How big and little?

June 4, 2010

~not to scale

I was listening to a talk on sustainability today, and the speaker showed a graphic depicting the Earth and a much smaller sphere that represented the amount of water on the Earth.  I don’t know if he meant it to be just freshwater or not, but that’s what is usually talked about.  Nobody cares how much saltwater there is to drink!

The point of his image was to impress upon us the relative scarcity of water as a resource for human life.  He did the same thing for the Earth’s atmosphere, presumably to show how small its total volume is so we take care about polluting it.  But, I wondered, how significant is this?  The fact that potable water is small in volume in comparison to the volume of the Earth should come as no surprise at all.  Is this just some enviro-sustainability scare story?  I did a few calculations:

Radius of the Earth ~ 6,400 km

Radius of the a sphere containing the Earth’s freshwater ~ 202 km

Radius of the liquid ‘biomass’ of the Earth’s human population ~ 0.4 km

That last one may have you puzzled, but I was just wondering how this hypothetical drinkable sphere compared to the mass of those that drink it.  I took the population of the globe, assumed an average weight of 60 kg (probably conservative, considering how many children and malnourished people there are in the world today) and assumed that 60% of their weight is water – estimates vary depending on age:  it seems to decline as we get older.  That’s how I determined our liquid biomass.

What does it mean?  Well, it’s very small compared to the water-sphere.  Of course, the water is not uniformly distributed, nor is it necessarily found where we need it.  For instance, a good proportion of it is locked into glaciers and the polar ice sheets.  But what does telling us that the water makes a sphere much smaller than the Earth tell us?  Not much there either.

Scale and perspective are key, and they are always in short supply.


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