>Trees certainly work as a buffer in the sense that carbon in plants is carbon not in the atmosphere, but the amount you could hold is tiny in comparison to the amount that must be sequestered
Is it? Let's do the math.
Absorbing the 350 teratonnes of human emitted carbon[1] over the 8.4 billion hectares of Earth's non-tundra / non-desert land surface area[2] works out to... 42 tonnes per hectare. If we can green deserts[3] that drops to 30 tonnes.
That's 30-40 trees per hectare, at 1 tonne per tree.[4] Or 3 kg per square meter of soil carbon, the equivalent of 25-35 cm of topsoil. More likely some combination of the two. Soil carbon is stored up to 40 meters down by deep rooted plants.[5]
Most of our land management is via agriculture, so agriculture seems to be the only lever long enough to make a dent. Practically this implies transitioning from soil destroying tillage to soil building cover crops, long-distance imported fertilizer to in-situ fertility produced by soil organisms, and ecologically unstable monocultures to resiliant polyculture, agroforestry, and rotational grazing systems.
The "gotcha" question is not whether we can replace our existing agriculture with these systems. The question is whether we can replace our existing agriculture with itself. Any unsustainable system is, by definition, not a viable replacement for itself.
Note that big farmers have already replaced "soil destroying tillage" with "soil building cover crops". Farm news is full of stories about early adopters 10 years ago bragging about how they are now building soil organic matter with a corresponding reduced need for fertilizer. The big farmers recognize the impact this has on the bottom line. Your mid sized farmers see that extra $10/acre in fertilizer as a cost of business and don't realize they could reduce it.
Unfortunately "long-distance imported fertilizer to in-situ fertility produced by soil organisms" cannot work if food is exported off of the farm. Every gram of food taken off the farm includes some micro-nutrients - various minerals that are required for life that have been removed from the soil and taken elsewhere.
Agreed, we're making a lot of progress on this front!
>Unfortunately "long-distance imported fertilizer to in-situ fertility produced by soil organisms" cannot work if food is exported off of the farm.
It's not like there's a shortage of rocks (where soil bacteria and deep rooted plants harvest and dissolve micronutrients) or air (the source of carbon, as well as nitrogen fixed by rhizobium bacteria in root nodules). Soil tests only look at dissolved nutrients, not those yet to be released from the minerals in rock, which range in size from bedrock to clay. This is detailed in the video I linked.
But it certainly helps if you recycle human waste into fertilizer, rather than just releasing those nutrients into waterways (and ultimately, the ocean) or landfills. The key is to close the fertility loop while also interrupting the fecal-oral route. Thermophilic composting or biogas digesters can do this on a small scale, and systems like those used by Milorganite can do this on a city scale.
Is it? Let's do the math.
Absorbing the 350 teratonnes of human emitted carbon[1] over the 8.4 billion hectares of Earth's non-tundra / non-desert land surface area[2] works out to... 42 tonnes per hectare. If we can green deserts[3] that drops to 30 tonnes.
That's 30-40 trees per hectare, at 1 tonne per tree.[4] Or 3 kg per square meter of soil carbon, the equivalent of 25-35 cm of topsoil. More likely some combination of the two. Soil carbon is stored up to 40 meters down by deep rooted plants.[5]
Most of our land management is via agriculture, so agriculture seems to be the only lever long enough to make a dent. Practically this implies transitioning from soil destroying tillage to soil building cover crops, long-distance imported fertilizer to in-situ fertility produced by soil organisms, and ecologically unstable monocultures to resiliant polyculture, agroforestry, and rotational grazing systems.
The "gotcha" question is not whether we can replace our existing agriculture with these systems. The question is whether we can replace our existing agriculture with itself. Any unsustainable system is, by definition, not a viable replacement for itself.
[1] http://petrolog.typepad.com/climate_change/2010/01/cumulativ....
[2] http://cnx.org/contents/TWFXbERo@1/The-main-biomes
[3] https://www.youtube.com/watch?v=2xcZS7arcgk
[4] http://cabiblog.typepad.com/hand_picked/2011/06/ever-wondere....
[5] http://www.soilquality.org.au/factsheets/organic-carbon