Climatic Change

Isotopes, Heinrich events, bond cycles, foram’s and Milankovitch – the science

behind global environmental change

Before we start

• There is one fundamental point to understand about the ocean- climate system – IT CHANGES

• Not only on a seasonal basis, but on a decadal and millennial cycle.

• Change is normal for earth, the problem is that humans haven’t been here long, and we are only just beginning to understand our existence depends on an a static environment that is not real.

So the problem with climate change is…

• We are making it happen faster than it has ever happened before*

• Note * (it has happened this fast, its just that not much else has survived when it did – think dinosaurs)

Timescales of different changes

Some context – why study ice ages?

• If the Earth’s history is compressed into one year, with the planet forming at one minute past midnight on 1 January, the Great Ice Age began at about 7 pm on 31 December. This short period of geological time is important for several reasons.

• There is much laterally widespread evidence in sedimentary deposits and ice cores for the climate changes that occurred in the relatively recent past. Going back in geological time, much evidence has been destroyed through erosion of the rocks and plate tectonic recycling.

• These climate changes can be studied to a very high degree of resolution. • Humans evolved during this period of climatic change and occupied every continent, and

virtually every type of environment, whether hot or cold, wet or dry. • The physical, biological and chemical conditions have not changed drastically over the past

2.6 Ma as they had done previously, so modern day processes and conditions can be used to inform us about the last 2.6 Ma.

• Much of our natural heritage of landforms and wildlife is a relic of the last glacial period that ended some 10 ka ago.

• The Great Ice Age is not yet over, so understanding the past may help us predict future climatic and ecological changes.

The key relationship

• Weather systems exhibit chaotic behaviour. By contrast, seasonal changes are triggered by latitudinal variations in solar insolation caused by the fact that the Earth’s axis of rotation is inclined with respect to its orbital plane around the Sun.

• In simple terms, there is a broadly linear relationship between radiation received by each of the Earth’s hemispheres and seasonal changes in temperature.

What this means for the surface

The last 21 000 years (and beyond)

• The last 2.6Ma are referred to as the “Great Ice Age”• This does not mean it has been cold for the last 2.6Ma• There have been a series of cyclical fluctuations between the “Greenhouse”

world we live in today, and the “Ice house” world dominated by Glaciation in the Northern Hemisphere

• Picture shows the maximum extent of the ice sheets

How do we know about glacial’s?•Glacials leave a distinct mark on the landscape, and these marks can be seen all over the northern hemisphere•The first are the different types of mountains – if you think about the UK, the glaciers never reached the hills around us, they did affect Wales and Scotland, and they still cover mountains in Northern Europe – classify those mountainscapes using the diagram here.

When the ice retreats

• It leaves behind other indicators in the lower regions, you don’t necessarily need to know all of them, but morraines are easily identifiable

Other Evidence

• Ice cores• Before we can discuss the

records, you need to understand about isotopes

• There are two dominant isotopes of Oxygen – 16 and 18

• The heavier (18O) preferentially condenses and falls back into the ocean during an ice age, leaving 18O enriched water, and 16O enriched ice.

Ice sheets

• Cores have been taken through the Greenland ice sheet and near Vostok.

• Analysis of the ice, and its dust and gas content, yields information concerning variations in: surface temperature at the drilling site as snow fell; the storminess of the atmosphere as indicated by the amount of dust and sea salt preserved in the ice, and the ‘dryness’ of continental areas at lower latitudes from which the dust was blown; the content of greenhouse gases in the atmosphere.

Isotopes in the Ice• We can measure

the % of CO2, Isotopic concentration to give the relative temperature and dust

• This is the Vostok core which extends back 420 000 years

Deep Sea Cores• In polar regions today, snow fall and ice have

δ18O values of–30‰ to–50‰.• Therefore, the larger the volume of land ice

and ice sheets, the higher the relative proportion of 18O in seawater.

• During glacial stages, when the maximum extent of glaciers and ice sheets covered approximately three times their present area, about 3% of ocean water was abstracted, enriching the ocean watersin18O.

• Higher or less negative,δ18O values in deep-sea sediments indicate larger ice caps and lower global sea-level at the time the sediments were deposited.

• The Oxygen is absorbed by Foraminifer and Coccolithophores

The deep sea record

• By now you should be able to see that there is some very accurate proxy data that provides quantifiable evidence of past climatic change

Plant and animal remains

• If we know where animals like to live now, we can infer paleoclimate from their remains

• Some living organisms are small, evolve rapidly and have a hard exoskeleton meaning they preserve well

• There are two you need to know about

• Pollen and Beetle

Beetles Pollen

• Are mobile• Rapidly evolving • Abundant coleoptera (all

beetle like insects)• Therefore beetle

assemblages in the fossil record can be used to identify paleoclimate.

• I.e. they share a mutual climatic range

• The best way to interpret quaternary environments

• Easy to identify and categorise

• Durable exine (outer shell)

• Thousands of remains per site makes interpretation more reliable

Paleo-environmental reconstruction

of South America based on fossil Pollen

spores

Dust and Caves

• These are so called Proxy Data that can be used to calibrate and support the evidence from the geological record.

• In China, there are Milankovitch related dust deposits known as Loess (Ice sheets = wind = blown dust)

• The Devils Hole Cave in Nevada yielded a 36cm long, 500 Ka record of climate change through Oxygen Isotope variations

What does this tell us?

• Firstly, there are some obvious patterns extending back over many thousands of years

• The first is that carbon and temperature are related.

Ice ages

• You only need to know about the last 21000 years, during which time we have been in an ice age, come out of one and gone into an interglacial

• There are new words to learn!

• Ice age: period of net growth of ice sheets and glaciers

• Interglacial: period of net ice sheet decay

• Stadial: a colder period within an interglacial

• Interstadial: a warmer period within a glacial

• (fluctuations are of the order of 1000’s not always millions of years)

• Holocene: time period of 10 000 years ago to present

Ice ages

• Over geological time (for the last 2.6 ma at least)there are a series of regularly spaced ice ages

• Why would the graph have a pattern?

Recent History (21ka to present)

• There are some stages you need to know:1. 21000 = last glacial maximum2. 15ka = rapid late glacial warming3. 12.9ka to 11.5ka = younger dryas

stadial4. 8.2ka= significant cooling event5. 5-1ka = holocene climatic

optimum6. 1.25-0.7ka = medieval warm

period7. 400-150 years ago = little ice age

Numbers do not relate!

Potential Causes of climate change

• Asteroids• Volcanoes• Solar output• Milankovitch cycles• Ocean circulation• Anthropogenic change

Milankovich• Milankovich theory is now widely accepted and

is understood as an orbital/astronomical forcing mechanism for northern hemisphere ice ages

• The essence of his theory is that changes in the intensity of the seasons in the Northern Hemisphere controlled the waxing and waning of northern high-latitude ice-sheets.

• In particular, he believed that Northern Hemisphere high-latitude summer temperatures hold the key to the onset of glaciations.

• If the summers were cold enough, winter snows would not completely melt, and so permanent snow fields would grow into glaciers.

• High latitudes between about 60° and 80° N are now regarded as being ‘Milankovich sensitive’.

• This is because there is a solar insolation minimum in this zone from the combination of the poleward diminution of insolation received per unit area, and the poleward increase in day length (up to 24 hours) in this direction during the summer months. Changes in the tilt of the Earth’s rotation axis (obliquity) change the amplitude of this minimum.

Three elements to Milankovich

• There are three ways orbit of the earth changes as it rotates around the sun

• Obliquity• Eccentricity• Precession

Obliquity• The variation in the Tilt of the earths

axis with respect to the plane of orbit• It shifts between a tilt of 22.1° and

24.5° and back again.• These slow 2.4° obliquity variations

are roughly periodic, approximately 41,000 years.

• When the obliquity increases, the amplitude of the seasonal cycle in insolation increases, with summers in both hemispheres receiving more from the Sun, and the winters less.

• As a result, it is assumed that the winters become colder and summers warmer.

Eccentricity

• Our orbit is not circular• Eccentricity measures how far from a perfect circle the

orbit is• It varies over a 95k and 123k cycle which combine to

produce the 100k• When the earths orbit is at its most elliptical, the

winters will be coldest and insolation lowest.

Currently the difference between closest approach to the Sun (perihelion) and furthest distance (aphelion) is only 3.4% (5.1 million km). This difference is equivalent to about a 6.8% change in incoming solar radiation. Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most elliptical, the amount of solar radiation at perihelion is about 23% greater than at aphelion

Precession

• Think elliptical hula hoops• There are two elements to

precession• The earths axis sweeps out a

cone in space – this is axial precession

• The elliptical orbit varies over time (like the furthest point of a hula hoop) this is Aspial Precession

• This variation is on a 26k cycle

Translation..

Look familiar???

There is a close correlation going back 400k between Milankovich cycles

• There are some problems with the theory

• But these are best left for another time if you are interested!

Milankovitch and Isotopes

• The pattern is visible in the foram data, the ice cores and dust/cave deposits

• It is however still being heavily researched and solutions to the problems being sought

• However, 400k of correlation is pretty significant

Why do Milankovitch variations force ice ages?

• For this think about:1. Plate tectonics2. Distribution of

landmasses3. Seasonality4. Snow/ice albedo and

feedback cycles

Other forcing factors

• Research:• Volcanoes• Solar variations• Asteroids• Changes to ocean

circulation patterns• (note a forcing factor is

any process that creates a change in the earths climate system)

Is it all true? (notes not necessarily needed)

• Yes• Remember, to use a cliché,this really is an “inconvenient truth” (haha, witty I

Know!)• We have identified a pattern of cycles, the stadials and interstadials ad longer

term cooling cycles known as bond cycles – shown by a drop in 18O• Ie:• More than 20 interstadials (warmer periods) and stadials• (colder periods) during the Weichselian Ice Age.• • Dansgaard-Oeschger (D-O) cycle: Sequence of stadial• and interstadial, average periodicity ~ 2-3 ka.• • Typical course: Rather gradual cooling followed by rapid• warming.• • Bond cycle: Longer cooling cycle, consists of several D-O• cycles with a successive drop of δ18O minima.

The end of Bond

• Most Bond cycles culminate in a partial collapse of the North American ice sheet (“Heinrich event”).

• Massive release of icebergs into the North Atlantic, leads to 5-10 m global sea-level rise

Interpretation

• The earth has its own well established patterns

• Long term cooling, rapid warming

• We are accelerating warming when we should be heading into a 23k precessional ice age

• Any clues?

Larsen B

• Over two months the ice sheet breaks up

• The land based ice behind is now breaking up

• What could this be? • There is an argument

this is a Heinrich Event