The Global Footprint Network every year calculates nature's supply in the form of biocapacity ie the amount of resources the planet generates, and compares that to human demand: the amount it takes to produce all the living resources we consume and absorb our carbon dioxide emissions. Earth Overshoot Day is a concept devised by the new economics foundation (nef) and marks the day when demand on ecological services begins to exceed the renewable supply.
Last year it was 25th September - the jump almost a month ahead this year is partly due to the availability of new data, meaning that previous Ecological Debt Day dates were almost certainly too conservative. As nef say: "The fact that we're blowing our annual ecological budget in less than nine months is not based on need, but on overconsumption. As in previous years, nef has found examples of ecologically wasteful 'boomerang' trade: the UK currently exports 131,000 tonnes of chewing gum to Spain, only to import 125,000 tonnes back again. We send 3,300 tonnes of cuddly toys to New Zealand, only to bring another 2,400 tonnes back again."
"We need a Great Transition to rebuild the economy and free us from the habits of overconsumption." Andrew Simms, nef policy directorRead more here about the day - but also read here about why chasing Gross Domestic Product hasn't served us well.