any alternative to growth remains unthinkable, even 40 years after the American ecologists Paul Ehrlich and John Holdren made some blindingly obvious points about the arithmetic of relentless consumption.

The Ehrlich equation, I = PAT, says simply that the impact (I) of human activity on the planet is the product of three factors: the size of the population (P), its level of affluence (A) expressed as income per person, and a technology factor (T), which is a measure of the impact on the planet associated with each dollar we spend.

Take climate change, for example. The global population is just under 7 billion and the average level of affluence is around $8000 per person. The T factor is just over 0.5 tonnes of carbon dioxide per thousand dollars of GDP – in other words, every $1000 worth of goods and services produced using today’s technology releases 0.5 tonnes of CO₂ into the atmosphere. So today’s global CO₂ emissions work out at 7 billion × 8 × 0.5 = 28 billion tonnes per year.

The Intergovernmental Panel on Climate Change (IPCC) has stated that to stabilise greenhouse gas levels in the atmosphere at a reasonably safe 450 parts per million, we need to reduce annual global CO₂ emissions to less than 5 billion tonnes by 2050. With a global population of 9 billion thought inevitable by the middle of this century, that works out at an average carbon footprint of less than 0.6 tonnes per person – considerably lower than in India today. The conventional view is that we will achieve this by increasing energy efficiency and developing green technology without economic growth taking a serious hit. Can this really work?

With today’s global income, achieving the necessary carbon footprint would mean getting the T factor for CO₂ down to 0.1 tonnes of CO₂ per thousand US dollars – a fivefold improvement. While that is no walk in the park, it is probably doable with state-of-the-art technology and a robust policy commitment. There is one big thing missing from this picture, however: economic growth. Factor it in, and the idea that technological ingenuity can save us from climate disaster looks an awful lot more challenging.

¨Growth¨ 1750 to 2000 Click to enlarge