Ancient Climate Change & the Human Role

I’m a student of environmental history. I’ve also long been interested in how humanity, society, and the environment have coevolved. Let’s take the example of fire as one of the major agents of change. As William Cronon writes in his introduction to Steve Pyne’s Fire: A brief history:

The process of fire’s coevolution with humanity was the invention of agriculture and the very different fire dynamics it necessarily entails: fire to clear fields, fire to change the composition of wild and domesticated vegetation, fire alternately bound and released in cycles that sometimes seemed increasingly under human control, and sometimes devastatingly, not. The consequence of the fires that have burned under this second, agricultural regime have brought a complex remapping of the Earth’s surface, extending fire’s reach in some regions and habitats while suppressing it in others. The consequences of this human manipulation of terrestrial fire ecology have been so subtle and profound that we are only now beginning to understand them.

This leads me to an article in Nature on ancient climate change and the prehistoric human role:

Scientists have come up with new evidence in support of the controversial idea that humanity’s influence on climate began not during the industrial revolution, but thousands of years ago. Proposed by palaeoclimatologist William Ruddiman in 2003, the theory says that human influences offset the imminent plunge into another ice age and helped create the relatively stable climate that we are familiar with today. It has been repeatedly panned as implausible by palaeoclimate researchers, but eight years on, Ruddiman and others say that they have the data to support early anthropogenic climate change.

As much as I’ve been intrigued by Ruddiman’s hypothesis (laid out in this book), I’ve not given it much consideration, largely because it hasn’t been taken seriously by his colleagues. But I’ve kept an open mind because of my familiarity with the field of environmental history. So I’m looking forward to this:

Ruddiman and several other researchers will present their supporting evidence in a series of papers scheduled for publication in a special issue of The Holocene journal later this year. Researchers presented some of the work this week at the American Geophysical Union’s Chapman Conference on Climates, Past Landscapes and Civilizations in Santa Fe, New Mexico.

“I’m of course hopelessly biased, but this year is going to be a good year for the early anthropogenic influence hypothesis,” Ruddiman said as he presented his overview study.

If he’s right, this will no doubt add an interesting wrinkle to the climate debate

32 Responses to “Ancient Climate Change & the Human Role”

  1. NewYorkJ says:

    Critics say that human populations were probably too small to support such a hypothesis, and recent studies have raised serious questions about early anthropogenic carbon and methane emissions.

    That’s where I’m at with the Ruddiman hypothesis.  We know the massive amount of anthropogenic greenhouse gases spewed into the atmosphere are having a significant effect today.  I’m not sure the pre-industrial magnitude from agriculture is that high, but I look forward to Ruddiman’s findings.

    RC had a guest post from Ruddiman some time ago.

    and an analysis of the political reaction:

    There are related hypotheses, this one focusing on the LIA.

  2. Tom Fuller says:

    I personally think that mankind’s initial changes to the environment may have been among the most drastic. (Not trying to play down our emissions of GHGs at all.) If deforestation today is estimated to account for 20%-25% of all CO2 emissions and the effect of initial emissions is stronger than that of later emissions, then it seems quite reasonable to think that the grand clearing that accompanied the spread of humanity might have had a dramatic effect.

  3. Eli Rabett says:

    <a href=””>Cronin</a>

  4. Ruddiman’s ideas seem to tie in well with Roger Pielke, Sr.’s on the importance of land-use changes.
    Back in 2005 or so, I read William F. Ruddiman’s _Plows, plagues, and petroleum : how humans took control of climate_ (2005). A very cool book, highly recommended. Some of his conclusions are speculative (even highly speculative), but the tide is running in his direction, I thought then, and still do.

    Another fine read on the surprisingly-large environmental controls by ‘primitive’ humans is Paul Martin’s _Twilight of the mammoths : ice age extinctions and the rewilding of America_ (2006). Again, Martin’s conclusions are controversial, and his “Mammoth Park” idea is truly over-the-top, but he’s been thinking about this stuff for a long time. Sharp old guy.

    Finally, I can’t recommend Charles Mann’s _1491 : new revelations of the Americas before Columbus_ (2005) too highly. His key insight: Passenger pigeons & bison as classic population ‘booms’ following removal of their keystone predator, the Amerinds after their 95% dieoff. The “primeval wilderness” the European settlers found wasn’t that at all, but an abandoned garden, running wild after the gardeners were killed off. Lots of other neat, unobvious stuff. Fascinating reading.
    Happy reading–
    Pete Tillman
    Professional geologist, amateur paleoclimatologist

  5. Keith Grubb says:

    Yes, I believe all previous civilizations have had an effect on climate, especially local climate. Guess what, man and the globe survived, without the help of climate scientists dictating policy.

  6. Keith Kloor says:

    New York J (1)

    Thanks for the links. For the moment, let me just say that new archaeological findings increasingly surprise to the degree prehistoric cultures shaped the landscape. See, for example, this story about the Everglades tree islands, which has made international news this past week. (Quick caveat: the AGU press release was rather breathless and the media coverage, for the most part, has just echoed it, instead of doing much actual reporting. I’m written about the tree islands before, am familiar with their paleoecological history and have also looked into this new hypothesis, which I find fascinating. More coming up.)

    Eli (3),

    MT has written about this news of the Republican attempt to intimidate Cronon, which I find highly disturbing. MT has a good overview with all the relevant links. If I have time, I’d like to post on this, as well, over the wknd.

    Peter (4)

    All great recommendations. Mann’s book is brilliant. I reviewed Martin’s Twilight of the Mammoths here.

    For a really neat regional environmental history that was pioneering, I’d also recommend William Cronon’s Changes in the land:Indians, Colonists, and the Ecology of New England

  7. kdk33 says:

    I wonder how wise it is to speak of a) the environment and b) us.  Aren’t we part and parcel of the environment?  Doesn’t everything evolve together?  Seems to me, the environment acts on us just as surely as we on it.

    So, perhaps population won’t grow until it runs into a resource constraint.  Perhaps population is always constrained and only grows when those constraints are relaxed.

    Just a different way a thinkin’

  8. Dean says:

    It is worth pointing out that there is nothing new about manmade climate change. The new part is the global part. Regional climate change from things like deforestation is not a new concept.
    And to Keith Grubb #5, yes, the globe survived and mankind did not go extinct. But plenty of people died. There is extensive evidence for the decline and collapse of many civilizations due to climate change – The Maya, Anasazi, numerous others, possibly Angkor. Much of the western hemisphere and eastern Asia suffered extensive drought during what we call the Medieval Warm Period.
    I’m not saying that these were all manmade climate changes, but the idea that adaptation to climate change, manmade or not, is a relatively benign process is not borne out by the history. It can and has been a brutal process that has destroyed many a civilization.

  9. Keith Grubb says:

    Indeed, as we have just witnessed in Japan, nature can be very brutal. Whatever mankind attempts to do to control nature will be met with failure. When CO2 levels were @280 ppm, there were plenty of natural calamities that happened. We may or may not be able to adapt to future climate change, but I’d be willing to invest in science that comes up with ideas for adapting to a hot or cold planet. Seems like common sense to me.

  10. Dean says:

    We’re not trying to control nature with AGW issues, we’re trying to control ourselves.
    And I’m not against investing in science to come up with ways to adapt, but I would also like to invest in ways to minimize what we have to adapt to.

  11. Keith Grubb says:

    To control ourselves would be a volunteer action, I’m OK with that. For instance I recycle everything I can, I turn off my lights if not needed, If I’m walking down the street and see trash, I pick it up, all volunteer actions. What climate scientists tell us needs to be done, requires me to be controled, I’m not OK with that. When you tell me that electricity rates necessarily needs to skyrocket, that we have to get used to using electricity when it’s cheap and abundant, not necessarily when we need it, that tells me you want to control me. I’m not interested in being a serf. Back to adaption. Where are my adaption scientists? They seem to be poorly funded.

  12. Dean says:

    Keith – I’m not debating what is the appropriate policy here. I do draw a strong line between the science and the policy. My point is that if whatever policies we do end up adopting, if any, fail to stop us from changing the climate, the results could be very serious. And connecting that to the topic of this thread, history demonstrates this many times over. Warming climates have not necessarily been benign in the past, whether or not we were the agents of change. And we have been on regional scales it seems. Warming climates have repeatedly been accompanied by drought and collapse.
    On a separate issue related to this thread, books like 1491 discuss a lot of recent evidence that pre-technological human societies did indeed modify the environment a lot more than people knew. Of course the degree of change is still vastly smaller than the change that modern technological society has wrought. But pre-industrial human populations, including in North America, changed the place a lot more than we used to think. Going with that – their populations were a lot higher than we used to think as well. The process by which they adapted to European arrival, though in this case not an issue of climate, nonetheless provides one example of a very painful case of adaptation.
    I was fascinated to learn, for example, that the vast herds of bison that greeted many settlers were there because of the decimation of Indian populations, who thus were not hunting them. Apparently before the arrival of European diseases, the populace of North America kept the bison population much lower. Stable, but lower. Only with the collapse of human population did their numbers soar.

  13. Keith Grubb says:

    As Menth would say, I’m going to puncuate this thread with a song.

  14. thingsbreak says:

    The “early anthropocene” idea is a compelling one, but it’s been subjected to some pretty convincing challenges.
    I do not agree with Keith’s claim that “it hasn’t been taken seriously by [Ruddiman’s] colleagues”. Certainly, Wally Broecker’s reaction was dismissive if not disrespectful. However, that was the exception rather than the rule, IIRC.
    The major complaints I remember were that the present interglacial more closely resembled MIS 11 or one of the older interglacials rather than the most recent ones; that realistic carbon cycling modeling adequately explained the CO2/CH4 increases without invoking anthropogenic sources, etc. I believe that Ruddiman addressed the former directly. I don’t know that I’ve seen a response from the “early anthropocene” crowd to the latter, e.g. Kleinen et al. 2010. If not already, I would imagine that Ruddiman and co. will address such criticisms this time around, as they generally tackle challenges head on rather than playing them down.

  15. John Mashey says:

    re: 14 TB
    Note: “early anthropocene” really includes several hypotheses, and it is good to separate them.
    A: Humans changed CO2 via deforestration/agriculture
    B: Humans changed CH4 via rice paddies & livestock
    C: A possible reglaciation was stopped
    D: Some CO2 wiggles in last 2000 years were contributed to by human plagues-reforestration-CO2 dip, of which the strongest example, if true, was the 1550-1600ish dip in CO2 (the red part in, due to 50M deaths in Americas.
    1) You will want to look for the Ruddiman, Kutzbach, Vavrus paper, which I think addresses the MIS 11 and carbon cycle issues.  There is a lot of discussion about alignment, and some great graphs that sure seem to make the current interglacial look very different, with Ch4 and CO2 inflections when Ruddiman proposed.  Kleinen, et al (2010) is included.
    2) Over last few years, there has been a great deal of interesting archaeology regarding deforestration and rice paddy growth in China, and it fits.  Likewise, it appears that people have long been underestimating the land clearance rates from early agriculture. Finally, the estimates of pre-Columbian native populations have risen over the years,
    People have found increasing evidence (such as via better aerial instrumentation) of large-scale agricultural civilizations across Amazonia and Central America, long-since covered by jungle regrowth.
    3) Look for the paper by Nevle, et al, regarding interesting archaelogical evidence from charcoal records in Amazonia and Central America, which bears strongly on the conjoined issue of land-use, die-offs, and carbon sequestration possible from hihgly-productive land.
    4) Of the 4 hypotheses, I think the evidence is piling up for A, B , and D, and I just haven’t looked enough at C to have an opinion.
    5) Indeed, Broecker has been dismissive, but others certainly have taken this seriously, whether they agreed or not, or thought “not proven” was the appropriate state.  All this is good science as usual, and when the dust settles on all this, I think it wil lbe a wonderful case study of:
    – hypotheses
    – counter-arguments
    – modifications
    – other researchers get interested
    – people go back and look at data they had and find it bears on the problem
    – others are stirred to look for new confirming/disconfirming data
    – sequences of dueling papers appear in journals
    Anyway, I think it will make a classic story of real science at work, with the several attributes useful for instruction:

    – the hypotheses are actually understandable to the lay person
    – the arguments will likley have spanned no more than 10-15 years
    – they are now (i.e., not things already well-known)

  16. Keith Kloor says:

    kdk33 (7)

    The dualism (humans and nature as separate) you mention has been a major problem in these kinds of debates, but thankfully it’s getting better.

    Dean (12)

    Right you are about the less than benign influence of climate on prehistoric civilizations. Archaeologists, however, are loathe to be deterministic, so they always make a strong push for socio/political forcings, as well. The interesting thing to ponder is whether the same exact debates will be playing out in 500 years–about the period we live in now. Of course, the difference is, which Julio Betancourt, a terrific USGS paleoecologist, has pointed out numerous times, is that we possess knowledge about the past that ancient cultures, like the Maya, didn’t have during their time.

    TB (14), John Mashey (15),

    Thanks for the added context on Ruddiman and the “early anthropocene.”

    To Mashey’s list of examples, I’ll also add that the revisitionist picture of the Amazon forest that is emerging via archaeology is astounding, as David Gann wrote in the New Yorker:

    “The latest discovery proves that we are only at the outset of this archeological revolution””one that is exploding our perceptions about what the Amazon and the Americas looked like before the arrival of Christopher Columbus.”

  17. Dean says:

    My strong hunch is that we (they?) will be having that debate in 500 years. Our knowledge of the past doesn’t affect much that we have varying perceptions among us of the current.
    As to the discoveries of large-scale civilizations in Amazonia, I wonder what we will find if we are ever able to develop advanced techniques for searching the ocean shallows. Remember that most people live along the coast, and in the early interglacial, most people lived on land that is now under water. Being a diver might end up being the primary qualification to being a field archaeologist.

  18. John Mashey says:

    KK @16
    re: Amazonia:
    yes, there is in interesting intersection between the archaeology and the sort of technology used <a href=””>here</a>

  19. JohnB says:

    Firstly to agree with TB #14. You have to go back to MIS 11 to get a good comparison to the Holocene, the cycles are wrong for a comparison to other interglacials. (Big thanks to Gavin for telling me where to start looking for more on this.)

    The theory assumes that there should have been a descent into an Ice Age that “something” stopped. However the assumption is based on comparisons to recent interglacials and not the one (MIS 11) that it should have been compared to. This being the case there was no “imminent plunge into another Ice Age” coming for human influences to “offset”. So the basic assumption of the theory is unfounded and the concept flawed.

    For a second point the theory simply doesn’t pass the “smell test”. If a mere 5,000,000 humans could effect the global climate so dramatically as to offset an Ice Age then the 300,000,000 around by the time of Christ should have produced far greater climatic changes. It is almost insane to believe that the land use changes to feed 4M people are greater than the land use changes to feed 300M. It simply makes no sense. This is in essence saying that clearing 100 acres of land has more impact than clearing 100,000.

    I have to add that even though hundreds of millions more people are doing the things that should have been warming the earth (remember those early guys “offset” an impending Ice Age, so the actions must warm the planet) the trend for the last 8,000 years is to a cooler world.

    This is not to say that humans might not have had some effect on the very local weather, but “local” is the operative word. Distribution methods were to say the least primitive and crops couldn’t be taken long distances. All farming was local to the population centre that it fed. The speed at which the food is transported gives a limit to how far from the markets the farms can be. I would also point out that the wheel wasn’t invented until circa 3,200 BC so there were no carts. We are talking mule trains here, caravans at best.

    Note that this also limits the size of towns and cities. A city can only grow as large as there is farmland available to feed it. This is why most trade was in non perishable goods, it wasn’t really until the Romans that trade in perishable goods was commonplace due to vastly improved transport system.

    Yes, history is littered with civilisations that have passed, and many of them from climate change, but to suggest that they caused it?

    Since he first evolved mankind has had a mental problem with the idea that we are at the mercy of the planet. So we populated the sky, fields, forests and rivers with Gods and spirits. There had to be a reason why the rain didn’t come, it must be the Gods are angry. Later we simplified things to a single God but still needed a reason for the rains not coming, so it became the fault of witches and demons and curses. Now, after the “Enlightenment” we still look for reasons, but this time the answer is always the same. “Man” did it.

    There seems to be a psychological imperative to show that man is a force to be reckoned with, rather than an insignificant mote vulnerable to the vagarities of the planets systems.

    It’s an outgrowth of the Euro-centricism of the standard historical texts. Nothing happened unless a white european did it. Now, nothing happened in history unless “man” did it or caused it.

    Why even think of invoking man made climate change for the destruction of an early civilisation? Aren’t natural climate change, p*ss poor farming techniques, bad water, poor sanitation and non existent medical knowledge enough? Seriously?

    In many ways I see this as the mirror image of people like Von Daniken. He believed early cultures were so stupid, primitive and powerless that it was impossible for them to build the things they did. So aliens must have done it for them.

    The new anthropocentricism believe early cultures to be so powerful that rather than being at the mercy of the climate and weather, they could actually influence it. Thus placing mankind in the position historically reserved for Gods. 🙂

    A final thought about carrying this “theory” to it’s logical conclusion. Climate is the average of weather and global climate is the average of global weather over time. Therefore if we can control local weather for long enough we can control local climate. If we can control enough areas of local weather then we can control global weather. If we can control global global weather for long enough then we control global climate.

    Does anybody really believe we can control global weather? 

  20. John Mashey says:

    re: #19

    Arguments from personal  incredulity don’t work in real science.  This is the wrong place to have a detailed discussion, but:

    The MIS 11 argument *sounds* impressive, it’s just that it’s 5+ years old, and research has moved on.  People really interested in this can read:

    Rohling, E.J., Braun, K., Grant, K., Kucera, M.M., Roberts, A.P., Siddall, M. and Trommer, G. 2010: Comparison between Holocene and marine isotope stage 11 sea level histories. Earth and Planetary Science Letters doi:10.1016/j.epsl.2009.12.054, a copy of which is <a href=”

    Rohling%20et%20al%20MIS11%20EPSL.pdf”>here,</a> a recent paper that references many of the earlier ones on the topic of alignment.  MIS 11 overall increasingly looks like a bad analog, but read the paper.

    It also helps to have read some of the other key papers that have been published in the last few years, such as <a href=””>this one</a>:
    Ruddiman, W.F. and Ellis, E. C 2009: Effect of per-capita land-use changes on Holocene forest clearance and CO2 emissions. Quaternary Science Reviews 28doi:10.1016/j.quascirev.2009.05.022

    That analyzes clearance rates from published population estimates and integration of recent research/archaeology on farming practices and rice cultivation, with key papers in last few years.  Early farming practices cleared more forest/person than current usage.  Read the paper, a bit less dense than the previous one.

    Alternatively, people can wait for the special issue of The Holocene on this.

  21. JohnB says:

    I’ll give them a read, thanks.

    You missed my point though. Farming did clear more land per capita then than it does now, very true. However farming practices didn’t change much between say 6,000 BC and 0 AD while the population became much larger.

    Unless you are going to postulate a phenomenal improvement in farming techniques that has left no trace in the archaeological record then you have to agree that for 100 times the population there was 100 times the land clearance.

    This means (as a first approximation) 100 times the climatic effects.

    This has nothing to do with personal incredulity, it has to do with the logical conclusion of a hypothesis.

    I doubt that KK would worry about an archaeological conversation on this thread, so I’ll get back to you after reading the papers.

  22. Keith Kloor says:

    JohnB (19)

    Good argument, but there there is a reasonable middle ground position between the two extremes you juxtapose. Also, you ask at the end: “Does anybody really believe we can control global weather?”

    I don’t believe anyone is suggesting as much, only that it can be and has been influenced by man’s actions.

    RE #21: “I doubt that KK would worry about an archaeological conversation on this thread, so I’ll get back to you after reading the papers.”

    Indeed! I welcome such discussion.

  23. Brandon Shollenberger says:

    I don’t want to jump in in the middle of a conversation, but I had to comment on something JohnB said:
    However farming practices didn’t change much between say 6,000 BC and 0 AD while the population became much larger.
    This is a strange statment, especially when paired with a later reference to the archeological record.  The archeological record actually shows enormous changes in farming practices in that time period.  I’m sure a simple Google search or visit to Wikipedia would turn up numerous examples.
    I’ll give a couple examples off the top of my head.  First, the first earliest known use of field farming was in 5,500 BC.  The use of farming in fields is surely a major change in farming practices.  Second, irrigation wasn’t used until after 4,000 BC.  That is obviously another major change in farming practices.  These are just a couple of examples; I’m sure there are many more.
    I’ll butt out of the conversation now.

  24. thingsbreak says:

    I just wanted to clarify that I wasn’t endorsing the idea that MIS 11 is the “correct” analogue for the Holocene. There is no perfect 1:1 analogue, and I’ve seen better arguments for MIS 19, as an alternative example.
    I also wasn’t trying to endorse the idea that “avoiding an ice age” was the only basis for the early Anthropocene hypothesis.
    As I said before, although I am not up to speed on Ruddiman’s (and other early Anthropocene supporters’) most recent papers, I am aware that many of the rebuttal claims made against the ’03 paper have been addressed, and I would expect significant remaining challenges to be acknowledged and addressed (if not necessarily thoroughly refuted) in their most recent work.

  25. JohnB says:

    TB. MIS 11 isn’t the perfect analogue, I doubt that there is one. However most people look at the ice core reord for the last 400k years or so and the difference between the Holocene and previous interglacials is stark.

    The graph most are familiar with is probably this one.
    or a variant thereof. Three sharp, short peaks and the Holocene roughly level for 10k years at a slightly lower temp. Anybody looking at this graph would be asking themselves “Why is the Holocene so different?” It is reasonable to assume that “something” changed.

    However, by going back to the better analogues, be it MIS 11 or 19, this requirement vanishes because they aren’t that much different from the Holocene.

    I’ll freely admit that my first thought when I started looking at the record was “WTF? What the hell changed?” The answer was “nothing” because I wasn’t looking at the right records.

    The “avoiding an ice age” bit was from the article KK quoted in the blog post. Sorry if it came across as though it came from you.

    Brandon, by all means barge in. While I read extensively on history, I’m always happy to be corrected in mistakes or misconceptions. One of the nice things about an archaeological conversation is that while climate may enter into it, there isn’t the same emotional baggage as the debate about the present has. 🙂

    While field or “open” field farming wasn’t adopted in Europe until the middle ages, it was pretty much the standard for the fertile cresent for a simple reason. There weren’t many trees.

    To create farmland in Europe, America and Australia large tracts of forest were felled, thereby creating a huge difference. However in the early civs in the fertile crescent and later in Egypt this wasn’t the case. There were very few trees and the land use change was from grassland to farmland rather than from forest to farmland.

    (Slight aside.) The dearth of timber is apparent in early Egyptian architecture. When the Pharonic Egyptians were working in stone, they copied what had gone before in the same way that “clockwise” mimics the shadow on a sundial. This picture from Luxor
    is a great example.

    It’s not a decorative fluting on those columns. The columns mimic the previous method of tying bundles of papyrus reeds together and then tying the bundles together to form posts. Sort of scary to think that large buildings were held up by posts made of bound grass. 🙂

    Back on track. So the introduction of field farming in Mesopotamia was more about the regulation of land use than a change in farming practices as it was all pretty much open field anyway.

    Irrigation did make a difference by allowing for an increased yield for the farmland but let’s not get too exicted by the word. There were no wheels. Irrigation was a guy using a long levered pole to dip a bucket into the river and then dump the water into a ditch. This method is however more effective than carrying buckets of water from the river to the field.

    Both of these factors aided the farmer to regulate and increase his yield, but didn’t change his technique. Farming was still a man with a pointed stick for a plough pulled by an Ox. (That’s if he had a plough and wasn’t using a hoe.) Seeds were then either spread or poked into the ground by hand. 

    And to keep the archaeological perspective, the Early Bronze Age 1 (EB1) didn’t start in that region until circa 3,300BC. So until that point all farming impliments were either made from wood or stone.

    Another thing to consider is reaping and winnowing techniques. Once a crop ripens the time for harvest is often quite short. There is no point to a farmer increasing his yield to 10 tons if 6 tons of it will go to seed before it can be harvested and stored. Personally I think the invention of the reaping hook was a major player in removing this particular bottleneck.

    Just to recap WRT the OP. The land use changes in this early period were from grassland to farmland and not from forest to farmland. I would add that technically wheat, etc are forms of grasses so the land use change was from grassland to grassland and unlikely to create even local environmental changes.

    Some may point out that the Middle East was once extensively forested and it was. However those forests were pretty much long gone due to climate change before the Sumerians and Babylonians came along.

    KK. Controlling the weather is the logical conclusion from the assumption. If by changing the land use we change the weather, then by controlling the way we change the land use we control the way the weather changes.

    In a general fashion this could be the case. I’ve read of some reforestation programs in the sub Sahara that appear to have led to more rain in the area. Whether this is due to more trees or simple rainfall pattern changes is still unknown. I have read of a “minority” opinion that planting more trees will bring more rain, but the jury is still out I think.

  26. Brandon Shollenberger says:

    JohnB, agriculture has never interested me so getting too involved in a conversation wouldn’t be a good idea.  However, I know a few things worth sharing.  First off, you seem to mostly miss the point I was getting at.  You had made a strident comment regarding land use:
    Unless you are going to postulate a phenomenal improvement in farming techniques that has left no trace in the archaeological record then you have to agree that for 100 times the population there was 100 times the land clearance.
    I made my response due to this.  Improved efficiency (which you agreed about) means less land needs to be cleared per person.  This means the ratio would not be 100:100.  That’s all I was getting at.
    However, since I’m making another response, I should point out another issue.  The amount of land clearance needed would not be 100:100 even if farming practices stayed completely the same.  The first people farming would gravitate to the land most suited for farming.  It is only as that land gets used up that land clearance becomes more necessary.  This means you don’t have a linear relation between land clearance and population.  Those two effects would work in opposite directions, and I won’t pretend to know what the net result is.  All I know is there is no linear relation here, and there is no way you’d get a 100:100 ratio.
    The second thing I know which is worth sharing is farming practices were very different in different areas in 6,000 BC (plenty didn’t even use it). This issue is important because of areas like China (actually, all of south-eastern Asia).  China uses paddy fields for a great deal of its farming.  This didn’t come about until after 4,000 BC.  Now then, paddy fields are a great source of methane.  They also led to domestication of the water buffalo.  Both of these effects caused a major ecological change in China.
    I don’t know enough to draw any real conclusions on the issue of historical anthropogenic effects on the climate, but I am confident in saying you’re far too dismissive of the idea.  Your references to the difference in population are far too simplistic, and they ignore all sorts of confounding factors.

  27. Brandon Shollenberger says:

    Oh yeah, I forgot to comment on something which struck me in your response.  You talked about ploughs.  Ploughs weren’t around in 6,000 BC.  Back then, the most you’d have is a guy with a hoe.  The introduction of a plough and domesticated animals dragging it was a major thing.
    As for irrigation, irrigation was a major change because it allowed farming in otherwise inhospitable areas.  More importantly, your description of irrigation is horribly misleading.  Describing it as basically a guy filling a ditch with buckets is in no way accurate.  All one has to do is look at things like norias and qanats to see irrigation was a huge change.
    You really seem to be understating the amount of change which took place in those six thousand years.  I don’t know just how important the land use changes were, but they were far more extensive than you portray.

  28. Atomic Hairdryer says:

    The thing that puzzles me about early anthropogenic influences on climate is does it scale?
    Couple of things that intrigue me. One is this:
    where modern land clearing has uncovered evidence of what may have been substantial civilisations. Today we’re told cutting back the Amazon is destroying the environment, yet there seems to be evidence that we may be restoring it to a state it was 800 or more years ago. It seems we’re repeating history, but does it have any significant effect on climate? The populations that previously maintained those settlements uncovered disappeared or collapsed for some reason, but was that due to prior climate change, or other anthropogenic influences? The forest grew back and hid what was there until modern clearence uncovered it.
    Then there was the guest articles about the Pueblo settlements. I’d not realised they were so large and again presumably in their day supported a fairly large population, but the current climate/geography can’t now. Did those die out due to prior climate change, or other pressures?
    Over in Europe we had land use changes due to clearance for agriculture, and also possibly military requirements. Timber used to be a strategic resource and forests grown to provide timber, somtimes stripped for war efforts, or cleared in peace time when it was surplus to requirements. I’ve seen suggestions that the plagues may also have had an anthropogenic effect due to population die-off and regrowth, but does it scale to a level where it could affect the climate?

  29. Brandon Shollenberger says:

    Atomic Hairdryer, I suspect some of the civilizations you have in mind wound up suffering the same effect seen on Easter Island.  There, a lack of resource management led to scarcity of certain things, and that helped caused the civilization to collapse. 

    As for how the things you listed affect climate, it is obvious they have some effect.  For example, cutting down forests can affect wind and rainfall patterns.  Removing hills (or producing artificial ones) can do the same, as can having large population centers.  Building dams, moving rivers and draining lakes all have effects too.  And of course, desertification speaks for itself.

    The trick is quantifying these effects.  Unfortunately, I have no idea how one would do that, so I can’t say if any of them “matter” for climate in general.

  30. John Mashey says:

    #28 The most obvious case is:
    a) About 50M native Americans died off from smallpox, etc post-Columbus, and jungles covered the areas in Amazonia and Central America where they used to keep the land clear by fire.  That reforestration was ~1550-1625AD and it soaked up CO2, and seems increasingly likely to have accounted for a Co2 drop unique in the last millenium.  See the red part of:
    Now, that could be coincidence, but the maunder Minimum was later, and people are doing a lot of work to compute the insolation->Co2 feedback sensitivities, and calculate CO2 sequestration from biomass regrowth, which of course is much higher in those areas, than say, in Kansas.
    The forthcoming Nevle paper I mentioned in #15 talks about some this.  If the Americas die-off contributed a major portion of this (which is what the current estimates seem to say), then it is quite plausible that earlier plagues did also, although with lesser effects, for various reasons.  (For example, plagues in China would not likely cause reforestration.)
    Hence: for 550 years, CO2 jiggled around between roughly 280-285ppm.
    Then, in ~75 years, it sharply dropped by 8-9ppm.
    Then, from 1600-1750, it jiggled around ~275-278ppm., and of course there were solar minima and volcanoes.
    Note of course that CO2 changes have different regional fingerprints than solar changes or volcanoes.

  31. Atomic Hairdryer says:

    #30 Yes, that’s the kind of thing I’m thinking of, although 50m may be too high a casualty number and the disease spread in stages, ie 1520 for Mexico, 1560 for Chile and spreading into N.America in the 1600’s. There have been similar debates regarding whether the Black Death had a similar effect, but again the timing might not quite fit.
    The Aztec die off may also not quite fit a regrowth hypothesis either, if chinampas was a substantial part of their agriculture as records seem to indicate. Many of those were artificially built in the lakes, so would not necesarily have resulted in forest regrowth. Other areas were artificially irrigated or flooded and changed when the Spaniards destroyed the dams.
    The good news I guess is that if the theory is true, then allowing a relatively small land area to reforest could mean a fairly rapid reduction in CO2 levels.. And hopefully not another LIA :p Then again, if people could ever agree on exactly where and when things like the MWP and LIA occured, correlation may become a lot easier.

  32. John Mashey says:

    Bill has a post @ RealClimate:
    That’s no on the later CO2 jiggles, but on fixing some broken assumptions about early land-use patterns, which is one part of the hypotheses that humans reversed the usual CO2/CH4 slide.
    I.e., early farmers used far more land/person than current, which does a much better job of aligning the numbers.

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