Humans are making more demands on the Earth than it can cope with, scientists believe.


They say humanity's footprint on the planet has increased by half in under 40 years.
Their analysis suggests that by 1999 the human economy was absorbing 120% of the Earth's productive capacity.
But while they think the trend will probably intensify, the scientists say solutions already exist which will maintain high living standards.
The claim that we are exhausting the planet's resources is made by an international team of authors in the Proceedings of the National Academy of Sciences (PNAS), published in the US.
The lead author is Mathis Wackernagel, of Redefining Progress, based in Oakland, California.
Their paper, Tracking The Ecological Overshoot Of The Human Economy, uses existing data to translate human demand on the environment into the area needed for producing food and other goods, and for absorbing wastes.
No constants
In 1961, the authors say in their "preliminary and exploratory assessment", humans were using 70% of the capacity of the global biosphere. By 1999, that had risen to 120%.
The assessment is based on several assumptions:
it is possible to keep track of most of the resources we use and the wastes we generate
most of these flows can be measured according to the biologically productive area needed to maintain them
the planet can be assessed in terms of "global hectares", representing the average productive hectare on Earth for that particular year
the natural supply of ecological services can be measured in the same way.
The natural world itself changes year by year, as does the human demand on it. Technology can reduce "the ecological overshoot", and may ultimately eliminate it.
The authors compared humanity's demand for the Earth's "natural capital" for each year since 1961.
They say: "The calculation provides evidence that human activities have exceeded the biosphere's capacity since the 1980s.
"This 20% overshoot means that it would require 1.2 Earths, or one Earth for 1.2 years, to regenerate what humanity used in 1999.
"The global average per capita area demand for 1999 adds up to 2.3 global hectares per person.
Breathing space
"This is significantly lower than the area demands in industrialised countries such as the US (9.7), or the UK (5.4) and Germany(4.7)."
Human demands
Growing crops: needs most productive land Grazing animals Harvesting timber Fishing Infrastructure for housing, transport and industry Burning fossil fuel
The authors say the Earth must provide an "insurance policy" or buffering system to protect other species and their homes.
If 12% of the Earth's "bioproductive area" were set aside to safeguard other species, they say, our activities would have exceeded the planet's carrying capacity a decade earlier, in the early 1970s.
They say there is hope of bringing human demands in line with the Earth's ability to regenerate itself.
One new technology, known as Factor Four, promises to halve resource use yet maintain service levels in transport and housing.
One of the authors is Professor Norman Myers, of Green College, Oxford, UK.
He told BBC News Online: "The overshoot will continue to increase if we do nothing, because of rising population and rising living standards.
Sceptic unimpressed
"But we can solve this without austerity or hair shirts, by using technology and avoiding waste."
But Julian Morris, of the UK's Institute of Economic Affairs, told BBC News Online that the PNAS paper was flawed.
"The study attempts to do too much in too little space with too many assumptions and too little data," he said.
"The claim that we have overshot the biosphere's regenerative capacity is a fiction based on inappropriate assumptions and poor data.
"The study is of little value either as an assessment of humanity's impact on the environment, or as a guide to action."