Major events in Earth's climate history that should be memorized are

Early Atmospheric Composition and Climate

In spite of the early faint sun,  the planet is thought to have been warm to maintain liquid water at the surface for most of its history and it was probably often warmer than it is at present. We have many reasons to believe this is true. The text cites the continuity of life for the past 4 billion years as important evidence  (p230).  Yet we think life survived many severe glaciations (Huronian, Ordovician, etc) and even complete glaciation during the Neoproterozoic. There are plenty of other compelling reasons to believe that the earth was not seriously glaciated for most of the deep past - such as an

• Absence glacial deposits
• Sedimentary carbonate rock cycle and their carbon isotopes
• Absence of BIFs (deep ocean acquired oxygen via surface exchange)
• Paleosols
• Redbeds
• etc

Most likely the Earth was kept warm by high concentrations of CO2. The heavy impact period supplied CO2 as planetesimals vaporized on impact. Geologists think volcanism was greater in the past and weathering was less effective at drawing down CO2 because the total area of continents was smaller.

Calculations of the amount of CO2 necessary to just barely keep the ocean from freezing over shown in Fig 12-3 indicate the partial pressure (pressure in the absence of all other gases) of CO2 would need to be about 0.1 bar in the first billion years or so. Probably the temperature was much higher, which means CO2 would have to be more like 1-10 bar. Carbonate rocks could provide enough carbon if something like 10% of the carbon that is currenty in  carbonate rocks had been in the atmosphere reservoir instead. This seems like a lot though. Furthermore paleosols (ancient soils) give an upper estimate of the concentration of CO2, which limits it to more like 0.01-0.1 bar. This means CO2 was probably accompanied by another greenhouse gas. Methane is a likely candidate because we know early bacteria produced methane (some bacteria still do). This could have caused a postive feedback where methanogens that are also hypotherphiles flourished. Methane is probably not able to run away though owing to its anti-greenhouse effect in very high concentrations.

The rise in oxygen 2 billion ybp was probably still enough to have driven away methane. Recall that methane is produced by bacteria in an anaerobic (low oxygen) environment. In fact methanogens don't survive in oxygen (most if not all - I'm not really sure). Furthermore methane breaks down in oxygen. In fact, the rise in oxygen probably occurred simultaneous with a reduction in methane levels. There is speculation that the Huronian glaciation dated back to about the same time as the rise in oxygen may be a result of the resulting reduction in methange.

Geological evidence of oxidized minerals tells us that Earth's atmosphere was oxidizing for the past 2 billion years, yet advanced multicellular life began with the Cambrian Explosion. It has been hypothesized that the atmosphere may have had enough oxygen to oxidize minerals but it was still limited enough in oxygen to prevent life from becoming complex and diverse. There are competing theories that will be discussed under the snowball earth heading below. Evidence in support of the low oxygen hypothesis is that early multicellular creatures were flat - so they could asborb oxygen effectively through a large surface area.

It is thought that eventually oxygen reached very high levels during the Carboniferous and early Permian. The evidence comes from carbonate sediments. Sediments form when organic and inorganic carbon materials settle to the sea floor. Inorganic sediments are high in ¹³C when photosynthesis is prevalent because photosynthesizers prefer ¹²C and so leave the atmosphere rich in ¹³C, which then makes its way to form inorganic carbonate sedimentary rocks. Photosynthesis also produces oxygen, so evidence of a high fraction of ¹³C to ¹²C indicates that oxygen is abundant. A high "fractionation" of ¹³C is seen in carbonate rocks that date to the Carboniferous and early Permian (see Fig 11-19 in the text). In addition a great quantity of coal can be dated to this time, which indicates organic carbon burial was high at the time.

Oxygen is thought to be regulated by the activity of photosynthesis and also by exchange with the ocean. Dissolved oxygen levels at the surface of  the ocean are a function of temperature. Colder water can hold more dissolved oxygen, so the high latitudes draw oxygen from the atmosphere and deep water formation in the high latitudes takes this relatively high concentration of oxygen down deep in the ocean. Thus the world ocean water has higher oxygen levels at depth compared to the surface. There is an oxygen minimum in this profile at about 1 km resulting from high levels of decomposition of organic materials that rain down from the surface because oxygen is consumed by decomposition.  

It is thought that if atmospheric oxygen decreased, this oxygen minimum might fall to much deeper levels, perhaps even to the point where decomposition is incomplete altogether and ocean sediments would contain higher levels of organic material. This is however not seen.

There is evidence of another mechanism involving organic carbon burial. Apparently bacteria in ocean sediments store Phosphorus (P) when oxygen is plentiful and release it when the ocean is anoxic (low in oxygen) at depth. Phosphorus is a part of nutrient cycle that influences phytoplankton. If the deep ocean became anoxic and P is released, it then becomes available to "fertilize" phytoplankton, which increases oxygen production and serves to return the atmosphere and ocean to higher oxygen levels

• Complete ice cover is an essential ingredient - the paleo evidence of BIFs and cap carbonates demands ice covered ocean (to make the ocean anoxic) and land (to shut off weathering)
• Complete ice cover is essential to get out of the snowball state too. Shutting off the sink of CO2 (weathering) but still allowing a source from volcanoes caused a super greenhouse to build.
• To make a snowball, sea ice must advance to about 30 deg N and S from the poles. Once it does, a small additional advance in the sea ice causes an instability and sea ice covers the planet rather quickly. This assumes that the whole ocean is near freezing - not a trivial matter. Ocean innertia may have slowed the advance somewhat. Ignoring this ocean innertia, the advance of a snowball is a balance between positive feedbacks from ice-albedo feedback and water-vapor feedback and negative feedback from temperature-OLR feedback.
• It is probably also important that land was mostly in the tropics, where cooling lagged that of the poles, and so weathering carried on strong while ice was advancing toward the tropics.
• Our ancestors - the eukaryotes - survived the snowball earth.  So they were only algae, but we think they were photosynthesizers, which adds to the fun of trying to find a habitable place with liquid water and sunlight.

Eukaryotes have been around for about 2 billion years, but they didn't evolve much (or at least theose that did advance didn't survive the snowball earth) until the end of the snowball earth period.  However soon after (?) the last event, eleven members of the eukarya phyla emerged quite suddenly!  There are two theories about why the eukarya diversified so suddenly: (1) Isolated populations around hot springs could have mutated into new species (this happens on islands today  with such odd creatures as the Komodo dragon and the dodo). Each population might have had low diversity, but taken together after a few million years while they diverged in isolation could account for a high degree of diversity. (Do you buy it?) (2) Hardships brought on by a rapidly changing environment favoed the emergence of new life forms. Recall that we learned tropical regions that experience a modest degree of disturbance tend to be diverse than  very stable regions.