An anonymous reader quotes a report from CNN: Human actions have pushed the world into the danger zone on several key indicators of planetary health, threatening to trigger dramatic changes in conditions on Earth, according to a new analysis from 29 scientists in eight countries. The scientists analyzed nine interlinked “planetary boundaries,” which they define as thresholds the world needs to stay within to ensure a stable, livable planet. These include climate change, biodiversity, freshwater and land use, and the impact of synthetic chemicals and aerosols. Human activities have breached safe levels for six of these boundaries and are pushing the world outside a “safe operating space” for humanity, according to the report, published on Wednesday in the journal Science Advances.

The nine boundaries, first set out in a 2009 paper, aim to establish a set of defined “limits” on changes humans are making to the planet — from pumping out planet-heating pollution to clearing forests for farming. Beyond these limits, the theory goes, the risk of destabilizing conditions on Earth increases dramatically. The limits are designed to be conservative, to enable society to solve the problems before reaching a “very high risk zone,” said Katherine Richardson, a professor in biological oceanography at the University of Copenhagen and a co-author on the report. She pointed to the unprecedented summer of extreme weather the world has just experienced at 1.2 degrees Celsius of global warming. “We didn’t think it was going to be like this at 1 degree [Celsius]” she said. “No human has experienced the conditions that we’re experiencing right now,” she added.

Of the three boundaries that scientists found are still within a safe space, two of them — ocean acidification and the amount of aerosols in the atmosphere — are moving in the wrong direction. There is some good news, however. The ozone layer was on the wrong side of the boundary in the 1990s, Richardson said. But thanks to international cooperation to phase out ozone-depleting chemicals, it is on track to recover completely. Crossing planetary boundaries does not mean the world has reached a disastrous tipping point. Hitting one does not mean “falling off a cliff,” Richardson said. But it is a clear warning signal. The significance of the planetary boundaries model is that it doesn’t analyze climate and biodiversity in isolation, the report authors said. Instead, it looks at the interaction of both, as well as a host of other ways humans are affecting the planet. Breaching one boundary is likely to have knock-on effects for others.