Labor , as used to make physical changes, seems to disappear completely as a production factor. While about 150 years ago the labor power, ‘LP’ , was the major force in society, besides the Horsepower, the HP. Maybe even you remember… . There was even a lively trade in LP’s, slaves where brought from Africa to America, which has been described as a result of the first big energy crises. .
Discovering ways to makes fossil resources useful as energy, made slavery obsolete. Some small pockets of real Labor Force resist, mainly in construction, ( and some smell like slavery as in constructing football stadiums in Qatar, ) But even construction work is moving towards industrialized prefab factories. So strange enough, all Labor Power is slowly phased out as a production factor. Which is not really smart, if at the same time we are outphasing fossil resources….
Labor Force brings more as laborers cost, is a often heard. Not completely unexpected: Someone can grow vegetables on a piece of land, and yield more then he needs personally. From the rest others can be exempted from growing food and start doing other things ( in the Japanese EDO period the people were called the “un-productives”) . However physically its impossible: that more comes out as goes in. It conflicts with all physical laws, and mainly thermodynamics: Otherwise we would not have an energy problem. Energy , the power to change, is always lost in a conversion process . There are losses in the conversion, and there is always a maximum to what can be converted. Then how about Labor? The evaluation starts at the wrong moment: Not the output of labor is the measure, but the food that went int the laborer: that does not come free of impact, it required land, including a year of solar radiation input. The growth of plants is proportionally to the amount of solar radiation received. Thats the input . And via labor less comes out as went in. Thats the issue. But then, is labor still an interesting source? Yes it is. Namely, the solar radiation energy comes form outside the earth, and if the earth is the evaluated system, labor has a net contribution:. Pure the energy conversion via humans costs energy, but via the combination with land and food grown its solar radiation that has been converted and made available: a net profit within the limited system of the earth. So its renewable energy. ( which involves land as converter, but thats for another time)
There is another side to this: people exist, and don’t go away. So food is anyway needed, whether that human being is providing some labor power or not. ( natural selection for human beings has already been overruled…) And to abolish people, is not an option, the whole energy and environmental crises would not exist if people that consume were absent. What we want is to maintain and sustain the system, with people included. And then it becomes a smart strategy to accept the existence of people, and to make their labor work, providing a net contribution to that system. Otherwise it makes no sense at all. ( nevertheless, its wise to limit the growth of population somewhat…) So make Labor Power work, I would say.
Strange enough we cant find labor as a standard factor in most energy and environmental assessment calculations, as for instance in Life Cycle analyses , ‘LCA’. In fact that could result in having more labor input giving a better LCA result. It should be brought in as a positive contribution. But none of that. To the contrary, if labor is studied in assessments, its usually seen as a negative impact. For instance the food intake as load.  But abolishing people is no option, as we argued. If the labor is replaced with machine power, the food load remains… In another case its the energy consumption of a labor hour thats counted ( in offices etc).  And we find a study that tries to calculate the social load , in terms of workers needed.  The most interesting study I found was quit extreme. The human work impact was split into three kinds of labor: HL1 is ‘qualified worker’, HL2 = ‘technician’, HL3 =’manual worker’ . And the impact is calculated as what the person consumes in daily life. The qualified worker has the largest impact. No wonder: earns the most money, consumes the most. Now remember, this is the same guy that was exempted from agricultural labor, the unproductive one in Edo, now causing the largest damage… Which I must say, is not a result of qualifying labor, but from the use of a artificial and cripple financial value system. Not a results of Labor Power analyses. Of course in that valuing system, the manual worker has the least impact, and probably earning the least, he is the least valued…. But still: he is calculated as having negative impact. His energy contribution has not been included. 
If you extrapolate this, that Labor Power is no longer a production factor but only a net environmental load, then this leads automatically to eliminating as much labor as possible: in other words: scientifically you strive for a ‘unemployed’ society. (which socially is already a trend, people are bothersome and robots patient) . But now scientifically you run all energy by the human conversion system useless. That is: the energy is probably used to think behind a desk or in the sofa of new technologies, that need less labor and consume more resources . Again, their load does not disappear, and you refrain from their solar energy accumulated power.
On the other side: if Labor always would be a positive contribution, there is the risk that to create a counter effect: that we start making more babies, as production factors, as impact free contributors. Thats not true of course, and should be guided by birth control . ( see blog…. )
So then how to deal with labor, how to get back with two legs on the ground? The answer might be in agricultural studies as labor-food system. As for instance advocated by Via Campesina, a NGO concluding that “ energy in agriculture was converted from something that originally was the main product of agriculture to something that became a main input into agriculture “.
This is a better starting point since the system evaluated is a ha of land , the net receiver of out of system resources, and not limited to a person. 
Its indeed not very smart to put more and more fossil fuels into agriculture. There are calculations that it requires around 10-15 calories to invest in agriculture, to get 1 calorie out.  This has all to do with the fact that the yield per hectare is raised, but at the cost of immense burden to other resources, and therefor just shifting burdens. (Again, a results of our artificial financial system, not from a whole system analyses) Its double disaster: at one side agriculture gets less effective and exhaust other resources, which runs more and more people obsolete in agriculture, exempted from work, and consuming more resources as qualified worker, which raise the need and wants again…
Following that arguing, its clear how in LCA’s this can be seen as a load by a worker. However that is a result of consumption that got out of hand, not related to Labor Power itself. Which requires as much food as 150 years ago ( except for the cases of obesity that is…)
Its somehow understandable, if we would not have pushed agriculture output that much, we might have run out of food long before  ( reinstating natural selection) .
In fact the conclusion is that land is the most scarce source we have , more as the fossil resources,which have been put in excessively. Which in turn now run low as well, and which we want to ban to prevent a too big climate change. Thats the crucial point.
So labor Power has to make a come back. As a positive contribution*. And as a side effect its more healthy as sitting all day in an office, and good to combat unemployment. Only, we have to get the physical assessments right, as well as to develop a new financial valuing system, that highly values labor. And meanwhile try to avoid that we run the other way, and grow LP-units again since they will become so valuable…
- To put things in perspective, Labor has too little power to sustain our current industrial society , to supply all energy for all our luxury. However in Nepal, it has been calculated that if every inhabitant would bicycle two hours a day and generate and store the energy , that would be enough for the current energy demand of the country! Which the more shows that our industrialized society has completely gone out of hand . More about this you can find at lowtech magazine, from which I learned this fact : its a beautiful source for the real powers in our society 
 Something new under the sun, An Environmental History of the Twentieth-Century World , McNeill, 2001, book: W. W. Norton & Company, Inc.
 The energetic metabolism of the European Union and the United States decadal energy input time-series with an emphasis on biomass. Haberl H, et all, (2006) J Ind Ecol 10(4):151–171
 Energy use per working hour : calculating the contribution of labor to manufacturing energy use.
Teresa W.Zhang and David A. Dornfeld page 189 in proceedings CIRP conference on life cycle engineering Tokyo 2007
 Is Labor a Suitable Input in LCA + DEA Studies? Insights on the Combined Use of Economic, Environmental and Social Parameters Diego Iribarren and Ian Vázquez-Rowe , Social Sciences journal. 2013, 2, 114–130; doi:10.3390/socsci2030114
 An input–output based framework to evaluate human labour in life cycle assessment
Rugani, B., et all, 2012 Int J Life Cycle Assess (2012) 17:795–812
 The EROI of agriculture and its use by the Via Campesina, Joan Martinez-Alier (2011) The Journal of Peasant Studies, 38:1, 145-160, DOI: 10.1080/03066150.2010.538582
 Effects of industrial agriculture on global warming and the potential of small-scale agroecological techniques to reverse those effects , A report to Via Campesina, by The New World Agriculture and Ecology Group, (November 20, 2009), Coordinator John Vandermeer, Gerald Smith, Ivette Perfecto and Eileen Quintero
 Count down, by Alan Weisman, Hardcover, 528 pages,Published September 24th 2013 by Little, Brown and Company (first published 2013)
 Nederlands: http://www.lowtechmagazine.be/2011/05/fietsmachines.html