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Carbon dioxide might fertilise plants, but they still need water

It sounds logical. Plants fix carbon dioxide into sugars using light and water in the process known as photosynthesis. Therefore, extra carbon dioxide should equal more plant growth. Plants benefit from…

Plants might thrive with more carbon dioxide, but only if there’s enough water. Flickr/blueforce4116

It sounds logical. Plants fix carbon dioxide into sugars using light and water in the process known as photosynthesis. Therefore, extra carbon dioxide should equal more plant growth.

Plants benefit from the increased levels of carbon dioxide humans have released into the atmosphere. But this doesn’t cancel the negative effects of climate change.

Recently the argument of “CO2 as plant food” has resurfaced, suggesting that the benefits of increased CO2 to plants and agriculture outweigh the costs of climate change. But as climate science shows, the picture isn’t quite so simple.

Why do we think CO2 is fertilising plants?

The new research led by CSIRO revealed that parts of the globe are greening. To show that CO2 was responsible for more plant growth, satellite observations were analysed, removing the effect of variations in rainfall. The research found the 14% increase in CO2 between 1982 and 2010 led to a 11% increase in green foliage.

The green in this image shows where plant growth has increased over the past three decades NASA

Photosynthesis, though, is a complex process. It depends on many factors other than CO2, including sunlight, temperature, rainfall and nutrients. Some of these factors are also changing under climate change.

To start with, for every 1°C rise in temperature, relative humidity rises by 7%. Satellite data indicate that the total atmospheric moisture content over oceans has increased since 1988.

This extra moisture could see the greening of arid zones like the Atacama and Namib deserts, where specialised communities of plants rely on coastal fogs for water.

But working against humidity and increased CO2 are aerosols like sulphur dioxide. These decrease the amount of light available to plants for photosynthesis, and are also on the rise.

The bigger picture: the tropics are expanding

So far we’ve focused on relatively minor factors. But the greatest effect of climate change will be the movement and expansion of climate zones, and the process is accelerating.

The tropics are expanding north and south into the subtropical and savanna regions of the Sahara, Mexico and Western Australia. This greening is consistent with a shift in overall climate conditions to those that were previously seen in the Pliocene (5.2-2.6 million years ago). Average global temperatures were 2-3°C higher than before the Industrial Revolution, when humans starting increasing CO2 levels.

Five different studies show that the tropics are expanding. Diane J. Seidel

At the same time temperate climates are expanding into the tundra zones of the Northern Hemisphere. Thawing of the tundra and increased rainfall might allow agriculture to expand into the region.

During the Pliocene places that we now know as deserts (like the Sahara, and the Gobi in Mongolia) were largely grassy savannas. Greening of these zones on the edges of the tropics might benefit pasture.

But the shift of desert zones into temperate zones, where the bulk of the crops and livestock takes place, is leading to more droughts. Extreme weather events of all kinds are also increasing in every zone.

Overall, the effect of climate change on agriculture is likely to be negative. CO2 might fertilise plants, but it can only work where there is enough water. And as the deserts shift and droughts increase in the temperate zones used to regular rainfall, agriculture will suffer.

Join the conversation

84 Comments sorted by

  1. Robert Smart

    retired

    I think the reason CO2 might help vegetation in dry areas is this: Plants need to open pores to get CO2 in. This lets moisture out. If there is more CO2 they don't need to open their pores so much.

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    1. Tyson Adams

      Scientist and author

      In reply to Robert Smart

      Kinda. What actually happens is an increase in water use efficiency both due to stomata open times and increased photosynthesis. But this is tempered by a number of things, not least of which is lead temperature and what type of plant (C3 vs C4).

      The big thing that people miss is that CO2 wasn't limiting at 280ppm, so at 400ppm the problem is still water, nutrients and light.

      http://www.nature.com/scitable/knowledge/library/effects-of-rising-atmospheric-concentrations-of-carbon-13254108
      http://jxb.oxfordjournals.org/content/60/10/2859.long
      http://www.skepticalscience.com/co2-plant-food.htm
      http://www.newscientist.com/article/dn11655-climate-myths-higher-co2-levels-will-boost-plant-growth-and-food-production.html

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    2. Tyson Adams

      Scientist and author

      In reply to Tyson Adams

      That should have been "leaf" temperature....

      On behalf of all my fellow plant scientists who have spent hours standing around waiting for the LECO to equilibrate, I profoundly apologise for that typo. There is nothing worse than a sunny day with intermittent cloud to change leaf temperatures and photosynthesis rates to ruin a day of sampling.

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    3. Felix MacNeill

      Environmental Manager

      In reply to Tyson Adams

      Aha - an example of Liebig's law of the minimum? Thanks Tyson - I hadn't been aware of that.

      A very powerful point indeed!

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    4. trevor prowse

      retired farmer

      In reply to Tyson Adams

      Tyson----I am led to believe that green- house farmers put extra co2 to increase production. Would that mean that at 280ppm that CO2 was still lacking and it has been a benifit to most plants to have 400ppm . Some research looking at leaf stomata in fossils has shown that the CO2 levels when the fossils were formed showed that the stomata were smaller, creating the theory that the level of CO2 was higher than today..Most data that I have seen suggests that CO2 can go up to 2000ppm before production drops. Also vigorous growing crops in dead calm conditions are thought to slow the growth rate because they use up all the available CO2.
      The devlopment of the salt tolerant legume to be called MESSINA is an exciting prospect for Australian agriculture---far more benifit than the carbon tax!!!!!!!

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    5. Tyson Adams

      Scientist and author

      In reply to trevor prowse

      The 280ppm figure is the rough value of atmospheric CO2 that our current plant life has evolved to use. Atmospheric CO2 has varied over time, but on scales that are irrelevant to current plant species, especially those that we use as crops and fodder.

      Greenhouses are completely irrelevant to the majority of agriculture. Firstly you have the problem that most agriculture is rainfed, so water (and nutrient) limited situations. Secondly the glasshouse is a closed system where microclimates can be…

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  2. Mark Lawson

    senior journalist at Australian Financial Review

    Forget the article, this point interested me.
    "To start with, for every 1°C rise in temperature, relative humidity rises by 7%".

    The point about relative humidity in the atmosophere is quite vital. The models depend on this to cause all the big temp increases we keep hearing about but don't see. The reference is to a 1991 paper which says, as I understand it, that relative humidity can increase when the temperature of the atmosphere increases, which is quite right as I understand it..

    But has the relative humidity increased by the degree required by the models for the heavy duty warming we've come to expect? Andrew points to a 2007 paper which finds some sort of increase, but doesn't say whether its anything like the model requirements. Its just more of this nonsense about comparing model results with or without climate forcings to "prove" that human influence is worming the globe and so on..

    We're still a few papers short of anything useful ..

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    1. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Hansen

      Mike, thanks for the heads-up re Mark's book, "A Guide to Climate Change Lunacy". I've just ordered a copy and look forward to comparing it with your evaluation.

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    2. Henry Verberne

      Former IT Professional

      In reply to John Phillip

      Don't expect a peer-reviewed scientific analysis John and you will not be disappointed!

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    3. Mike Swinbourne

      logged in via Facebook

      In reply to John Phillip

      Goodonya John. Have a read of Mark's book and have all your ideological views confirmed by another crank.

      Heaven help that you should expose yourself to any science.

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    4. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Mike Hansen

      "A guide to climate lunacy"? I had no idea that Marks ideological musings were so hypocritical.

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    5. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to Mark Lawson

      "Andrew points to a 2007 paper which finds some sort of increase, but doesn't say whether its anything like the model requirements."

      Well if he didn't say so then why didn't you try to find out?

      I think we all know the answer to that question.

      Santer et al's 2007 paper pointed out that observations showed a 0.41 kg/m2/decade increase in water vapour over the oceans since 1988. I don't have the figure for average water vapour over the oceans but over the whole world it is 25 kg/m2. Assuming…

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    6. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mark Lawson

      Jeez Mark, I am glad I bought a copy of your book. Judging by the bilious attacks from the supposedly liberal thinkers below, you must have written (said, or even thought) something that bothers them. They all seem to be upset with what you've written but I wonder how many of them have actually read it. I guess that's what they mean when they're talking about informed decision making and the like. Cheers

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    7. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Felix MacNeill

      Well I don't know, I've been googling his "tome", it's much admired in the deniosphere. I did read a review which was interesting. http://petergallagher.com.au/index.php/site/article/led-by-lunatics
      But have in conclusion, decided it is not I who is an "activist"
      but those who have a pathological inability to understand the big picture, so need to deny and argue trivia. The must be a neurone function difference.

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    8. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Hansen

      Mike, I truthfully did not know the book existed until you metionjed it so "Well done" and thanks.

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    9. Mark Lawson

      senior journalist at Australian Financial Review

      In reply to Mike Hansen

      Mike - I see you've stopped to crude abuse. I must be winning the argument. Now go back and look at what I wrote. I was not doubting the Clausius-Clapeyron relationship. I said specifically it was right. The argument is not about whether the atmosphere CAN hold that extra water vapor but there is any evidence that it is going towards that limit.. the tiny piece of evidence you point to is interesting, but its impossible to point to anything as specific as a 7 per cent increase from rain fall - "extreme rain fall events intensifying" is hardly precise, assuming its right.. ..

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    10. Mark Lawson

      senior journalist at Australian Financial Review

      In reply to John Phillip

      John - I'm so glad you've bought a copy. Its a bit dated now, particularly as the mega-drought still ruling at the the time I wrote it has broken well and truly, but much of it is still relevant..

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    11. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Swinbourne

      Mike S, I have repeatedly asked you to state your rebuttals of several commentators and yet you offer nothing other than ad homs. Now, I have actually read the IPCC report that was leaked as well as the 2007 version and I have read Mann's hockey stick paper and several of the works around that, amongst others. Additionally I've read a couple of books by some of the people that you regularly deride. I realise that you don't care about this but my point is that the only place I find the kind aggressive…

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    12. Mark Lawson

      senior journalist at Australian Financial Review

      In reply to Byron Smith

      Byron - no, sorry .. only had time to glance at your first link but it does nothing of the kind. Go back and look at it. The paper is a response to a skeptic paper which found that there had been a decline in atmospheric water vapor. They had to respond. Now note the very cautious language used. It only says that there has been an increase, which I don't necessarily dispute. the problem is that you need a BIG increase to make the models work. This point has been kicked around quite a bit and that's all they can come up with.. You'll have to do a lot better than that.

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    13. Mark Lawson

      senior journalist at Australian Financial Review

      In reply to Chris O'Neill

      Chris - go back and look at your figures. 0.15 degrees per decade! So you agree that global warming isn't happening. That increase is tiny and probably even below measurement error. Now I don't necessarily dispute your calculations.. although you'll probably find the figure for water vapor over oceans is higher which will throw out your ratios .. but look at the figure you give for temperature.. the point about water vapor is that its supposed to cause the warming.. not the CO2.. so we're getting this additional water vapor plus the COs and no warming? You may want to re-think your approach.

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    14. Mark Lawson

      senior journalist at Australian Financial Review

      In reply to John Phillip

      John - thanks for that. Actually I doubt whether any of then know anything at all about the skeptical case.. you will still find even scientists telling you its about CO2 when its CO2 plus water vapour.. but thanks for the support..

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    15. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to John Phillip

      John, I wouldn't, because I am happy reading the reports done by nasa, csiro, climate commission, etc . Mark is not a peer reviewed scientist, but a journalist with AFR. If he submitted his book to some climate scientists (peer review), and the book was found to be credible, I would be interested, but really it's probably dated, and I wouldn't want to waste the money. More to the point.
      This article and catalyst last night seems to be more like the most current scientific analysis, (what I read), as obama said.....

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    16. Felix MacNeill

      Environmental Manager

      In reply to John Phillip

      John, he has written things that bother me. They're called lies. They do bother me. liberalism doesn't extend to acceptance of lies. And informed decision making relies strongly on REJECTING falsehoods, not glibly accepting everything dished out by anyone who can find a publisher.

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    17. Felix MacNeill

      Environmental Manager

      In reply to Alice Kelly

      Alice, the correct term, I think, for Mr Lawson and his chums is 'dysactivist'.

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    18. Felix MacNeill

      Environmental Manager

      In reply to Mark Lawson

      Being unable to distinguish between weather and climate never dates, Mark.

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    19. Felix MacNeill

      Environmental Manager

      In reply to Mark Lawson

      Mark, I see you have once again resorted to repeating the same old discredited comments over and over - one of the subtlest forms of abuse known.

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    20. Felix MacNeill

      Environmental Manager

      In reply to Mark Lawson

      Hey Mark, I just re-read Henry's comment:

      'Don't expect a peer-reviewed scientific analysis John and you will not be disappointed!"

      You call that crude abuse? It's a simple statement of fact: your 'scientific' analysis is not peer reviewed.

      Still, we've already noted that you have trouble distinguishing between facts and abuse.

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    21. Mike Swinbourne

      logged in via Facebook

      In reply to John Phillip

      John P

      And I have repeatedly asked you and the other deniers around here to stop talking nonsense - thinking that you know more that the real experts because of what you have read on a denier website.

      You complain that I aggressively deride you and others. You betcha! Its because I am sick and tired of hearing the same old zombie denier memes raising their heads over and over and over again. It's a conspiracy. There has been no warming for the past XX years (since 1998). There is no consensus…

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    22. Michael Hay

      retired

      In reply to Mike Hansen

      Mike, it would seem to be a logical conclusion to observe that the changing climatic events call for a massive increase in water storages. Tropical storms and cyclones deliver far more water than this country could ever fully utilise, but at present this asset is ignored, and thereby, wasted. Surely it is time to encourage evaluation instead of always accepting fixed ideologies.

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    23. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to Mark Lawson

      "So you agree that global warming isn't happening."

      Way to move those goal posts, Mark!

      For your information (something with which you have little familiarity), 0.15 deg C/decade is not "global warming isn't happening", however you try to spin it.

      "the point about water vapor is that its supposed to cause the warming"

      You just don't get it, do you? Feedback, that is. You obviously have zero understanding of what a feedback loop is. Being a feedback, water vapour is both increased by…

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    24. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to Mark Lawson

      "the skeptical case"

      You mean the schizophrenic skeptical case. You never know what it is from one day to the next.

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    25. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Felix MacNeill

      The emperor has no clothes. Usually people don't understand, today crude abuse. Really a long relaxing perusal of the stock market with particular attention to the margins, and a nice cuppa tea fixes things.

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    26. John Campbell

      farmer

      In reply to Mike Hansen

      Interesting, but I thought unnecessary. It was clear to me even before your input that Mike Lawson was talking garbage.

      It would be helpful though if such people were to indicate they were anti-science.

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    27. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to Mark Lawson

      "only had time to glance at your first link but it does nothing of the kind"

      and the second link?

      and Andrew Glikson's Santer et al link?

      So many links, so many ignored.

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    28. Grant Burfield

      Dr

      In reply to Mark Lawson

      "To start with, for every 1°C rise in temperature, relative humidity rises by 7%".
      Is this the new consensus in Climate Science? Well presumably it is as Dr Glikson has said so and published it at the Conversation.
      And there I was thinking that at any given site on any given day, a rise in temperature led to a decrease in relative humidity. Back to the drawing board for me.

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  3. John Nicol

    logged in via email @bigpond.com

    Hi again Andrew.

    First I would hope that this interesting discussion may continue for much longer than the recent articles on Climate change which have been closed for comment almost as soon as they opened. I do not know why this was but hope that you and the moderators will respect the fact that we don't all receive our emails advising us of topics until at least 24 hours after they have been posted - it appears.

    First let me say that I don't blame Andrew Glikson for this article, as he…

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    1. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Mike Hansen

      Mike, thanks for the skepticalscience link. You've noted John Nicol's links with an organisation in somewhat derisive tones. Readers may wish to examine the Australian Climate Science Coalition website in order to establish the veracity or otherwise of their position for thjemselves. The url is http://www.auscsc.org.au/ .

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    2. Fred Payne

      retired

      In reply to John Nicol

      Does it really matter where the increased levels of CO2 are coming from?
      It seems to me that the problem for farmers will be knowing when to plant crops in the face of increasingly erratic weather.
      So even if the .8 of a degree which is reported to have caused the dramatic increase in catastrophic weather events is produced by non anthropogenic factors, putting millions of tons of extra CO2 is unlikely to mitigate the effects of Increased green house gasses and consequent temperature increases of over 2 degrees

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    3. Mike Swinbourne

      logged in via Facebook

      In reply to John Nicol

      "....However, the units at ANU and other universities, continue on their relentless campaigns to promote Anthropogenic Global Warming...."

      Just in case you weren't aware John, it is actually the function of universities to promote science.

      But maybe, just maybe, you could think about what you said for a minute. All the universities in the world are endorsing the science of climate change. That should tell you something.

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    4. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to John Phillip

      "You've noted John Nicol's links with an organisation in somewhat derisive tones."

      Well deserved of course.

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    5. Felix MacNeill

      Environmental Manager

      In reply to Mike Swinbourne

      Mike, I would have thought it would tell him either that he was talking complete crap or that the conspiracy is so powerful that the black cars will almost certainly be turning up outside his secret hideout as we speak. But that would be a simple, commonsense, rational kind of take on the situation and we can't expect that from Mr Nicol.

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    6. John Nicol

      logged in via email @bigpond.com

      In reply to Mike Hansen

      Mike,

      I think you might like to compare the claims made by the meteorological advocacy group with reality. As in the case of every "extreme" weather event, for the last three years, research has showed that individually, none was exceptional. The IPCC study over two years provided extremely low probabilities of the order of 5% in some cases, that any of the periods they looked at with regard to these "extreme" events, showed a connection with Global Warming, meaning that they had all occurred…

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    7. Felix MacNeill

      Environmental Manager

      In reply to John Nicol

      John, in which alternative reality do you actually expect that anyone will believe you over the World Meteorological Organization...and (how many times does one have to say this) all the world's science academies, etc.?

      Publish your material in the peer reviewed literature then get back to us.

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    8. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to John Phillip

      John, are you more or less a denier? You need "to get out more often", and pay more attention to any of the credible sites on offer here day in day out by not just the authors of "the conversation", but mike & mike felix, (sometimes me). The argument so devotedly proposed by deniers is pseudo argument. There is no "debate" amongst the climate science community. They earn their money. I've only been using a computer for a few years, but it's not really hard to find good science. As homework, try reading nothing but the best for one week, start with the conversation today, (Andrew Glixon) and these four... then just keep going
      http://www.nasa.gov/centers/langley/science/climate_assessment_2012.html
      http://nsidc.org/greenland-today/
      https://theconversation.com/a-storm-of-stupidity-sandy-evidence-and-climate-change-10492
      http://carbontracker.live.kiln.it/Unburnable-Carbon-2-Web-Version.pdf

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    9. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Alice Kelly

      Thanks for the links, Alice. I'm on a couple of weeks leave, so I will have a chance to look at them over the next week or so. Cheers.

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    10. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to John Phillip

      And another thing John, remember that Mark Lawson started (his) conversation with... "Forget the article....." pretty rude really, you are instructed to not read his book for two weeks, Alice

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    11. John Phillip
      John Phillip is a Friend of The Conversation.

      Grumpy Old Man

      In reply to Alice Kelly

      Can't promise I'll leave it for two weeks, but it'll take a couple of days to get here. Cheers :)

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    12. John Nicol

      logged in via email @bigpond.com

      In reply to Mike Hansen

      Mike Hansen,

      Sorry to be so long in getting back to you Mike, but this has been a busy day.

      Yes, I am sceptical that the measured increases in carbon dioxide, which somewhat mysteriously, represent only about half of the carbon dioxide emissions accounted for through auditing the fossil fuel usage world wide. I accept the "criticism" of being a "denier" which of course is a quite meaningless insult so carries no stigma whatsoever, to the disgust i suppose of its users!

      However, just to…

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    13. Gary Murphy

      Independent Thinker

      In reply to John Nicol

      "Also, the Arctic Ice, as you will see from the links I have given you, is at the highest level in ten years."

      Arctic sea ice volume is tracking well below 2007 levels
      http://neven1.typepad.com/blog/piomas/

      Ice Sheet Mass decreasing
      http://www.nasa.gov/mission_pages/Grace/news/grace20121129.html

      "In a landmark study published Thursday in the journal Science, 47 researchers from 26 laboratories report the combined rate of melting for the ice sheets covering Greenland and Antarctica has increased during the last 20 years."

      Oceanic heat content has increased significantly in the last 15 years:

      http://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/

      "little Alice"

      Do you think physical stature is relevant or are you just being sexist?

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    14. Alice Kelly
      Alice Kelly is a Friend of The Conversation.

      sole parent

      In reply to Gary Murphy

      Gary, What a long letter John wrote. If he calls me "little" it's probably less of an assumption than some make. Don't worry, in real life I'm much more intimidating.

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    15. David Rennie

      IT Consultant

      In reply to John Nicol

      John you state that no one seeks any other possible explanations for temperature increase such as solar variations. Your claim of no temperature increase for 15 years is based precisely on such a variation which you know is theENSO affect. Deniers cherry picked this 15 year period because of the extreme El Niño in 1998 and the extreme La Nina in 2011. It is the deniers that refuse to recognise these other factors.
      Going from an El Nino to a La Nina will have a very negative effect on a temperature…

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    16. John Nicol

      logged in via email @bigpond.com

      In reply to David Rennie

      David Rennie,

      Dear David,

      First you say “You acknowledge that temperatures are hotter than at any time in the last 150 years however you ignore the fact that they are hotter than any time in the last 1500 years which is much more significant.” I do not know of any one except Michael Mann who has tried to argue that case, for a recent maximum in 1500 years - so I am wondering if you could provide a reference. I pick this up again below. However, what if it is 1,550 years - what caused it…

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    17. John Nicol

      logged in via email @bigpond.com

      In reply to David Rennie

      David,

      You may also be interested in this article from the CSIRO website:
      http://www.csiro.au/Portals/Media/Deserts-greening-from-rising-CO2.aspx
      "Deserts 'greening' from rising CO2

      Increased levels of carbon dioxide (CO2) have helped boost green foliage across the world’s arid regions over the past 30 years through a process called CO2 fertilisation, according to CSIRO research.

      3 July 2013

      In findings based on satellite observations, CSIRO, in collaboration with the Australian…

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    18. David Rennie

      IT Consultant

      In reply to John Nicol

      John,
      Here are a few references on temperatures the last 2000 years
      http://en.wikipedia.org/wiki/List_of_large_scale_temperature_reconstructions_of_the_last_2,000_years
      http://en.wikipedia.org/wiki/Temperature_record (Refs 5,6,7,8,9, 10)
      http://www.nap.edu/catalog.php?record_id=11676#toc
      None of these suggest that it was as warm 1500 years ago, as it is now, they simply stop the analysis back that far.

      You can find the data for sea ice extent here. Check the data yourself other people have…

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    19. David Rennie

      IT Consultant

      In reply to John Nicol

      John,
      You seem to ave missed the point of this statement:
      "Our work was able to tease-out the CO2 fertilisation effect ... to take out the observed effects of other influences such as precipitation, air temperature, the amount of light, and land-use changes".

      What the CSIRO is saying is, if you IGNORE the temperature increase, and the change in rainfall and other matters increased CO2 increases output.

      However the increase in CO2 causes increased temperature and changed rainfall so the real world effects are more complicated and can not be inferred from this paper.

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    20. John Nicol

      logged in via email @bigpond.com

      In reply to David Rennie

      Dear David,

      Thank you for your quick response to my post last night. I am still to read the links you have given me but will work my way through them as soon as possible.

      My first comment is on your suggestion that: “You claim that all the temperature records since 2002 show a temperature decline, this is untrue only three of the five do so.” The random variation of temperature, up and down, will always occur for many different reasons, but will include the fact that convection and very…

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    21. David Rennie

      IT Consultant

      In reply to John Nicol

      John,
      you write a great deal ,however do try to concentrate on my original point; that you claim that 'no one seeks any possible explanation such as solar variations, to explain the changes in global tempertatures, other than "We believe that most of the increase in global temperatures during the second half of the 20th Century was very likely due to increases in the concentration of atmospheric carbon dioxide." from IPCC AR4 2007 and CSIRO "Climate Change in Australia - 2007".'

      I have demonstrated…

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    22. John Nicol

      logged in via email @bigpond.com

      In reply to David Rennie

      David,

      You have said: "I have demonstrated that it is YOU not the scientists who igore these variations"

      Presumably you are saying that I am ignoring the variations in solar effects, and other natural factors such as La Nina which are causing the temperature to be falling at present. Is that a correct interpretation of what you are meaning here? I have not seen anything from any of the climate units or in the writings of the IPCC which acknowledge these phenomena in a quantitative way to…

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    23. David Rennie

      IT Consultant

      In reply to John Nicol

      John
      I am saying precisely that you are ignoring the variations in the solar effects, that you are ignoring the ENSO effect and you are ignoring the effects of aerosols.
      Scientists have identified these relationships in the past.

      If you admit that the impact of all three of these since 1998 or 2002 to the present would lead to a decline in temperatures then we have a starting point for discussion.
      Once you acknowledge that based on these variations we would predict a decline in temperature…

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  4. Chris O'Neill

    Victim of Tony Abbotts Great Big New Tax

    "To start with, for every 1°C rise in temperature, relative humidity rises by 7%."

    I think that should be absolute humidity rises by 7%.

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    1. Grant Burfield

      Dr

      In reply to Chris O'Neill

      Correct, and it's great to see that at least one person here has an understanding of thermodynamics and atmospheric physics. You must feel like Robinson Crusoe.
      However the percentage increase is still temperature dependent. For a fixed relative humidity and a 1°C rise from the following ambient temperatures in °C (0, 10, 20, 30) the percentage increases in atmospheric water vapour content at the sensor are (7.1, 6.6, 6.0, 5.6).
      http://carnotcycle.wordpress.com/2012/08/04/how-to-convert-relative-humidity-to-absolute-humidity/

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    2. Chris O'Neill

      Victim of Tony Abbotts Great Big New Tax

      In reply to Grant Burfield

      "a 1°C rise from the following ambient temperatures in °C (0, 10, 20, 30) the percentage increases in atmospheric water vapour content at the sensor are (7.1, 6.6, 6.0, 5.6)."

      I'll bear that in mind when the world's average surface temperature reaches 20°C.

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  5. John Campbell

    farmer

    If any of us really wants to read denialist crap why doesn't 'the converstaion' have some articles that specifically allow it with annotation to that fact and the rest of us can then avoid this 'flat earthist' nonsense.

    After all there is no debate about AGW only spin and disinformation, which to say the least is boring to the extreme. Real science is interesting and I for one are very interested in hearing about the latest findings and clarifications. Would I do find objectionable thought is having to wade through masses of childish nonsense in trying to find a gem or two.

    Real skepticism based on facts and science is fine, but denialism based on nothing much more than opinion and wishful thinking is something altogether different.

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    1. John Nicol

      logged in via email @bigpond.com

      In reply to John Campbell

      John Campbell,

      Since you know us all by our first anmes by now, you just have to flick past what we write. BTW this gem of a post just above, by you, has really had me glued to my monitor. I could barley take myself away long enough to post this little response.
      John Nicol

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