millenial scale vriability (younger dryas n heinrich events)

polar ice cores neccesity for millenial scale variability

deep-sea sediment cores give smoothed, low frequency representation of global ice volume bcs low sedimentation rate, however polar ice cores give sub decadal high res records. Greenland ice cores reflect high lat N hem atmospheric n regional conditions, whereas antarctic track S hem conditions. Records show that rapid warming in greenland corresponds to slow cooling in antarctica and vise versa, this is bcs of AMOC variations and interhemispheric heat distributions.

Lowe and Walker, 2014

Younger Dryas (YD) Stadial

key characteristics

changes it caused

trigger mechanisms

12.9-11.7 Ka BP, abrupt intnese cooling event, reversed warming of deglaciation, named frm arctic wildflower that re appeared across N europe sed layers in this interval, showing shift to tundra.

• Rapid onset, greenland ice core show sudden temp drop within decades n transfer out occured faster (potentially within a decade).

• Glacial re-advance in Scotland. Coined Loch-Lomond re-advance with extnesive ice caps n valley glaciers during the cold stadial (Dawson, 1992).

• polar front migration. Marine proxy reconstructiosn show pre YD winter-sea ice limit n polar front were high lat, during peak YD polar front shifted south drastically, pushing winter sea ice limit down to the med, capping the N atlantic w ice to block solar radiation. This also cut off heat transport to west europe. Post YD the polar front retreated rapidly northward back to its modern high-latitude configuration. (Ruddiman and Mackintyre, 1981)

expand dawson and add extra source for YD

triggers:

Laurentide ice sheet warming created proglacial Lake Aggassiz, lake outburst injected lots freshwater via Mackenzie River to Arctic n N atlantic. lots freshwater lowered surface density/salinity, weaker downwelling, weaker AMOC, stopped northward heat transport. Murton et al, 2010 identifed 25km long canyon cut into devonian limestone, paleo-waterfalls wider than niagra falls, extensive gravel fields indicative of major outburst as evidence of the megaflood.

Heinrich events, represent major ice-sheet instability thru anomalous coarse lithic fragments n dropstones in fine-grained pelagic ooze (CaCO3, SiO2) recorded in distinct heinrich layers in sediment horizons. Isopach maps show equal thickness of sediment, showing a plume that thins out frm coarse labrador sea seds. This tracks the ice bergs frm the hudson strait as a source, which drained from the core of the Laurentide ice sheet. (Lowe and Walker, 2014)

source for the isopach maps (Ruddiman, 1977) identifed the ruddiman belt frm labrador to hudson.

These r linked closely to global millenial scale variations: correspond to peaks of cold-seeking planktonic forams thus colder sst at time, also see bond cycles emerge when superimpose onto greenland delta o18 record. progressively colder Dansgaard-Oeschger (D/O) cycles. Weakend AMOC creates cold D/O stadial after meltwater pulse (heinrich event), as fresh pulse fades circulation rebounds to allow stronger deewater formation n flip into warm DO interstadial. These feature under Bond cycles which encompass ocean-ice coupled feedbacks driving surface variations (Agosta and Compagnucci, 2016)

whats the anthropocene

markers of anthropocene

early humans impact

new geological epoch dominated by human activity within the holocene, which leaves permanent signature on earths stratigraphic record, starting at the Industrial Revolution for major change but humans left an impact since first presence.

enough concrete cover surface 2mm layer n enough plastic to wrap in film (Lewis and Maslin, 2018)

farming began 11,000yr BP southwest asia then global , industrial revolution 1760 in NW europe to global, (Lewis and Maslin, 2015)

Lewis and Maslin, 2015 proposed anthropocene start dates thru global stratotype section and points (GSSP) identifying Orbis GSSP where drop atmospheric co2 frm 270-280ppm w collapse of indigenous americas populations n widespread forest regeneration 1610 AD. And Bomb GSSP (1946 AD) where nuclear weapon detenations n peaks atmospheric radiocarbon and persistent industrial chemicals

early humans.

• australia fire. Humans in austrialia 50-70ka BP used fire to change vegetation n modify resource availability, impacting ecosystems. Turney et al, 2001: high res lithostratigraphic and pollen/spore data frm volcano craters shows dramtic transition frm rainforest to taxa to surge in fore tolerant eucalyptus trees n increase in charcoal. However the works of Mooney et al, 2011;Bird et al, 2024 argue charcoal is inline w natural climate cycles of MIS and human burning didnt occur on that scale till post 11ka BP.

• megafauna extinction: between 50-10 Kyr BP loss of 178 large mammals. Some argue efficient human hunters did this, w sites of mass kills in Europe where over 100k hordes dead in burgundy France where ran off steep cliffs (Combier, 1955) Other research shows extinctions mirror human migration routes, w australuia n N america reaching over 80% extinction rates when humans arrived, whereas africa only lost 20% (Andermann et al, 2020) w statstucal mdoels that show 96% accuracy for human models to predict extinctions. However radiocarbon dating suggests megafaunal extinctions staggered over millena not instant, w more relation to stadials n interstadials (Stuart et al, 2004) who found this via radiocarbon dating on megafaunal bones, showing wooly mammoth n others died w changing vegetation balances.

• agricultural revolution. 11-13ka BP. shift frm mobile hunting to sedentary farming due to population pressure n advance in knowledge. feedack loop where once start farming remain bcs more mouths to feed bcs more kids when more food available. Fertile river crescents were hotspots for this. creating land use changes on widescale (Ruddiman, 2003) who showed link between rice farming and co2/methane spiking across southeast asia 8000 years BP. linked to massive clearning of primary rainfoests and emissions from rice paddys.

historical data records for climate reconstructions

workflow

indirect: documentary proxies (environmental observations in diaries, crop harvest dates, pictures depecting environment)

direct: instrumental observations, usually systematic/quantitative (ship logbooks, Central England Temperature (CET) record)

CET frm Manley 1974, that began in 1659 to capture events like little ice age.

workflow:

• standardisation via content analysis, where descriptive language is ranked n converted to create clear numerical index that defines weather conditions and severity.e.g unbearable summer heat vs hot weather.

• calendar corrections, account for shifting calendar styles across diff regions, with correction from julian to gregorian calendar across europe which gave a 10 day shift.

• check for bias/false claims, some records focus on extremes to justify tax exemptions or aid requests or reflecting recorders emotions.

• source reliability, artists copy others work rather than visiting themselves.

• calibration. historical descriptive entires r ranked and categories via a grading scale from warmest to coldest, then look for periods where documentary records overlap w instrumental climate measurements. Modern instrumental data r converted to the same simplified category and regression models calculate the relationship between records and actual values to help reconstruct back to pre-instrumental era.

(Brazdil et al, 2005)

Ship logbooks

phenological data

pictures

naval officers in English East India Company would record detailed weather observations at set times for navigation and safety. These have high spatial n temporal precision w co-ordinates n set calendar dates/times. These help fill spatial data gaps as record across remote ocean basins w no land-based proxies, using all quantitative data. Hannaford et al, 2014 used wind data from climatological database of worlds oceans and english east india company databases as most logbook observations were around trade routes (following global wind patterns). Rainfall over south africa heavily linked to wind, found link between wind direction/force n rainfall in modern era and used that to reconstruct past rainfall. Found evidence of El Nino patterns with wet eastern sites n dry western that reversed short time later.

• however moving ships dont give continous measurements from the same location, thus hard to judge change at a single site and also requires time expensive stitching of records from thousands of journeys from different vessels.

• in an extreme storm the ship crew focus on survival rather than measurements, thus noon records are blank.

• spatially constricted to trade routes.

french grape harvest dates (Labbe et al, 2019), used records of harvest dates frm local agricultural tax records that date back to 1354. warm/sunny years allow early harvest whereas cold/wet delays harvest dates, thus reconstructions of records shoed stable harvest dates apart from 1998-2018 where shifted 13 days earlier bcs of rapid human induced warmings.

• However local authorities controlled when farmers could harvest grapes, with dates depending on labour, religous holidays, military conflicts, changes of governance etc which skew the climate timeline

• Also changing farming practises or plant diseases can completely change harvest dates like the insect infestations in 19th century europe,

• harvest dates for grapes depend on growing season temps (april-august) thus winter and autumn weather has no relation and cant be reconstructed.

Collection n dating of over 300 historical pictorial documents to compare them agaainst modern topographical features n map exactly how far glacier tongue advanced/retreated in peak of the little ice age. Used realistic pictures to see speciic rock formations, terminal moraines, position relative to buildings etc. this gave high resolution data on little ice age glacial dynamics and allowed cross verification to modern moraines. Allowed them to map out the rapid short term advances in consecutive years from cold summers (Maunder minimum and volcanic eruptions that made the little ice age) that moraine dating cudnt show. Identifed precise long term glacial fluctuation curves.

• However artistic subjectivit made some artistis in roman era exagerate scale of nature to evoke a sense of awe.

• Dating uncertainties w the paintings, most werent time stamped at the time of observation. picture created at one point could be based off a sketch from 20 years prior, creatibg uncertainties in dating timeline.

• many art was copied from other glacial art rather than visiting themselves, to create non-accurate portrayals of the environment.

• also this only works for dramatic geographic features in regions w history of landscape art, it cant be used for remote polar ice sheets or remote ocean locations.

• (Zumbuhl et al, 2008)