Last week in London I delivered a presentation on sustainability to about 120 middle and senior managers from within BT.  Afterwards we explored climate change, at the new and very impressive Atmosphere Gallery  in the Science Museum.

Recently I have taken to introducing my presentations using the measure of how many planets worth of resources humanity is using and how that number is growing. If you are not familiar with it, this web site from Global Footprint Network explains the concept.

Another analogy I like to use to illustrate the concept is a bathtub with water running in from the tap and out through the plughole. The water running out the plughole represents the resources we are using. The rate of water running in through the tap represents the earth’s ability to replenish those resources.

Currently the water is running out the tub at 1.5 times the rate it is coming in (the world average). Many developed economies are running the water out the drain at 4 or 5 times the rate of the water coming in through the tap. Emerging economies are much more sustainable, but they are growing in population and affluence and aspire to be like developed economies. This has the effect of making the plughole bigger and it is getting bigger at an accelerating rate.

I hear many people say that science has been crying wolf for too many years and nevertheless everything has been OK. Using the bathtub analogy, the water in the tub is especially important in response to this challenge.

I think of humanity as a small child sitting outside the tub, able to see the water run out the plug hole, able to see the water running in from the tap, but unable to look over the edge of the tub to see the level of water inside.  The child has been watching the water run out from the plug hole for many hours. It has never stopped and she is lulled into a false sense of security that all is OK.    But actually during those many hours (or in the case of humanity, many millennium) the level has gone from very high to very low. We, like the child, are naively reassured by history. Perhaps we are perilously close, in terms of length of human life on earth, to the last gurgles as the stored water in the tub runs out.   We will be left with a dry tub where the water running in from the tap goes straight to the plughole and there is no reserve to support our lifestyle.

Of course in practice, different resources will exhaust at different times. And we have found replacements historically. But the pace of change is such that historic out turns are no indication of future performance.

We need to make a steep change in our actions to respond to this challenge.