Earth's environmental systems are nearly at "their biophysical limits," portending irreversible and possibly cataclysmic world changes, a U.N. report warns.

Environmental disasters ranging from arctic and antarctic sea ice and ice sheet shrinkage to Amazon rainforest deforestation, and from ozone depletion to an escalation of planetary toxins, are pushing the planet to an ecological tipping point that could change life as we know it, the U.N. Environment Program said in a report released in Rio de Janeiro.

"Scientific evidence shows that Earth systems are being pushed toward their biophysical limits, with evidence that these limits are close and have in some cases been exceeded," the Global Environment Outlook report said.

If humanity does not urgently change its ways, several critical thresholds will likely be breached, causing abrupt and generally irreversible negative changes to human well-being and planetary life-support functions, the report warned.

A result will be that "governments will preside over unprecedented levels of damage and degradation," U.N. Undersecretary-General and Environment Program Executive Director Achim Steiner said in releasing the report.

The 550-page report -- produced over three years and involving 600 experts who built up a detailed picture of the world's well-being -- identified areas of progress, including the expansion of protected areas such as national parks. But it detected little or no progress in 24 of the 90 most important environmental goals and objectives areas, including climate change, fish stocks, and desertification and drought.

The report said there was hope, if current policies and strategies are sharply changed and strengthened to meet an ambitious set of sustainability targets by mid-century.

It pointed to policy initiatives it considers responsible, including public investment, "green accounting," sustainable trade and technological innovation.

The United Nations is to convene a Conference on Sustainable Development, known as Rio 2012 or Rio+20, in Rio de Janeiro June 20-21.

The conference is a 20-year follow-up to a historic 1992 U.N. Conference on Environment and Development also held in Rio de Janeiro.

Steiner said he hoped the conference -- whose preparations have so far been politically charged -- would lead to decisive action so the planet can manage and reverse negative environmental trends and avoid the worst results of environmental degradation.