人類太超過？ 研究：地球9大「限度」破表4件

Human activities have "dangerously compromised" four of the nine processes that are crucial to maintaining the stability of the planet, warns an international team of researchers .

In a study published online today, the 18 authors, from universities and research institutes around the world, provide new evidence of changes to the systems which regulate the resilience of the Earth, placing current and future societies at risk.

Climate change, the loss of biosphere integrity, land-system change, and altered biogeochemical cycles such as phosphorus and nitrogen runoff have all passed beyond levels that put humanity in a "safe operating space," the scientists report.

Their study fixing new "planetary boundaries" was published today in the journal "Science." The boundaries represent thresholds or tipping points beyond which there will be irreversible and abrupt environmental change.

Crossing boundaries raises the risks to current and future societies of destabilizing the Earth System – the complex interactions of land, ocean, atmosphere, ice sheets, life and people.

"Transgressing a boundary increases the risk that human activities could inadvertently drive the Earth system into a much less hospitable state, damaging efforts to reduce poverty and leading to a deterioration of human well-being in many parts of the world, including wealthy countries," said lead author Will Steffen from the Stockholm Resilience Centre at Stockholm University and the Australian National University, Canberra.

The report, an update to previous studies, is titled "Planetary Boundaries: Guiding human development on a changing planet."

Two core boundaries – climate change and biosphere integrity – have been identified, each of which has the potential on its own to drive the Earth system into a new state should they be substantially and persistently transgressed, warn the study’s authors.

The internationally agreed upper climate change limit of two degrees Celsius above the planetary temperature in pre-industrial times lies beyond the climate change boundary, the authors warn.

This "makes two degrees a risky target for humanity, and therefore an absolute minimum target for the global climate negotiations," they say.

One of the affected systems is the nitrogen-phosphorus cycle, which is essential to all life and important to food production and clean water.

Altered biogeochemical cycles from the use of fertilizers such as phosphorus and nitrogen is risky, warns Professor Elena Bennett from McGill University’s School of the Environment in Montreal, who contributed the research on the nitrogen-phosphorus cycle.

"People depend on food, and food production depends on clean water," said Bennett. "This new data shows that our ability both to produce sufficient food in the future and to have clean water to drink and to swim in are at risk."

One of the problems is that phosphorus, used as a fertilizer for fields and lawns, is in limited supply, and that supply is geopolitically concentrated, explains Bennett. Nearly 90 percent of all known phosphorus reserves are found in just three countries – most of it in Morocco, with the rest in China and Algeria.

Further, the excess of phosphorus-based fertilizers that drain from fields and lawns into neighboring lakes can have disastrous effects on the surrounding water. It can lead to the sudden growth of algae that can cause the decline or death of other lake organisms and produce toxins that are dangerous to people or animals that swim in the lake or get drinking water from it.

"About half a million residents of the city of Toledo found out that their tap water had been contaminated with a toxin called microcystin last summer. And in 2007 the Quebec government declared that more than 75 lakes were affected by toxins produced by blue-green algae," said Bennett.

"This kind of problem is likely to become much more common. We will see more lakes closed, will have to pay more to clean our water, and we will face temporary situations where our water is not cleanable or drinkable more and more frequently," she warned. "That’s what it means to have crossed this planetary boundary."

The planetary boundaries concept, first published in 2009, identifies nine global priorities relating to human-induced changes to the environment.

The nine planetary boundaries are:

• Climate change
• Change in biosphere integrity, biodiversity loss and species extinction
• Stratospheric ozone depletion
• Ocean acidification
• Biogeochemical flows
• Land-system change
• Freshwater use
• Atmospheric aerosol loading, microscopic particles in the atmosphere that affect climate and living organisms
• Introduction of novel entities, such as organic pollutants, radioactive materials, nanomaterials, and micro-plastics.