The dynamics of abrupt climate change

Question 1

Sub-Milankovitch Events (stadials and interstadials) -> climate oscillations which take place separate to orbitally forced Milankovitch cycles

Answer 1

they tend to be stronger across the NH in the North Atlantic Ocean as it has an antiphase relationship with the SO

Question 2

Dansgaard-Oescherger (D-O) Events -> periods of rapid warming 8-16 (Capron et al., 2010) followed by slower periods of cooling every 1.5ka (Dansgaard et al., 1989)

Answer 2

e.g. Allerød interstadial was proceeded by the Younger Dryas Stadial (12.5kyr) (Lowe and Walker, 2015)

Question 3

Henrich Events -> influx of meltwater into the North Atlantic Ocean from e.g. Laurentide Ice sheet -> weakens the THC causing cooling -> later followed by warming back to normal temperatures

Answer 3

led to the Younger Dryas in Henrich Event 0 (H0) (Andrews et al., 1995)

Question 4

evidence for Henrich events

Answer 4

iceberg rafting debris sediment in marine cores around the North Atlantic Ocean (Henrich, 1988)

Question 5

Bond Cycles -> sequence of D-O events until the warming causes a Henrich event by destabilising ice sheets (Bond et al., 1992; Bond and Lotti, 1995)

Answer 5

MIS 3 (60-28kyr) -> there were multiple D-O events (Capron et al., 2010)

Question 6

Periodicity not fixed for D-O events

Answer 6

MIS 5 = interglacial but possessed multiple stadials and interstadials which did not fit the 1.5ka e.g. D-O events 13 and 17 -> e.g. methane concentrations in NGRIP (Capron et al., 2010)

Question 7

Rapid Climate Change Events -> occur between stadial and interstadial events

Answer 7

identified in the NGRIP ice core (Anderson et al., 2010)

Question 8

Younger Dryas 12.5kyr -> stadial triggered by Henrich 0

Answer 8

Laurentide Ice sheet led to meltwater influx into the North Atlantic (Anderson et al., 2010)

Question 9

8.2ka -> 2-4 cooling over the NH and impacts of the SH

Answer 9

Laurentide ice sheet melted -> Lake Agassiz formed and overflowed into the Northeast Atlantic Ocean = outburst sediment from the lake as evidence -> THC changes (Wiersma et al., 2011; Li et al., 2012; Lewis et al., 2012)

Question 10

Little Ice Age -> cooling across the 16-19th century -> irregular intervals

Answer 10

thought to be a result of sunspot activity and volcanic activity (Miller et al., 2012)

Question 11

causes -> sub-orbital forcing on a 1.5-2kyr cycle

Answer 11

no real evidence (Bond et al., 1997)

Question 12

causes -> binge-pure hypothesis (Capron et al., 2010)

Answer 12

Stable ice sheets increase friction by pinning the ice to bedrock but as geothermal heat is produce ice sheets become unstable leading to basal melting, lubrication and surging (Ruddiman, 2013), or ice sheet compression leads to ice sheets eroding to sea level causing the ocean to pull it off its bedrock pinning point -> surging increases as sea level increase = positive feedback

Question 13

causes -> tropical driver hypothesis (Rahmstorf, 2002)

Answer 13

changes to planetary waves across the tropics alter SSTs and oceanic circulation

Question 14

causes -> wind field oscillator hypothesis (Schmidt and Hertzberg, 2011)

Answer 14

modelling of the start of a D-O event with deglaciation and warming simulates a shift in the subtropical Jetstream over the Rocky Mountains from a zonal to a meridional flow contributing to the sub-Milankovitch climate oscillations (Schmidt and Hertzberg, 2011).

Question 15

causes -> salt oscillator hypothesis/see-saw mechanism (Broecker, 1998; Bluiner and Brook, 2001; Clark et al., 2002; Gottschalk et al., 2015)

Answer 15

Brine rejection, evaporation and the supercooling of water under ice sheets leads to downwelling off the coast of Antarctica producing the Antarctic Bottom Water (ABW), and in the North Atlantic produces the North Atlantic Deep Water (NADW) (Clark et al., 2002; Gottschalk et al., 2015). The Atlantic Meridional Overturning Current (AMOC) forms as warm, fresh water replaces the salty, cold downwelling water in the North Atlantic and Southern Ocean (Clark et al., 2002). The influx of meltwater into the North Atlantic decreases the salinity of the water, causing the NADW to halt as downwelling ceases = cooling across Greenland and warming across Antarctica since the AMOC cannot activate without the NADW forming, meaning warm, equatorial water accumulates in the Southern Ocean -> increased evaporation across the SO eventually increases allowing the ADWC to restart and then the AMOC/NADW causing localised cooling across the SH and rapid warming over the NH resulting in D-O events (Bluiner and Brook, 2001; Broecker, 1998).

Question 16

evidence for see-saw mechanism (1)

Answer 16

(Ganapolski and Rahmstorf, 2001) -> modelled the asymmetrical response through inputting freshwater into the North Atlantic

Question 17

ice cores - multi proxies -> useful for validating sub-Milankovitch events

Answer 17

Antarctic Ice Cores = 800,000yrs, Greenland Ice Cores = 130,000, Vostok Ice Cores = 420,000

Question 18

evidence for see-saw mechanism (2)

Answer 18

GRIP and Vostok = methane concentrations -> Antarctica reacts faster to climatic changes with Greenland lagging behind by 1.5-3ka and 8th and 12th D-O event Antarctica warmed prior to Geeenland (Bluiner and Brook, 2001)

Question 19

evidence for see-saw mechanism (3)

Answer 19

GISP2 and Byrd -> methane concentrations = rapid warming and rapid cooling in Greenland but in Antarctica there was slow cooling and then rapid warming (Bluiner and Brook, 2001)

Question 20

evidence for the see-saw mechanism (4)

Answer 20

Antarctica would warm when Greenland would cool -> GRIP and Droning Maud Land ice cores = methane concentrations (EPICA members, 2006)

Question 21

evidence for the see-saw mechanism (5)

Answer 21

research used O-isotope ratios to reconstruct ocean temperatures -> MIS 3 -> AMOC was found to be the driver of the climatic change but could not determine what caused the addition of freshwater (EPICA members, 2006).

Question 22

Marine sediment cores -> lengths of 0-60Ma and are readily available -> multiproxy containing isotopes and other indicators

Answer 22

North Atlantic Ocean favourable for Milankovitch evidence as sedimentation rate is higher

Question 23

evidence for Heinrich events (1)

Answer 23

in the NA ice rafted sediment debris tended to increased periodically -> -> 1,400yrs, 2,800yrs, 4,200yrs, 5,900yrs, 8,100yrs, 10,300yrs and 11,100yrs -> VM 29-191 marine core was a key piece of evidence with ice-rafted debris records (Bond et al., 1997)

Question 24

evidence for Heinrich events (2)

Answer 24

sediment tended to be found 40-50N indicative that the Laurentide ice sheet played a role (Bond et al., 1997; Bond and Lotti, 1995)

Question 25

evidence for Heinrich events (3)

Answer 25

coupled atmosphere-ice interactions = identified that a drop in atmospheric temperatures causes the Greenland ice core to contain more soluble impurities -> occurred for the Little Ice Age, 8.2ka and Younger Dryas (Bond et al., 1997).

Question 26

evidence for Heinrich events (4)

Answer 26

AMOC behaviour by analysing carbonate production -> lower Circumpolar Current near the ABW is low in carbonate while the NADW is high in carbonate -> possible to analyse carbonate build up to then determine which currents dominate through which periods -> D-O events have strong carbonate concentrations = NADW re-establishing -> Heinrich events have carbon disillusion = NADW weakening (Gottschalk et al., 2015)

Question 27

evidence for Heinrich events (5)

Answer 27

Pollen records imply environmental shifts in line with Heinrich events and ice-rafting debris e.g. in Florida pine pollen was more copious during warmer, wetter conditions, while grass and oak pollen was more common during drier, colder conditions (Ruddiman, 2013)

Question 28

Lake cores -> vary in length between 100-10ka to 2Ma, tend to be temporal and are most informative when located in closed systems

Answer 28

sedimentation rates are variable but average around 1cm/10yrs, they contain O-isotopes and C-isotopes.

Question 29

Tree rings -> very small range, tend to be located where tree species last longer e.g. Sequoias

Answer 29

1 ring = 1 year producing a high temporal resolution, ring no/thickness informs precipitation rates while C14 produces dates for environmental shifts

Question 30

Hyrax middens -> 20ka, low latitude regions e.g. southern Africa, pollen, and geochemical elements in the excrement

Answer 30

local and regional environmental conditions informed

Question 31

future climate change

Answer 31

there are no yet certain conclusions regarding the future of the THC -> speculated that greenhouse gas concentrations will lead to a weakening as the ocean will be expected to hold more energy -> unsure how this will materialise (Clark et al., 2002).

Question 32

Teleconnections -> North Atlantic Ocean is leading to the weakening of the relationship between ENSO and the Indian Ocean Monsoon from the 1970s (Chang et al., 2001).

Answer 32

When ENSO impacts the Indian Ocean Monsoon -> impacts of Indian Ocean Dipole either amplify or oppose ENSO -> therefore important to analyse future changes (Chang et al., 2001).