A few weeks ago I was asked to share some brief thoughts on Rio+20, an assignment I tried to turn down on the grounds that my thoughts on Rio were unpublishable, even in this modern era. But I was prevailed upon. "What advice would you give them? We really want you to use your voice." Who could resist? But my instincts proved right; it was unpublishable, although for different reasons. Their decision. No problem, not since everyone can be their own publisher. Here goes.

Rio +20 is all about sustainable development. We can argue about what exactly “development” might mean, but at least the conference isn’t about sustainable growth. You still hear economists and financial types talking about sustainable growth, especially now that growth is seen as one of the few ways to repay national debt. To a biologist, which is what I consider myself, the very notion of sustainable growth is misguided, at best.

Of course agriculture, which is no more than biology harnessed to human needs, has plenty to learn from economics and finance. “Diversify your portfolio to protect against risk.” That’s the advice on which hundreds of millions of small-scale farmers base their existence. They wouldn’t dream of putting all their eggs in one basket.

“Avoid investments so complex that you cannot really hope to understand them, like credit swap defaults and collateralized debt obligations.” OK, so some big financial bosses still haven’t taken that entirely to heart, but the advice remains sound. If your food security requires a complex set of factors – rainfall, temperature, soil structure, pests and diseases – all to be at the right level at the right time, and you have very little control over any of them, then it isn’t a very secure system.

The advice I’d give economists and others at Rio+20 is that “Invest for the long term” doesn’t mean what they think it means. Long term, for them, is, like, a couple of decades, max. For me, it is of the order of geological eons, or at least a good few generations, and the limit I keep bumping up against is primary productivity. That’s the amount of stuff produced by living things on Earth, and all of it depends on plants, sunlight, and photosynthesis (ignoring giant tube worms and other weird things at the bottom of the oceans). Sunlight – captured by plants – powers human existence. For too long that existence has depended on fossilized sunlight, and we all know where that has brought us.

For the people at Rio, and those who care about what the people at Rio decide, I want to share this one thing from biology: real sustainability means we cannot use more than primary productivity gives us each year, at least not over the long term. There’s a research paper, Human appropriation of the products of photosynthesis, that I’ve ranted about since 1986, when it was published. It estimates that back then people were appropriating 40% of global primary productivity. Global population in 1986 was below 5 billion. It is now 7 billion. Rio+20 needs to figure out how to nourish 9 billion. Sustainably.

Why bother publishing these rejected thoughts? Because I need to refer to them over at the other place.

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