Human impact on Earth so great, it's 'too late to be a pessimist'

Humanity has dangerously overstepped three of the planet’s nine key natural thresholds, and are on track to cross the remaining six in coming decades, according to a new warning from an international team of scientists.

Taken together, the potential threats make it “too late to be a pessimist,” according to one of the researchers.

The three areas in which people have passed the limits of safe operating space are climate change, biodiversity loss and the nitrogen portion of the nitrogen/phosphorus inputs to the biosphere and ocean. The world is also approaching the safety limits for land system change, ocean acidification, global freshwater use and the phosphorus portion of the nitrogen/phosphorus inputs to the biosphere and ocean.

Two other thresholds — chemical pollution and atmospheric aerosol loading — have yet to be quantified by the research team.

“Until now, the scientific community has not attempted to determine the limits of the Earth system’s stability in so many dimensions and make a proposal such as this,” said Sander van der Leeuw, director of the School of Human Evolution and Social Change at Arizona State University and a co-author of the study published this week in the journal Nature. “On a finite planet, at some point, we will tip the vital resources we rely upon into irreversible decline if our consumption is not balanced with regenerative and sustainable activity.”

The groundbreaking study, undertaken by 29 European, Australian and US, not only identifies the critical tipping points for nine vital planetary systems but examines how stresses in one system affect other systems as well. The research aims to answer the question: ‘How much pressure can the Earth system take before it begins to crash?”

Scientists have dubbed our current era the Anthropocene, the first in which the actions of 6-plus billion humans have become the main driver of global environmental change.

In their Nature article, “A safe operating space for humanity,” the international research team proposes a limit for each boundary that would maintain the conditions for a livable world. For biodiversity, for example, the limit is less than 10 extinctions per million species per year. The extinction rate today is 100 species per million per year, compared to the pre-industrial value of 0.1 to 1 species.

While that approach doesn’t offer a complete roadmap for sustainable development, it does provide an important element by identifying critical planetary boundaries, the researchers say.

“We expect the debate on global warming to shift as a result, because it is not only greenhouse gas emissions that threaten our planet’s equilibrium,” said van der Leeuw. “There are many other systems and they all interact, so that crossing one boundary may make others even more destabilized.”

The team represents the threats to each natural system using an illustration of a polygon made up of nine wedges (one for each vital system); the green inner core of the polygon indicates the safe operating space for each system, while overlaying red wedges show how far humanity has pushed each system.

“We must make these complicated ideas clear in such a way that they can be widely applied,” said van der Leeuw. “The threats are so enormous that it is too late to be a pessimist.”

“Human pressure on the Earth system has reached a scale where abrupt global environmental change can no longer be excluded,” said lead author Johan Rockström, director of the Stockholm Resilience Centre at Stockholm University. “To continue to live and operate safely, humanity has to stay away from critical ‘hard-wired’ thresholds in Earth’s environment, and respect the nature of the planet’s climatic, geophysical, atmospheric and ecological processes. Transgressing planetary boundaries may be devastating for humanity, but if we respect them we have a bright future for centuries ahead.”

Co-author Diana Liverman, a professor of geography and development at the University of Arizona, added, “Our attempt to identify planetary boundaries that, if crossed, could have serious environmental and social consequences has a special resonance in the southwest where pressures on biodiversity, land use, and water are likely to intersect with climate change to create tremendous challenges for landscapes and livelihoods.”

Liverman is currently attending an international climate conference at Oxford, where participants are discussing the implications for humans and Earth ecosystems of a 4-degree Centigrade global temperature rise.

In addition to Liverman, Rockström and van der Leeuw, the group of authors includes Hans Joachim Schellnhuber, Will Steffen, Katherine Richardson, Jonathan Foley and Nobel laureate Paul Crutzen.