Assume that, in past geologic time, there were much higher concentrations of CO2 in the air. The Earth started not with O2, but with CO2, so that's quite intuitive.
Now look at the entire organic history of the Earth as being one long process of "sequestering Carbon".
"Fixing" Nitrogen is the process of extracting it from the inert, stable N2 molecule into compounds that can be used by plants. Nitrogen forms 79% of the atmosphere, and is needed for plant growth, but plants can't use it in that N2 format because they have no way of supplying enough energy to break it up into nitrous or nitrate compounds. Nitrogen fixation in nature takes energy, which it is said comes from lightning strikes. We fix N artificially in fertilizer plants.
Sequestering Carbon would be extracting, or sequestering, C from ambient, almost-inert and stable CO2 into multiple-Carbon-bond organic compounds. Such compounds are capable of burning, and are a storehouse of potential energy. The energy we get from creating CO2 is large, so also, the energy it takes to break up CO2 is also large. Plants use the energy of sunlight in the long process of photosynthesis to make Carbon useful to them by extracting it from CO2 in ways that they cannot do with N2. Stable compounds tend to form, that's entropy, and that's why breaking up 2H2O into 2H2 and O2 takes so much energy. In nature, nothing comes without cost, and nothing comes from nothing.
Such compounds as cellulose, decaying organic matter, "tar sands", coal (C137-H97-O9-NS for soft coal or C240-H90-O4-NS for better coal with more multiple carbon bonds), crude oil (mostly a mix of hydrocarbons, a junk pile of organic debris formed under extreme pressure, ethane (H3C=CH3), methane (CH4), etc.) then, at deeper levels, into "natural gas" deposits, then, after aeons, dissipated into carboniferous rocks.
Now assume this entire cycle results in more Carbon being deposited deep in the Earth's crust, and that volcanic eruptions then result in redeposit of this C into the atmosphere.
This picture envisions a gigantic, cosmic Carbon cycle, from high CO2 after volcanic events (we know there were huge events in prehistoric times), then the slow sequestering of the C in that CO2, back via plant growth into organic compounds, using the sun's energy and the "plant" of Earth's CO2-eating vegetation growth. Followed by the slow sinking of this sequestered Carbon back into decaying materials, tar, coal, oil, natural gas, and then, at great depths, back into rock.
Now assume that CO2 concentrations of 2000 ppm, about four times current levels, were common in ancient times, resulting is plant growth on a scale we can only imagine today.
The awful consequence is that by taking coal, oil and natural gas out of this cycle, we are interrupting this cosmic C recirculating cycle, and returning to the times of old, when CO2 was at very high levels.
This would make it anomalous and dangerous to artificially to un-sequester all this sequestered Carbon, stored in vast underground pools of oil, natural gas, coal fields, and peat bogs, etc., the work of untold hundreds of millions of years. Energy storage undone in a few hundred years.
One consequence would be the folly of trying to replace the burning of all this oil and coal, which is basically sequestered Carbon, by one-year's plant growth to make ethanol or bio-diesel.
A recent study by the American Solar Energy Society, presented in a huge and important series of technical papers entitled "TACKLING CLIMATE CHANGE IN THE U.S.", points out that the U.S. ALONE would have to offset, or "sequester", at least 1,100 million metric tons of Carbon PER YEAR just for CO2 levels to stay at current elevated 550 ppm numbers.
This vast quantity of Carbon, "1100 million metric tons," is a lot, but it gets worse. That's only for the US. Moreover, in CO2 form, it's three times the weight (CO2 has an atomic weight of 44, Carbon is only 12 parts of that weight).
It's a challenge to believe that we could get enough energy, or make some sort of re-sequestering plants large enough, to recapture that amount of Carbon. Hence, Branson's recent challenge prize to sequester Carbon would be a futile endeavour.
One interesting number to calculate would be
HOW MUCH PLANT GROWTH WOULD IT TAKE TO SEQUESTER 1100 MILLION METRIC TONS OF C, OR 3300 MILLION METRIC TONS OF CO2?
Intuitively, it seems this number would be many times the current level of plant growth on the entire Earth. If so, it means that there is NO way that planting trees, crops, or human sequestering of C, could possibly overcome the huge effects of interrupting that cosmic Carbon cycle.
