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Explaining Quaternary Climate Change: Sub-Milnkovich Phenomena (Rose…
Explaining Quaternary Climate Change: Sub-Milnkovich Phenomena
Introduction
The Vostok ice core provides evidence of shorter period fluctuations within the 100,000 year glacial cycle (the ‘stadials’ and ‘inter-stadials’)
Suggests the influence of shorter wave-length Milankovitch Cycles (especially the ones at c80kyr and 100+Kyr before present).
However, there's also individual or repeated short-term fluctuations preserved in ice-core records, sometimes dramatic that
cannot be explained by reference to the Milankovitch Mechanism
These are called Sub-Milankovitch Phenomena
Greenland provides good information on these - take, for example the dramatic Younger Dyras fluctuations (11.5Kyrs BP) which led to a prolonged cool period - NOT part of the 100 or 40 thousands year cycles
The significance of these cores is that they have all revealed a detailed picture of the complex pattern of fluctuations in temperature during the last glacial: changes in temperature that are repetitive, often extremely rapid and of significant amplitude
Greenland:
Ice cores here go back up to 140,000 years
E.g. GRIP: 120,000 years (Rose, 2010, Figure 9)
Penetrated up to 3km in some cases
In the past 120,000 years, there are around 25 different interstadial warmings - temperatures must have fluctuated by between 9 and 15C in Greenland, within the space of a few decades - The initial warming at the start of the holocene may have taken less than 20 years (
Walker and Lowe, 2007
)
E.g. GISP (Archer, 2007) shows the past 100,000 years, and details the DO events occurring between around 60Kyr BP and 25Kyr BP (NB: They pin Holocene climate on the development of agriculture)
Greenland versus Antarctica:
While such phenomena are present in the ice-core records for the last Glacial – Inter-Glacial Cycle from Vostock, Antarctica, they are far better developed in the Greenland cores (e.g. GRIP, NGRIP, GISP2) due to the more
rapid rate of ice accumulation
--> time is more stretched out in cores --> more detail to observe
The Greenland cores show the same gross pattern as the Vostok core but with much more
internal variability
, indicating that Greenland experienced a different and
more dynamic climatic regime
during the last glacial than did Antarctica. This was due to the more
variable influence of the Atlantic Ocean
What the Greenland (and Antarctic) Cores Show
The evidence for
frequent abrupt changes in temperature
Evidence for rapid temperature oscillations (Dansgaard – Oeschger or
DO Events
where there's a rapid warming followed by a more gradual cooling) which can be grouped together into Bond Cycles (shown in the GRIP core)
2 cycles between 44Kyr BP and 30Kyr BP - 4 DO events in each
NB DO events only really seen in N. Hemisphere
Evidence of quasi-periodic, dramatic, iceberg-rafting events associated with the coldest atmospheric temperature conditions immediately prior to a rapid warming (
Heinrich Events
) of around 5C change in ocean temperature in a matter of decades (shown in GRIP and aided by sediment cores around Greenland showing 'dropstone' events from melting icebergs
isotopic temperature:
37% of variance explained by 100kyr (eccentricity)
23% explained by 41kyr cycle (obliquity)
11% explained by precession
Variability of insolation ~7% here
Above equates to 71%
Sub-M Phenomena in the N. Atlantic
DO Event and Heinrich Event Relationship
Proved problematic, because the ice-berg rafting events occur just prior to abrupt atmospheric warming episodes
recent research into why some Antarctic ice-shelves are disintegrating may supply the answer (
Bintanja et al. 2013
)
In western Antarctica,
This destabilises them and leads to the generation of flotillas of ice-bergs within a cold surface layer of melt-water
This keeps the ice-bergs from melting away by insulating them from the warm water at depth
This process now appears to be endangering many of Antarctica’s ice-shelves
This model (see stages 1-9) can be
applied to Greenland
during the last glaciation to explain why ice-berg formation (H) preceded air temperature rise (DO). Periodic
surges of the Gulf Stream would have brought warm water northwards
beneath the extended Greenland ice-shelves, melting the base of the ice to create ice-bergs floating in a surface layer of cold water (Heinrich or ‘H’ events)
Only subsequently, when a greater influx of warm sea could warm the air and so influence air-flow temperatures over Greenland, was a D-O event created
Heinrich Events
Glaciers essentially cast rivers of ice that slowly flow down hill to the sea - drawn by gravity and weight of ice accumulating inland
Rate of flow slowed by friction of rough ground beneath
Eventually meets sea - here, because ice is less dense than water, it floats --> forms an ice shelf
Point where it leaves ground = grounding line
Without the slowing effects of friction, the glacier's flow accelerates
New evidence shows grounding line has been steadily retreating for decades in Antarctic locations (Pine Island Glacier - responsible for 25% of melting)
Pine Island Glacier's grounding line has retreated >40km since 1992
Currents of warmer water melt the ice from beneath --> undermine and thin the glacier
NB: at this stage, warm water from the Pacific penetrates 100m below SL and the stress is compounded as the SL tide rises and falls
As less of the glacier is in contact with the ground, less friction --> ice held inland above SL spreads forward and thins --> raises SL
Further worsening the issue, the land behind the grounding line in West Antarctica is BELOW SL
Therefore, when the low-lying valleys are breached, the undermining will be unstoppable
Because the ice is thickest furthest inland, there's more mass to propel the glacier forward
There's also no large ridge nor mountain to act as a barrier to block the glacier's flow
Eventually, the glacier will have nothing to 'cling' to and will break up entirely
The Younger Dryas Period (dated at 12,846 BP in Greenland but 12,670 BP in Europe)
Lasting for about 1300 years and resulted in considerable changes to vegetation and hydrological conditions
including the re-formation of glaciers and a small ice-sheet in parts of upland Britain (which was free from ice at 14.5kyr BP)
Recent studies suggest its onset was just 1 year (2009 evidence from N. Ireland)
Previous studies thought >6C temperature decrease per century
Cause:
Believed to be a suddenmajor reduction in the warming influence of the Gulf Stream / NADW due to the
catastrophic discharge of cold melt-waters into the North Atlantic from North America
Strahler and Strahler, 1996
Impounded pro-glacial lakes were suddenly released to decant into the nearest ocean
Broecker et al (1989) outline the release of flood waters via the Gulf of St Lawrence which may have led to the decline in temp.
From this, it is clear that some of the changes identified in ice-cores do not have to be synchronised across the Globe, because local/regional changes in past conditions did not have to have uniform impact over the entire Globe.
Hence, the North Atlantic Region was probably prone to greater climatic variability during “Glacial Phases”, “Glacial – Interglacial Transitions” and possibly even at other times.
More Widespread Implications
Much of the regional evidence for climate change was fragmentary in the past, but the new paradigm based on the study of ice and ocean cores is beginning to provide the basis for a coherent interpretation:
Tjallingii et al (2008)
constructed a “humidity index” for the western parts of the Sahara and Sahel for the past 120,000 years
By measuring the variation in the relative coarseness of sediments (the coarser, the more arid) preserved off the Mauritanian Coast, they were able to construct a pattern of humidity change
This was then linked to evidence of changing sea surface temperatures (SST) provided by another core off the coast of Portugal, and then back to the Greenland ice-core NGRIP and associated North Atlantic cooling events (Heinrich Events)
This revealed that the changes in humidity, and therefore vegetation cover, in NW Africa in the later Pleistocene were the product of the interplay of ocean cooling events and the Milankovitch Mechanism
Should there be major melting events along the coastline of Greenland in the near future, then the same could occur
Tjallingii et al also found sea surface temrpature drops occurred in time with Heinrich events - therefore, made the link that the sheared ice bergs cooled water which was then carried down to the Eastern Atlantic
The cores of the coast of Africa show increased sediment during wet phases where rivers and streams carry sediment to the sea - during dry phase, you get sand, during wet phase, you get finer silt --> look at ratio of coarse versus fine sediments to map humidity
From this work, we can now link the change in ice sheets to what might happen to African vegetation levels in the future
it's the ratio of aeolian dust and hemi-plagic mud; if there's low EM2 to EM3 - indicates a positive humidity value - you need humidity and precipitation to crete streams to deposit EM3
15cm of dust represents about 1000 years in the cores
Heinrich events --> shutdown of AMOC --> no longer moist water vapour coming from west tropics to Africa --> increased aridity
How do Milankovitch Cycles impact humidity?
Periods of maximum precessional insolation --> ITCZ migrates north --> increased monsoon strength
Greenland versus Antarctic Picture:
A reflection of geographic location:
Antarctica is an isolated “Island Continent” surrounded by cold oceans
Greenland is located in a “Continental Hemisphere” and subjected to the variable influence of the North Atlantic Ocean and the products of volcanic events
NB: Such short-term variations appear elsewhere over the Globe where there are fine resolution records (Alley, 2000), indicating that short-term fluctuations are a feature of the Pleistocene climate due to:
Variations in atmospheric dust load due to volcanic activity
Changes in the strength and pattern of warm ocean currents.
Periodic cooling of ocean surface waters due to melt-water discharge from adjacent land areas (NB as meltwater is lighter than sea-water it forms a surface layer).
Rose (2010)
Introduction
Aims to outline causes, scale and style of ice ages
Describes the Milankovitch forcing mechanisms and used to explain why climate variation within an ice age is relatively predictable
Describes how the sub-M climate variations are superimposed on these predictable changes
Describes the relatively constant climate of the past 11.5ka (Holocene), via human's input of GHGs to the atmosphere
History:
Cooling began a little before (2.75Ma) the start of the Quaternary (2.58Ma)
Work from Ruddiman (2001) provides analysis of ice sheet formation in history compared with the distribution of the continents
There have been 3 intervals of glaciation in the past 500 million years
The most recent period of cooling began 35Ma when Antarctica froze over - but didn't strike more temperate regions until 2.75Ma
Causes of Present (2.75Ma) Ice Age
Ruddiman (2001) notes that factors are much debated
Greenhouse gases are the most effective way to control the earth's climate
What, then controls GHGs on a 10s of millions of years timescale?
We also need a process that can sequester CO2 from the atmosphere
Explanation 1
: Much favour due to the increase in mountain building in the past 50Ma due to collision of Indian and Eurasian plates --> Himalayas
CaSiO3 in the rock reacts with CO2 in air --> Lime (CaCO3) and Silica (SiO2) --> transported to sea in solution --> used to form carbonate secreting organisms e.g. corals --> therefore locked in sea bed sediments
Explanation 2
: Transfer of heat from tropical oceans to high latitude oceans has become less effective
Led to consequential cooling of the poles
Land bridges between the Arctic and South America broke 20Ma ago --> Drake's Passage and created between North and South America (Isthmus of Panama) around 8Ma ago
Drake's passage allows strong circumpolar currents isolate Antarctica from from warm waters
In Panama, warm, salty water is now diverted to the North Atlantic --> more moisture within polar regions --> snow --> ice sheets and glaciers
Explanation 1 is favoured as the 2nd doesn't match modelling forecasts
Scales and Styles of Ice Ages
Rose includes a good diagram of cooling trend starting 70Ma to present - shows that cooling start 55Ma but the ice age was switched on 35Ma
Note the severe cooling at 15Ma and 2.58Ma
Amplitude of CC increased 2.58Ma
Learn the 70Ma figure to draw
Milankovitch Forcing of Climate - the Yardstick for CC during human time
Up to Quaternary period: 23,000 year cycles (precession dominated)
Up to 900,000 BP: 41,000 year cycles (obliquity dominated)
Up to today: 100,000 year cycles (eccentricity dominated)
Reference Ruddiman, 2006 or Imbrie and Imbrie, 1979
Link between climate expectations and cycles first recognised by Croll (1864) but elaborated on by Milankovitch (1920)
Therefore, pre-quaternary, they explain why there were no repetitive ice ages (other than some polar ice), and why post-900,000BP, we experienced more extreme climate variations
In the past 900,000 years, conditions were colder than now for 90k out of every 100k cycle
Sub-M Forcing of Climate - Complicating Milankovitch Regularity
M cycles don't identify smaller and more irregular changes
These changes are still very important for us
Shackleton and Opdyke, 1973, first vindicated the M hypothesis through testing of ocean cores
Cores showed M cycles but failed to identify S-M forcing due to their short lived and irregular nature which wasn't picked up by the low-resolution ocean cores
Ice cores show a much more detailed record of CC (Alley, 2000) - e.g. from GRIP - shows changes of around 10C which are almost instantaneous (on a geological timescale)
Jouzel et al 2007 produced a graph of temperature based on GRIP
Sub-M factors include:
Variations in atmospheric transparency caused by volcanic activity
Changes in the position of ocean currents caused by massive freshwater deposits into the ocean
Human-induced global warming
Holocene CC (11,700-0BP)
What has caused the relative lack of CC during the Holocene?
Ruddiman (2003) and Ruddiman and Thompson (2001) outline how natural CH4 trends diverge from expected trends at around 5000BP and that CO2 trends diverge about 8,000BP
Learn the graphs - they suggest that human agriculture pumped out these gases above natural levels
Learn the IPCC 2000 graph of predicted CO2 levels by 2100AD - put together Oldfield and Aversion (2010)
Since the IR, GHG levels have risen above any previous levels (Zalasiewicz et al, 2008)
Questions
Could link to questions on what evidence we gain from ice and sea cores
What are DO events, Bond Cycles and Heinrich and why has their identification been of importance? 2011 - unlikely
Most likely is linking it to ice and sea core evidence