Carbon Neutral, Mass Balance and other such stuff

If there is anything more confusing than "climate change" it is the carbon cycle.

The US EPA has a handy dandy "Global" carbon emissions article.  "Globally, emissions by sector look about like that.

Regionally, i.e. the US emissions by sector look like this, notice there is no Forestry aka land use piece of pie.

Total Emissions in 2013 = 6,673 Million Metric Tons of CO2 equivalent 
* Land Use, Land-Use Change, and Forestry in the United States is a net sink and offsets approximately 13% of these greenhouse gas emissions.
All emission estimates from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2013

That is because in the US forestry produces a net carbon sink.So if by some odd chance the world changed their land use/forestry practices, "global"land would be a net carbon sink reducing emissions by about 25%.  That would be elimination the 17% emissions and producing a 8% sink.  That could actually be as high as a 35% net reduction since agriculture could add another 5%.  So let's say that the "world" land changed from a 17% net carbon source to a 13% net carbon sink.  That would mean that roughly 30% of the human related carbon emissions would not cycle through the oceans.

"Globally" "nature" is a net carbon sink.  What should be "natural" though is a bit obscured by a few thousand years of human civilization.  Human "civilization" flourished in the "fertile crescent" which is the middle East.  Now that "fertile" crescent looks a lot like desert. Ancient agriculture which is still practiced to some extent in the ROW that has net land use emissions was a bit rough on the land.  'Civilizations" died out due to "climate change" which could easily be related to deforestation for energy and agricultural expansion resulting in the desert regions which could have been lush tropical rain forests at some time in the past.  All that would not have changed "nature" from a carbon sink to a carbon source, but it would have changed the carbon cycle path way.

Around 5000 years ago, which happens to be around the time of one of the major "fertile crescent" "civilization" collapses, atmospheric carbon did reverse from slight downward trend to a slight upward trend.  That "fertile crescent" region could include most of Indian,mainly the Indus Valley, and changes in Indian Monsoon patterns are a big deal as far as "global" climate goes.  This blip in the atmospheric carbon concentration is not one of the more common "climate change" talking points and land use in general takes a back seat to the demon "fossil fuels".  That is probably because it is easy to account for fossil fuels and not so easy to account for land use change.

Land Use, Land-Use Change, and Forestry (17% of 2004 global greenhouse gas emissions) - Greenhouse gas emissions from this sector primarily include carbon dioxide (CO2) emissions from deforestation, land clearing for agriculture, and fires or decay of peat soils. This estimate does not include the CO2 that ecosystems remove from the atmosphere. The amount of CO2 that is removed is subject to large uncertainty, although recent estimates indicate that on a global scale, ecosystems on land remove about twice as much CO2 as is lost by deforestation. [2] 

From the EPA link above, the amount of CO2 removed by land use is subject to large uncertainty.  The minions of the Great and Powerful Carbon are not all that great with uncertainty.  They tend to think they have a handle on it and accuse the non-believers of using uncertainty to muddy the waters.  So they use things like the "carbon cycle mass balance CONSTRAINT" to impress their loyal followers with some creative BS.  The CONSTRAINT basically just indicates that "nature" is a net carbon sink but doesn't provide any indication on what "normal" sink efficiency should be.  Henry's law is a "LAW" that provides considerable information on the ocean part of the sink, but as far as land goes we are pretty much shooting in the dark.

From ocean ph, it is pretty obvious that altering the carbon cycle pathway from stronger land sinks to relying more on the ocean sink is having an impact.  Removing a couple of gigatons of carbon from the oceans each year would also have some impact.  So we are at a point where land and ocean use could have as larger or larger impact on atmospheric carbon than demon carbon from coal and such.  One of the reasons man switched to coal was because "natural" "sustainable" sources of energy were not sustainable and had a negative impact on the local nature.  Trying to go back to "sustainable" energy that involves land use will likely make things worse, it has in the past right?

update:

Since the biggest part of the Mass Balance debate is "what's natural", this is a look at "natural" assuming everything prior to 1750 had to be natural.  Fossil Fuel wise there wasn't much anthro going on pre 1750.  There was land use and deforestation going on though.  Fire was used to clear land and got out of hand I imagine plus wood was the primary fuel.  If you ignore that, this chart of the Indo-Pacific Warm Pool SST and Antarctic CO2 from the Law Dome ice core would be all natural variability.  If you consider that CO2 in ice cores are "naturally" smoothed over a fair long time scale and little plankton shells are also "naturally" smoothed over a different time scale, these two are a remarkably good match.

As I have shown before, the Oppo 2009 IPWP also compares well to the Lamb climate reconstruction which had the Medieval Warm Period and Little Ice Age periods.  The timing isn't perfect, but you should expect some shift with different smoothing time scales.  The selected "pre-industrial" period, 1750 happens to be close to the deepest part of the Little Ice Age anomaly.  "Natural" variability during this all natural assumed period is about +/-0.75 C and +/- 6.5 ppmv CO2.  6.5 ppmv is small compared to current ACO2 impact, but 0.75C is not small compared to current temperature anomaly.

The simple Mass Balance calculation requires estimates of ACO2 to be very accurate which requires inclusion of Land Use impact since it is estimated to be about one third of total emissions.  Land Use "emissions" would also have an impact on the "natural" carbon sink.  For attribution you need to consider both Land use emissions and Land Use sink impact which requires a baseline or "normal" sink efficiency.

If you use the Mann et al. 2015 version of the past you create the impression that all temperature change is created by ACO2.  Climate isn't simple though.  There can be multi-century lags and the solar precessional cycle is about 21,000 years.  Ocean and ice core reconstructions operate on the millennial time scales meaning you have to consider how you smooth your instrumental data to avoid spurious eureka moment spikes.  You may be able to slice paleo to annual resolution, but that will never account for millennial scale natural smoothing already a part of the proxy.