In the early 1950s in the middle of a hot July day, young William Rees jostled with his cousins to get lunch, a hearty meal served on their grandmother’s porch to fuel them through another several hours of hard labor on the farm.

William realized he’d helped grow everything on his plate: beef, chicken, new potatoes, spinach, baby carrots, and the large leafy salad. The hard work of the past months – an eternity in a pre-teen’s life – was tangibly manifest in the food he and his cousins were about to eat.

“And as I stared at that plate just waiting for grandpa to say grace, I felt as if I was in an elevator in freefall… It was such an overwhelming experience of connectedness to the earth.”

That epiphany – visceral understanding of the connection between his consumption and the resources required to support that consumption, guided William Rees in his education and career, as he tried to create a methodology for understanding the impact humans have on their environment.

As a professor, Rees would challenge his students to determine the amount of time a city could continue if it were suddenly isolated, as though a huge bell jar had encapsulated the city. He called this a “regional capsule,” and used this experiment to illustrate the land area required to sustain populations. The answer to Rees’ exercise was “two days,” a city could only survive two days if cut off from imported products.

While the west argued that Rees’ conceptual basis was flawed, China put in place laws and measures to ensure her cities could be self-sufficient. For China it is viewed as a matter of national security. China remembers what happens when her fishing fleets are bombed and commerce between cities is prevented by warfare.  America, Canada, and Australia have no such memory. Britain had for so long been dependent on the goods imported from its once-vast empire that  World War II was a mere aberration where imports were impeded by enemy submarines but never truly interrupted.

In 1992 William Rees was issued a new computer. Unlike his previous computer, which rested flat on his desk like a squat toad, the new computer was a tower. The old computer likely took up 2-3 square feet of his desktop. The new tower required less than 1 square foot.

On that day, William was simply enjoying the computers new features and rearranging his desk to take advantage of the tower configuration.

“I really like the smaller footprint of this computer,” William Rees mentioned to a doctoral student.

Then the lightbulb went off. Rees went back to his paper on regional capsules and re-wrote it to use the term “ecological footprint” – a measure of the land resources across the globe required to support human activity. How many global acres per person is required?

In 1992 Rees was able to compute the number of acres across the globe it takes to support a single person in any of a variety of countries. Canada, the US, and Australia, nations whose communities were formed based on abundant availability of fossil fuels, came in at about 20 acres per person.  Old world industrialized nations like the UK, Spain, Germany, and France came in at about 12 acres per person. The lifestyle in developing nations (e.g., Togo, Afghanistan, Laos, China, India) usually required less than 3 acres per person.

If our world had access to multiple “earths,” how many earths would it take to sustainably resource human activity? In 1992 the answer was 1.25 – with the excess being provided by past earths in the form of fossil fuel. In 2012 the answer is 1.5.

The graph below shows the total ecological footprint of the world population, including the amount of that footprint we are using from past earths in the form of fossil fuels.

Our non-carbon footprint still falls within the capacity of this earth to sustain. But increasingly our global acres are spread across the entire globe, meaning transportation is required to get the products of our “acres” to us. We have become dependent on that carbon footprint to function.

I could have wished William Rees had spent time defragging his hard drive, and had an “Aha” moment that could help people understand how fragmented their global acres are across the planet, their vulnerability were global travel to be seriously disrupted.

As it turns out, there is a more direct measure we can use. We can figure out where our acres are in the world by tracing the location of the rain, irrigation, and water pollution required to support our lifestyles. This is the water footprint, a little-discussed concept that I think holds the key to understanding our immediate impact on the world.