April 22 is Earth Day

Yet today, the environmental challenges we face dwarf those that touched off that first celebration. And the planet we honor on Earth Day is a far different place from that of just four decades ago.

World population has almost doubled, from 3.7 billion to 6.9 billion. The amount of land paved over to build houses, cities and roads has increased 75 percent, from 228 million global hectares to 400 million global hectares, according to Global Footprint Network's 2009 National Footprint Accounts. The amount of productive forest land required for fuelwood, paper and timber products, has gone up 53 percent to close to 2 billion global hectares. The productive land and sea area we need for food - for fishing, crops and grazing our livestock - has increased 69 percent, to 5.6 billion global hectares.

But the most staggering increase is reflected in our carbon Footprint: the amount of productive land area that would be needed to absorb our carbon emissions. Since 1970, our total carbon Footprint has more than tripled, from 2.9 to 9 billion global hectares. Carbon has also gone from being a smaller part of humanity's total Footprint than cropland, to outstripping every other area of demand by a significant margin.

The result of this ecological overspending is clear from the crises we are confronting now - most prominently climate change, but also biodiversity loss, deforestation, fisheries collapse, soil erosion and other problems. Earth has changed from a place where we could operate as if resources were limitless, to one in which resource constraints are becoming a pressing and increasingly decisive concern.