How big and little?

~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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6 Responses to How big and little?

  1. zeusiswatching says:

    Clearly, we have every reason to use water more efficiently and to do a better job of harvesting rainwater before it runs into the sea, but I appreciate your skeptical analysis of the model above.

    • lichanos says:

      There are a lot of things we could and should do to use water more sensibly, that’s very true.

      What gets me about this sustainability stuff – and I hear a lot about it even in my engineering work, believe it or not – is that it’s so vague. What does it MEAN? We always come back to this general sense that we are wasting and degrading and messing up everything. It seems to me to be a return to the original sense of the word “pollution,” which was religious.

      Freeman Dyson has said that environmentalism is the new dominant secular religion in the west, and that for the most part, it’s a pretty good thing. I agree, but this sort of stuff, basically an emotional and unthinking perception, passed off as analysis, rankles me.

  2. troutsky says:

    I believe environmentalism is dead, meaningless. Forty Earth Days have done nothing. Around here, sustainability is a dirty word propagated by the UN and other anti-free market forces to undermine our way of life.

  3. tonyricochet says:

    Hi, nice blog.

    It’s interesting that you say “Scale and perspective are key, and they are always in short supply” because the graphic used to illustrate this post is misleading (whether it’s the original graphic from the presentation you describe, or whether it’s a mock-up isn’t clear in the post).

    The sphere of water shown in your graphic is about 30% of the size of the Earth – therefore a radius of approx. 2,000km. It should really be shown 10 times smaller than that (radius of only 202km) – or about the size of the head of the stick figure!

    I agree that the relative sizes don’t signify very much – but the discrepancy is certainly bigger (and therefore more surprising?) than you suggest in your graphic!

    Best wishes

    • lichanos says:

      I was aware of the inaccuracy of my spheres’ radii, but I was too lazy to do it right. I make these header images quickly. Also, true scale would have been so small as to be invisible. The graphic is just to illustrate the general concept.

      I sometimes say, in a weird sort of backhanded acknowledgment of your points, that in cartography, it’s more important to look right than be right! If it looks wrong, even if it’s right, people will ignore and distrust it. If it looks right, and is generally right, they might get some useful information. In a sense, decent maps are all wrong, but they look right.

      Fascinating and endless topic…I have added a note to the image.

      Thanks for commenting!

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