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Geography 311 – The Atmosphere
Summary Notes.
Part V: Climate Change
Long-term Climate Change
Faint early sun paradox (sun was 30% dimmer at beginning of Earth’s history) – temp
should have been -7°C or even colder but in fact it appears that the Earth has not been
frozen for most/all of its history. Why not?
Connection between CO2 levels and climate – CO2 higher -> higher surface temp.
There are also positive feedbacks (higher surface temp -> more wv evaporating ->
more GH effect -> even higher temp).
CO2 concentrations would need to have been a lot higher than today – maybe
even 1000x higher.
What regulates atmospheric CO2 concentration?
on a short-term have balance between photosynthesis and
respiration+decomposition (about 60 Gt/yr) in and out of atm.
on a longer-term, CO2 is regulated by exchange between atm and ocean
(surface phytoplankton consume CO2, so where there is a lot of algae at
surface – at high latitudes, CO2 level is depleted in surface ocean waters –
this causes CO2 to diffuse from atm to ocean. Where surface CO2 is high
due to low concentrations of phytoplankton, ocean becomes a source of
atm. CO2).
on even longer time scale atm. CO2 is regulated by rock-cycle and
weathering processes. – CO2 dissolves in water to produce carbonic acid
(acid rain) which reacts chemically with rock to decompose it. When
carbonate rocks weather they liberate calcium and bicarbonate ions which
wash into ocean. There they get ingested by sea organisms to generate
their shells (of calcium carbonate). Silicate weathering uses an extra C
atom from atm. (in form of CO2) to do the same thing so causes a net flux
of C from atm. into limestone rock. If left unbalanced this flux of CO2
would use up all CO2 in atm over a period of 20,000 yrs. Over a long time,
this flux is replenished thru’ plate tectonic activity. CO2 escapes from deep
mantle to atm thru’ sea floor spreading, volcanic venting and
metamorphism of carbonate rocks.
Silicate weathering is a CO2 and climate stabilizer. It provides a negative
feedback. (increasing CO2 -> higher temp -> more w.v in atm -> more silicate
weathering -> reduces CO2). Also (decreasing CO2 -> lower temp -> ice
formation -> higher albedo -> lower temp -> ice coverage over earth -> no silicate
weathering -> CO2 gradually increases back up until temps increase and ice
melts……..).
Over 4.6 billion year history of earth we have had dramatic coolings and then
temp has warmed again, then other coolings etc… Earth is thrown in and out of
these by changes in CO2 concentration regulated by silicate weathering.
Glacial cycles (Pleistocene)
8 ice ages in past million years
global temp. drops by 5 – 10 °C
last ice age (19,500 – 14,000 years ago)
glacial cycles driven by astronomical cycles – Milankovitch theory – 3 components
tilt of earth’s spin axis – period of 41,000 years
shape (eccentricity) of earth’s orbit around the sun – period of 100,000 years
precession – period of 23,000 years
Milankovitch calculated effect of these 3 components on summer sunlight
reaching earth at each latitude
evidence (timing) of ice ages
foraminifera – single-celled marine organisms in shells of calcium carbonate
shells in ocean sediment preserve seawater characteristics of when they lived
measure isotopic ratio of 18O : 16O in shells (δ18O)
heavy isotope more likely to fall back into ocean, lighter isotope more likely to
reach land (meteorological distillation – heavier isotopes of water tend to
condense to form rain before lighter isotopes)
ratio of O isotopes (δ18O) in seawater closely tracks proportion of world’s water
locked up in glaciers – higher δ18O in ocean -> higher volume of ice in
sheets/glaciers
cores drilled from ocean sediment show isotopic ratio rose and fell roughly with
Milankovitch cycles, but amplitude (magnitude) of cycles not consistent with
theory (100,000 year cycle is dominant, but should have weakest effect on
sunshine)
changes occurred in both hemispheres at same time
but seasonality in southern hemisphere should be different from northern
hemisphere
appears that northern hemisphere response to changes drives the southern
hemisphere – why?
evidence suggests that link between atmosphere and ocean is important
and that ocean circulation changed
ocean changes
evidence – carbon dioxide trapped in air bubbles in Greenland ice cores show that
CO2 levels during last ice age (19000 yrs ago) were only 2/3 of
interglacial levels -> changes in ocean because this large a change cannot
be accounted for in atmosphere (ocean holds 60x amount of CO2 as
atmosphere)
look at nutrients in foraminifera shells – shows phosphate and nitrate content of
oceans:
today – Atlantic’s deep water has only half phosphate and nitrate of Indian
and Pacific oceans
in glacial times – nutrients were more evenly distributed throughout
oceans
what do nutrients tell us? – nutrients are biologically depleted at surface of
ocean (where there is plant life). As water sinks down in the ocean
it picks up sinking phosphate and nitrate. Low levels of nutrients in
water -> water has recently been near surface.
Today – ocean circulation is such that warm water flows near surface to N.
Atlantic then gets chilled and becomes dense (it is also very saline)
and sinks to ocean bottom. This is “North Atlantic deep water” and
is the origin of much of the deep water in the world’s oceans
(Atlantic conveyor). From here it flows south around Africa and
joins current around Antarctica. (Thus nutrients are low in Atlantic
because water has recently been near surface.)
In glacial times – worldwide circulation must have been very different,
Atlantic conveyor was shut down until end of last ice age
How can ocean circulation be changed by atmospheric changes? – theory involves
distribution of salt (thermohaline circulation) –
where water vapor evaporates from ocean it leaves ocean more saline,
where rain falls ocean becomes less saline
surface water sinks when saline (more dense)

possibility that changes in atmospheric circulation changed
salinity distribution, which changed ocean circulation
(Aside: (Younger Dryas is an example of how salinity can change ocean
circulation
Younger Dryas – 11,000 years ago glaciers were retreating and
temperature rising, then sudden reversal to glacial conditions
foraminifera shells show Atlantic conveyor had shut down again – why? –
meltwater from N. American ice sheet which had been flowing
into Gulf of Mexico changed and flowed in Atlantic. Salinity in N.
Atlantic was reduced and water didn’t sink. Deep water flow was
re-activated when ice meltwater changed flow direction again.))
Theory of glacial cycles:
sunshine in northern hemisphere is reduced, this changes global circulation of air
and precipitation patterns, this changes patterns of ocean salinity, which
changes ocean circulation. Changes in ocean circulation affects Southern
as well as northern hemisphere causes global ice age.