Magnitude and Impact

[Music] all right hi my name is brandon drake i want to thank uh in particular uh jan dressen simon just threatened all the other folks who've worked very hard to bring us all together here today today i'd like to talk about magnitude and impact with regard to the late bronze age collapse specifically trying to get a perception as to the scale of the climate changes we see here as has been discussed earlier in some of the comments the term climate change is very nebulous you need some kind of reference point to really your footing to understand what kind of changes are occurring so to begin i want to take you back to the year 1620 where in turkey there were significant problems this is in the midst of the little ice age we see at the beginning a series of droughts effect at the toward the end of the 16th century and then well into the 17th century in response to these droughts we start to see lots of internal rebellions uh uh primarily the chile acts in this case as these rebellions incur more and more and more that you see the ottoman empire investing more and more resources to combat these revolutions and thus dividing out royal power and on the periphery we start to lose control and here nomadic groups which include bedouins and tatars began to nibble away at former farmland and what you see is a large degradation of ottoman authority this quote from a chronicler at the time notes that there was such a cold the istanbul boss first froze and without ships many of the men crossed over ice to ushkandar galata and kasampasha on foot in that same city some men froze from the severe cold and died the earth was covered in snow famine invaded and the man who could get any bread for dirham counted himself lucky the historian sam white noted that uh uh climatic events were the uh or the one of the primary motivators for these events for the ottoman empire they noted that first you start with uh with unusually cold weather then you see increased turidity then you see famine as agricultural produce production drops you see the farmland less able to supply urban centers with food you see more destabilization of central control and then the periphery starts to nibble away and this effect actually and the fact and the effects of this period lasted for centuries one of the reasons the ottoman empire started so strong in the 15th century and lost a lot of its control in world affairs later on was because of the population stress from these time periods population in anatolia never really recovered from this point while populations soared in the rest of europe anatolia was left a little bit behind now i want to now direct our attention to the publication of the little ice age this is based on a quick entry into isi web of knowledge to see how many articles contain a little ice age in the title here we see the number is 7158 this makes sense we have lots of historical records it's a it's an event that has multiple climate climate records that overlap and agree with each other so we see lots of research on it a similar a much more severe event the younger driest much deeper in antiquity has about 9805 articles now let's come to the late bronze age collapse what we're here to discuss today 59 an isi web of knowledge this does not include many of the edited works by by colleagues in this room that are in that are in books but in terms of peer-reviewed articles we don't see the same kind of research focus on this event which is quite surprising because we see some of the greatest climatic impacts associated with this event so there's a clear demand for a lot more research and i think one of the reasons we see so much ambiguity today about this subject is so little primary work has been done it's really only the past five years that we see folks like kanyowski and like and lego deriving primary empirical data that identify this period rather than just the theoretical so just to give you a sense of scale and we'll see if this is a laser pointer it's not a laser pointer does this have a laser pointing function by chance uh it has to be youth okay never mind we'll be all right so in any case this is the uh uh just two temperature record for the past two thousand years you see the younger driest there at about eleven thousand and then the greek dark ages are one of those small little peaks there so here's an example where climate reference points are important in the vast scheme of things the events we see in the holocene are not very large climatic events certainly not in the context of the greater pleistocene so understanding what a little wiggle in the holocene means for human civilizations is a very important task and it's not one that's directly evident based on the climate records now typically speaking we see climate studies focusing on one of two types of ends first we see a focus on historically documented climatic events second we see focus on extremely severe climatic events that show up in multiple proxy records the things that fall in the middle the mid to low magnitude events that might have affected past human societies don't receive the same kind of attention particularly when historic primary historical records are not available now i want to turn to one of the ideas of our own current time which is anthropogenic global warming and uh based on the international panel on climate change one of the key problems is this how big does a climatic event have to be to severely impact a human society and this is a surprisingly difficult question to answer because there's no one data source that addresses this this requires integration from multiple fields now one thing i want to comment on specifically regarding what simon joseph said earlier even a small earthquake can cause tremendous damage to poorly constructed buildings so even a small period of climate change can have a severe impact on a population that is not prepared for any variation so it isn't just a question of how strong the climate event is based on our paleoclimate proxy records but how well prepared that society is to handle it whether it's through its agricultural system or its own infrastructure now the ipcc differentiates these terms out using two terms first magnitude this refers to the severity of the climatic event or process based on its own merits in a paleoclimate proxy record or in a global circulation model impact however refers to the effect that a given climate event has on a human society so we have magnitude and we have impact what i want to stress today is that magnitude is a question addressed via paleoclimatic records impact is one addressed by archaeology and understanding the cross-section between these two is critically important it is very possible to have a very low magnitude magnitude climate change have an outsized impact on a human society that is vulnerable to either that change of precipitation or that change in temperature now there's no question that an event like the younger driest would have a severe effect on human societies but unfortunately we don't have a comparable event affecting any kind of agrarian-based civilization so as such we have to drift our attention towards different types of records the problem is the kind of holocene fluctuations we see that have affected human societies are not the easiest ones to pick out in regional records so we run into a dilemma local climate records always provide better insight than global records specifically when we're talking about specific human societies however these records tend to be low resolution somewhat ambiguous and difficult to reconcile with human chronologies on the flip side we have global climate records that present high resolution and clear impacts throughout the northern hemisphere but these are not as easily relatable to specific areas microclimate is an extremely important factor and so it's really hard to go from the extremely general broad global models to something small such as a specific tell in israel or in greece for today i want to focus on comparing and contrasting the events of the late bronze age and greek dark ages with other holistic fluctuations to at least get a benchmark as to what kind of scale of the event we're looking at so as a consequence i'll only discuss global climate records today but this is not to suggest that they are in any weeds superior or even adequate to the question of specific archaeological impacts now as i'm sure will be covered quite extensively the next two days we see in the 12th and 11th centuries bc the eastern mediterranean is subject to abandonment and lots of population migrations uh you'll see a couple terms bandied around the first is collapse of individual cities and abandonment the other is migrations of people known as the sea peoples these two events are about separate at least the sea peoples are introduced about 50 years before we start to see wholesale abandonment but nonetheless it's generally agreed that they're that they're linked somehow uh and so as a consequence that opens up our scale for the time period we're looking at now the earlier work i i relied on when i first got interested in the subject was looking at sea surface temperature records this is mainly because most cyclogenesis in the mediterranean occurs within the mediterranean that is to say the mediterranean is its own moisture source as a result the the system is highly dependent upon sea surface temperature records if you have so to discuss mediterranean climate you have hot dry summers and cool wet winters in a hot dry summer you'll have warm sea surface temperatures but you also have a warm air that can retain that moisture and transport it some distance there one of the interesting phenomena is that the sahel of africa seems to get a lot of the precipitation that is evaporating out in the summer however if you have colder sea surface temperatures well in any way before i get too ahead of myself in the winter however the air pressure cools but the sea surface temperatures stay relatively resistant so as a consequence the air drops its moisture load a little bit more quickly and a little more locally so this is one of the reasons you see one of the one of one of the many reasons you see increased precipitation in the winter now this is predicated on the difference between evaporative potential of the sea and the atmosphere if we have colder sea surface temperatures we see less evaporation just as a colder cup of coffee is going to evaporate less than a warm cup of coffee and that has long-term impacts over precipitation cycles so a sustained decrease in sea surface temperatures would presumably based on what we know the mediterranean today affect its overall precipitation the problem that's not right because most of the temperature lost from the mediterranean is due to evaporation i'm not it's latent heat transfer so you you would expect your evaporation is potentially quite high even if your air temperatures are low but the the actual drop in temperatures related to the heat transfer to the atmosphere so it's an evaporative that's a that's a fair point that's a fair point thank you i'd also like to point out that even in making this kind of interpretation based on this data is very difficult for this time scale because if we look the distance in these data points we see entire abandonment of cities between two points of data so it's extremely hard to reconcile these chronologically point being these are potentially very insightful for climate in the region but the resolution isn't sufficient again this is one of the dilemmas we see with local climate change proxies being used in archaeological questions the resolution sometimes doesn't quite catch up so this is one of the many reasons i wanted to look at global climate change proxies for two reasons one they're sufficiently high resolution second we at least get a benchmark of comparison for better documented uh time periods in particular the little ice age here colored are the various events of the of the late bronze age collapse we see the introduction of the sherdana and the uh uh a carniac temple inscription we see the battle of kadesh these occur right about there and then we see the relatively rapid final occupation layers of multiple cities mapped out what you'll notice is that these cities occur during a period of rapid temperature loss in the northern hemisphere at least as recorded in greenland again this is greenland not the mediterranean so this is only an associational identification nonetheless we do see that in the northern hemisphere we do have relatively rapid temperature loss and as a fat as an interesting point if you measure from the top of that point of change at the bronze age climatic optimum to the bottom that is the sharpest temperature drop in the past 6 000 years in fact it's the largest magnitude event that has occurred in the past 6 000 years period in terms of beginning and ending temperatures so there was clearly climatic fluctuation going on in the context of the late bronze age collapse one of the things i'd like to highlight about this time period here colored blue is our period of the late bronze age collapse and in green the period associated with the greek dark ages in orange we see our descent from the medieval warm period optimum down to the little ice age which is colored in light blue you notice that both time in both cases you have a sharp temperature decrease but then you also have a plateau of lower temperatures following it what i'd like to point out is that human societies specifically those relying on urban development are very dependent on agricultural production so as a consequence relative changes in temperature can have a huge impact on them and at least in this regard the little ice age and greek dark ages share this as a similarity one other record i'd like to highlight is the intertropical convergence zone uh this is a band of cyclones mentioned by simon joserat that is driven by the warmest sea surface temperature records on earth it has been proposed that southward movement of this zone is one of the factors which weakens the indian monsoon one of the best records for this is the karyako basin which is an anoxic marine zone its varved sediments record variations in atmospheric 14c used in radiocarbon dating curves the reason for this is in an anoxic marine basin basin the primary input for sediments is often plankton which fall and die the photosynthetic plankton will incorporate 14c while they're alive and thus an anoxic basin means low carbon recycling which means you've got a decent marine you've got a decent body of evidence to look at atmospheric fluctuations in 14c however these same environmental parameters preserve other other records as well i'd like to highlight titanium concentrations in particular titanium concentrations in the karaoke basin have been argued to reflect river discharge the more rain you have over venezuela the more river discharge you have and thus the more titanium is transported into the karaoke basin conversely the less rain you have the less river discharge you have and thus less titanium the primary precipitation force in the zone is the itcz so uh uh folks have argued that this is one of the the ways to look at changes in the itcz so here we look at the record of the itcz our x-axis is titanium concentrations in the basin what we see are there are two periods when we have a systematic at least according to the authors southward displacement the itcz the first is during the little ice age the second is associated with the greek dark ages now this is systematic decline there's also another point uh in the uh around 400 to 600 a.d but not quite as severe so here too we see another similarity between the late bronze age collapse and the little ice age both see a systematic change in the intertropical convergence zone and once again to clarify same colors dark blue represents late bronze age collapse dark green indicates greek dark ages orange indicates the medieval warm period and then light blue indicates the little the little ice age again what we see is we see in addition to a sharp temperature drop and a plateau for northern hemisphere temperatures we also see a strong a sharp decrease in titanium deposited here which has been argued to be reflective of of southern placement of the itcz so in this case the collapse of lebron jade societies does occur in the context of yet another uh climatic signal an additional record that has been highlighted by uh by rolling minford and other folks is the ratio of potassium uh and just two ice records this reflects non-sea salt being deposited the argument is that um and correct me if i'm wrong the argument is that increased non-sea salt or potassium is reflective of an increased intensity for the siberian high what i want to focus on exclusively here is simply that we see two peaks for this non-sea salt being deposited in the just two record the first is during the time of the greek dark ages the second is during the uh the little ice age so again we see a parallel between these two periods i think it's north atlantic storminess that's been it's essentially picking up yeah no no the the potassium is is coming in this guy's work quite strongly it comes from china so the potassium it's the it's the sodium that's the north atlantic southern storm when it's in the sea ice cover there's sea salt and non-sea salt and they both reflect different yeah so in any case um the argument here is that this reflects an intensification of the siberian high specific to the potassium record and again this dynamic may have been similar between the greek dark ages and the little ice ages and a little ice age now the big missing data set and i would be thrilled if someone can point me in this direction is a solid proxy for the north atlantic oscillation that can extend back to 3000 to 3000 bp or the middle and late bronze age this is one of the largest climate forces in the mediterranean region but it lacks a really good long-term proxy record at least any that i'm familiar with now in lieu of that we can look at glacial advances in europe at least in the uh greater luche glacier glacier in the alps here we see the little ice age along the y-axis shows a more lower latitude placement of these glaciers so you so the higher these peaks the more gla the the stronger the glacial advance we see strong glacial advances associated with little ice age uh the early iron age of the greek dark ages and also that same period we saw earlier in 400 600 a.d so the same periods where we see the itcz shifting south based on the karako basin there's an overlap with these records and if you look at the jsp2 record we also see generally lower temperatures as well so i do think we're seeing broad changes in atmospheric circulation that tie this events we see in the little ice age with the events later seen earlier in the greek dark ages so in this i would argue that the greek dark ages are similar to the little ice age in that they represent low temperature plateaus relative to a preceding warm period they both occur contemporaneous with an unusually self-replaced position of the itcz and they both happen during intense activity of the siberian high now just for to compare i decided to line these two periods up we see at zero at the very top of the y axis is the absolute peak in just two temperatures and light blue for the media for the medieval warm age and dark green for the bronze age climate optimum now these are relative to each other the real difference between these two is about three degrees centigrade there's actually quite the bronze age optimum was considerably warmer than the medieval warm period at least based on just two but nonetheless we see the same basic pattern and interestingly a little bit similar timing where you have a sharp temperature drop followed by a plateau in this case the little ice age lasts last longer but then again we're defining the greek dark ages not based on cli on climate but rather based on human education on on archaeology of the region in any case what i'd like to point out is based on a measure of severity which is rate of change per year the greek dark ages are much sharper and higher magnitude than was the decline from the medieval warm period so if we consider the historical effects of this transition in europe especially in light of anatolia in the in the eastern mediterranean i think we have a rough analog for what we may have seen in the great dark ages with one important and major exception i would argue that societies in the medieval era were much better adapted and able to handle this than their bronze age colleagues primarily because the agricultural technology was far superior in the medieval era so i would argue that the lba collapse can be thought of as a rapid transition to the cooler greek dark ages relative to the warmer bronze age optimum same comparison this with the itcz just for a frame of reference we have a lot lower resolution in the in the late bronze into greek dark ages but we see that the greek dark ages were home to some of the sharpest changes in itcz position at least as reflected by karaoko basin i think more research will help clarify if this magnitude is an artifact of the data source or if it is measuring a real climatic factor but nonetheless you can see that these sharp changes would have had implications for precipitation and whether we're talking about more arid or more wet conditions the rapid transition to any climatic to any any any precipitation regime has implications for human infrastructure so i would argue that it isn't necessarily whether it's wetter or whether it's more arid that's more important but rather if the change is too rapid it's harder for populations to adapt their technology and their subsistence strategies to cope so in this case change not necessarily warm or wet or dry or wet is the enemy so provided that the karaoke basin titanium record accurately reflects itcz movement the late bronze age in greek dark ages showed considerably more severe variation precipitation patterns potentially so as a consequence the greek dark ages are generally colder and i would argue arid but i think uh hopefully there'll be a lot more clarity after these next two days regarding this but i would also argue that the rapid changes we see had negative impacts for human populations in this in the area now of course in the context of the holocene these are not high magnitude events these are very normal average changes we see throughout the holocene which is one of the most stable climatic periods we see in the past 800 000 years at least based on the epica record however they were they did have severe impacts on populations living in the eastern mediterranean again a perfectly small scale climate magnitude event would could easily be missed by a climate researcher because but be very noticed by archaeologists in the region now i want to also comment many sites show evidence for earthquakes being the source of destruction destruction relatively unambiguously but i think one of the key questions of the late bronze age is not how the sites were destroyed but why they were not rebuilt after their destruction whatever the source of the destruction humans earthquakes the inability to build back suggests there are significant changes to the way people are living now finally a discussion of process and event an event is a discrete is a discrete uh set of conditions a process is a longer term pattern of change i would argue that what we see with the late bronze age and the greek dark ages doesn't really qualify as an event per se at least not climatically certainly with regard to the archaeology the problem is we see an overall holocene trend towards cooling temperatures and so as a con and humans adapt to these cooling temperatures with the way they interact with the landscape so an event becomes a troublesome thing it's very hard to find evidence of an event but i would argue that we have clear evidence for a longer term process that humans are responding to so in any case not necessarily a specific event but rather the overlapping of several processes that create that unique perfect storms perfect storm as dr klein stays that had high impact on human societies at the time so historian sam white suggested that the climatic fluctuations of the little ice age resulted first in droughts then in rebellions and finally in sustained long-term depopulation within the ottoman empire and this is a permanent hint to the permanent hit to the ottoman empire it's not hard to imagine that the more severe conditions we see earlier during the late bronze age collapse and greek dark ages would have had similar if not more severe effects on those human populations now of course this isn't just an academic question the ipcc has proposed a warming of two degrees centigrade as a warming target for the world to meet based on our current rate of anthropogenic projections i would also like to note this is far too optimistic this assumes immediate changes to our carbon infrastructure i want to point out that the temperature change associated with the bronze age collapse was approximately two uh it was about two degrees centigrade as well so our most optimistic outlook for human response to anthropogenic climate change corresponds to one of the most severe impacts we've seen with regard to a climate event so it's worth considering uh that archaeology has should have a role to play with regard to how we set targets and what is acceptable in terms of our own climate changes in our own time and this is a key part of the discussion that i think is missing if we judge climate by magnitude alone we miss the factor of impact and impact is what's most important to us and just again to give a sense of scale here's the famous co2 record of epica combined with monoloa here you see the peaks are the interglacials the bottoms of the glacials this variation is what is normal for the pleistocene the uh the late bronze age collapse occurs at about zero roughly zero we're looking at a very large scale here but you can see there's very little happening there climatologically compared to what has happened in the past the co2 emissions we've seen in just the past hundred years take us well past even pleistocene variability so again with regard to our actions and our industry it's worth considering what kind of climate changes aren't just possible but can we actually handle as a society in any case thank you very much for your time and i'm hoping any questions you might have [Music] you

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Length: 26min 42sec (1602 seconds)

Published: Wed Sep 16 2020