Variations in External Forcing

1
Variations in External Forcing

• All related to changes in solar input to Earth.
• Solar Cycle
• 11-year cycle 0.2 in Flux
• gives variation of 0.1K in Teff
• Faint Young Sun
• Young sun believed 20-40 dimmer
• Milankovitch Cycles (orbital changes)
• correlated to ice ages

2
The Faint Young Sun Problem
3
Faint Young Sun Paradox

• Higher atmospheric greenhouse gas levels are a
  good way of compensating for the faint young
  Sun.
  
• During the Archean Eon (3.8 to 2.5 billion years
  ago), before photosynthesis, O2 levels were very
  low.
  
• In todays atmosphere, methane (CH4) has a
  relatively short lifetime (10 years) due to
  reaction with OH. Oxygen is required to form
  OH.
• In the anoxic Archean atmosphere, there was no
  OH. CH4 from small biological sources
  (fermentation) would have built up over time in
  the atmosphere.
• CH4 probably made a significant contribution to
  the Archean greenhouse effect.

4
CH4-Climate Feedback Loop

• Archean life, thermophilic methanogens,
  generated more methane as temperatures increased,
  further warming the climate
  
• But
• If CH4 becomes more abundant than CO2, organic
  haze begins to form, cooling climate.

Titans Organic Haze Layer
5
Archean Climate Control Loop
CH4 production
()
Surface temperature
Haze production
Atmospheric CH4/CO2 ratio
()
CO2 loss (weathering)
6
Snowball Earth

• Earths climate is stabilized on long time-scales
  by the carbonate-silicate cycle.
  
• Earths climate is theoretically susceptible to
  episodes of global glaciation. It can recover
  from these by buildup of volcanic CO2.
  
• The first such Snowball Earth episode at 2.4
  Gy ago may have been triggered by the rise of O2
  and loss of the methane component of the
  atmospheric greenhouse.

7
Snowball Earth Glaciations

• Geological data indicate tropical glaciers at sea
  level 2.3, 0.75, and 0.6 Gy ago.
  
• The first of these (Huronian glaciation, 2.3 Gy
  ago) may be triggered by the rise of O2 and the
  corresponding loss of CH4.
  
• The later glaciations may have been triggered by
  changes in carbon burial.
  
• (Hoffman et al., Science 281, 1342, 1998)

8
The Carbonate-Silicate Cycle
9
The Carbonate-Silicate Cycle Negative Feedback
Loop(Stabilizing)
Rainfall
Surface temperature
Silicate weathering rate
(-)
Atmospheric CO2
Greenhouse effect
10
Late Precambrian Geography
Hyde et al., Nature, 2000
glacial deposits
11
Triggering a Snowball Earth Episode

• Continental rifting created new shelf area,
  thereby promoting burial of organic carbon.
  
• Usually there is a negative feedback weathering
  of carbon-containing silicates decreases as
  temperature decreases, slowing the loss of
  carbon.
• Clustering of continents near the tropics may
  have allowed silicate weathering to proceed even
  as the global climate gets cold.
  
• Alternate theory evolutionary invention of fecal
  pellets may have increased carbon burial.

12
Glacial Tipping Point
13
Caldeira and Kasting, Nature, 1992
14
Recovering from a Snowball Earth episode

• Volcanic CO2 builds up to 0.1 bar.
• Ice melts catastrophically (within a few thousand
  years).
  
• Surface temperatures climb briefly to 50-60oC
  (hot!).
  
• CO2 is rapidly removed by silicate weathering.

15
How Did Photosynthetic Life Survive?

• Near volcanic vents, i.e. Iceland?
• Some models show glaciation limited to the land
  (open ocean at the equator).
  
• Rate of precipitation adding to ice caps may have
  slowed as ocean covered.
  
• Interpretations of rocks from ocean sediments is
  subject to heated debate.
  
• Snowball Earth or Slushball Earth?