CO2 and Long-Term Climate Change

Question 1

Earth’s Temperature

Answer 1

The Earth’s temperature
has varied over time from
very cold glacial worlds,
possibly even a Snowball
Earth during the
Precambrian, to very warm
worlds with palm trees
growing in high latitudes.

Question 2

Faint Young Sun
Paradox

Answer 2

• Early sun ~20-30% weaker
• First evidence of ice ~2.3 Byrs ago
• possible “Snowball Earth” between 850-550 Myrs
  ago…
  ‣ sun only ~5% weaker at this time
  need an internal thermostat
• i.e. a negative feedback mechanism

Faint Young Sun Paradox
(Fig. 4-2)
Sun was 25 to 30 % weaker
in Earths early years. Enough
to freeze the earth with
today’s GHG concentrations
However, the Earth was
never totally frozen for
billions of years. Some early
life forms existed.

Question 3

Snowball Earth Hypothesis

Answer 3

Snowball Earth hypothesis, in geology and climatology, an explanation first proposed by American geobiologist J.L. Kirschvink suggesting that Earth’s oceans and land surfaces were covered by ice from the poles to the Equator during at least two extreme cooling events between 2.4 billion and 580 million years ago.

Question 4

1. Prologue

Answer 4

Breakup of a single landmass 770 Ma
leaves small continents scattered near
the equator.
Formerly land-locked areas are closer
to oceanic sources of moisture.
Increased rainfall => increased
weathering => decrease in
temperature => large ice-packs form
in polar oceans.
Ice has a higher albedo than water.
Ice-albedo feedback cycle => planet
engulfed in ice within a millennium.

Question 5

2. Coldest state

Answer 5

Global average temperatures plummet to -50
deg C after runaway freeze begins.
The oceans ice over to an average depth >1
km.
The only heat source is from the Earth’s
interior (Sun still faint).
Most microscopic marine organisms die, but
a few cling to life around volcanic hotsprings.
The cold, dry air arrests the growth of land
glaciers, creating vast deserts of windblown
sand.
With no rainfall, CO2 emitted from
volcanoes is not removed from the
atmosphere -> gradual warming -> thinning
of sea-ice.

Question 6

3. Thaw stage

Answer 6

After 10 million years of ‘normal’ volcanic activity,
concentrations of CO2 in the atmosphere
increase 1000 fold.
Ongoing greenhouse warming effect pushes
temperatures to the melting point at the
equator.
As planet warms, moisture from sea ice
sublimating near the Equator refreezes at higher
elevations => feeds the growth of land glaciers.
The open water that eventually forms the tropics
absorbs more solar energy and initiates a faster
rise in global temperatures.
In a relatively shorter time-period of a few
hundred years, an extremely HOT and wet world
will supplant the deep freeze.

Question 7

4. Hothouse aftermath

Answer 7

As tropical oceans thaw, sea-water evaporates
and works along with CO2 to enhance the
greenhouse conditions.
Surface temperatures soar to more than 50
degrees C. The hydrologic cycle of rainfall and
evaporation is intensified.
Torrents of carbonic acid rain erode the rock
debris left in the wake of the retreating glaciers.
Swollen rivers wash carbonate and other ions in
the ocean, where they form carbonate sediment.
New life forms populate the world as global
climate turns conducive.
Life engendered by prolonged genetic isolation
and selective pressure.

Question 8

The Rock Carbon Cycle

Answer 8

• CO2 input to the atmosphere
• CO2 removal from the atmosphere
• ⇒ over time these balance each other

Question 9

Volcanic CO2 Input

Answer 9

• Current rate: 0.15 GtC/yr
• balanced by removal at same rate
• current anthropogenic emissions: ~10 GtC/yr
• What if all volcanic activity stopped?
• how long before all CO2 in the atmosphere was
  consumed? (600 GtC)
• all near-surface carbon? (3700 GtC)
• all non-rock carbon? (41,700 GtC)
• Can volcanoes act as a thermostat?

Question 10

Chemical Weathering

Answer 10

• Hydrolysis
• H2O + CO2
• CaSiO3 + H2CO3 ➙ CaCO3 + SiO2 + H2O
• rocks carbonic acid shells of organisms - Continents are mostly silicate minerals (quartz, feldspar…)
• Hydrolysis makes up 80% of total (0.15 GtC/yr) flux
• Net removal of CO2 from the atmosphere
• Dissolution
• H2O + CO2
• CaCO3 + H2CO3 ➙ CaCO3 + H2O + CO2
• rocks carbonic acid shells of organisms
• No net removal of CO2 from atmosphere
• Thermostat?
• must be climate sensitive
• Climatic controls on weathering
• temperature
• precipitation
• vegetation

Question 11

Thermostat?

Answer 11

• Climate sensitive? Yes.
• Warmer climate ⇒ increased weathering
• Colder climate ⇒ decreased weathering

Question 12

Weathering as negative feedback

Answer 12

initial change — warmer climate — increase temperature, precipitation, vegetation — increased chemial weathering — increased CO2 removal by weathering — reduction of initial warming

Question 13

Weathering as negative feedback

Answer 13

initial chnage — colder warming —— decreased temperature, precipitation, vegetation — decreased chemial weathering — decreased CO2 removal by weathering — reduction of initial warming

Question 14

Life as Thermostat?

Answer 14

• Hydrolysis ⇒ 80% of carbon ➙ rock flux
• Life ⇒ 20% of flux
• GAIA Hypothesis: Life may be responsible
  for long-term climate regulation
• organic carbon subcycle
• influence of vegetation on weathering rates
• First life ~ 3.5 Byrs ago ➙ CO2 drawdown
• O2-rich atmosphere at ~2.3 Byrs ago
• first land plants ~400 Myrs ago
• Progressively larger influence on weathering rates
  over time
• Counter some of increased sun strength?
• First shelled animals ~ 540 Myrs ago
• Chemical CaCO3 precipitation before then

Question 15

Flowering plants (Angiosperms)

Answer 15

Did not exist before 130 Ma
Rise of Angiosperms
between 130 Ma and 80 Ma
Believed to increase rate of
weathering.

Question 16

Atmospheric
Carbon Dioxide (CO2)

Answer 16

Ancient soils and fossils of
plants and animals provide
evidence that atmospheric
carbon dioxide levels have
varied considerably over the
last 600 million years (top)
with glacial periods generally
corresponding to times with
low levels of this greenhouse
gas.