Today, August 8th, is Earth Overshoot Day (EOD) for 2016, the day when humanity supposedly has consumed the natural resources available from the planet for the year 2016; we're now running a deficit, somehow. At least if your believe the calculations of the Global Footprint Network (GFN), the think tank behind this concept. EOD is well reported by media every year, and perhaps it does some good as an ecology awareness thing. But is the idea scientifically sound?

EOD is based on the ecological footprint concept. The ecological footprint of a person (measured in hectares) is the area of the earth required to extract the natural resources needed to sustain that person: to grow crops, keep farm animals, obtain natural resources for consumables, etc. It's a very complex thing to calculate, and of course people argue about what the correct formula should be. One article by Willis Eschenbach (2010) goes into some depth and argues that the formula used by GFN vastly overestimates the footprint. I don't know who is right, but it does seem like the footprint is very difficult to estimate, so we should probably be careful with drawing strong conclusions.

But I have a more fundamental problem with EOD. Regardless of how it's calculated, the claim that we have today "overshot" our yearly earth-budget, at 8 months out of 12, means that we are consuming about 1.5 of the available food and natural resources available; or that the footprint of humanity is 1.5 Earths. This is not some kind of parable: GFN really claims that we have exhausted our natural resources, by a wide margin, and that this has been going on since the 1970's.

My question is, what are we all living on then?

As of today there should be no more food around, or any other natural-derived product for that matter. If GFNs estimate is correct, shouldn't humanity be long dead? Shouldn't we expect 1/3 of the earth's "surplus" population to die off pretty quickly? Doesn't the fact that we're not dead prove that EOD is wrong? Isn't it physically impossible for our total footprint to exceed 1 Earth?

EDIT: Nice to see that this question stirred a lot of debate! :) I think several answers bring up one key point that resolves the problem: GFN defines the footprint not as the actual area required for production at present (which must be < 1 Earth), but the area required for sustainable production (which is larger). Exactly how the sustainable area is defined is still mysterious to me, but I guess it is at least theoretically possible.

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    $\begingroup$ Why do you think humanity would be dead? I guess the point of the EOD is, that humanity now starts killing other species off, not itself. If we used up our resources, we now would be taking them from others. Taking more than earth can sustainably recover. $\endgroup$
    – skymningen
    Aug 8, 2016 at 8:30
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    $\begingroup$ The point of EOD as I understand it is to present a bleak picture to the general public, so that they buckle up. Anyone who is a scholar or in one of the sciences or has pursued some sort of advanced education knows that calculating a foot print is very hard. Example, I am from India, where people routinely go hungry and die of hunger. Is it because India wants for food? No, we have been in surplus production for a long time, but the logistics is lagging behind. $\endgroup$ Aug 8, 2016 at 8:36
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    $\begingroup$ You've grossly misinterpreted it. Imagine a company that has a turnover of £1M per year. This would be the equivalent of the natural resources produced each year. Now the company spends £1.5M in a year. This would be the equivalent of using the natural resources produced for the year, the turnover, by August. However, the company has been around for 4.5 billion years and has some money in the bank becuase it was only recently that a stupid new CEO (humans) came in and started spending more money than they were making. The company's best hope is that the CEO dies before he destroys the company. $\endgroup$
    – rg255
    Aug 8, 2016 at 8:53
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    $\begingroup$ "EOD is well reported by the media each year" Really? This is the first I've ever heard of it. $\endgroup$
    – TylerH
    Aug 8, 2016 at 17:47
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    $\begingroup$ Forests take many years to fully recover. If we use up the right fraction of forest land every year, we can continually utilize a different forest area without reducing total forest area. If we use a larger fraction, the world's forested area will get smaller every year, eventually effectively being wiped out. Using "1.5" as the excessive fraction, it means we use up half again as much forest as can naturally replenish itself "forever". It doesn't mean it's completely gone after 8 months; it means we use as much in 8 months as should only be used in 12. $\endgroup$ Aug 9, 2016 at 4:16

4 Answers 4


TL;DR: it's a simplified measure of sustainability, but accurate enough to be useful for public engagement

EOD is hosted and calculated by Global Footprint Network (GFN), an international think tank. The GFN estimates national and global net supply and demand for renewable resources, specifically:

  1. food and fiber products,
  2. livestock and fish products,
  3. timber and other forest products,
  4. space for urban infrastructure, and
  5. carbon sequestration.

From the website:

Global Footprint Network measures a population’s demand for and ecosystems’ supply of resources and services. These calculations then serve as the foundation for calculating Earth Overshoot Day.

On the supply side, a city, state, or nation’s biocapacity represents its biologically productive land and sea area, including forest lands, grazing lands, cropland, fishing grounds, and built-up land.

On the demand side, the Ecological Footprint measures a population’s demand for plant-based food and fiber products, livestock and fish products, timber and other forest products, space for urban infrastructure, and forest to absorb its carbon dioxide emissions from fossil fuels.

The results are reported in 'global hectares', the area which would be required using global average productivity, to aid intercomparability.

The methodology was published in 2013 (Borucke &al 2013).

So, to answer the question in your title ("Earth overshoot day: is it a sound concept?"), data quality may be an issue (I haven't pored over the paper in detail) but the basic methodology is arguably appropriate for the question they're asking. The only issue I might have is that they effectively consider land and sea use to be interchangeable, but I'm not sure the alternative would be an improvement.

To answer the second part of your question ("what are we all living on then?"), it is possible to harvest more from a system than it can sustainably produce. Exceeding this harms the future productivity of the system. To take the specific examples named for EOD:

  1. Overharvesting plant-based food and fibre products can result in soil acidification, soil erosion, and/or soil salinification;
  2. Overharvesting livestock and fish products can result in overgrazing and/or overfishing;
  3. Overharvesting timber and other forest products results in deforestation;
  4. Using more space for urban infrastructure reduces the availability of land for the other examples on this list;
  5. Producing more carbon emissions than can be absorbed by its forest leads to an increase in atmospheric carbon, resulting in climate change.

Ultimately, while 'Earth Overshoot Day' is a public engagement activity, there is a reasonably robust and transparent methodology behind it which is good enough for the job. Remember: all models are wrong, some models are useful.

  • $\begingroup$ Thanks, this is very helpful. I will look at the Borucke et al paper. I take it that the key number is the amount of resources that can be sustainably produced. But I think that fits poorly with the "footprint" concept, unless you define it as the hectares required for sustainable production, not as the hectares required for actual production. $\endgroup$
    – Roland
    Aug 8, 2016 at 14:00
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    $\begingroup$ @Roland I'd say sustainability is implied in most similar questions. For example, consider "how much do you need to earn to rent a flat in London?" - most people would infer that the asker is interested in sustainable renting (i.e. a salary that would allow you to rent indefinitely), and wouldn't consider an answer that says 'you can rent a flat in London with zero income, assuming you're willing to eat into savings or go into debt" very helpful. $\endgroup$
    – arboviral
    Aug 8, 2016 at 14:45
  • $\begingroup$ Yes, I understand you point, but I don't think that's so obvious in the case of the "footprint" idea, because "sustainable" is hard to define here. $\endgroup$
    – Roland
    Aug 9, 2016 at 12:16
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    $\begingroup$ Upvote for correct use of "pored over" $\endgroup$
    – user151841
    Aug 9, 2016 at 13:15
  • $\begingroup$ @Roland Why is it harder to define here? We have a source (the earth's resources) and a sink (our consumption of those resources). If the two are in long-term balance, we have sustainability. $\endgroup$
    – AkselA
    Aug 10, 2016 at 10:57

I think that your question comes from a misunderstanding in the definition of the overshoot day, here is the definition in your question:

Regardless of how it's calculated, the claim that we have today "overshot" our yearly earth-budget, at 8 months out of 12, means that we are consuming about 1.5 of the available food and natural resources available; or that the footprint of humanity is 1.5 Earths.

And here is the definition from the website:

Earth Overshoot Day, falling on 8 August this year, marks the date when humanity’s annual demand on nature exceeds what Earth can regenerate in that year

(emphasize mine)

The difference is that you define resources as available while the website defines availability on a yearly basis.

Like you wrote, it's impossible to consume 1.5 Earths, that would mean that we harvested more fish than are in the ocean, for example.

But if we take it on a yearly basis (as the website do), it's easier to tackle that we harvest more fish each year than fish can reproduce and grow during the same time. So we are harvesting in the fish stocks that developed during the previous years.


Technically, an overshoot means the amount by which a certain dynamic variable shoots above its final steady state value. So, to know overshoot, you need to know the steady state. In this case, as you said, the steady state is just predicted and not really known.

Regarding your question:

what are we all living on then?

Assuming that the overshoot concept holds in this case then we are actually living on the available resources. However, if we are using more than a certain limit then we would deplete the resources in future. This will lead to decline in our population because we need resources to survive. Because of declined population, the resources would accumulate (because of the feedback) and we may finally reach the steady state via a damped oscillatory dynamics (compare with the fox-rabbit situation in the simple Lotka-Volterra models). There are other possible solutions too (sustained oscillations, extinction, etc).

Note that overshoot happens because of a delay and it would not happen (at least using deterministic models), if there is no feedback i.e. a fixed amount of resource is present and we just consume it.

Now, the main issues are:

  • How accurately do we know the rate and mechanisms of resource production?
  • How accurately can we calculate the steady state, or in other words, how accurate is our model of resource-population dynamics?
  • $\begingroup$ "Assuming that the overshoot concept is true, we are living on the available resources." how can we deplete food resources? Food production has a nonfinite limit (I mean a cap value which cannot be determined atm). How is it possible that we will exhaust natural finite resources such as iron? wouldn't we find newer sources with the advances of technology? We are already in the early stages of transitioning to renewable source. How is it that we will deplete available resources? $\endgroup$ Aug 8, 2016 at 11:15
  • $\begingroup$ @KoustavPal As I said, there are many complexities. I am not commenting on whether whatever model is used, is correct. I am just explaining the concept of overshoot. However, food production is an easy concept that can be nicely explained using Lotka-Volterra. Renewable resources are resources that are formed constantly in nature. If the energy consumption rate is more than production, then the population will be affected again. In a timespan of million years even oil is renewable :P $\endgroup$
    Aug 8, 2016 at 11:21
  • $\begingroup$ That's a nice connection to dynamic systems. But this particular overshoot has been going on since at least the 1970's according to GFN ... $\endgroup$
    – Roland
    Aug 8, 2016 at 13:54
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    $\begingroup$ "Food production has a nonfinite limit" How can you say this? Food is not just agricultural. There are already the beginnings of protein wars (fighting over food from the sea.) Cod is a good example: cod populations are at record lows despite once being so plentiful that Explorers wrote that one could walk on the surface of the water (off Maine/Newfoundland) by stepping on the backs of cod. We are hunting many species to the brink of extinction. That's not non-finite. $\endgroup$ Aug 8, 2016 at 14:04
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    $\begingroup$ @WYSIWYG Koustav said it in the first comment, not you. $\endgroup$
    – reirab
    Aug 8, 2016 at 21:01

Consider an investment earning interest x.

If I spend x dollars a month I cover the interest and will have spending money forever.

If I spend less then x the interest adds to the principal and the investment grows giving me potentially more interest to spend.

If I spend more than x I decrease the principal and so reduce the interest I get the next month. Even if I switch to only spending x I will still eventually run out of principal . To be stable I have to switch to spending x' a smaller limit.

They have modeled the whole earth as a group of investments and expenses using hectares as currency.

Note: in a mortgage when you are paying about 1/3 into principal you are about 2/3 through repayment. Counting about 40 years to get here might mean maybe another 20 till we are "paid off" if a string of assumptions are true.

  • $\begingroup$ While it's not as direct an answer as some of the others, I find this metaphor to be extremely helpful in understanding the concept in clear mathematical terms. $\endgroup$
    – Xodarap777
    Jul 18, 2019 at 18:47

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