Let’s suppose a billionaire has given you 1 million US$ with the sole instruction that you live wisely from it. Thus, you decide to put all the money in the bank at the highest annual interest rate in the market (5%) and live only from the interest, which means you have 50,000 US$ a year or 137 US$ per day to spend.
The first year you spend your allotted money and nothing more. However, by the second year you have had children, desire a higher standard of living and buy a bigger house, two cars, and membership at a sports club. You spent 400,000 US$ of your original capital and were not willing to reduce your standard of living to remain within the 137 US$ per day budget. Consequently, by August 7th you run out of interest money to live from. To sustain your standard of living the rest of the year, which you have already become quite accustomed to, you must tap into your remaining capital (600,000 US$). As a result you deplete your capital since you were no longer able to live from the interest payments. August 7th was your overshoot day. And if you repeat the same spending pattern the following year, you will overshoot again on March 3th of your third year and be left with no capital by your fourth year.
Earth Overshoot Day is calculated by the Global Footprint Network and refers to the date “when humanity’s demand for ecological resources and services in a given year exceeds what Earth can regenerate in that year”. According to these calculations over the past four years earth’s overshoots are occurring sooner in the year: in 2012- August 22; in 2013 – August 20th; in 2014 – August 19th; and in 2015 – August 13th. The overshoot day calculations derive from the ecological footprint methodology, assembled in 1992 by William Rees and his PhD student Wackernagel, to assess the amount of land (in global hectares) needed to sustain an individual’s consumption and waste assimilation requirements (Rees, 1992; Rees and Wackernagle, 1994).
As you can tell from the bank allegory, to calculate your overshoot day you need to know how much capital you have (natural capital), your interest rate, and your consumption/waste patterns. To upscale this information to the entire planet requires vast amounts of information collected with a high degree of certainty. The Global Footprint Network has been collecting data on natural resource endowments and use for some countries for roughly two decades and is making great strides in improving their calculation (Kitzes et al. 2007).
The intention of Earth Overshoot Day is to send a clear message: humans need to reduce their consumption and waste patterns within the limits set by nature. However, this limit is very difficult to calculate and giving a specific date as an overshoot day gives a false sense of certainty that, in my opinion, diminishes the credibility and the efforts to educate the population about earth’s limited carrying capacity.
© Global Footprint Network 2015. National Footprint Accounts, 2015 Edition. Licensed and provided solely for non-commercial informational purposes. Contact Global Footprint Network at www.footprintnetwork.org to obtain more information or obtain rights to use this and/or other data.
Kitzes, J., Peller, A., Goldfinger, S., Wackernagle, M. 2007. Current methods for calculating National Ecological Footprint Accounts. Science for Environment and Sustainable Society 4 (1): 9.
Rees, W. 1992. Ecological Footprint and appropriated carrying capacity: what urban economics leaves out. Environment and urbanization 4(2): 121.
Rees ,W. and Wackernagel, M. 1994. Ecological footprints and appropriated carrying capacity: measuring the natural capital requirements of the human economy. In: Investing in natural capital. Jansson, A. et al. Washington DC.
 At a 5% interest rate, the bank will pay you 30,000 US$ on a 600,000 US$ deposit. With living expenses of 137 US$ per day that amount of money will only last for 219 days out of 365 days in a year.
 Not taking into account your expenditures to finish the second year.
 http://www.overshootday.org/about-earth-overshoot-day/ (visited on August 18th, 2015)
 “A global hectare is a hectare that is normalized to have the world average productivity of all biologically productive land and water in a given year” (Kitzes et al. 2007, p. 1)
* The author is a researcher at INESAD and Ph.D. candidate in Environmental and Natural Resources Policy at the State University of New York at the college of Environmental Science and Forestry (SUNY-ESF), firstname.lastname@example.org – email@example.com.
The views expressed in this article are those of the authors and do not necessarily reflect the views of Fundación INESAD.