Video
Top-Down Evidence of Large Early Anthropogenic Changes
Transcript
Karen: Good afternoon, everyone, and welcome to ArcheoThursday. This is your host, Karen Mudar. I'm an archeologist in the NPS Washington Archeology Program. This is the third year of a webinar series devoted to examining topics in archeology. The first year, we took a tour of advances in North American research topics and last year we heard about advances in archaeological prospection and recordation. This year we're exploring the Anthropocene and climate change in archeology. The Anthropocene is generally characterized as the period during which human activity is the dominant influence on climate and the environment.
Leaving aside question of geological implications, it's the time in which the human species shifted from being one among many to becoming the dominant species on the planet and affecting not only other animals and plant species, but the atmosphere and the oceans as well. While we've been long familiar and even proud of the human ability to alter the landscape, our impacts on the biosphere are more disturbing. Changes in climate and biological conditions that include acidification of the ocean, extensive and wide-spread pollution on several scales, from cellular to system-wide. The addition of alarming amounts of green house gases to the atmosphere have brought about the extinction of 800 species.
In short, human activities have triggered the greatest planetary biological and atmospheric changes in over 65 million years. This is pretty alarming when you think that the last mass extinction took down the dinosaurs! Labeled the sixth extinction, the Anthropocene has the dubious distinction of being on the same scale as the Chicxulab meteor and eclipsing the most recent Ice Age. Although this intimately concerns humans, especially since the younger people in this audience will be witness to profound environmental changes within their lifetime, what does this have to do with archeology? Archeology is both contributing to our understanding of the causes of these global changes and is being significantly affected by the effects of climate change, a characteristic of the Anthropocene Era.
After much wrangling by politicians and a huge amount of data amassed by the Inter-governmental Panel on Climate change, it now seems irrefutable that climate change is occurring and is caused by human actions over time. How long a time period? Did it begin with the Industrial Age? Did the point of no return occur sometime in the 20th century, when our population skyrocketed? Obviously, archeology has the potential to contribute to this discussion. Our first speaker in this series, William Ruddiman, will lead us through some of the data assembled to date and more about bill in a minute.
Our growing recognition and acknowledgment of the wide ranging effects of human activities also influences our scientific paradigm and analytic perspective. Two presentations in this series will address landscape-scale data to examine the effects of past human activities. I think that the scale of change both in describing the effects of the Anthropocene has encouraged archeologists to think on a larger scale than they’re usually accustomed to, despite the contributions made by scaler and landscape archeology.
Craig Allen with USGS has been studying the effects of pre historic timber removal, followed by 19th century cattle grazing on plant communities, that has modified the rate of erosion of archaeological sites in Bandelier Wilderness. I'm looking forward to this talk very much. He spoke to us about four years ago on some of his research and it was ... The evidence was very provocative. Ellen Wohl will discuss her research on Anthropogenic activities that have shaped Rocky Mountain National Park Wilderness. I'm sorry if I mispronounced your name, Ellen; we haven't talked yet.
Wilderness continues to be arenas for long-term effects of activities such as fire and hunting are down played or dismissed. This dismissal affects our abilities to identify and manage archaeological resources in wilderness. We're also going to hear several presentations about the effect of Anthropogenic climate change on archaeological sites. This facet of the Anthropocene does, and will continue to, influence the ways that we manage our resources. I'm sure that you're aware that our fire seasons are getting longer and that many fires are more intense. The lengthening fire season is largely due to earlier spring snow melts and later snow fall and altered rain fall patterns play a role as well.
We have a presentations about these changing fire regimes to provide context for research that NPS Archeologist Jay Sturtevant will present to us about identifying archaeological sites that are vulnerable to fire. Shelby Anderson at Portland State University is going to talk to us about work that she's been doing in Alaska on coastal settlement, also in Park Service units, which are at risk from sea level rise.
I'd like to include a presentation of sea level changes, which will not be manifested in the same way all over the world, if I can identify a suitable speaker. If you have any suggestions, please let me know. Marcy Rockman, the NPS Cultural Resources Climate Change Coordinator, will give us information about recent Federal efforts to manage and anticipate the effects of climate change. She's been involved in the development of a number of planning documents and other projects and will give us an update on her activities.
In a glass-half-full-of-dirty-water sort of way, climate change also provides new opportunities and ice patch archeology is a recent development within the field. We have climate change to thank for melting the ice that discovered Ötzi, the 5,000 year old man who was found in the Italian Alps. On November 13th, Craig Lee will talk to us about the results of a project to monitor melting ice patches here in the United States. This is going to be our next webinar, the one on November 13th. I hope that you'll join me for a wonderfully diverse set of presentations this fall and tell your friends about them, too. The schedule is still evolving and I'll send out revisions as more information is available.
Today, I have the pleasure of introducing William Ruddiman, who has graciously agreed to give our first talk. Doctor Ruddiman is a paleoclimatologist and professor Emeritus at the University of Virginia. He earned a PhD in marine geology from Columbia University and worked at US Naval Oceanographic office from 1969 to 1976, and at Columbia's Lamont Doherty’s observatory from 1976 to 1991. He's a fellow of both the Geological Society of America and the American Geophysical Union. Ruddiman's research interests center on climate change over several time scales, but his recent research has concentrated on the ways farmers may have affected climate over the last several thousand years.
I have to admit that I'm not as familiar with his Anthropocene work as I am with another area of his research. He's also know for his hypothesis that tectonic uplift of Tibet created the monsoonal circulation pattern that dominate the Asian climate today, which Southeast Asian archeologists ignore at their peril. When I went to look for his vita, I was directed to a long version and a short version, which gives you some idea of his publication record. He's written at least 137 articles and 3 books and edited 7 more. His 2006 book, Plows, Plagues, and Petroleum won the Phi Beta Kappa award for the best book in general science in 2006.
I want to finish my introduction by reading an excerpt from a review that appeared in Science the same year. The author James Flight says “Plows, Plagues and Petroleum is excellent reading for scientists and non scientists alike. Whether or not one agrees with Ruddiman’s most recent hypothesis, he has much to say and his ideas provoke thought. Ruddiman's ideas have not been greeted with open arms by the scientific community. Both scientific hypotheses should be poked and prodded, tested and re-tested and made to stand up to the available observations. Ruddiman embraces this process cheerfully. At a time when some scientists seemed to fear that open criticism will give the public the impression that we disagree about the facts on climate change, it is good to read a Ruddiman's faith in the scientific method and his willingness to let the process unfold as it should. Even if it means he has to take a few lumps a long the way.”
I'm going to ... Sorry. Thanks very much for being with us today, Bill. You've set a high bar for yourself and we anticipate that your presentation will provoke much thought.
William: Thank you, Karen. This is a little bit odd. I've never given a talk wearing a sweatshirt and my slippers, but it's very comfortable. The other thing that struck me is, when I give a talk, I usually tend to focus on one or two types of people in the audience. I either notice somebody smiling and nodding approval of things I'm saying and, of course, look at them often, because that's very encouraging. Sometimes I notice somebody falling asleep and then I focus my attention on them and try and keep them awake. This time, I'll just have to stare out my window while I'm talking and try to imagine your reactions.
The first slide, which is probably up right now, I can't see it by the way, is the title for the first third of the talk, on this hypothesis that anthropogenic changes were large prior to the Industrial Revolution. I call this kind of evidence top down evidence, because it's comparing this inter-glaciation the last 10 or 11,000 years with previous ones. The cartoon shows orbital elements: the tilt of the vertical axis and the eccentricity of its orbit which change over cycles of 20,000, 40,000 and 100,000 years. This means that we get a new inter-glaciation about every 100,000 years. We can compare the current to the previous ones.
The comparison I'm going to show you is the comparison of green house gases during those different inter-glaciations. We can do this because geochemist working on ice cores has isolated bubbles of ancient air and analyzed carbon dioxide and methane in those bubbles and applied a time scale to their result.
The next slide show variations, kind of a spaghetti-diagram plot. It should show variations of measured green house gases, methane at the top, carbon dioxide at the bottom. The time scale for the current inter-glaciation is shown in red at the top of both slides. The red circles are the measurements of methane and measurements of CO2 for this inter-glaciation. The measurements of methane and CO2 for the previous inter-glaciations which are referred to by stage numbers shown on the slide, are shown in different colors in both of these slides. These are the times in the previous inter-glaciations, which are equivalent in age or in timing to the last, in this case, 15,000 years of the glaciation leading into the current inter-glaciation which has lasted since about 11,000 years ago.
If we look at methane at the top, you can see that the ... I'm going to call the present inter-glaciation Holocene times. The present inter-glaciation comes up from low glacial value in concentrations of methane and a peak between about 13,000 to 10,000 years ago, with an interruption which I won't talk about, then they decrease until about 5,000 years ago before increasing straight on to the present. Now previous inter-glaciations is a little hard to tell, because they're all somewhat different, but they show the same basic pattern: low values rising to a peak somewhere in the range of maybe 12,000 to 10,000 equivalent years ago and then decreasing.
You'll notice that upward turn of methane in the current inter-glaciation, after 5,000 years ago, you don't see that in the previous inter-glaciation. There's one inter-glaciation that has brief two point oscillation there around 2,000 years ago, then the methane concentration comes down and continues it's downward path. This is what got me started on this whole hypothesis. The Holocene or in turn inter-glacial trend rise for the last 5,000 years ... does not look like previous inter-glaciations. I call it wrong way methane trend. The same is true, not quite as obvious, but true for carbon dioxide at the bottom. You see the same basic trends for the current inter-glacial values come up to a peak to around 10,000, 11,000 years ago.
They start to decrease, but then about 6,500 years ago, they begin to increase. The other inter-glaciations also show an increase to the peak, somewhere in the broad range 12,000 to 9,000 years ago, then decreases. Some of the decreases are very small, some of them are larger. One inter-glaciation which I won’t discuss here, but you may want to ask me about later, Stage 15 ... We'll call it ovals does show an increase throughout. The carbon dioxide trend generally looks anomalous. All these complications can be simplified with the next slide, which repeats the methane trend for this inter-glaciation in red. It combines all of the previous inter-glacial gas trends into an average shell in the dark blue circles and a standard deviation around that average.
You see for methane that the average of the previous inter-glaciations peaks at 10,000 years ago, about the same time as it does for the current inter-glaciation, then both trends decrease through about 5,000 years ago. The current inter-glacial methane goes up and the previous inter-glacial average continues down. That wrong way trend of the current inter-glaciation stands out quite strongly on that plot. The same is true and even more so, for carbon dioxide. The carbon dioxide trend for this inter-glaciation, sits right almost on top of the average of the previous inter-glaciation issue.
Then after 6,500 years ago it departs and heads up, whereas it continues down for previous inter-glacials. The methane trend takes a wrong way turn after 5,000 years and the CO2 trend takes a wrong way turn after 6,500 years ago. If you want to be truly objective , you can call these wrong way turns anomalies, but I made the case immediately that I thought they were anthropogenic, because if the natural trend is down and the current trend is up, then the only thing that is new about the climate system in the last 10,000 years is the presence of humans. Those convergences could mark the beginning of human influences on climate and the second two thirds of the talk will document that.
There is one inter-glac ... two of these previous inter-glacial stages, 11 and 19, have been posed as analogs to the Holocene because of the variations in the earth's orbit. If you look at the Part B procession, (I assume we’ve advanced to the next slide) the procession trend for Stage 11 is shown with a black plot and it has about the same amplitude and for about similar timing to the same trend in Stage 1 shown in red. There's another inter-glacial stage 19 in green, which shows also a very similar trend. Based on the similar procession trends, the stage 11 was initially put forward as a good analog for this inter-glaciation. But, it turns out, if you look at the tilt, or the obliquity of the earth's axis, as Stage 11 trend in black at the top is offset
to the left, there's two trends, Stage 19 and Stage 1. Stage 19 and the current inter-glacial Stage 1 were much more similar, somewhat lower amplitude of stage 19, so Stage 19 is the best analogue. There's no perfect analogues, but it's the best analogue. Part C in this slide shows the stage 19 CO2 trend, which peaks at an equivalent time of about 10,000 years ago, which perhaps will be about 787,000 years ago. Then it goes steadily downward like that average that I calculated for the CO2 trends. The Holocene trend peaks at a similar value, slightly higher but the record sampling is much denser, then it decreases in a somewhat similar way, but then again around 6,000 to 7,000 years ago the Holocene, or Stage 1, trend starts to increase, instead of going down.
When I published in 2003, this hypothesis, I projected that CO2 trends would have gone down to about 240 to 245 parts per million, had it not been for humans. That red dashed line shows that projection and that projection falls right on top of what actually happened in Stage 19. This is data that the team is still working on. My point would be, that Stage 19 is the best analogue, in terms of the changes in the earth's orbit, for what should've happened to earth's climate during this inter-glaciation and humans are certainly not a major factor in global green house concentrations back then. So Stage 19 is an experiment that nature has already run and it says that CO2 should have been decreasing for the last 10,000 years. Well, it did decrease for about 4,000 years and then it rose. That's more support for the early anthropogenic hypothesis.
The next slide (which is the map) shows an experiment run for that inter-glacial Stage 19, at the time equivalent to the present day, which is 777,000 years ago. It uses the orbital configuration for that time and the green house gas concentrations for that time. The scale at the bottom in various colors, shows how many months snow cover would have laid on the ground in the northern hemisphere, under similar orbital variation and lower CO2. All the white areas are areas that would be permanently snow covered. Of course, it includes Greenland, which is high standing in cold features, but it also includes a huge area, Siberia and also Alaska, the Brooks Range, the northern Rockies and the broad area in the Canadian Archipelago, including Baffin Island, which is a feature about the size of California and Oregon.
What this experiment does, based on known orbital values and carbon dioxide measured from ice cores, what this the experiment suggests is that everything shown in white would have 12 months snow cover. If the snow cover doesn't melt in September, late August, or early September, it's going to be added to the next year. This is a map of areas that could potentially have been glaciated at the time most similar to today.
The next slide shows - at the bottom it repeats that map I just showed you of the most similar time in inter-glacial Stage 19. At the top it shows you an estimate we did previously. This is work with John Kutzbach and Steve Alverson, Feng He at the University of Wisconsin. It shows the same thing. Months of snow cover for the projected values of CO2 and methane that I used in the original early anthropogenic hypothesis. You get almost identical snow cover, a little more in Alaska and northern Canada, but otherwise nearly identical. That's just another way of saying that the Stage 19 time most like today, looks like the projection of where we would be right now for early farmers.
I’m going to skip this, because it's not that important. Here's the cartoon that summarizes that finding. It shows you global temperature on the left and the 20,000 years of record in the current inter-glaciation at the bottom. You start off with very cold glacial temperatures, perhaps 4 to 5 degrees centigrade colder then modern temperatures. Then you have the rise out of the glaciation peak in warmth in global temperature around 8,000 years ago. Then in top trend, the observed trend is a schematic version of what temperatures have been doing for the last 8,000 years, oscillating by going very slightly colder before launching off into the present-day Industrial Era anthropogenic warming.
Down below, that trend is plotted and what I call the natural trend, which is the trend that would've happened, would've occurred had it not been for the green house gases that farmers put in the atmosphere. Those are shown ... the early anthropogenic agricultural green house effect is shown in orange. My claim 10 years ago was that we would have gone through the threshold that needed to start the next glaciation sometime in the last several thousand years but we didn't, because green house gases from agriculture kept us out of it.
We can skip now to slide 10 which shows a global map. This is the second part of the hypothesis. I call this bottom up evidence of early anthropogenic changes and this gets into the area that all of you work in, which is heavily reliant on archeology. This is global scale big picture archeology. What's shown here is a map of the spread of agriculture, starting from the most prominent of the source regions of agriculture beginning to spread - that's shown in dark green - and beginning to spread out of those areas, at the time shown in white numbers, which indicate thousands of years. If you look at the Fertile Crescent in Southwest Asia it's 9,000 yeas ago. If you look over to the Americas, it's more like 4,000 years ago out of Mexico.
China was a bigger source of domestication of cost and livestock. This is the pattern of the spread of agriculture. You'll note that the spread of agriculture is happening pretty much within that interval in which the green house gases are going the wrong way. That is, that they're increasing when they should've decreased. There's lots of data, much which has just been very recently assembled, there on the timing and the spread. The next ... I’m going to repeat the methane patterns to remind you that somewhere between 5 and 4,000 years ago, the methane trend bottomed out at the value around 550 parts per billion and then it's been increasing ever since.
The slide after that shows a map of southern and southeastern Asia, put together by Dorian Fuller who works at University College London. He has compiled all the available data from archeology and archaeobotany. Those are his areas of specialty. What he's mapped here is the initial arrival of irrigated rice agriculture across that entire region. Dorian is an archeologist but most especially an archaeobotanist. He can tell whether he's looking at remains of dry rice or wet rice. He can tell if it's been domesticated, or if it's still wild. This is very nicely constrained map. What it shows you is irrigated rice, agriculture develops in the Yangtze River Valley of China, around 5,000 years ago, very small initial spread 5,000 to 4,000, a few darkest colors on the map. Then by 1,000 years ago, it spread all the way down into Southeast Asia, Java, Sumatra, and across into India.
Basically by 1,000 years ago its being irrigated. Rice is being grown everywhere it’s grown today. At least the initial entry of irrigated rice has reached every area where rice is grown today. Dorian took those data and he assumed that as rice first arrived in a given area, it would spread - would become more densely farmed within that area, based on population, on estimated population. He compiled a block of the total area under irrigated rice culture from 6,000 to 1,000 years ago, which is shown in green. He converted those to that area of rice agriculture to estimated methane emissions and to estimated methane concentration in the atmosphere, which are supported by estimated emissions with basically no significant lag.
He used modern relationships to do that, and what he got was an estimate that between 5,000 and 1,000 years ago, the methane concentration in the atmosphere would have increased by 70 parts per billion, because of the spread of irrigated rice. The observations in the ice core shown at the bottom of this slide, suggests that the actual increase over that interval was about 100 parts per billion. Irrigated rice could explain about 70% of the wrong-way rise in methane. If you think back to slides, the two slides I've already showed you, the full anomaly in methane has two parts. One part is the rise in methane, which has only happened at this glaciation, inter-glaciation. The other part of it is, that in previous inter-glaciations, it fell. The real methane anomaly is the sum of those two and it's going to be larger than that 100 parts per billion observed methane increase shown at the bottom of the slide.
Irrigated rice, in that case, is not similarly explained 70% of the total anomaly, but something less. However, in this paper, which appeared in the journal Holocene in 2011, Fuller also mapped the first appearance of livestock, which are notorious methane emitters, again, using archaeological data. You see that between 7,000 and 5,000 years ago, livestock occur in the Sahara, what's now the Sahara Desert. Back then it was a grassland, because the monsoon ran stronger. Livestock from the Iranian plateau which is an arid region and western China, which is also arid. There were domesticated livestock in fairly broad areas. None of them had high carrying capacities, so they're not likely to have had huge effects on climate, on methane emissions.
After 5,000, in that interval 5,000 to 3,000 years ago, the livestock start to spread down into East Africa, all across India and across most of China and Southeast Asia. There's some big holes in this map and a lot of work to be done, but basically again, that same number, 5,000 years ago, methane emissions from this source, you could only imagine would've gone up, and by 3,000 to 1,000 years ago you have livestock - domesticated livestock - in all these regions. Livestock are going to contribute a significant amount to this story, once they're fully mapped. That's the methane story and I would say there's pretty good evidence from archeology that methane emissions from agriculture are very promising for explaining the wrong-way methane trends.
The next slide shows that –repeats that - CO2 trend, again this inter-glacial in red and the previous inter-glacials in blue. There are lots of data on this as well, although much has yet to be compiled - synthesized. One paper published in the 1990s by Zohari and Hopt shows the spread of agricultural remains out of Southwest Asia across Europe and if you look at the scale at the top right, you can see that, prior to about 9,000 years ago, most of the evidence is in southwest Asia coming into Turkey and a little bit of Greece. The big jump across Europe in the evidence ... This is all reliably carbon 14 dated archaeological sites. The big jump is after 8,400 years ago to 7,000 and then another jump 7,000 to 5,500. By 5,500 years ago, agriculture is present in every part of Europe where it's used today,
obviously not as densely as it's used today, but it's there and it's beginning to fill in. What's remarkable about this to me is that 5,500 years ago we're still in the late Neolithic, the Late Stone Age, and the Bronze Age is still 1,000 years or more in the future. People with axes made of flint are chopping down trees, burning or gurgling to kill them and then burning the debris and then planting crops in between the stumps. Even before the Middle Ages we were doing major clearance. Again, this pattern, this timing of the big jumps, the light green and the yellow colors, is about the same or brackets the 6,500 years ago reversal of the carbon dioxide trend. Here's a recent synthesis of data from England, which is far from Southwest Asia as you can get in the agricultural part of Europe. You see the time scale on the left.
The synthesis of pollen remains shows that somewhere around maybe 6,000-5,000 years ago, you start to get hints that the woodland coverage - woodland pollen are decreasing. That becomes very evident by 4,000. The two lighter plots, light green and yellow plots show semi-open pasture and pasture and meadow increasing, after maybe 5,500 and certainly by 4,000. Also this study, this is Woodbridge et al., compiled the density or the number of sites with C14 dates, reliable archaeological work, and you see a big jump around 6,000. Again, that's right after the reversal of the carbon dioxide trend. All of this says we're in the ball park to explain why carbon dioxide might have started climbing instead of dropping after 6,500 years ago, because of agriculture.
The same is true in China. There's a study by Zee et al. a few years ago who then compiled these archaeological sites, in this case I don't know the detailed reliability about these sites. I suspect some of them are very reliably Carbon 14 dated and many of them may simply be correlated into the reliable dating by using the Chinese cultures, which succeed each other every several 100 years, usually less than 1,000 yeas. If you have these cultures very nicely dated at several sites, then you can use places where you only have the cultural data and link into the Carbon 14 time scale. That's just a guess on my part. It's hard to imagine all of this has reliable C14 data.
Anyway, the top map from looking at China, 8,000 to 7,000 years ago, there's a sprinkling of sites in the Yellow River area and just a few in the upper Yangtze River. 5,000 to 4,000 year ago, the Yellow River is covered with sites and the Yangtze River is beginning to become dense with sites. Remember that it's just in that lower map interval that irrigated rice begins to spread out of the Yangtze River area south. That lower part of that map in the next 1,000 and 2,000 years is going to get very dense with sites.
This slides shows that in that 3,000 year interval between those two maps, there was something like 30 fold increase in archaeological site density, which means that population must have doubled every 600 years, if archaeological sites are linked to population, which you think they would be in some way or another. Once again evidence from major human presence on the land. All these people are farmers. Major increase in agricultural activities sometime between pretty much 7,000 to 5,000 years ago which, again, brackets the reversal of the CO2 trend 6,500 years ago. There's a very recent study in ... I’ve forgotten what the source is, by Dobson, John Dobson shows in the Yellow River where it goes upwards and off the map in that perhaps densest blob of pre ... It shows that people were burning coal for fuel and home heating, because they had deforested that entire part of China.
That dates to 4,500 years ago. Coal crops up in surface mines. It was readily accessible and that's what they had for heating, so that's what they used. All this data so far says things were happening, major things were happening, at the time that the carbon dioxide reversal occurred. There's one problem, one criticism of the hypothesis, I think has been made which has affected more people then any other criticism and that is, that there couldn't have been enough people with this archaeological evidence I'm showing you. There couldn't have been enough people to drive the carbon dioxide increase. This is particularly true between 6,500 years ago and about 3,000 to 2,500 years ago. You see that ... I meant to say, advance to the next slide, this would be number 19, slide number 19. The carbon dioxide trend comes up pretty rapidly, 6,500 years ago to 2,500 years ago,
whereas the conventional estimates of population lies much later, mostly after about 1,000 years ago. You have this problem, if humans are causing this CO2 increase, how come the CO2 increase occurs way before the population increase? That doesn't seem to make any sense. I think there are two answers to that question, one of which has emerged and one of which is emerging. There is historical data from China and from Europe, very spotty but enough to catch the trend I think, of land use going back 2,000 years. The top slide, we're now on slide 20, shows hectares per person in China. This is hectares per person, this is cultivated land, actual crops, not pasture, not hay fields, not other areas cleared just for the wood.
This is crop products per person. You see a downward trend from the year 5, I think is the first data point, during the Han dynasty down to much lower values in the very late industrial era, the values of .25, .2, or less coming into the Industrial Era. What that means is, people were using about 4 times as much land per person for their crops 2,000 years ago as they were using in the very late pre-industrial times. The bottom plot shows a similar but different index, which is hectares cleared per person, in other words, deforestation in Europe. In this case, this index does include pastures and meadows and land burned for other purposes. There's many more hectares per person then the plot at the top, but the trend is quite similar. You go from 6 or 7 hectares per person deforested in Europe 500 years ago to about 1 or 2 in the last 200 years,
again, about a factor of 4 decrease. What this means is that even though populations may have been lower, or were lower, back in time prior to the last couple of thousand years, people were clearing at least four times as much, using at least four times as much land. As you get farther back to 2,000 years ago, we don't have data, but you can only imagine that it remained at least as high, or was perhaps there was even more clearance. Someone who has played as the ...
We go to the next slide which shows a cartoon of trees. The person that first really shined a spotlight on this issue was Ester Boserup who was an economist that published two books in 1965 and 1981. She looked at modern populations, hunter gathers and people in various stages of cultural development. She concluded that early agriculture was very inefficient. People used much more land per person, because of the lack of efficiency which developed later. So, back 5-6,000 years ago, total well watered valley soils were available for farming. Whoever came in took as much as they needed and used that land inefficiently, because they didn't need to be efficient. The way they used it, probably suggested by this cartoon, that you start with mature forest,
you clear off the trees and burn the residue. You can farm for 2 or 3 years, especially because the soil is enriched by ash from the trees. You can farm for 2 to 3 years, but soil plays out and then you have to move on. You move on and you clear another ... This will be maybe a hectare or two to farm that. Meanwhile out behind you, the shrubs and saplings start to come in, then larger and denser saplings and then, later on, small trees. The big grey arrows show that these people moved around, they were leaving a trailing footprints of trees that had not yet grown back. Those grey arrows represent carbon that is in the atmosphere and have not yet gone back into the cleared land.
So, that's one of the things that make for footprints much larger. Later on, people discovered things like manure and much later crop location and that ... You go from people moving around constantly every 2 or 3 years to, in the last few centuries, people farmed the same land every year and maybe get 2 crops a year out of it. That's called intensification and what it represents is much less land use per person. The effect on this, on the amount of clearance is only becoming evident in the last couple of years.
The next slide shows a map, a couple of maps. At the bottom, we will see, is a map based on the assumption of people have always cleared the same small amount of land as they did in the year 1800 or 1850 or 1900. If that's true, there hasn’t been much forest clearance at all, so you get a map that shows 80 to 90% natural vegetation across almost all the continents. You see a slightly lighter green color in Europe and southwest Asia and in northern China, very little clearance. If you use that historical data of higher clearance, in the past, decreasing towards the present, you get a very different picture. That's the map at the top, which shows lots of clearance in Europe, southwest Asia, India, China and lots of clearance in -this is a map of 2,000 years ago, I forgot to say-
lots of clearance occurred in Mexico and in the Inka empire in South America. The map at the top, again this is a model based on an assumed amount of clearance per person, looks much more like a map published in the Cambridge Encyclopedia of Archaeology in the 1990s. All the dark green areas having what is called complex stratified agriculture. Those dark green areas are a pretty good match to the areas that the top simulation shows as deforested As you come forward to 1800 - I forgot to mention this, this work by Jed Kaplan and his associates in Switzerland -
as you come forward to 1800, you bump up against an interesting constraint. If he made that assumption that everybody ... Farmers always cleared the same amount of land per person and it was small, you end up with an estimate that Europe was, as of 1800, no more then half deforested, that’s a possible effect, except for a few areas, including Spain. That most of it was 50 to 70% forested at 1800. The reason this is strange, or odd, or impossible, is that there is good data showing that countries in Europe began to reforest after 1800, because of the increasing efficiencies of intensive agriculture. Countries in western and central Europe reforested in the last 200 years. If they hadn't deforested by 1800 and they have subsequently reforested, then you ought to see predominant forest all over Europe, well,
of course you don't. Most countries, till you get up into part of Russia, or maybe around the Baltic, most countries are 80, 85% deforested. The map on the left shows what you get if you assume that early clearance was intense. It's shown in those plots I showed you earlier. You get much more clearance by the time you get to 1800. 90% to maybe 60-70% deforested and those numbers are reasonably consistent with the fact that Europe has, if anything, deforested, or reforested in the last 200 years. These maps pretty much require that early clearance was much more intense then the first, than these early land simulations suggested.
One last subject or issue is here I ... The next slide repeats that carbon dioxide trend rising early and the population trend rising late. The population trend is a schematic. There’s actually fairly decent population data back to 2,000 years ago. Any more zigs and zags in that population curve. All population curves prior to about 2,000 years ago are projections and the classical technique is to use a geometric population increase, which I can describe in two ways, one way quite simply means the population doubles every 1,000 years. Most of the rise occurs right in the curve. Another way of saying is that, that the fractional rate of population growth is constant all the way through those 7,000 years that are shown here.
Whatever the population is at a given moment, a fractional population growth in the next year, or century, or what have you is the same. If you think about it, that's a rather strange assumption. I mean 7,000 years is a lot of time, or 7,000 years that are pre-industrial. That's a lot of time and it's hard to believe that population that the environment would be consistently conducive to exactly the same rate of population growth. There's another view, and this is finding some support in DNA evidence.
The next slide repeats the geometric cartoon, somewhat different time scale, but it also shows the logistic, the trend that's labeled logistic. You noticed that population comes up much earlier and then levels off. The logistic population curve is one that's widely used by ecologists and paleoecologists and what they assume is that both rates are fastest early on when resources are abundant, in the case of farmers that there would be fertile soils, well watered soils in valleys. As slow as people ... as population increases, the growth rate is slow because people are now forced to farm up the hillsides, less well water, less fertile soils and also disease starts to play a role, that whittle away at the population. These logistic curves are going to push the rate of early ...
If they turn out to be correct and I mentioned that there's DNA evidence that supports that that's the case, they’re going to push the population growth farther back in time which is going to push the CO2 curve that they can ... The carbon emissions curve that they can support back in time. This perhaps the major issue I'm dealing with right now and trying to work with some colleages with the expertise to do that, to see what is the most reliable population curve and will explain the early CO2 rise. The higher per capita land use reconciles part of that mis-match, but the earlier population rise should also help.
One last slide. This is a related subject. This is slide now 26. The carbon dioxide curve from a high-resolution ice cores and now you're looking at the last 1,800 years and also methane at the bottom from 1,000 to 1,800, shows very coherent oscillations in both green house gases and they both have dips. Now remember in both cases the curves have been rising for thousands of years to get into this area of high CO2 and high methane. Then they start to show some dips at 200 to 600 AD for the CO2 curve. Again 1200 to 1400 for the CO2 curve, a really sharp drop between about 1525 and 1610 for CO2. Then the methane dips shown at the bottom. In the middle, I plotted the major depopulation events, that is, major crashes of global population over that same interval of time. You'll see that both Europe and China had population decreases that were quite sizable of 5 and 7.5% of global population,
during that same interval 200 to 600 when the CO2 values were going down. Another decrease in China after 1200. That's Genghis Khan and his successors. The Black Death in Europe there in 1350. Then the biggest disaster of all, the 50 million people who died in the Americas because of contact with European diseases, to which they did not have immunity, along with some loss of life in China. Some pretty remarkable correlations between losses of people and dips in the green house gases. My plan here was and is, that let's take the Americas. If almost all Native Americans, not all, but have used agriculture along with hunting and gathering, if they're clearing large areas of, say, the Mississippi and Tennessee River Valleys, but larger areas also in Central and South America, if then 80 or 85% of them die due to diseases, between 1525 and 1600, then forest start to grow up where they used to farm, and
it takes CO2 out of the atmosphere. The methane decreases probably involve mostly in part things like Genghis Khan ripping up the irrigation infrastructure in China and having livestock starve as a result of people dying and livestock not being fed. There's a lot of plausible links between millions of people dying and green house gas decreases. As I said that's almost another story, but it's a very interesting one and at a more recent time scale. You get the sense from my tone that I think the data are coming in to test hypothesis and the book review in Science that Karen read earlier, that's the process of science working itself out. My sense, of course I'm biased, but my sense is the data that are coming in, are coming in behind the early anthropogenic effect on climate and not behind the criticisms of the hypothesis. With that, I throw myself open to any questions.
Karen: Bill, thank you for a fascinating talk. Are there questions? You folks are not on mute, you know. You can speak up!
Speaker 3: Bill, I'm wondering If the loss of buffalo herd would've had any effect on methane in particular
William: I guess that's something I don't think about much. What I remember would be, there were, of course, 12,500 years ago, there was a mass extinction of large mammals in the Americas. A lot of those were methane emitters. When they were gone and as I understand it, what probably happened was, that the survivors like the Buffalo, then proliferated. You basically replace one kind of methane emitting wild animal with a more selective group of methane emitters. What that would do to the global climate, I don't know. I think if you look at the numbers of livestock around today, I should have a number at hand, but I don't, it's a huge number. I don't know exactly how it compares with the buffalo.
Speaker 3: The American bison I guess, they would be replaced by domestic cattle?
William: For a while, when the live stock go down into Africa, it's probably just a replacement scenario, where livestock edge out the wild animals and there's no net increase of methane. After a while, people have huge livestock herds and you start getting the excess of methane. An area like China, which used to be forest and now becomes suitable for livestock, that's really pretty much almost all of the livestock that are in that increase over previous levels, because there weren't that many grazers -browsers in the forest.
Speaker 3: This is fascinating. Thank you so much.
William: Thank you.
Speaker 4: Bill. This is Brendan Carter calling from Sitka, Alaska. I read some of those papers you wrote back in early 2000s as part of my dissertation research and it's good to see that you're following up on that, because it seemed very compelling. Some of that early work was very compelling. What I was wondering is what ... The thing that popped in my mind and has kept with me for the last 10 years is the problem of the stochastacy in that when you destabilize a system the way that it seems like it's been destabilized, or it's been artificially stabilized, I guess, at a high level and seems to be going in an opposite direction of where the Milankovitch curves would normally take us, whether that's a problem, like, if the carbon sinks catch up and we all of a sudden get plunged into a new Ice Age.
William: I thought you were heading in one direction and then you went in another. In effect, my original hypothesis and still my claim is that, we would be in the very early stages of a new Ice Age, if we had left nature alone and those maps of simulated months of snow cover show that the areas in the northern hemisphere with 12 month snow cover which is, to me, the same as incipient glaciation, they were 30% larger then the present day Greenland ice sheet. It's not a huge area, but it's not a small area, either. I say it would've happened sometime in the last 2,000 years that we would have those areas glaciated. The farmers stopped that. Now of course, I think we've stopped any natural glaciation which should have happened 2,000 years ago. We stopped that for 10s or 100s of thousands of years in the future.
Speaker 4: We may skip the next cycle?
William: I think we already have begun to.
Speaker 4: Oh, wow, okay.
William: The insulation curves, the summer insulation curves guide glaciation in the northern hemisphere. They don't really get any lower then they have been for the last 1,000 years until about 60,000 years from now.
Speaker 4: Oh my God.
William: We have a 2.75 million year record of glacial cycles and there's never been a 60,000 year long inter-glacial in all that time. I think we've already done it. It's still quite remarkable, these glacial cycles. 2.75 million years and we've already stopped them.
Speaker 4: Yeah, that seems pretty dramatic.
William: Yeah. I'm not a dramatic person, but that's a conservatively thought out conclusion. It doesn't sound like it, but it is.
Speaker 4: Addressing the stochastacy part of it, do you see any increase in the potential for having rapid broad scale climate switches like what we saw in the Younger Dryas? Beginning and the end of the Younger Dryas?
William: Yeah, I skipped over that. The Younger Dryas looks like to me like one of those ... Many time when large ice sheets are present, you get the fairly large flips in northern hemisphere climate in particular. The Younger Dryas is the last big one. Once you get into the full inter-glacial, you just don't see them anymore. Temperature hasn't changed much, certainly less than a degree centigrade in the last 8,000 years. I don't see them in the future, there are some people like to talk about that possibility. It's impossible to rule out, but I think the future looks ... Even if it's just slow and gradual warming without any dramatic warming, it looks bad enough and some kind of flip.
Speaker 4: Yeah, that was the thing that really sort of freaked ... I wouldn't say freaked me out, but the data seems like ... Essentially at this point it's almost like a majority human-influenced environment. We just don't know how stochastic the system is going to be on a short term basis over a period of human experience, single generation human experience. I don't know that we've been there in recorded history.
William: Yeah, it is hard to predict. I would say that on the last 200 years of climate change and on projections of climate change, I'm amazed that climate scientists are not in any way a skeptic. The only thing I'm adding to the story is that, when we started to warm things up after 1850, we were building on 6 or 7,000 years of prior effects on green house gases and climate. In my view, it started from a different lower base line, but I have nothing to say that would change the main stream community view on the current global warming and the future of global warming.
Speaker 4: Yeah. You've seen Bill Webb’s stuff on palynology and charcoal deposit in the eastern United States, his palynological work?
William: Who did you mention?
Speaker 4: It's Webb, William Web.
William: Oh, yeah.
Speaker 4: I recall that he ... You see a big blip in carbon at I want to say 8,000 or 8,500. It's almost like there's some intensive forest going on in the eastern United States. That’s really complementary to this, the early [crosstalk 01:16:11] ...
William: I was a member of the CoMap that Tom Webb and John Kutzbach were heads of. I don't remember anything like that. What I remember is pretty much a slow steady march towards warmer inter-glacial climate. I think they had a little bit of a reversal to cooler climates about 2,000 years ago, because if I remember right, spruce trees moved back south. They had been retreating north during the deglaciation and then they stabilized and then they came a little bit south. It got a little cool or a little wetter, something like that, but it was very small.
Speaker 4: I would say more of the charcoal data seemed to show early forest clearing. [crosstalk 01:17:02] ...
Karen: If I can interrupt you guys. There was an article in Science within the last five years that talked about an increase in carbon with the extinction of megafauna. I don't know if that's the blip that you might be thinking of.
Speaker 4: Yeah, it probably is.
William: I thought he was talking about the last 80,000 years ago and that's back about 12.5 thousand.
Karen: Oh, okay.
Speaker 4: I have to go back and look at the Webb papers.
William: Okay. Anyone else want to chime in?
Marcy: This is Marcy with the National Park Service, Marcy Rockman. That was a really interesting presentation. I'm still trying to wrap my head around some of the comparisons. I think, so feeling my way through my question, I'm still trying I think trying to wrap my head around the comparison between the inefficiency of early agriculture and what that would have done in terms of carbon contributions to the atmosphere, compared with, say, our modern capacity to contribute carbon to the atmosphere, sort of per person or per acre. I'm trying to remember if you mentioned any publications or analyses of that. Do you know of any studies that have been done? Could you point me in a direction of anyone who might be looking at this? I'd be really curious to look at it, what might we estimate from early agricultural carbon contributions compared to modern carbon contributions per person, or per acre.
William: I showed the name Kaplan, K-A-P-L-A-N. He published a paper. It was also in the Holocene. It’s got a 2010 date on it, because that's the date that it was accepted online. It's in the same Holocene volume as the Fuller work on the rice agriculture. He's the one that used the historical ... He actually went back and dug out some of the historical data that people had assembled in the late 90s and the name escapes me, but somebody from the UK had compiled a lot of data on early land use. His name is Jed Kaplan. If you get access to that Holocene volume, he's got maps starting at, I don't know, 10 or 11,000 years ago and some right through to the present of percent land clearance and carbon emissions region by region and subregion by subregion. It’s a very comprehensive paper.
Marcy: Thank you.
William: Sure.
Karen: Are there any more ... Are there comments or questions for Bill? Bill, I have something. I'm going to reveal my ignorance here. When you talked about the methane contributions of domestic crops, could you talk for a minute about what exactly the mechanism is? Is it the difference between C3 and C4 plans?
William: This is one of those talks I always forget something and I left that out. The methane, anthropogenic methane, it comes from irrigated rice, because what rice paddies are, is basically anthropogenic wetlands. Methane comes out of natural wetlands as swamp gas, or bubbles up from stagnant water. Rice paddies are anthropogenic wetlands. They're quite rich in carbon. They feed methane into the atmosphere. Methane comes from livestock, I'd like to say at both ends, from both ends. It comes from ...
Karen: That part I understand!
William: It also comes from a burning crops and weeds. If you have a pile of debris and you set it on fire and if the bottom layers are not getting any oxygen, then the carbon goes into methane instead of carbon dioxide. Is that okay?
Karen: That's why you're also ... Even though, obviously, people in Europe are not growing crops at any one time, you're tracking the methane releases, a by product of farming.
William: Yeah, you do get some farming that way. I would guess that, since livestock came into Europe, a long list of crops, particularly cattle that I would guess, that the domesticated livestock was a major emitters of methane in Europe. It's cattle and goats and sheep and pigs, all of them domesticated in Southwest Asia.
Karen: Uh huh. Thank you for that.
William: You're welcome.
Karen: More comments? More questions? Bill thank you very much for a fascinating time.
Description
William Ruddiman, 10/30/2014, ArcheoThursday
Duration
1 hour, 12 minutes, 25 seconds
Credit
NPS
Date Created
10/30/2014
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