Ecological footprints offer a great tool for quantifying and understanding the ecological impacts we have as individuals and communities -- which is why we've talked about them so often. London's City Limits is a couple years old now, but it bears mentioning now because the project helped define the cutting edge of footprinting research. The team, led by the group Best Foot Forward, came as close to comprehensively describing the entire ecological footprint of a major city as anyone, anywhere in the world. Essentially, their work shows how much planet it takes to keep London thrumming along. Much of their research is available online as PDFs, and, if you're even half as much an eco-geek as I am, they make provocative reading (and there's plenty of meaty analysis and methodology explanations for those of you who are geekier than I).

One of their key findings? If Londoners are to live globally equitable and sustainable lifestyles, they will have to figure out ways of reducing their fossil fuels and materials consumption by 35% by 2020, and 80% by 2050. In other words, a bright green London will be one in which people live at least as well as they do now, but using 1/5th as many resources. (A comparable analysis of a typical North American metropolis would no doubt show that the efficiency gains needed are much higher: we might need to slash our impacts by 9/10ths or more.)

The study lays out three general scenarios for meeting the challenge: a business-as-usual approach (in which things continue more or less as they are), an evolutionary approach (in which gradual change is made towards existing targets), and a revolutionary approach, where Londoners embrace major paradigm shifts in the provision and use of energy and materials. Only the revolutionary scenario actually leads to a bright green London.

If I have any serious criticism of the project, it is this: having demonstrated the extent of the change needed, why waste time imagining scenarios which won't work to create that change? A much better use of scenarios would be to imagine a variety of bright green Londons based on changes in external forces (the speed at which technology progresses, say, or the rate at which climate change makes progress more difficult) and local decisions. That would give people a sense of the range of options London could pursue in its efforts to become sustainable: better yet, done right, the scenarios themselves could be aids to foresight and bold decision-making, perhaps even inspire change on their own. As we've said often before, we cannot build what we cannot imagine: imagination engineers the future.

But that point aside, this is a really supurb piece of work, useful for anyone who's thinking about cities and the future. We look forward to seeing more from this gang.