history of our planet week 9 A

Quaternary interglacials and rapid climate change

Rapid quaternary climate change:

• Defined as: “occurring when the climate change system is forced to transition to a new climate state at a rate that is more rapid than the rate of change of the external forcing”: Committee on abrupt climate change – national research council 2002

• Glacial interglacial cycle driven by orbital (i.e. external) forcing c. 100 ka, Milankovitch cycles

Glacials and interglacials punctuated by sub-Milankovitch millennial scale events interstadials (and stadials)

Last glacial interglacial transition (LGIT) – not a smooth transition

Cold snap – about 1000 years cooler

The Younger Dryas:

• Abrupt climate change event

• Named after dryas octapelia – alpine/ tundra wildflower

• Found in Scandinavian lake sediments

• Indicates shift to very cold conditions

• Shift occurred in c. 40 years or less

• Duration c. 1100 years

• Temperature decrease: Greenland = - > 15*C, British isles = - c. 5*C

• Enough for remnant parts of ice sheets to start regrowing

• Glacial readvance (e.g. loch Lomond stadial) – till, moraines, erratics etc.

younger dryas recording

• First recorded in north Atlantic

• Other records: ‘global event’

• ‘glacial like’ conditions: Strong winter monsoon, Weak summer monsoon, Southwards ITCZ migration

• Sub-Milankovitch timescales

• May be internal forcing

Deglaciation and the younger dryas:

• Proglacial lakes formed along margin of Laurentide and cordilleran ice sheets (from the meltwater)

• Staged collapse of north American pro-glacial lake Agassiz. YD – at least 4 outburst events

• Freshwater influx, reduces north Atlantic deep water (NADW) production, reduces meridional (AMOC) heat transfer

• Cooling in high altitudes

• the influx of water went into Hudson bay etc.

• the salinity of the water is important – it disrupted the salinity levels of the north Atlantic

the younger dryas: an alternative hypothesis:

• Kinzie et al (2014, journal of geology)

• 25 sites containing metal spheres, shocked quartz, iridium and black mates of carbon – impact evidence?

• Coincided with onset of younger dryas

• Air burst or impact from one (or more) comets caused younger dryas

• Very controversial – misinterpretation, not reproducible (Pinter et al, 2011, ESR)

• Demise of Clovis culture (a group of people) – sudden disappearance from archaeological record ( 13,000-11,000 yr BP) – coincides with younger dryas

• Large scale megafaunal (typically mammals over 40 kilos) extinction

• Overkill? (by the clovis people – so ran out of food – but problematic and unscientific)

• Climate change?

• Links with younger dryas?

• Something else e.g. meteor

Rapid quaternary climate change: summary:

• Glacial interglacial cycle (orbital forcing, external) punctuated by rapid/ abrupt climate changes (suborbital forcing, internal?)

• Rapid reorganisation of ocean and atmospheric processes

• LGIT- centennial/ millennial scale changes driven by meltwater pulses (e.g. younger dryas)

• Societal timescale

Holocene:

The current interglacial c. 11,5000 to present

Why study the Holocene:

• We are in it

•   Period of relative stability

• Punctuated by series of climatic events

• Context for recent anthropogenic change

• Evolution of modern civilisation

• Palaeolithic, 2.6 ma – 10 ka

components of holocene

• Neolithic revolution

• Dawn of agriculture

• Explosion of culture, technology, art

• Dramatic population increase

• Dawn of Anthropocene?

Range of potential forcing mechanisms:

• Orbital – Milankovitch: Insolation – amount of solar activity reaching a point of earth due to changes in Milankovitch cycles

• Suborbital (sub-Milankovitch): Solar activity, Volcanic activity, GHGs (natural vs anthropogenic), Comets

 

Characterising the Holocene: 1

• LGIT: increased summer insolation at northern latitudes

• Rapid warming – warm summers, cold winters

• Thermal maximum – warm summers, cool winters

• Cooling – cooling summers, warming winters

• System is very simplistic but this broad pattern is seen in climate records globally (wanner et al, 2008)

Characterising the Holocene: 2

• Milankovitch forcing – smooth

• What drives rapid, abrupt climate change beyond the YD?

• Sub-milankovitch timescales

• Internal forcing? Atmosphere-ocean circulation

• External forcing? Solar activity, meteors

The 8.2Kyr event:

• Post younger dryas: absupt climate change event - one final glacial meltwater pulse

• Within Holocene – established interglacial

• Significant cold snap in Greenland ice cores

• Daley et al (2011, Glob, Plan. Change)

• Evidence across north Atlantic region

• Consistent with meltwater event

• Greenland = - 6+- 2*C

• Duration of 150 +- 30 years

• 15% reduction in CH4

Abrupt Holocene climate change:

• Holocene – a period of relative stability? Considerable climatic variability

• Absence od orbital forcing changes? Absence of deglaciation?

• What drove changes/ events

  In Pleistocene:

• Dansgaard-oeschger warming events

• Heinrich cooling events

In Holocene:  

• Glacial meltwater pulse events (e.g. younger dryas, 8.2kyr event)

• Ice rafted debris events, known as ‘bond’ events, after Gerard bond

Bond events:

• Periodicity? 1470 +- 500 years (like D-O events)

• Associated with colder prevailing conditions in north Atlantic region

• Linked with several Holocene climate events (e.g. 4.2kyr event)

The 4.2 kyr event:

• Pronounced ‘dry’ event across mid to low latitudes of Europe, chine, Africa. North America (Weiss and Bradley 2001, science); wetter in tropical south America and Asia? North Europe?

• Associated with societal disruption and collapse of ancient civilisations

• Timing coincides with Bond (IRD) event 3?

Holocene climate drivers:

The last 1000  years:

• the hockey stick

• Growing influence of humans over the last 100 years (or more)

The Holocene summary:

• Appears relatively. Climatically stable

• Complex regional patterns of climatic change

• Large number of records (proxy, instrumental) enable comprehensive study

• Range of orbital (Milankovitch) and sub orbital drivers

• Large, short term variation – possibly cyclical – solar?

• Notable climatic events – effects on early civilisations

• Last 1000 years – growth of anthropogenic climate forcing

Quaternary interglacials:

• Glacial interglacial cycle initially dominated by obliquity (tilt, 41ka cycle)

• Last c. 1 million years dominated by eccentricity (shape, 100ka cycle)

• Last c. 1 million years – glacial interglacial cycle became more pronounced, more cyclical, particularly in last 600,000 years

• Most recent = most evidence; reflected in quaternary stratigraphy

MIS 5E – the last interglacial:

• MIS 5 – subdivided into 5 substages

• MIS 5e – ‘ipswichian’/ ‘eemian’ (130-115ka)

Evidence for past interglacials:

• MIS 5e – ‘ipswichian’ interglacial

• Lake and peat records etc. are rare

• Archaeology and sedimentology evidence

• Distribution across southern Britain

• Generally beyond maximum ice extent at LGM

• River terraces: Rivers respond to interglacial-glacial cycle: i) inclusion, ii) aggradation, Law of superposition, Contain archaeology, pollen, Trafalgar square, London: classic last interglacial (ipswichian) site (thames terrace deposits, hippos and hyenas)

Ipswichian flora:

• Pollen preserved in variety of sediments (including river terraces)

• ‘indicator’ species provide evidence of warm continental climate (pollen and plant macrofossils)

• Najas minor (brittle waternymph)

• Trapa natans (water chestnut)

• Acer monspessulanum (Montpellier maple) – mediterranean species

Humans in MIS 5E: Hominin evidence:

• Hominin fossils

• Archaeological industries

•   Evidence of butcheries

Humans in MIS 5E:    So where were the humans?

• Homo sapiens

• Homo neanderthalensis

• (homo sp. Altai/ homo sapiens ssp. Denisova)

MIS 5E: a good analogue?

• Compilation of global temperature records, 130-116kyr

• Average 1.5*C warmer than today; regional patterns

•   Potential analogue for future 2*C climate stabilisation scenario

• MIS 5E was: Warmer, Wetter, Higher sea levels, Smaller ice sheets, Lower atmospheric CO2

• Why?: Greater axial tilt (obliquity), Perihelion in NH summer (today- NH winter)

• Laster c. 1.5ka, Holocene: 11.5 ka

MIS 11: a better analogue?

• Eccentricity: 400ka cycle

• Orbitally, MIS11is a better analogue than MIS 5e

• Low eccentricity: ‘rounder’ orbit; coupled with reduced processional amplitude: Fewer cold stages within the interglacial, Resulting in prolonged warm period

• MIS 11: 428-397kyr ago – 31,000 year interglacial

• No anthropogenic influence, so can only tell us what ‘should happen’

• Would the Holocene have been a ‘long’ interglacial regardless of humans?

• Less evidence from MIS 11 (i.e. older)

Summary; MIS 5e

• partial analogue for a “2*C world”

• Higher temperatures, especially in the arctic

• Higher sea levels – 8m higher

• Significant impact  on biodiversity/ biogeographic boundaries

• but: Different orbital forcing conditions

summary: MIS 11:

• a better analogue?

• Closer to Holocene orbital forcing conditions

•   Long interglacial?