Since the oceans provide a great deal of food for humans and their livestock, some portion of that carbon is more rapidly cycled than if it were not disturbed. Same with land where food sources are more rapidly cycled due to a few billion people and their livestock. If it were not for mankind, some portion of that carbon would still be cycled, but what is "normal" isn't all that easy to determine.
Obviously, burning the very long term sequestered carbon in fossil fuels is part of the change in the carbon cycle, but land and ocean abuse also play a significant role. Since a very small change in land and ocean carbon sinks can completely offset fossil fuel emissions, it should also be obvious that changes in agricultural, ocean harvesting and general construction have a large impact.
Estimates of land use impacts, specifically soil carbon, are wild ass guesses. For example, "pre-industrial" is vague and soil carbon estimates are limited to at most the top meter of soil. Replacing deep rooted native plants with shallow rooted crops could have twice the impact of current estimates. In the southeastern US, former cotton and tobacco fields have been replaced with tree farms and orchards. Today the yield per acre is a factor of ten or more greater than it was at the turn of the 20th century, so more of these crops can be grown on less acreage freeing considerable land for "conservation" of soil uses. Most likely because of that transition, the US is now a net carbon sink, meaning that US land area is taking up more carbon than it is releasing.
Globally, land carbon balance is negative, meaning it is a source rather than sink and is contributing about a third to the atmospheric carbon increase. If that land area were managed to produce an equal carbon sink, then roughly two thirds of the atmospheric carbon uptake would be reduced. "Science" tends to select when it wants to use gross values and net values in a haphazard manner greatly complicating understanding of the situation. From what I have been able to determine so far, land use has roughly twice the estimated impact, but that really depends on what "pre-industrial" condition is selected.
Potential damage of increased fossil fuel use appears to be over estimated by a factor of two which depends on what "normal" is selected again invoking "pre-industrial" definitions. If you pick 1700 to 1918 as "pre-industrial" you have potentially more warming than if you select 1000 to 1200 as "pre-industrial". The is a growing "war", if you will, in paleo-science with some factions claiming a "pre-industrial" nirvana that never changes, the hockey stick crowd and the new guard of ocean paleo-climatologists indicating considerable variability in past climate.
So despite the claims of "consensus" there are large areas where the climate science is far from being resolved. Politically, the advantage in this uncertainty goes to the precautionary principal crowd. It is really to their advantage to keep large uncertainties all the while playing the Merchant of Doubt uncertain card, belittling the realists with rational questions. Actually solving some parts of the climate problem reduces the precautionary urgency, so don't expect many simple cost effective mitigation attempts to be very well publicized.
Conservation agriculture for example has a positive impact of soil carbon and water retention but requires evil Monsato products like "Roundup" brand weed killer. Like any product, Roundup can be over used and since it isn't "natural" it doesn't have much "green" support. Pesticides which also make life bearable can be over used and aren't on the "green" happy face list along with antibiotics, genetically modified crops, radiation and a surprising large amount of scientifically developed "solutions" to feeding the world and making the world more bearable. Use of these scary science developments can reduce land abuse increasing the acreage that can be set aside for "conservation" which is really a much longer time scale version of crop rotation.
To add insult to injury, the Green Police have started alienating themselves from the third world counties they profess to care for. Coal is one of the least expensive energy sources for many nations that are not allowed to dabble in nuclear and the new Asian Bank initiative is providing funding for projects the Green Police cannot stomach. Every one of these developing nations will run into the same environmental problems faced by the developed nations and will turn to the same "solutions" used in the developed world with the appropriate 10 to 30 year lag time. Instead of trying to force "solutions" on the third world the developed world is better off actually solving problems by losing the not in my backyard mentality knowing that those solutions will be copied, likely without appropriate attribution of intellectual property, by the poorer nations of the world.
All this basically means is that global "de-carbonation" is an incredible myth for at least the next 30 years so focus on the "low hanging fruit", land use improvement, is about the only viable option until some of the energy of the future technologies arrive and are then predictably pirated by the third world.
We live in a hand me down world folks. Leading by example is more than a cliche, it is business as usual. If we use it, properly, they will too. You cannot force "solutions" on a cost conscious world.
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