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First, astrophysicists have long held that our sun should have an evolution though time similar to those inferred from the study of other stars of similar size in the universe.

Our sun should have begun rather small and dim and grown in diameter through time. The amount of sunlight reaching the Earth should thus have increased by from 15% to 30% since the earth formed some 4.5 billion years ago.

If nothing else was different than today, this would mean the surface of the earth world have changed in temperature tremendously, and no liquid water could have been present on the Earth prior to 2 billion years ago.

However, we see instead by looking at the geological record, that there has been liquid water on the earth since it its crust solidified, and in general the Earth's surface seems to have remained within a surprisingly narrow range.

This is called the Faint Young Sun Paradox.

Second, CO2 as measured in our atmosphere shows a remarkable seasonal cycle.

The so called "Keeling curve" from the Mauna Loa observatory shows this spectacularly as well as dramatic upward trend in CO2 caused by fossil fuel burning.

Where does this cycle come from?

Obviously plants exert a great effect on CO2 levels.

First, lets look at some numbers.

These numbers tell us a lot about the nature of the system.

Two kinds of biogeochemical cycles maintain the Earth's atmospheric levels of CO2: fast and slow.

The fast cycle operates on time scales of hundreds to thousands of years.

The second operates on hundred of thousands to millions of years.

Both are essential.

The critical chemical reactions are:

Photosynthesis and Respiration:

CO2 + H20 + e = CH2O + O2

Carbonation:

CO2 + H2O = H2CO3 = H+ + HCO3-

Calcium Carbonate dissolution and precipitation:

Ca2+ + 2HCO3- = CaCO3 + H2O + CO2

Carbonate equibrium in seawater:

H2CO3 = H+ + CHO3- = H+ + CO32-

Note also that any carbon not immediately respired results in the accumulation of O2 in the atmosphere. We have O2 in the atmosphere because of the C buried as organic matter in sediments and rocks.

A negative feed back loop keeps O2 levels from getting to high:

If O2 levels get to high, land biomass will burn and photosynthesis will go down,
and O2 will go down.

Also the more carbon is buried, the more nutrients are buried, putting another break on the system.

Over hundreds to thousands of years, adding more CO2 to the atmosphere is just sucked up by the ocean, lowering the pH and thus producing more bicarbonate to neutralize it from carbonate thus driving the equibrium equation back towards the acid side. Lowering atmospheric CO2 has the opposite effect, and results in the precipitation of CaCO3. This effect was spectacularly observed in the water pool in the lung in Fall, 1995.

Because the ratio of ocean C to atmospheric C is about 50 to 1, doubling or tripling atmospheric CO2 does little to the oceans or the net atmospheric CO2 on the long run. The only reason we are having an effect on the atmosphere is because the RATE of the input exceed that of the removal by the oceans! Over thousands of year our contribution to the atmosphere via fossil fuel burning would be nil.

Note also that if we look just at the fast cycle, the precipitation of CaCO3 is a source of CO2!

OK, but why settle on say 250 ppm instead of other amounts. Well, this must be a function of the amount of carbonate in the oceans.

That is controlled by the long term cycle of carbon.

Here the critical relationships are termed the UREY reactions, which are:

Calcium and magnesium silicate weathering and metamorphism:

CaSiO3 + CO2 = CaCO3 + SiO2

MgSiO3 + CO2 = MgCO3 + SiO2

With some intermediates this is for Ca silicates:

CaSiO3 + 3H2O + 2CO2 =
Ca2+ + 2HCO3- + H+ + Si(OH)4 =
CaCO3 + SiO2 + H2O + CO2

So there is a net loss of one molecule of CO2 for every molecule of CaSiO3 weathered and the precipitation of carbonates is a net sink for CO2 not a source!

Because of plate tectonics nearly all of this buried carbon is returned via subduction and metamorphism over about 200 million years

In total about 0.2 x 10^15 g of C is buried each year and just about that is returned by outgassing.

So, its chemical weathering that controls the flow of Ca2+ and HCO3- to the oceans where it ends up being buried as carbonate.

The most obvious one is temperature.

If CO2 in the atmosphere goes up, temperature goes up,
But if temperature goes up, chemical reaction rates go up
If chemical reaction rates goes up, chemical weathering of Ca and Mg minerals goes up
CaCO3 precipitation goes up
and so the CO2 goes down.
and the temperature goes down

But,

If CO2 goes down,
temperature goes down,
weathering rates go down,
CaCO3 precipitation goes down
and CO2 accumulates in the atmosphere
and the temperature goes up.

But there are other feedbacks involved:

Plants add CO2 to soils via respiration and their ultimate decay and respiration by bacteria.

They also have organic acids of their own which results also in added chemical weathering.
They also hold the water in the soil longer
Thus plants are said to fertilize chemical weathering
But plants use CO2 as their source of carbon, so more CO2 makes plants grow faster,
which makes weathering go faster too.
This is an another negative feedback

But, faster plant growth is limited by nutrient availability,
but that is a positive function of weathering.
which could compensate for the grater rate of plant growth
This is a positive ffedback for the plants.

But a positive feed back on temperature is that: higher CO2 leads to warmer temperatures,
which leads to more evaporation,
which puts more of the greehouse gas, water vapor in the atmosphere,
which makes it warmer

Well you would need about 1000 x present CO2 levels to compensate.

Or some other greenhouse gasses, such as methane (CH4)

This is possible and has been proposed as the solution.

Methane may have especially important in the early Atmosphere because of the lack of lots of O2.

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