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Samenvatting colleges Quaternary Climate and Global Change
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[Meer zien]Voorbeeld 4 van de 40 pagina's
Voorbeeld 4 van de 40 pagina's
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Voorbeeld van de inhoud
Lecture 1: IntroductionTime scales events in the course: Pleistocene (2.58 Ma) – Holocene (last 1000 years) – Anthropocene
(last 100 years). We will look at the past, present and future. We will talk about ice ages, interglacials
and human impacts. Lastly there is a focus on climate change and global change.
We will look at glacial and interglacial, methods of reconstruction, rapid climate change, recent,
causes and mechanisms, human impact, and future projections.
There are differences in the perception of geological time, as present can be defined as decades,
centuries etc. There are many different timesteps between the present and past.
Age methods: Preservation and dating method limits to resolution
- Instrumental records (300 years)
- Proxy data: historic sources (103), dendro-climatology (104), varved lake records (104), pollen
analysis (106), isotope analysis cave deposits (105), isotope analysis continental ice sheets
(105), isotope analysis ocean cores (106), and geological/geomorphological research (infinite)
Quaternary = youngest geological time period of the last 2.58 Ma
Climate = atmospheric conditions prevailing over areas. It varies over historic and geological time and
over the earth surface (i.e., zones). The general conditions of the atmosphere are defined over a set
of time windows. The longer time period, the less constant the climate is. Climate is the total of
weather conditions at a certain location and gives the expected weather. There is a difference is
meteorology (30 years) and geology (non-strict time window, so longer than 30 years).
Global change = all changes owing to human activity, broader than climate effects alone.
Climate change = change in average or variability of the atmospheric conditions. It is the systematic
deviation in running statistical properties of climate.
Climate variables: Temperature, precipitation, wind speed, air pressure, humidity, cloudiness, and
sunshine. You can use the averages, variability, extremes, frequencies, and intensities. It can be used
on spatial and temporal variations.
With the climate system we can understand and explain climate variations. But with climate
variations we can also reconstruct and investigate the climate system.
Quaternary and particularly last 1 Ma: big ice ages. There is a mode switch from glacial to interglacial
as rapid climate changes. It has occurred repeatedly with 10 5-106 period: climate cyclicity. This allows
to count, correlate, subdivide and intercompare cycles.
Shorter duration climate variations occur superimposed (millennial and centennial scale). They are
inside the last ice age (in the Holocene). There is irregular recurrence: feedback tipping points, event
response.
,Climate system exists of solar radiation budgets, hydrological cycles, climate zones and weather
systems, and main components of the systems.
Albedo reflects radiation (white reflects much, black
reflects little). The ocean is very dark, so low albedo. In
the winter there is snow, which has a high albedo, so
there is seasonality. In the southern hemisphere there
are no continents at the boreal latitudes, which causes
the asymmetry in seasonal albedo change.
There is an unequal heating of the Earth Surface, because
of the earth’s rotation etc. There is more heat at the
equator which is converted to the poles. This unequal
heating leads to circulations and different cells. This causes
global atmospheric circulations, which effect monsoons in
the low to mid latitudes.
Great circulations mix all the ocean waters of the world (about 400 years). In the North Atlantic
Ocean, it turns from a shallow water and sinks to a lower place (3-4 km) and flows back. If you want
to heat up the ocean, there is a lot of energy needed and it takes a long time, so there is a long
climatic change. It affects the asymmetry of climate cycle. Oceanic responses and climate feedbacks
affect land response and feedbacks.
On land there are tree rings, pollen vegetation history, soils volcanoes and loess.
Cryosphere: ice as a component of its own. It is a source of long atmospheric records and a player in
asymmetry of climate cycles and global sea-level change.
The biosphere affects the climate system by albedo, evapotranspiration, carbon storage (biomass
takes in CO2 and deltaic C trapping), vegetation (roughness to wind fields) and soil protection (dust
particles). The biosphere responds to climate change with some lag.
8 glacials in the last 800 thousand years. The interglacials are warm and have a high CO2 value and
dO18. They are instable while the glacials are more stable.
,Lecture 2: Milankovitch forcings
Milankovitch cyclicity (orbital forcing): earth received radiation budget in W/m2 latitudinal variation
(orbital parameters). High latitude ice sheet response in the Northern hemisphere:
- small ice (progressive effect of glaciations in 41 ka regular cycles > 1 Ma)
- big ice (revolution towards 100 ka saw-tooth cycles in the last 1 Ma).
Milankovitch influences when the northern hemisphere or southern hemisphere has its summer
close to the sun. Obliquity is most important cycle.
1. Obliquity (tilt): variation of 22.2 to 24.5 degrees and cycles of 41.000 years. This affects the
seasonality and higher latitudes. It enhances differences between summer-winter insolation
(weak or strong seasonality) and the effect of variations increase towards the poles. Changes
the seasonality for both the N and H hemisphere.
2. Precession (wobble): strong: 23.000-year cycle and weaker 19.000-year cycle. This affects the
low-mid latitudes (monsoons, sapropels). It determines which season is enhanced/reduced
(seasonal timing) and has opposite effects for N and S hemispheres.
3. Eccentricity (shape orbital ellipsoid): distance varies between 153 and 168 x 10^6 km. It has a
cycle of 100.000 year and 413.000 years. The eccentricity is the modulator over long term. It
modulates effects of precession (even stronger monsoons) and enhances/reduces
seasonality (pacing of ice build-up). It produces just 0.2% radiation difference on its own.
Earth orbital parameters
- Total radiation remains almost constant. 0.2% variation due to eccentricity changes
- Variations over latitudes and seasons changes: opposite effect of precession and eccentricity.
Minimal poleward of 70 degrees, where obliquity dominates radiations.
- Lower seasonality favours ice sheet growth. Warmer winter results in more snow
precipitation and a cooler summer allows more snow to remain until next winter. Strong
equator-pole temperature gradients: intensified poleward general circulation/moisture
transports, feeding ice sheets.
The different graphs can be added to create a total graph of the
changes. This can be done to create changes over time in insolation.
So, then the different time periods can be recognized. It can also work
the other way around, you can for example look at the sapropels in
sediment and see if they belong to a 20/40/100 thousand year cycle,
so which type of forcing caused the sapropels to form.
60-65 degrees north is often taken because:
- that is the place where land accumulates snow during winter
- albedo-feedback sensitive region
- It is where past major ice sheets have formed because snow was stored long enough at
periods of low irradiance + albedo feedback
- Latitude where ice sheets melt away at the end of glacial, when irradiation is on the increase
There is a difference between the northern and the southern hemisphere because of the angle of the
axis of the Earth with relation to the sun (ap and perihelium). When it is summer in the southern
hemisphere, the Earth is closer to the sun (so warmer) than when it is summer in the northern
hemisphere.
, Store mass of water on land: lower ocean
volume/global sea level
LGM
LIS 40 Gone totally
SIS 25 Gone totally
GIS 6 +6 m max to go
WAIS 10 +6 m max to go
EAIS 10 +15 m potential
tipping point
Thermal 5 +2 at +3 degrees
warming
In total 120 meter.
Last Glacial Ice Sheet Mapping: Global Maximum is not the Regional Maximum.
Different types of dating may contradict each other. Offshore limits: more difficult/slower than on
land. Maximum stages: more difficult than deglaciation stages
- Mapping and relative dating since 19th century
- Radiocarbon dating (14C) since 1950
- Luminescence dating (OSL) since 1990
- Cosmogenic isotope dates (Be) since 1990
Termination is melting of the ice sheet. Big Ice
Ages only exist since 400-800 ka. Mid-Pleistocene
revolution ca. 1.2-0.8 Ma). Early Pleistocene and
Pliocene were different.
Every 41 ka, ice sheets grew to about the size of the picture, after which they melt again. The ice
sheets are domes with 1 km thickness in the centre. In the 100 ka cycles, the ice sheets grew bigger
with a thickness of 2.5 km in the centre. Winds go from North America towards Europe, which is
amplified in ice ages because of a large gradient between poles and the equator.
Ocean-recorder volume of ice sheets. Ocean record of continental ice voluwe shows a slow cooling
trends tarting in Neogene, interglacials modest cooling and glacials dip deeper and deeper. For very
long time there were 41 ka cycles. The last 100 cycles were 100 ka.
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