Download Carbon Dioxide and Long

CO2 and Long-Term Climate
Greenhouse Worlds:
Venus and Earth
Why is Venus so much warmer than 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
Venus is closer to the Sun
• Mean distance from the Sun
– Venus:
– Earth:
108.2 million km
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 Insolation
• Amount of insolation received varies
inversely with the square of its distance
from the Sun.
Earth
Venus
(1)2
(0.72)2
=
1
=
1.93
0.581
• Venus receives nearly twice the solar
radiation as Earth does.
• But, this isn’t the reason . . .
Venus
• Upper atmosphere
– Thick cover of sulfuric
acid clouds
– High albedo (80%)
– Only 20% of insolation
reaches the surface.
Earth
• Clouds reflect 26% of
insolation
• 74% of insolation
reaches Earth’s
surface.
Less Insolation Reaches the
Surface 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
0.20
0.74
= 0.52
The Cause . . .
The 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 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
Paradox
Earth’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 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
Should 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
Throughout 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
Throughout Geologic Time
3.2 to 3.5 Bry old
Procaryotes from
Australia
Cambrian marine life
• 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.
Why then, with a weak Sun
wasn’t Earth completely frozen
for the first 3 billion years of its
existence?
A Warming Process Must Have
Been 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 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 Gases
Carbon Exchange between
Rocks and the Atmosphere
Over long periods, slow exchanges
can produce large cumulative changes
in atmospheric CO2
Carbon 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 CO2
Heat in Earth’s interior causes rocks to melt.
Volcanic Eruptions
Volcanic Sources of CO2
Heat in Earth’s interior causes rocks to melt.
Yellowstone National Park
A 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
Compensate
• 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 CO2 the
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
Continental 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
Hydrolysis
• 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 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 Mineral
Example Using Wollastonite
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
Example Using Wollastonite
wollastonite
Accounts for 80% of carbon buried
per year in sediments and rocks
Diatomite
• Silica deposited in the deep ocean
Limestone
• Limestone ridge
in the Canadian
Rockies
• Limestone in
France
Coral Limestone
Barrier Reefs
Great Barrier Reef
Australia
Skeletal Limestone
- Coquina
• Formed from wave-broken fragments of shells, corals,
and algae.
Chalk
• Fine-grained, light colored, and porous from microscopic marine
organisms (plankton).
White Cliffs of Dover
Kent, England
Coccolithophorids
(Coccoliths)
•
•
•
•
Primary constituent of chalk in the White Cliffs of Dover
Calcareous platelets
Secreted by yellow-green algae
Extremely small
Bioclastic Limestone
Coarse-grained with
shell and coral fragments
Fine-grained carbonate
mud from coralline algae
Dissolution 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
calcite
carbonic acid
Ca + 2HCO3
calcium bicarbonate
Dissolution of Limestone Forming Caves
Great Onyx Cave, KY
Howe Caverns, NY
Carlsbad Caverns, NM
Stalactites
Dissolution 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
Temperature
• 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
Precipitation
• Increased precipitation results in a greater rate of
chemical weathering
• Groundwater in soils increases
– Formation of carbonic acid increases
Effects of Temperature and
Precipitation are Linked
• Warm tropics
– High humidity and
rainfall
– Rapid chemical
weathering
• High Latitudes
– Cold and dry
– Little chemical
weathering
Effects of Temperature and
Precipitation 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
Vegetation
• 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:
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
Climate - 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
Climate - Negative Feedback
• Initial cooling is
reduced
• Less CO2 is removed
from the atmosphere
due to decreased
chemical weathering
Negative Feedback Explains the
Faint 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 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 Sun
• Warmer temperatures
• More
– Precipitation
– Vegetation
• More chemical
weathering
• More CO2 removed
from the atmosphere
• Offset warming from
the stronger Sun.
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 Hypothesis
• Proposed by
– James Lovelock
• Independent Scientist,
Environmentalist, Researcher,
Author
– Lynn Margulis
• Department of Geosciences
• University of MA at Amherst
Gaia
• 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 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 . . .
• 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-day
Bacteria Oscillatoria
Fossil Record is
poor or absent
Development of Life on Earth
Stromatolites (2.9 Byr ago)
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, Australia
Present-Day Stromatolites
Development of Life on Earth
Evidence is provided by
Banded Iron Formations (BIFs)
BIFs 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 rustlike 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 Gaia
by supporters
Development of Life on Earth
Fossil Record is
poor or absent
• “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
Development of Life on Earth
Fossil Record is
poor or absent
• 400 Myr ago more complex treelike plants appeared
• Similar to modern cycads
• 430 Myr ago simple land plants with roots and stems
• Similar to the modern-day Psilotum
Role of Marine Algae and Terrestrial
Microbes 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 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