Provocative new study warns of crossing planetary boundaries

The scientists also argue that as we spread fertiliser on farmland and burn coal, we are pumping far too much nitrogen into the environment. Human activity releases 121 million tons of nitrogen, much of which ends up polluting rivers, lakes and oceans and potentially pushing their ecosystems into irreversible changes. At most, the scientists argue, less than 35 million tons of nitrogen would be a safe boundary.

The rate at which species are becoming extinct is also far beyond a safe boundary, according to the scientists. During most of the history of life, species have become extinct at a slow, fairly regular pace. And as old species have become extinct, new ones have been evolving. There have been times when many species have become extinct at a much faster rate, and these pulses have sometimes ushered in a global collapse of ecosystems. The authors of the new Nature paper propose that to avoid collapse, the extinction rate cannot rise above 10 times the long-term background rate. Today, however, scientists estimate that the extinction rate is 100 to 1,000 times higher.

In five other areas, the scientists found, we have not yet crossed the boundary into the danger zone. As we release carbon dioxide, for example, some of it goes into the oceans and makes it more acidic. In acidic seawater, coral reefs have a harder time building skeletons, because the minerals they produce for their skeletons quickly dissolve. Invertebrates have the same trouble making shells. According to recent surveys, the ocean is now acidifying 100 times faster than at any time during the past 20 million years. Yet the Nature co-authors estimate that we have not yet reached the point where acidity may cause ecological collapse. But we are close.

While the paper makes for a sobering read, its authors think we should also find some cause for optimism in it. Humanity nearly crossed another threshold by destroying the ozone layer with chlorofluorocarbons. But we recognised the crisis in time and banned chlorofluorocarbons, allowing the ozone layer to slowly recover. If we had waited much longer we might have been too late to do anything. “We were able to avoid a global disaster,” says Rockstrom. He hopes we can do the same again, and keep human civilisation from falling off the environmental mesa.

“The authors make a strong case for their selection of key boundaries,” says the Carnegie Institution’s Christopher Field, “and the proposed locations for the boundaries are conceptually reasonable”. Field said he would not be surprised if other researchers argue for shifting the boundaries based on further research. “But most would agree with the general theme that we are pushing very hard on the Earth system, so hard that we should not be surprised if key parts begin to break.”

Other researchers agree with the basic concept of the new paper, but question whether we should be trying to pin down planetary boundaries. Michael Mann, a climate scientist at Penn State University, agrees that there probably is a dangerous threshold for climate change, but he thinks that 350 parts per million might be too strict a limit. And on a practical level, Mann points out that the policies being considered by the US Congress probably won’t even be able to keep carbon dioxide levels down to 550 parts per million in 2100. “I sometimes worry that there is the danger that if we dramatically move the goalposts and argue that 350 ppm must be the stabilisation target, policymakers will just throw their hands up in futility,” says Mann, or reach instead for a quick-fix geoengineering scheme, “which frankly terrifies me”.

But some critics question the basic concept itself. “The notion of a single boundary is just devoid of serious content,” says Stuart Pimm. “In what way is an extinction rate 10 times the background rate acceptable?”

One reason that the concept of planetary boundaries is so provocative is that it highlights how much scientists don’t yet understand about the thresholds built into our planet. “I think this is interesting and I’m glad the paper is coming out,” says Naeem, “but it could lead to the false sense that we understand the biosphere better than we do.”