COCO22 and Long-Term Climate and Long-Term Climate

Greenhouse Worlds:Greenhouse Worlds:Venus and EarthVenus and Earth

• Mean temperatures at surface– Venus: 460o C– Earth: 15o C

• At first glance, it would seem that distance from the Sun is a major factor

Why is Venus so much warmer than Earth?

Venus is closer to the SunVenus is closer to the Sun

• Mean distance from the Sun– Venus: 108.2 million km– Earth: 149.6 million km

• Venus is 72% (or .72 Astronomical Unit*) of Earth’s mean distance (1.0 A.U.)

* An astronomical unit (A.U.) is the average distance between Earth and the Sun and is used for distance measurement in the Solar System.

Venus Receives More InsolationVenus Receives More Insolation

• Amount of insolation received varies inversely with the square of its distance from the Sun.

• Venus receives nearly twice the solar radiation as Earth does.

• But, this isn’t the reason . . .

Earth

Venus

(1)2

(0.72)2 0.5811.931

= =

VenusVenus• Upper atmosphere

– Thick cover of sulfuric acid clouds

– High albedo (80%)– Only 20% of insolation

reaches the surface.

EarthEarth

• Clouds reflect 26% of insolation

• 74% of insolation reaches Earth’s surface.

Less Insolation Reaches the Less Insolation Reaches the Surface of VenusSurface of Venus

• Even though receives 1.93 times the insolation the Earth does– The amount reaching its surface is 52% of Earth’s– This is due to the high albedo of the cloud cover on

Venus

1.93 x 0.200.74

= 0.52

The Cause . . .The Cause . . .The Thick Atmosphere of VenusThe Thick Atmosphere of Venus

• It’s atmosphere is 90 times as dense as that of Earth.• 96% of the atmosphere is carbon dioxide• Venus is said to have a “runaway greenhouse effect.”

Venus and EarthVenus and Earth

• Are Greenhouse planets• Contain nearly equal amounts of carbon• The difference is where they store carbon

– Venus: Primarily in its atmosphere– Earth: Most is stored in rocks

• Limestones• Also in reservoirs of coal, oil, and natural gas

– On Earth the major greenhouse gas is water vapor • Greenhouse heating from atmospheric carbon is relatively

small (31o C)

– On Venus the major greenhouse gas is CO2• Enormous net greenhouse warming (285o C) even though

atmospheric water vapor is nearly absent

The Faint Young Sun The Faint Young Sun ParadoxParadox

Earth’s SunEarth’s Sun

• Formed from the solar nebula 4.55 Byr• “Shines” as a result of an ongoing nuclear reaction in its

core

Fusion in the Sun’s CoreFusion in the Sun’s Core

• Four hydrogen (H) nuclei (each with a mass of about 4.030 mass units) join to form a helium (He) nucleus with a mass of only about 4.003 energy units.

• The mass that seems to be lost is converted to radiant energy– 4 million metric tons of matter are converted into energy every second

An Expanding Sun

• The earliest Sun had 25% to 30% lower luminosity.• As nuclear fusion caused the Sun to expand,

– It became brighter

Hertzsprung – Russell (H-R) Diagaram

• Shows the relationship of a star’s mass to its luminosity• The Sun will eventually expand to a red giant and then end its life as a white dwarf

All Water on the Early Earth All Water on the Early Earth Should Have Been FrozenShould Have Been Frozen

• A decrease in the Sun’s brightness of just a few percent would cause all water on Earth to freeze.

• The geologic record shows that Earth has never been completely frozen.

Evidence of Liquid Water on Earth Evidence of Liquid Water on Earth Throughout Geologic TimeThroughout Geologic Time

• Sedimentary rocks are a prominent part of the rock record.• Most sedimentary rock indicate a liquid water depositional

environment.

Evidence of Liquid Water on Earth Evidence of Liquid Water on Earth Throughout Geologic TimeThroughout Geologic Time

• Primitive life dates back to at least 3.5 Byr ago.• Continued presence of life on Earth along with a

succession of increased complexity isn’t congruent with extreme cold.

3.2 to 3.5 Bry oldProcaryotes from

AustraliaCambrian marine life

Why then, with a weak Sun Why then, with a weak Sun wasn’t Earth completely frozen wasn’t Earth completely frozen for the first 3 billion years of its for the first 3 billion years of its

existence?existence?

A Warming Process Must Have A Warming Process Must Have Been PresentBeen Present

• There must have been a process that warmed Earth.

• But, it must not be doing so today– Combined with the strengthening of the Sun

Earth would be uninhabitable.

• Somehow, Earth has remained within a moderate temperature range during the period of the Sun’s increasing output.

A Thermostat ProcessA Thermostat Process

• A process that :– Warmed Earth when it otherwise would have

frozen– Reduced heat upon detecting increasing

warmth from the strengthening Sun

• Greenhouse Gases could have been part of the mechanism.– More abundant during early Earth history– Decreased as Earth warmed

Effect of Greenhouse GasesEffect of Greenhouse Gases

Carbon Exchange between Carbon Exchange between Rocks and the AtmosphereRocks and the Atmosphere

Over long periods, slow exchanges can produce large cumulative changes in

atmospheric CO2

Carbon ReservoirsCarbon Reservoirs

• Largest carbon reservoir is in rocks.

• Inverse relationship between size of reservoir and rate of exchange

• Over millions of years slow exchanges can result in large changes in atmosphere CO2.

Volcanic Sources of COVolcanic Sources of CO22

Heat in Earth’s interior causes rocks to melt.

Volcanic EruptionsVolcanic Eruptions

Volcanic Sources of COVolcanic Sources of CO22

Heat in Earth’s interior causes rocks to melt.

Yellowstone National Park

A Balancing ActA Balancing Act

• Rate of carbon input is roughly balanced by a similar rate of natural removal– Probably prior to industrial revolution

• Volcanic input of carbon is irregular because volcanoes don’t erupt on a “schedule.”

• If volcanic input of carbon stopped . . .– It would take 4,000 years for atmospheric CO2 to fall

to zero.• A geologically short period of time

Other Reservoirs Would Other Reservoirs Would CompensateCompensate

• Near surface reservoirs would lose CO2.– Vegetation– Soil– Surface Ocean

• They would take 24,700 years after end of volcanism to lose all their carbon.

• Deep-Ocean carbon reservoir would also lose.– With this reservoir it would take 278,000 years for a

complete termination of volcanic carbon input to completely deplete all reservoirs.

• This is 0.01% of all Earth history

Is the Volcanic Source of COIs the Volcanic Source of CO22 the the

Natural Thermostat?Natural Thermostat?• Volcanoes alone could not have delivered

the amount of carbon needed to:– Prevent the atmosphere from running out of

CO2

– But not overheat the planet

• Volcanic processes are driven by Earth’s internal heat.– Volcanism doesn’t react to external changes

and then act to moderate their effects like a thermostat.

Chemical Weathering of Chemical Weathering of Continental RocksContinental Rocks

• The major long-term process of CO2 removal

• Avoids long-term buildup of CO2 levels over time– Of the types of chemical weathering

previously discussed, two types are important in the carbon cycle.

• Hydrolysis• Dissolution

HydrolysisHydrolysis

• The main mechanism for removing CO2 from the atmosphere

• Three key ingredients

– Water derived from precipitation– Minerals in continental rocks– Carbon dioxide from the atmosphere

Continental RocksContinental Rocks• On the average, composition of granite

– Composed of silicate minerals– Typically cations (Na+, K+, Fe+2, Mg+2, Al+3, and Ca+2

are:• Chemically bonded to the negatively charged silicon-oxygen

tetrahedron (SiO4-4)

The Silicon-Oxygen Tetrahedron

Olivine – A Silicate MineralOlivine – A Silicate Mineral

Example Using WollastoniteExample Using Wollastonite

• CO2 dissolves in rainwater (and in groundwater)– Forms carbonic acid

• Carbonic acid reacts with wollastonite• Weathered products release Si+4, Ca+1, and HCO3

-1

– Eventually end up in the ocean and is deposited in shells of marine organisms.– Eventually forms limestone

wollastonite

Example Using WollastoniteExample Using Wollastonite

Accounts for 80% of carbon buried

per year in sediments and rocks

wollastonite

Diatomite

• Silica deposited in the deep ocean

LimestoneLimestone

• Limestone ridge in the Canadian Rockies

• Limestone in France

Coral LimestoneCoral Limestone

Barrier ReefsBarrier Reefs

Great Barrier ReefAustralia

Skeletal LimestoneSkeletal Limestone -- CoquinaCoquina• Formed from wave-broken fragments of shells, corals,

and algae.

ChalkChalk

• Fine-grained, light colored, and porous from microscopic marine organisms (plankton).

White Cliffs of DoverWhite Cliffs of DoverKent, EnglandKent, England

Coccolithophorids (Coccoliths)

• Primary constituent of chalk in the White Cliffs of Dover• Calcareous platelets• Secreted by yellow-green algae• Extremely small

Bioclastic LimestoneCoarse-grained withshell and coral fragments

Fine-grained carbonate mud from coralline algae

Dissolution of LimestoneDissolution of Limestone• Rainwater and CO2 combine in soils forming carbonic acid• Calcite in limestone is chemically dissolved• Dissolved ions flow to the ocean in rivers.

CaCO3 + H2CO3 Ca + 2HCO3

calcite carbonic acid calcium bicarbonate

Dissolution of Limestone Forming Caves

Great Onyx Cave, KY

Howe Caverns, NY

Carlsbad Caverns, NM

Stalactites

Dissolution of Limestone RatesDissolution of Limestone Rates

• Faster than hydrolysis of silicates

• Returns all of the CO2 to the atmosphere

– Within the relatively short time it takes dissolved ions to reach the sea and become incorporated into the shells of marine organisms.

• No net removal of atmospheric CO2 during the overall process.

Chemical Weathering may act as Earth’s thermostat

Rates are sensitive to climate

Climate Factors That Control Chemical Weathering

TemperatureTemperature

• Controlled laboratory experiments indicate– Weathering rates

double for each increase of 10o C.

• Lab studies are difficult to transfer to studies of the real Earth• Only a few silicate minerals have been examined• Natural rates are difficult to determine in the field• Rapid carbonate dissolution complicate studies – May dominate total dissolved ions in rivers - Do not control CO2 levels in atmosphere

PrecipitationPrecipitation

• Increased precipitation results in a greater rate of chemical weathering

• Groundwater in soils increases– Formation of carbonic acid increases

Effects of Temperature and Effects of Temperature and Precipitation are LinkedPrecipitation are Linked

• Warm tropics– High humidity and

rainfall– Rapid chemical

weathering

• High Latitudes– Cold and dry– Little chemical

weathering

Effects of Temperature and Effects of Temperature and Precipitation are LinkedPrecipitation are Linked

• Smaller-scale complications creating region variations– Hot regions may

have high evaporation rates

– Dry out soil– Evaporated

water may fall in another region

VegetationVegetation

• Plants – Extract CO2 from the air– Delivers it to the soil

• Combines with groundwater to form carbonic acid– Can increase the amount of chemical weathering by a factor of 2

to 10 over the rate on land without vegetation.– More vegetation increases rate of CO2 extraction from air and

increases amount of carbon in the biomass.

Chemical Weathering: Chemical Weathering: Earth’s Thermostat Earth’s Thermostat

• Mechanism involves two facts:– The state of Earth’s climate affects the rate of

global chemical weathering– Weathering can affect the state of Earth’s

climate• It regulates the rate at which CO2 is removed from

the atmosphere

– Chemical Weathering acts as negative feedback

Chemical Weathering for a Warming Chemical Weathering for a Warming Climate - Negative FeedbackClimate - Negative Feedback

• Increase in temperature, precipitation, and vegetation

• Increase in weathering rate

• More CO2 removed from the atmosphere

• Slows the warming

Chemical Weathering for a Cooling Chemical Weathering for a Cooling Climate - Negative FeedbackClimate - Negative Feedback

• Initial cooling is reduced

• Less CO2 is removed from the atmosphere due to decreased chemical weathering

Negative Feedback Explains the Negative Feedback Explains the Faint Sun ParadoxFaint Sun Paradox

• Favored over volcanism, which did occur at high rates on the early Earth– High rates could have produced enough CO2

to warm Earth– But, it’s highly unlikely that the slowing of

volcanism over a 4 Byr period was paced exactly the rate needed to counter the strengthening Sun.

Early EarthEarly Earth

• Earth was cooler– Less precipitation– Less vegetation

• Chemical weathering was slower

• Slower CO2 removal– 100 to 1,000 times as

much CO2 in the atmosphere as today

Strengthening SunStrengthening Sun

• Warmer temperatures• More

– Precipitation– Vegetation

• More chemical weathering

• More CO2 removed from the atmosphere

• Offset warming from the stronger Sun.

A Snowball Earth?A Snowball Earth?

• Evidence of several glaciations between 850 and 550 Myr ago– If these were at or near the poles, then climate was similar to today– If these were in the tropics then it’s possible Earth would have been

close to a frozen state

• This is unresolved . . .

The Gaia HypothesisThe Gaia Hypothesis

• Proposed by – James Lovelock

• Independent Scientist, Environmentalist, Researcher, Author

– Lynn Margulis• Department of Geosciences• University of MA at Amherst

GaiaGaia• Hypothesis that life

evolved in order to regulate Earth’s climate

• Named after the Greek Goddess known as Earth or Mother Earth (the Greek common noun for "land" is ge or ga).

• It is written that Gaia was born from Chaos, the great void of emptiness within the universe

Modern-Day Biologic ProcessesModern-Day Biologic Processes

• Cited by Gaia supporters• Important parts of the processes of chemical

weathering and carbon cycling

– Carbon is at the center of the CO2 cycle

– Terrestrial plants contribute CO2 to the soil and form carbonic acid

– Shelled ocean plankton extract CO2 from the ocean and store it in their calcium carbonate shells

Critics of Gaia Cite that . . .Critics of Gaia Cite that . . .

• Most of the active roles played by organisms in the biosphere today– Are a relatively recent development in Earth’s history

• The role of life in the distance past– Probably smaller– Or nonexistent

• Through Earth’s long history life has differed considerably from those of today– Life forms that existed for over 90% of Earth history

• Too primitive to have an effect on chemical weathering to drive Earth’s thermostat

Development of Life on Earth

Primitive organisms similar to the modern-dayBacteria Oscillatoria

Development of Life on Earth

Stromatolites (2.9 Byr ago)

Fo

ssil

Rec

ord

is

po

or

or

abse

nt

Stromatolites:Stromatolites:Builders of Limestone and Producers of O2

• Cyanobacteria – older incorrect term

is blue-green algae– Photosynthetic

bacteria

• Secret CaCO3 in daily cycles

• Traps sand and forms layers in various mound-like formations

Shark Bay, AustraliaShark Bay, Australia

Present-Day Stromatolites

Development of Life on Earth

Evidence is provided by Banded Iron Formations (BIFs)

BIFs and the AtmosphereBIFs and the Atmosphere• How are these rocks related to the atmosphere?

• Their iron is in iron oxides, especially hematite (Fe2O3) and magnetite (Fe3O4)

• Iron combines with oxygen in an oxidizing atmosphere to from rust-like oxides that are not readily soluble in water

• As photosynthesizing organisms increased in abundance, free oxygen was released into the oceans, causing the precipitation of iron oxides.

Cited as evidence of Gaiaby supporters

Development of Life on Earth• “Cambrian Explosion”

– 450 Myr ago– Sudden appearance of shelled

marine organisms– Cited by critics that life didn’t play a

role in transferring products of weather on land to the seafloor in preceding 4 billion years

Fo

ssil

Rec

ord

is

po

or

or

abse

nt

Development of Life on EarthF

oss

il R

eco

rd i

s p

oo

r o

r ab

sen

t

• 430 Myr ago simple land plants with roots and stems• Similar to the modern-day Psilotum

• 400 Myr ago more complex treelike plants appeared• Similar to modern cycads

Role of Marine Algae and Terrestrial Role of Marine Algae and Terrestrial Microbes on the Early EarthMicrobes on the Early Earth

• Gaia supporters argue that critics underestimate the role of these organisms in Earth’s early history.

• Claim that modern-day bacteria play a greater role in weathering than is recognized– Therefore, they must have been important in

Earth’s early history when they were the only terrestrial life-forms

The Gaia Hypothesis is UnprovedThe Gaia Hypothesis is Unproved

• While fascinating, “the jury is still out.”

• Better quantitative measurements are needed of separate contributions of the following factors to the rate of chemical weathering:– Biological– Chemical– Physical