THRESHOLDS AND BOUNDARIES

"As scientists learn more about how our planet works, both now and in the past, they are increasingly aware of the existence of environmental ‘tipping points’ – points of no return. These are thresholds beyond which environmental change becomes hard to stop.

Many of these arise from feedback loops – vicious circles in which changes that are small in their own right make future changes more likely. For instance, global warming is causing the carbon stored in polar soils to be released into the atmosphere, triggering further warming and releasing even more soil carbon.

Scientists are still trying to understand exactly where these tipping points lie, and what risks they pose. But even given the remarkable resilience of the Earth system, it’s increasingly clear that we’re heading into dangerous territory. Human society needs a stable environment; if things change too much or too fast, many societies may be unable to cope.

But we’re causing rapid changes in many areas. This has led a team of scientists to suggest nine critical boundaries we need to stay within to avoid unacceptable environmental degradation with serious consequences for societies. They suggest we’re close to or beyond at least four of them – ozone depletion, climate change, biodiversity loss and nutrient pollution. We’re also worryingly close to several other suspected tipping points.

And these thresholds are interconnected; crossing one makes it easier to cross others. For instance, clearing Amazon rainforest could affect weather patterns, endangering water supplies on the other side of the world, while unchecked ocean acidification will devastate biodiversity. This kind of change could put millions at risk of starvation.

The more we understand environmental tipping points, the better our chance of avoiding a cascade of changes that we can’t hope to reverse."