them. Where are the 'brave new world' kind of self promotions that used to dominate in the 50s and 60s?

    I often picture JFK making his speech about the adventure to take people to the moon. He set a date for completion. He wasn't around to see it completed within the timeframe he set, but it was done in the timeframe he set. And that needed rocket scientists and specialised engineers to get the job done.

    The technological changes we're looking at are not so demanding. We just need lots of businesses as well as governments to get going on the task.
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  37. Roger:
    Why not use this research to produce small walk-behind or ride electric-powered tractors with detachable instruments and drop-in battery packs charged by banks of solar cells or the grid?

    Sure, as long as we're able to maintain the enormous, globe-spanning industrialized infrastructure secondary batteries perch on top of. Same with the semiconductors used in any modern, efficient electric motor power control system, the motors themselves, PV panels, etc.

    These things can't be made by the folks staying at home in the village while the fields are tilled.

    I believe you're thinking more of Taiwan, not Cuba.
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  38. adelady@35, much has been done to demonstrate that abundant crops can be had without FF fertilizers. For more information see this excellent library located in Steve Solomon's home in Tasmaina, Australia. Specifically see Chapters 8 and 9 in his Organic Gardener's Composting book.
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  39. doug:
    "as long as we're able to maintain the enormous, globe-spanning industrialized infrastructure secondary batteries perch on top of."

    Electricity is the way of the future. Wind and sunlight are fairly uniformly distributed throughout the world. The cost of producing electricity locally is steadily coming down as volume increases. The biggest detriment to progress is oil companies who think they have the most to lose. Technology will follow in the wake of demand. See Hot, Flat, and Crowded for more elightenment.
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  40. Roger, the "way of the future" relies on things such as ample access to neodymium permanent magnets, high current semiconductors and a myriad of other requirements needing a substantially globe-spanning system of materials manipulation, with means of incentives for cooperation, etc.

    But we're swerving well off topic here; this thread is not the right place to explore technological adaptation. I've ransacked the cornucopia of topics here and I can't find an appropriate place to take this conversation so perhaps we should leave it be.
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  41. Doug's right, we are off topic but adelady @ 27 writes:

    Goats are brilliant at clearing weed infested land, but finding enclosures that are both movable and secure enough would be an issue.

    Goats are brilliant at clearing weed infested land because they rip plants out by the roots, ie, they literally eradicate. Consequently, they can be a huge problem with regard to deforestation.

    Coming back on topic, JMurphy @ 29:

    Identifying a clear MWP hotter than today would not be the final nail in anything. While it would change the parameters of our debate around climate sensitivity and forcings, I'd like to think that most of us who participate in these discussions do so not to score points but to come to a better understanding of the fascinating intricacies of the workings of the world in general and climate in particular. AWG may have been the catalyst to our musings but the science is never *settled* if you are a genuine scientist! :-)
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  42. Chris, the discussion was about agriculture.

    I very much doubt anyone with any brains would use goats for anything but reclaiming degraded growing areas. They're particularly good for areas within a farm infested with blackberry and the like. (Though I personally think pigs are better for this.)

    I know of no grain crop that grows under forest cover, especially not native forest.
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  43. As for "a clear MWP hotter than today", that *would* be the final nail in the coffin of the idea that climate sensitivity is a small or manageable number.

    The last thing anyone wants is this or any other confirmation that climate sensitivity is on the much higher side rather than the lower.
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  44. chriscanaris, you know very well that there are many so-called skeptics out there (including self-appointed auditors) who are obsessive about proving a global MWP. Why do you think that is ?

    And, don't forget that another self-appointed blog expert has only this year brought out a book referring once again to the 'hockey-stick'.

    Finally, you have only to read the threads about the 'hockey-stick' on this site, to see so-called skeptics claiming that Mann has been 'proved wrong' (which seems to suggest that, therefore, AGW is also proven false) and that McIntyre, etc. are heroes who have proved...well, who knows, but they keep coming up for adoration by some on here.

    Why the obsession ?
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  45. Way back yesterday, chriscanaris wrote:

    1) CO2 has been rising
    2) Temperatures have been rising
    3) (1) very likely has made a substantial but not exclusive contribution to (2)


    As you know, I'm always grateful to see sceptics agreeing with these points. It's nice to know there is some common ground here. I will try to reciprocate (at least a little!) below.

    continuing...

    4) We're not as confident that temperature rise is unprecedented - ie, we have some doubts about the palaeoclimate proxy record when it is 'spliced' onto the modern record

    I'm pretty confident that this is the warmest it's been in the past few millennia (and that unless we act soon we will exceed the peak warmth of the previous interglacial). But, from your perspective, can you explain why this question of "precedents" is important?

    I can see two possibilities. Some people seem to believe that if one or more previous intervals were as warm as today (for presumably "natural" reasons, ignoring Bill Ruddiman), then that would cast doubt on the anthropogenic origins of modern warming. An alternative reason for focusing on "precedence" is the idea that if societies and ecosystems survived an equally warm period in the past, the impacts of modern warming can't be too serious.

    IMHO there are obvious, serious flaws with the first alternative and more subtle issues with the second. (It's also possible you have some other reason for placing importance on the question of "precedents" that isn't obvious to me...)

    OK, back to chriscanaris:

    5) We don't want to overlook the role of other feedbacks which may be important whether as exacerbating or mitigating factors

    I appreciate the way you worded this. It's important to keep in mind that the range of values for climate sensitivity (2 to 4.5 C per doubling) is intended to cover the uncertainty on both the high side and the low side, around a best estimate of 3 C. It is frustrating to me that so many sceptics focus exclusively on one half of this range of uncertainty while ignoring the other half. That seems like a dangerous (if all too human) irrational mindset.

    It is very easy to justify a low value for climate sensitivity if one hunts for all possible lines of reasoning that would support a low value while (not necessarily deliberately) avoiding thinking about those that would support a high value. This, to my mind, is a rather common theme in the writings of a great many "sceptics" on this site.

    chriscanaris continues his list:

    6)We're not as confident of catastrophic outcomes even if temperatures and CO2 rises more or less as projected

    This, I think, is your strongest point, at least if you can guarantee that we don't switch over heavily towards burning coal or tar sands when the conventional oil starts to run out. The uncertainty on the question of impacts (economic and environmental) is real. To be perfectly honest, the tendency of many (most?) sceptical commenters to reiterate easily rebutted claims (yes, CO2 is a greenhouse gas; no, the CO2 rise is not coming from the ocean; no, there is no way that solar irradiance can explain modern warming) lets us supporters of the "mainstream climate science" position off the hook -- we get to provide easy answers, rather than discussing the more murky questions of impacts & consequences.

    I don't think the outcomes of a middle-of-the-road emissions scenario will be uniformly "positive" or "negative", but rather a mix of both depending on your location and your viewpoint. And although I use it myself from time to time, I'm not convinced that the "precautionary principle" argument is a really solid one. In balance, I think the costs of 21st century climate change will be greater than the benefits, but I recognize this is a purely qualitative interpretation on my part.

    Of course, there has to be a limit somewhere. We've burned around 300 GT of carbon since 1750. I agree that the impacts of this are likely mixed. The business-as-usual scenario would see us burning around 2000 GT carbon. If we burned all the reasonable coal reserves available, this would be 5000 GT carbon. I cannot imagine the impacts of the latter scenario being anything short of disastrous.

    Returning to chriscanaris's comments:

    7)Even if our reservations in (4),(5), and (6) prove to be correct, becoming much less dependent on fossil fuel and decarbonising our economies and our emissions is a very good idea anyway for lots of other reasons.

    Thank you for including that. I think it would be really helpful if our society could focus on identifying common grounds for at least some action, and go ahead with those actions while we debate whether concerns about climate change justify further action.

    8)However, we're much more likely succeed at (7) if we avoid a panicky response and scare the proverbial horses whilst triggering the laws of unintended consequences.

    I'll have to remain agnostic on this, I guess. We don't have parallel Earths to experiment on, so I guess we'll never know whether some hypothetical (different) approach to dealing with the energy/environment/climate nexus would have worked better. There is certainly a part of me that would like to believe that we've just handled this all badly and that our failure to take any significant steps on this issue is not a sign of inherent failings in human nature. Who knows?

    Anyway, thank you for your comments, chriscanaris -- as always they are a bit of a breath of fresh air after reading too many frankly silly dispatches from strangely unsceptical "sceptics" here and elsewhere.
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  46. adelady at 22:46 PM, I don't think that there is any viable alternative to feeding the masses other than the way agriculture is heading. Advances in technology now has twice as many people being fed off half the area of land compared to not so long ago, and that trend is still continuing.
    Where the really BIG advances can be, and MUST be made is to reduce the waste.
    Modern lifestyles means that only about half of the nutrients stripped from the soil and converted to food are actually utilised, the rest being lost as wastage along the way or simply being thrown out.
    Virtually none of that wastage, or any of the waste byproducts produced even if the food is consumed wisely, make it back to the point of origin to be recycled.
    I think it is a nonsense to expect that the problems can be fixed by changing farming methods when all such changes will do is allow the mindset that is oblivious to all the wastage to perhaps waste even more.
    In recent years we saw such mindset when Victorian politicians congratulated capital city dwellers on reducing water consumption by X litres per day.
    That left many country dwellers wondering what the city dwellers were doing with what they, the country dwellers, considered a precious resource, as their own normal daily consumption was somewhat less than the X litres supposedly being saved.
    Now that it has rained, the water saving measures are being relaxed and people are again free to quickly resume the wasteful usage of water once again.
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  47. Interesting graph. Doesn't look much like this one: http://www.ncdc.noaa.gov/paleo/pubs/mann2008/mann2008.html

    The most notable conclusion I can make from all the paleo reconstructions of temperature is that none of them provide much usable information. All of them show that changes of ~.5-.6 degC over periods of 10 years or so are to be expected. The bottom line takeaway is wait another 30-40 years and we will have a much better idea of what drives the climate.
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  48. Always worth remembering, while they may provide information on expectations, reconstructions of past climate do not presume to be explanations of what drives the climate.

    Philc, when you say no reconstructions of past temperature do not provide much usable information, what do you mean? Are you saying they provide no cues on likely boundaries? If your conclusion is that they show temperatures as generally having been constrained within a range of perhaps 0.6 degrees C, how is that not useful?
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  49. philc writes: The bottom line takeaway is wait another 30-40 years and we will have a much better idea of what drives the climate.

    Actually, we've already got a pretty good understanding of what drives the climate.
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  50. Our understanding of what drives the climate is so good we can say it's very likely we can't afford to wait another 30-40 years before we react to this threat.
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