Climate change, the rate of extinction of species and the challenge of feeding a growing population are among the threats putting humanity at risk, the UN Environment Program said in its fourth Global Environmental Outlook since 1997.
"The human population is now so large that the amount of resources needed to sustain it exceeds what is available at current consumption patterns," Achim Steiner, the executive director of the program, said in a telephone interview. Efficient use of resources and reducing waste now are "among the greatest challenges at the beginning of 21st century," he said.
The program described its report, which is prepared by 388 experts and scientists, as the broadest and deepest of those that the UN issues on the environment and called it "the final wake-up call to the international community."
Over the past two decades the world population has increased by almost 34 percent to 6.7 billion from 5 billion; similarly, the financial wealth of the planet has soared by about a third. But the land available to each person on earth had shrunk by 2005 to 2.02 hectares, or 5 acres, from 7.91 hectares in 1900 and was projected to drop to 1.63 hectares for each person by 2050, the report said.
The result of that population growth combined with unsustainable consumption has resulted in an increasingly stressed planet where natural disasters and environmental degradation endanger millions of humans, as well as plant and animal species, the report said.
Steiner said that demand for resources was close to 22 hectares per person, a figure that would have to be cut to between 15 and 16 hectares per person to stay within existing, sustainable limits.
Persistent problems identified by the report include a rapid rise of so-called dead zones, where marine life no longer can be supported because of depletion of oxygen caused by pollutants like fertilizers. Also included is the resurgence of diseases linked with environmental degradation.
The report is being published two decades after a commission headed by the former Norwegian prime minister Gro Harlem Brundtland warned that the survival of humanity was at stake from unsustainable development.
Steiner said many of the problems identified by the Brundtland Commission were even more acute because not enough had been done to stop environmental degradation as flows of goods, services, people, technologies and workers had expanded, even to isolated populations.
He did, however, identify some reasons for hope that pointed toward better environmental stewardship.
He said West European governments had taken effective measures to reduce air pollutants, and he praised efforts in parts of Brazil to roll back deforestation in the Amazon. He said an international treaty to tackle the hole in the earth's ozone layer had led to the phasing-out of release of 95 percent of ozone-damaging chemicals.
Steiner said more intelligent management of scarce resources including fishing grounds, land and water was needed to sustain a still larger global population, which he said was expected to stabilize at between 8 billion and 10 billion people.
"Life would be easier if we didn't have the kind of population growth rates that we have at the moment," Steiner said. "But to force people to stop having children would be a simplistic answer. The more realistic, ethical and practical issue is to accelerate human well-being and make more rational use of the resources we have on this planet."
Steiner said environmental tipping points, at which degradation can lead to abrupt, accelerating or potentially irreversible changes, would increasingly occur in locations like particular rivers or forests, where populations would lack the ability to repair damage because the gravity of a problem would be far beyond their physical or economic means.
Looking ahead, Steiner said parts of Africa could reach environmental tipping points if changing rainfall patterns stemming from climate change turned semi-arid zones into arid zones, and made agriculture that sustained millions of people much harder.
Steiner said other tipping points triggered by climate change could occur in areas like India and China if Himalayan glaciers shrank so much that they no longer supplied adequate amounts of water to populations in those countries.
He also warned of a global collapse of all species being fished by 2050, if fishing around the world continued at its present pace.
The report said 250 percent more fish are being caught than the oceans can produce in a sustainable manner, and that the number of fish stocks classed as collapsed had roughly doubled to 30 percent globally over the past 20 years.
The report said that current changes in biodiversity were the fastest in human history, with species becoming extinct a hundred times as fast as the rate in the fossil record. It said 12 percent of birds were threatened with extinction; for mammals the figure was 23 percent and for amphibians it was more than 30 percent.
"Scientists now refer to a sixth major extinction crisis that's under way," Steiner said.
The first mass extinction, about 440 million years ago, and the four succeeding extinctions were the result of physical shocks to the planet like volcanic eruptions and plate tectonic shifts.
The report said that annual emissions of CO2 from fossil fuels have risen by about one-third since 1987 and that the threat from climate change now was so urgent that only very large cuts in greenhouse gases of 60 to 80 percent could stop irreversible change.
The effects of global warming, like the melting ice in the Arctic are "accelerating at a pace that goes beyond the scenarios and models we've been using," Steiner said.
Climate change, however, was an issue that gained huge momentum over the past year, with governments, industries and citizens increasingly seeking solutions to the problem, Steiner said. The recent award of the Nobel Peace Prize to the Intergovernmental Panel on Climate Change and to former Vice President Al Gore was a sign of widespread scientific consensus that climate change is under way, he said.
Steiner called for an accelerated effort on a far wider range of environmental issues to build the same sense of urgency as shown on climate change over the past year to address the worsening situations of biodiversity, land degradation, fisheries and freshwater.
Many biologists and climate scientists have concluded that human activities have become a dominant influence on the planet's climate and ecosystems. But there is still a range of views on whether this could result in a catastrophic unraveling of natural resources as the human population heads toward nine billion by midcentury, or more of a steady diminution in diversity.