Course News

1
Course News
IMPORTANT

• First Hour Exam, Tuesday 21 September
• exam held in Packard 101, not here in PA 466
• exam held during regular class time (920-1035)
• exam covers lectures and reading material up
  through Chapters 1-4 in Ruddiman
• format is multiple choice, machine graded
• bring 2 pencils to exam

2
Lecture 8 Tectonic-Scale Climate Change/Faint
Young Sun Paradox

• Why care?
• We want to learn all possible states for Earth
• We need context for modern changes
• Distance in time filters record
• downside cant see short-term events
• upside clarifies long-term controls and feedbacks

3
Earth Has Been Habitable For 3.55 Billion Years

• Sediments amount to an ancient and long record of
  liquid water
• Life present at 3.55 byr or earlier
• Earth was never frozen (or boiled)
• Two main climate states
• icehouse ice sheets present
• greenhouse (hothouse) - no ice sheets present

4
Why Has Earth Been Habitable?
Venus compared to Earth Twice the solar
flux Same carbon, but all in atmosphere as
CO2 Higher albedo

• Venus stinking hot, Earth just right
• Factors? Distance from Sun? radiation balance?

5
Snowball Earth (aside)
Extensive (global?) glaciation 550-850 Myr Earth
almost frozen? Success of Life to blame (by
drawing CO2 too low)?
6
Faint Young Sun Paradox

• Suns output must have increased by 50
• Somehow the increase in solar output has just
  been compensated
• Implies a natural thermostat exists
• Greenhouse gases must be involved

Even with a modern atmosphere,Earth frozen for
much of its history
7
Carbon ReservoirsPossible Sinks and Sources
Relevance of reservoir proportional to time scale
8
Solid Earth as Long-Term Carbon Source

• Volcanoes are a variable source of on average
  0.15 gigatons/year of carbon
• Processes that balance volcanic flux operate at
  scales of thousands of years, so atmospheric
  composition vulnerable to long-term changes
• Hard to see how this could steer delicately
  balanced and consistent change over billions of
  years, especially given what we know about the
  variability of volcanic emissions

Rate of volcanic emissions cant be thermostat,
because there is no feedback involved
9
Chemical Weathering as the Main Long-Term
Thermostat
10
Climate Controls on Chemical Weathering

• Weathering reaction rates can vary by up to a
  factor of 10 due to variations in
• temperature
• precipitation
• vegetation cover

11
Weathering as Long-Term Thermostat
Change in rates of CO2 removal by chemical
weathering is a negative (regulating) feedback in
the climate system
12
Organic Carbon Subcycle
Its possible to physically bury organic carbon
far faster than inorganic carbon (for which the
rate of chemical weathering is the control)
At intermediate to longer time scales, changes in
organic productivity or burial rates could cause
significant changes
13
Resolution of Faint-Young-Sun Paradox

• Increase in Suns strength offset by reduction in
  initially much stronger greenhouse effect
• Offsetting mechanism is increase in chemical
  weathering

14
Gaia Hypothesis - Does Life Have its Hand on the
Thermostat?

• Global homeostasis regulation by and for the
  biosphere- James Lovelock
• Life regulates climate (environment) for its own
  good
• Carbon (weathering, deposition) albedo
  (cloud-seeding by algae-produced dimethyl
  sulfide cloud formation via water vapor and
  veggies)
• different flavors of hypothesis
• passive, like weathering, deposition
• more active (evolutionary responses)

15
Gaia Hypothesis - Does it Fly?

• Is it a scientific hypothesis?
• Cons
• Not testable
• (depends on flavor of hypothesis)
• Most of the time, early life too simple
• (simple is ok life had impactoxygen at 2.3 Byr)
• Big excursions imply poor regulation
  (extinctions, Snowball Earth, oxygen poisoning)
• (need to consider time scale, external forcings)