From the University of Cambridge, it makes you wonder how climate could just go and change abruptly on its own back then, with CO2 levels being in the “safe zone” and all that:
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Decline of Bronze Age ‘megacities’ linked to climate change
Climate change may have contributed to the decline of a city-dwelling civilization in Pakistan and India 4,100 years ago, according to new research
Scientists from the University of Cambridge have demonstrated that an abrupt weakening of the summer monsoon affected northwest India 4,100 years ago. The resulting drought coincided with the beginning of the decline of the metropolis-building Indus Civilisation, which spanned present-day Pakistan and India, suggesting that climate change could be why many of the major cities of the civilisation were abandoned.
The research, reported online on 25 February, 2014, in the journal Geology, involved the collection of snail shells preserved in the sediments of an ancient lake bed. By analysing the oxygen isotopes in the shells, the scientists were able to tell how much rain fell in the lake where the snails lived thousands of years ago.
The results shed light on a mystery surrounding why the major cities of the Indus Civilisation (also known as the Harappan Civilisation, after Harappa, one of the five cities) were abandoned. Climate change had been suggested as a possible reason for this transformation before but, until now, there has been no direct evidence for climate change in the region where Indus settlements were located.
Moreover, the finding now links the decline of the Indus cities to a documented global scale climate event and its impact on the Old Kingdom in Egypt, the Early Bronze Age civilisations of Greece and Crete, and the Akkadian Empire in Mesopotamia, whose decline has previously been linked to abrupt climate change.
“We think that we now have a really strong indication that a major climate event occurred in the area where a large number of Indus settlements were situated,” said Professor David Hodell, from Cambridge’s Department of Earth Sciences. “Taken together with other evidence from Meghalaya in northeast India, Oman and the Arabian Sea, our results provide strong evidence for a widespread weakening of the Indian summer monsoon across large parts of India 4,100 years ago.”
Hodell together with University of Cambridge archaeologist Dr Cameron Petrie and Gates scholar Dr Yama Dixit collected Melanoides tuberculata snail shells from the sediments of the ancient lake Kotla Dahar in Haryana, India. “As today, the major source of water into the lake throughout the Holocene is likely to have been the summer monsoon,” said Dixit. “But we have observed that there was an abrupt change, when the amount of evaporation from the lake exceeded the rainfall – indicative of a drought.”
At this time large parts of modern Pakistan and much of western India was home to South Asia’s great Bronze Age urban society. As Petrie explained: “The major cities of the Indus civilisation flourished in the mid-late 3rd and early 2nd millennium BC. Large proportions of the population lived in villages, but many people also lived in ‘megacities’ that were 80 hectares or more in size – roughly the size of 100 football pitches. They engaged in elaborate crafts, extensive local trade and long-ranging trade with regions as far away as the modern-day Middle East. But, by the mid 2nd millennium BC, all of the great urban centres had dramatically reduced in size or been abandoned.”
Many possible causes have been suggested, including the claim that major glacier-fed rivers changed their course, dramatically affecting the water supply and the reliant agriculture. It has also been suggested that an increasing population level caused problems, there was invasion and conflict, or that climate change caused a drought that large cities could not withstand long-term.
“We know that there was a clear shift away from large populations living in megacities,” said Petrie. “But precisely what happened to the Indus Civilisation has remained a mystery. It is unlikely that there was a single cause, but a climate change event would have induced a whole host of knock-on effects.
“We have lacked well-dated local climate data, as well as dates for when perennial water flowed and stopped in a number of now abandoned river channels, and an understanding of the spatial and temporal relationships between settlements and their environmental contexts. A lot of the archaeological debate has really been well-argued speculation.”
The new data, collected with funding from the Natural Environment Research Council, show a decreased summer monsoon rainfall at the same time that archaeological records and radiocarbon dates suggest the beginning of the Indus de-urbanisation. From 6,500 to 5,800 years ago, a deep fresh-water lake existed at Kotla Dahar. The deep lake transformed to a shallow lake after 5,800 years ago, indicating a weakening of the Indian summer monsoon. But an abrupt monsoon weakening occurred 4,100 years ago for 200 years and the lake became ephemeral after this time.
Until now, the suggestion that climate change might have had an impact on the Indus Civilisation was based on data showing a lessening of the monsoon in Oman and the Arabian Sea, which are both located at a considerable distance from Indus Civilisation settlements and at least partly affected by different weather systems.
Hodell and Dixit used isotope geochemical analysis of shells as a proxy for tracing the climate history of the region. Oxygen exists in two forms – the lighter 16O and a heavier 18O variant. When water evaporates from a closed lake (one that is fed by rainfall and rivers but has no outflow), molecules containing the lighter isotope evaporate at a faster rate than those containing the heavier isotopes; at times of drought, when the evaporation exceeds rainfall, there is a net increase in the ratio of 18O to 16O of the water. Organisms living in the lake record this ratio when they incorporate oxygen into the calcium carbonate (CaCO3) of their shells, and can therefore be used, in conjunction with radiocarbon dating, to reconstruct the climate of the region thousands of years ago.
Speculating on the effect lessening rainfall would have had on the Indus Civilisation, Petrie said: “Archaeological records suggest they were masters of many trades. They used elaborate techniques to produce a range of extremely impressive craft products using materials like steatite, carnelian and gold, and this material was widely distributed within South Asia, but also internationally. Each city had substantial fortification walls, civic amenities, craft workshops and possibly also palaces. Houses were arranged on wide main streets and narrow alleyways, and many had their own wells and drainage systems. Water was clearly an integral part of urban planning, and was also essential for supporting the agricultural base.
At around the time we see the evidence for climatic change, archaeologists have found evidence of previously maintained streets start to fill with rubbish, over time there is a reduced sophistication in the crafts they used, the script that had been used for several centuries disappears and there were changes in the location of settlements, suggesting some degree of demographic shift.”
“We estimate that the climate event lasted about 200 years before recovering to the previous conditions, which we still see today, and we believe that the civilisation somehow had to cope with this prolonged period of drought,” said Hodell.
The new research is part of a wider joint project led by the University of Cambridge and Banaras Hindu University in India, which has been funded by the British Council UK-India Education and Research Initiative to investigate the archaeology, river systems and climate of north-west India using a combination of archaeology and geoscience. The multidisciplinary project hopes to provide new understanding of the relationships between humans and their environment, and also involves researchers at Imperial College London, the University of Oxford, the Indian Institute of Technology Kanpur and the Uttar Pradesh State Archaeology Department.
“It is essential to understand the link between human settlement, water resources and landscape in antiquity, and this research is an important step in that direction,” explained Petrie. “We hope that this will hold lessons for us as we seek to find means of dealing with climate change in our own and future generations.”
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Not only there, “The Akkadian empire flourished for about 100 years until, at 4170 +/- 150 years BP, it suddenly collapsed (Weiss et al., 1993).”
http://www.ncdc.noaa.gov/paleo/abrupt/data7.html
I thought there was some sort of climate optimum 7000 – 4000 year ago?
I see that in the last sentence they talk of “dealing” with climate change rather than “controlling” it.
Is it wishful thinking or are we beginning to see a change of attitude?
This I can say as a fact, Dr. Michael Harner determined that the Aztec civilization in the new world failed because they exceeded the ‘carrying capacity’ of the environment there. Too many people and not enough protein because everything was hunted. That is why human flesh was used as a substitute in their diet. The tribe disbanded and one of the migrants was adopted by the today Crow.
David Larsen, the Spanish conquistadors had nothing to do with the collapse of the Aztecs?
I already learned of this hypothesis when writing The Book of Lilith (some of which takes place in Mohenjo-Daro during the Indus Valley civilization). It’s interesting that there isn’t so much as a blip on the CO_2 radar. It is also interesting to note the substantial discrepancy between the two ice-pack based “measurements” of CO_2 at different locations during their period of overlap. Finally, it is very interesting to try to find any sort of associated blip in any of the proxy-based temperature reconstructions or the mean reconstruction represented here:
http://commons.wikimedia.org/wiki/File:Holocene_Temperature_Variations.png
There is nothing interesting happening near 4000 BP, and only one of the various proxies (sedimentation that is a proxy for sea surface temperatures) shows a significant warming and COOLING during the immediately preceding century or two, culminating in a return to boring just around the right time frame. Given the resolution of the data, perhaps this was precipitated by a major cooling of SSTs, as the heating/cooling of the Indian Ocean is a major factor in the monsoon. That, in turn, might have been coupled to what?
The scary answer is a global shift in thermohaline circulation. That’s one of several nonlinear mechanisms that we do not understand, that is known to depend on things like the slowly changing topography of the ocean floor as the continents move around, and that is probably at all times a transient quasi-stable mode in which the system is stuck out of SEVERAL OTHER modes that would be equally quasi-stable if events caused a transition.
http://en.wikipedia.org/wiki/Thermohaline_circulation
Note that off the western coast of South India is one of the places where a cold, deep current rises, warms, and joins the ENSO-linked warm surface current that eventually becomes the Gulf Stream and warms the coast of NC and eventually all of Europe. Any sort of twitch of this deepwater rise and junction point could have enormously profound consequences on global climate. Indeed, twitch the warm current just a tiny bit south so that it no longer carries warm water to the North Sea, and the entire Arctic literally goes into the deep freeze. I’m guessing that this is the “critical” event that tips the Earth over into glacial episodes — it is one that is suspected of causing the Younger Dryas shortly after the Wisconsin glacial era (almost) ended.
Paradoxically, warming could easily trigger such a thermohaline shift. If a substantial melt of Greenland were to occur, the resulting freshwater flood would overlay the increasingly salty Gulf Stream and, since its descent into the depths off of the coast of Greenland depends on the density associated with increasing salinity, the warm water could be blocked and force the current to find a different “drain” for the high density, cooling surface waters further south. If that pattern then stabilized, the LIA would look warm within a century for most of Europe and all of the Arctic.
It is worth remembering that the thing that keeps the Arctic “warm” isn’t local CO_2 based warming — it is heat imported from the tropics, carried there primarily by the Gulf Stream. Isolate the poles and keep that heat closer to the tropics, and the world could easily cool even as the tropics warmed.
rgb
“it makes you wonder how climate could just go and change abruptly on its own back then, with CO2 levels being in the “safe zone” and all that:” This still has me laughing, thanks.
In 2002 a 10 sq. mile city was found off the coast of west India in 120 feet of water. Climate has probably changed since when it thrived as well.
A bit more about it: http://www.spiritofmaat.com/announce/oldcity.htm
This I can say as a fact, Dr. Michael Harner determined that the Aztec civilization in the new world failed because they exceeded the ‘carrying capacity’ of the environment there.
I think you mean the Mayans. The Aztecs were post-Mayan, and may or may not have been heavily influenced by remnants of Mayan culture. The Mayan civilization may or may not have collapsed for ecological reasons. It is one of several possible explanations (and there could be more than one explanation). There is evidence of some climate-shift or ecologically linked factors, but it is difficult to explain the completeness of the collapse without additional factors such as foreign invaders, a massive plague. Remember, the civilization was quite large — it is unlikely that the whole thing would exceed any magic number carrying capacity all at once, and after any sort of die-back one would expect such a dominant culture to re-establish itself (possibly slightly changed). The Mayans pretty much didn’t.
rgb
Surely these declines in Cities over the eons are scattered everywhere.
In Saudi many neolithic remains of long dried out villages remain where petroglyphs show the animals of the time etc.
The climate changes and with it the way we survive.
In 2002 a 10 sq. mile city was found off the coast of west India in 120 feet of water. Climate has probably changed since when it thrived as well.
This is still being disputed, of course. The evidence is not as clear as the article you link suggests. That’s not to say it couldn’t be true — in fact the climate HAS changed, and so has sea level, substantially over the last 10,000 years. But it isn’t clear that they’ve found a “city”, or even that the artifacts they’ve found are really artifacts (or if they are, that they aren’t carried there by currents from other locations and times).
At some point I’d guess that somebody will do a proper excavation of at least part of the site and find some unambiguous evidence, but at the moment not even the Indian researchers who did the carbon dating are uniformly convinced that they’ve found a pre-Harappan civilization.
The graph supplied is ridiculously scaled ….
I am undecided on climate change, and think this is a silly post
Your logic is, drastic climate change occurs naturally, therefore *adding* manmade change is not worse ???
it is just a basic logic fail: if the system is metastable, or variable, adding energy or heat or whatever is going to increase the variablilty, which by this very post caused really bad things to happen…not sure as skeptics that is the message you want out there, but , hey it is your blog
PS
congratulations to you for your list of pro/skeptic/anti blogs – you deserve an blog barnstar for that
The early Indus city-state civilizations such as Moheno Darro may have well died due to climate change, but not global warming. The warmistas are trying to throw dust into the eyes of the incogniscentii.
We know over the past few thousand years, sea levels have risen – don’t forget all that ice melting from the deep freeze of the ice-age.
There was a land bridge between the following in barely pre-historic times:
Ceylon – India
:British Isles – France (phew that was a close one! 😉
Australia – rest of the world
rgbatduke says:
February 26, 2014 at 2:51 pm
“But it isn’t clear that they’ve found a “city”, or even that the artifacts they’ve found are really artifacts”
I agree “city “needs more explanation, but have no trouble with artifacts.
The remnants of villages under the Black Sea flooded at the end of the last Ice Age, and the stone implements around Perth, Western Australia from a rock outcrop now more than 20 m below sea level should be well known.
I thought many would have read “Collapse” by Jared Diamond. He documents many “societies” that collapsed from environmental reasons (many self inflicted rather than natural).
It is known as the 4.2 kiloyear event:
http://en.wikipedia.org/wiki/4.2_kiloyear_event
Greenland was warmer in this period, meaning it was a very cold period in the temperate zone, hence the widespread droughts. It was very unlike the early mature period Harappan period from 2600 BC, which was much warmer and wetter, and much closer to our present climate:
http://snag.gy/BztF1.jpg
Are they sure?
Maybe it was not Climate Change what done it.
What if it was Water Wet?.
OK sarc off.
Given the multitude of things that can doom a city, what evidence of weather is really offered?
We interrogated the relics of snails.
May be..
“…project hopes to provide new understanding of the relationships between humans and their environment…”
gobbledegook! Do we need new understanding that we need water? This is the typical verbal diarrhea one sees these days from the is science.
David Larsen says:
February 26, 2014 at 2:24 pm
“This I can say as a fact, Dr. Michael Harner determined that the Aztec civilization in the new world failed because they exceeded the ‘carrying capacity’ of the environment there.”
As soon as I see ‘carrying capacity’ I know the research has been done by one of the green ideologue people hating politico-scientists. Present day Central America has 42M people ( http://en.wikipedia.org/wiki/Central_America ) and this doesn’t include the southern half of Mexico so lets say today we are “carrying” 75M people in this environment. Also, note this:
“…disease (small pox and typhus) is known to have ravaged the region; thus, the indigenous population of the Valley of Mexico is estimated to have declined by more than 80% in the course of about 60 years (after colonization by Spaniards.
http://en.wikipedia.org/wiki/Aztec#Colonial_period_population_decline
David, “facts” are very sacrosanct things not to be bandied about loosely.
Even if the graph is “ridiculously scaled” it is pretty clear that CO2 wasn’t driving anything dramatic. However, to then claim that there couldn’t have been a big shift in climate is to ignore shifts in climate due to other causes. (for example, a change in the more distant past from a SE monsoon over N and NE Africa changing to a SW monsoon over Asia, due to the rapid flooding of the Meditteranean with rapid drying of NE Africa). To say so also hands the CO2 meme to the warmists on a platter. Not that I accept this paper. I remember reading about the Harrappa culture
and 2 hypotheses relating to failure were 1. it was a burned brick architecture and effectively they destroyed the Indus valley forests to fire the kilns. 2. Their bronze weaponry did not change in any aspect for 1200 yrs, making them vulnerable to actively evolving attackers.
I wouln’t call estimates of rainfall from shell analysis, direct evidence. To then extrapolate to every bronze age culture in the near and middle east is really pulling a long bow! I seem to remember papers showing that the early Cretian and Mycenaen cultures were destroyed by the Thera (?)
eruption that gave rise to the legend of Atlantis.
Which came first the climate or the change?
I guess, when you are talking about anything that affects people it can be climatic.
That’s what climate change looks like?
Thanks for the interesting articles and comments.
The largest number of settlements of the “Harappan” civilisation were located along the Saraswati river. The river slowly dried up, as is well attested in the Vedas, Upanishads, etc, and confirmed by modern geology.
The primary culprit in the drying up of the Saraswati was geologic, however, not climactic. It lost its two main tributaries, the Yamuna and Sutlej, who’s channels shifted to the Ganges and Indus basins, respectively. This was around 1800 B.C. During this period, uplift in the Sialawik hills also cut the river off from it’s glacial source in the Himalaya. This reduced the mighty Saraswati from the greatest river of the subcontinent and center of the civilisation to a seasonal stream…the dislocation of population and impact on the civilisation must have been enormous. In the course of this period populations shifted to the Indus and Ganges basins.
Finally, even the seasonal stream dried completely with the changes in climate. This history can be traced through the Rig Veda and later literature, and has been recently confirmed and reconstructed by recent discovery and exploration of the pertinent paleo-channels, many of which are clearly seen in satellite photos. As early as the ’38-’39 Sir Aurel Stein documented over 1000 sites along the dried up bed of the Saraswati, vastly outnumbering the sites in the Indus basin.
rgbatduke says:
“Indeed, twitch the warm current just a tiny bit south so that it no longer carries warm water to the North Sea, and the entire Arctic literally goes into the deep freeze.”
There’s a problem with that, during colder periods when the mean position of the jet stream is further south, more warm water is transported into the Arctic. It is more likely that during extended extreme cold periods that the oceanic inflow to Arctic, and very importantly the outflow of sea ice off the east coast of Greenland is inhibited by ice shelves spreading out from the land.
More accurate assessment: “Upsurge in funding for useless academics linked to ‘Climate Change’ phase included in research proposal”
I remembered reading that climate change brought down the Egyptian empire of old and quickly googled it. Looks like there may be a “consensus” that climate change did them in about 4kyr ago or so (I am not an expert on this by any means). However, one link was interesting from Aug 28, 2012, which begins “The drought parching the United States is one of the worst in the nation’s history, but it hasn’t been as destructive as the drought that may have withered ancient Egypt’s Old Kingdom. Pollen and charcoal buried in the Nile Delta 4,200 years ago tell the tale of a drought of literally Biblical proportions associated with the fall of the pyramid builders.” That link is
http://news.discovery.com/earth/egypt-withered-under-drought-and-climate-change-120828.htm
This particular link goes on to mention other civilization-killing droughts 3,000 and 5,000 years ago and links to another discussion of Khmer and Mayans experiencing climate change : http://news.discovery.com/earth/khmer-collapsed-under-climate-pressure-120105.html
“Climate change” certainly looks deadly. If only the Egyptians, Mayans and Khmer had been able to curtail their CO2 emissions, we might all be speaking a different language.