Climate over the long term (Ch. 3 - 6 highlights)

• Long-term climate changes• Plate tectonics• What maintains Earth’s habitability?• Faint Young Sun paradox• CO2 : Earth’s thermostat *

• Past icehouse conditions• Past greenhouse conditions

* Critical idea

Long-term climate changes

Long-term: consider how Earth’s climate has changedover last few hundred m.y.

Why study long-term changes?

-- Helps us to fundamentally understand howEarth’s climate system works

-- If we don’t know this, we can’t evaluatehow climate might change in future

From before:

Natural climate variations

time scale type according to book

~few years “historical”

~10-1000 years “historical / millenial”

~10,000 years “orbital”

millions of years “tectonic”

Long-term climatechanges

Plate Tectonics

• Theory that the upper portion of Earth is subdividedinto ~dozen large pieces (lithospheric plates)that move relative to one another

• Most volcanic activity occurs at plate boundaries,either where plates are moving apart (divergentmargin), or where they are moving towardseach other (convergent margin)

“tectonic” means any large scale Earth movement

Map of Earth’s lithopheric plates

Seafloorspreadinghere

Subduction& mountain buildinghere

Plate tectonics can affect climate because:

(1) Continents can change position This strongly affects ocean currents.

(2) It controls the rate of volcanism(high when plates moving fast, low otherwise).

(3) It controls the rate of weathering (high when more continents collide andmore mountains formed).

Changing continent positions:Assembly of supercontinent Pangaea

Rate ofvolcanism

Changes in amount of uplift of continental rockcould regulate amount of weathering

“Upliftweatheringhypothesis”

Get uplift mainlywhen continentscollide

Why increased rock fragmentation leadsto more weathering:

Weathering depends on surface area

What maintains Earth’s habitability?

Earth’s climate “just right”

-- at present

-- mostly over geologic time

geologic evidence (e.g. sedimentaryrocks) & biologic evidence (fossils)indicates liquid water stable at surface for most of Earth history

-- not always true in past, however

Venus Earth Mars

avg. temp. 460 oC 15 oC -55 oC

avg. distance 0.7 x Earth 1.5 x Earth to sun

solar energy 2 x Earth 0.44 x Earth

input (flux)

Climates on three planets today

Venus Earth Mars

avg. temp. 460 oC 15 oC -55 oC

greenhouse 285 oC 31 oC 5 oC warming

avg. temp. 175 oC -16 oC -60 oC with no

greenhouse

Climates on three planets today

Too cold

Just right

Phase diagram for water

Venus, Earth, Mars with no greenhouse effect (& same pressure):

-16 C

Faint Young Sun paradox

(1) Astrophysical models indicate that sun’sbrightness should have increasedsignificantly over age of solar system

(2) So why wasn’t Earth frozen earlier?

Solar luminosity

-- what we mean by sun’s “brightness”not same as albedo!

luminosity = energy / (area * time) = Watts / m2

at surface of sun; we call this flux away from sun

-- flux decreases as distance from sun increasesbecause solar energy spread over a larger area(spreads over surface area of sphere = 4 * pi * r2)

-- models suggest sun’s luminosity increased by ~30% over age of solar system

Earth shouldhave beenfrozen before1.8 b.y. ago

CO2 is a greenhouse gas, helping to makeEarth habitable today

The amount of CO2 in the atmosphere mayhave varied in the past to keep Earthcomfortable

CO2 : Earth’s thermostat?

GCM results: the effect of different CO2 levels

CO2 as Earth’s thermostat

-- where is carbon (C) stored on Earth?

-- how is C exchanged between different reservoirs?

Carbon reservoirs today

Where is carbon stored on Earth?

Limestone(carbonate)rock: CaCO3

Carbon cycle

How is C exchanged between different reservoirs?

C exchange betweenrocks & ocean +atmosphere

Note: low rates

Focus on C exchange between rocks and atmosphere:

• volcanic eruptions add C to atmosphere(as CO2), remove it from rocks

• chemical weathering of rocks either adds or removes C from atmosphere, depending on type of rock weathered; we’ll considerremoval of C from atmosphere

Volcanic eruptions (Regulated by plate tectonics)

More volcanism earlier in Earth history?

-- Yes, more plate tectonic activity

-- Could get more CO2 in atmosphere, stronger

greenhouse

-- But unlikely that this alone exactly balanced variations in solar luminosity

No reason for volcanic activity on Earthto be related to solar luminosity !

Chemical weathering (hydrolysis):

-- chemical reaction of minerals with water to form different minerals

CaSiO3 + H2O + CO2 CaCO3 + SiO2 + H2Omineral rain atm mineral mineralin rock

Makes carbonic acid H2CO3

Chemical weathering (hydrolysis):

-- removes CO2 from atmosphere, puts it in

limestone (or carbonate) rock

-- proceeds faster if more precipitation, higher temperature, more vegetation

CaSiO3 + H2O + CO2 CaCO3 + SiO2 + H2Osilicate rain atm limestone /rock carbonate

(Why?)

Chemical weathering (hydrolysis):

-- removes CO2 from atmosphere, puts it in

limestone (or carbonate) rock

-- proceeds faster if more precipitation, higher temperature, more vegetation

CaSiO3 + H2O + CO2 CaCO3 + SiO2 + H2Osilicate rain atm limestone /rock carbonate

(Why?-- carbonic acid)

Temperature - weathering feedback:

Temperature - weathering feedback:

The amount of CO2 in the atmosphere mayhave varied in the past to keep Earthcomfortable

Chemical weathering (hydrolysis) was probablyimportant in regulating this

The weathering process involved a negativefeedback

CO2 : Earth’s thermostat?

Can weathering explain the Faint Young Sun Paradox?

If colder (lower solar luminosity), weathering rates should have been less...

… more CO2 stored in atmosphere, less in rocks...

… more greenhouse effect, higher temperature.

So: Yes, in principle.

But there were times in Earth’shistory when the (presumed CO2)

thermostat was not so effective...

This led to

icehouse & greenhouse conditions

Past icehouse conditions

evidence for multiple glaciations 3 glaciationslast 500 m.y.

Major glaciation 550-850 m.y. ago

Glacial striationsin Alaska

Formed by movement of iceover rock

Positioning of large landmasses over polar regions help cause glaciation

Note: Polar positioning is not the only reason we had past icehouse climates

Past greenhouse conditions

fossil evidence for warm conditions100 Ma ago (Cretaceous period)

Dinosaurs

Warm-climate flora

O-isotopedata, deepoceans:

~13 oCcooling inlast 50 m.y.

100 m.y. ago (Cretaceous):

• Supercontinent Pangaea breaking apart• High sea level

GCM models including changes in plate position and CO2 fail to fully explain Cretaceous climate

What led to greenhouse conditions in the Cretaceous?

Probably 2 factors important

(1) Higher CO2 in atmosphere

-- faster plate movement led to morevolcanic emission of CO2

-- there was less removal of CO2 from

atmosphere by weathering becausethere were few high mountains(no plate collisions)

(2) Heat was transported in oceans differently thantoday

Today

Then

Model simulation of Cretaceous ocean salinity

Highly saline water is dense andcan sink, even if warm

If heat inCretaceousoceanstransported moreefficiently,would tend toequalizetemperaturesmore…

...discrepanciesbetween models& geologicevidence wouldbe explained