With more than four months still left to go in 2015, humans already have spent all of the resources Earth produces to sustain us for the year, according to the latest in a series of annual reports on the demands the world's population makes on the planet.

Called "Earth Overshoot Day," it fell this year on Aug. 13, marking the point in the year at which humanity goes into ecological debt, based on a comparison between what we use up – calculated in terms of agricultural crops, fish stocks, cotton for clothing, carbon emissions and the trees we cut down in forests around the world – and what the planet's ecosystems can renew in a year.

"Take a lake full of fish: there’s a limit to the amount of fish that can be caught each year while still leaving enough to reproduce and build back the stock for next year," Saamah Abdallah at the . "Today we hit that limit, for the planet as a whole."

Aerial view of the Taim Ecological Station on fire, in Rio Grande do Sul state, southern Brazil, in March 2013.

(LAURO ALVES/AFP/Getty Images)

The day is the creation of the , which says that from this day forward in 2015, "we are drawing down the planet's principal rather than living off its annual interest. This overshoot leads to a and a buildup of carbon dioxide in the atmosphere."

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Human consumption of natural resources first started exceeding the planet's ability to regenerate them in the early 1970s, the group estimates, and the overshoot day has arrived -- this year's comes six days earlier than 2014, and two months earlier than in 2000, .

"For the remainder of the year, we will be living on reserouces ," the World Wildlife Federation notes.

"The big problem is not that our deficit is getting bigger, it is that it cannot be maintained in the long-run," said Mathis Wackernagel, the group's president, in an . "Even though we are in a deficit equation we are not taking measures to take us in the right direction."

The group estimates that it takes the equivalent of 1.6 Earths to support the world's population of more than 7 billion today, a number that's expected to grow to , and up to . If current trends in population growth and resource use continue, we'll need two Earths to meet the needs of the world's population every year, rather than just the one we actually have.

Key to reversing the trend are reducing carbon emissions and changing the way the world's cities are planned and developed, as well as changing to more sustainable forms of agriculture, including eating less meat and preserving the world's fisheries.

"Small actions ... make a big difference and all of us play an important role in ," the WWF says.

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Saint George Basin, Australia (JAXA/European Space Agency)