Sunday, October 30, 2011

When We Eat, We’re Eating Energy

Herewith some startling numbers. A report by the Congressional Research Service in 2004 (link), provides direct and indirect energy consumed in Agriculture, including crop and livestock production. The numbers are in quadrillion BTUs, and the answer is that we consumed 1.7 quadrillion BTUs of energy, 1.1 quadrillion BTU in direct uses, and 0.6 quads in indirect, thus for fertilizers and pesticides. A USDA Factbook (link) tells us that “The aggregate food supply in 2000 provided 3,800 calories per person per day.” The (late) 2012 Statistical Abstract (link, Table 2) tells me that the 2002 population of the United States was 288.1 million people. So let us combine these numbers. Below the CRS data shown graphically:

Now one food calorie, also known as a kilo calorie, is worth 3.968320721 BTUs. Put another way, it takes 0.251995761 calories to make a BTU. So we can convert calories to BTUs or BTUs to calories. A quadrillion is 1015. Here is what it looks like: 1,000,000,000,000,000. So let us proceed.

If we apply year 2000 calories per capita to the 2002 population, we discover that in that year agriculture provided 1.1 trillion calories per day or 399.6 trillion calories a year to the total population. We can render that number into BTU-equivalents by either multiplying by 3.968… or by dividing it by 0.25199…. In any case we will be 1.584 quadrillion BTUs. That was the energy output of our agriculture in 2002 measured in quads of BTUs. So what did we expend in energy to get that output? What was the energy input? It was 1.7 quadrillions of BTU.

To get a ratio here, we take the input and divide it by the output. The result is 1.072. In other words, we expended more calories in industrial energy to get 1 calorie that we could eat. But the energy ratio calculation is not yet done.

We learn from this story in the Atlanta Journal Constitution, by William G. Mosely, co-author of a 2010 National Academy of Science study, “Understanding the Changing Planet: Strategic Directions for the Geographical Sciences,” that in addition to direct and indirect energy uses, an equivalent amount is used in processing and packaging of food. These two are, according to Mosely, 7 percent each of national energy use. And then another 3 percent of national energy consumption is used in food distribution.

Using these additional inputs, it turns out that in 2002 we used total energy of 4.13 quads as input for that 1.58 quads of caloric output. The cost now turns out to be 2.6 units of energy in for each unit of edible energy out.

Therefore it is absolutely true that we are eating energy in this modern day and age. This should make us very alert when we think about World Population. It has now reaching 7 billion—projected to reach 8 billion in 2050—a time by which we shall have pretty much consumed currently know hard reserves and only shale stuff will be left to take us to the next billion-person addition to the human family.

Added later. Thanks to russel’s comment to an earlier version of this post, I realized that I was using the wrong kinds of calories, not the kilo calories used in food. This post has therefore been updated to correct that very big error. I appreciate the correction.


  1. In your calculations I do not see that you account for the fact that a food calorie is not the same as an energy calorie? Food is counted in Kcals, not Cals.

    Which would mean that your ratio is 1.073, not 1073.

    Still, combine that with food security data and you get really scary numbers.

    Many countries, particularly in Africa and the Middle East, do not produce enough food to support their populations and must import that food. In the case of Egypt and Saudi Arabia, that number is 80% of all food is imported.

    Long before 2040, the oil will be gone from the Middle East and/or the oil wars will stop food from flowing so easily or in such large numbers.

    Even today, that means 64.2m people in Egypt that cannot be fed to a minimum diet.

  2. Much appreciate this comment, russel. I've changed the post to make the correction. In retrospect I realize that I should have known that...

  3. Let's see if we can identify what that energy gains us...

    Going back to the 1700's when no fossil fuels were needed and no powered machines (water/windmills excepted) were used, it took ~3 acres to feed a person. One farming family could farm roughly 65 acres. Presuming ~8 people, that meant they fed an additional 16 people. One could say that for every Kcal put into food, we got three out. 90% of the population were farmers.

    Today, 1.2 acres supplies 3800 (double what we need) per person. I suspect that would be even lower if we ate more like people did the 1700s.

    The average farm size is ~418.
    I can't find how many people are needed to farm that, as we are no longer tying farms to families in most cases.

    What I can find is that now less than 1% now work as farmers.

    So we now free 89% of the population from the sort of work we describe as backbreaking.

    Is it worth the trade?