Climate Change summary and notes




Climate Change summary and notes


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Climate Change summary and notes


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.


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