Image from Phys.Org illustrating boundaries of the Earth system

These are estimates of how the different control variables for seven planetary boundaries have changed from 1950 to present. The green shaded polygon represents the safe operating space. Human activities have already pushed the Earth system beyond three of the planet's biophysical thresholds, with consequences that are detrimental or even catastrophic for large parts of the world; six others may well be crossed in the next decades, conclude 29 European, Australian and US scientists in an article in the September 24 issue of the scientific journal Nature. Credit: Image courtesy of Courtesy of Stockholm Resilience Centre

Human activities have already pushed the Earth system beyond three of the planet's biophysical thresholds, with consequences that are detrimental or even catastrophic for large parts of the world; six others may well be crossed in the next decades, conclude 29 European, Australian and U.S. scientists in an article in the Sept. 24 issue of the scientific journal Nature.

Scientists have been warning for decades that the explosion of human activity since the industrial revolution is pushing the Earth's resources and natural systems to their limits. The data confirm that 6 billion people are capable of generating a global geophysical force the equivalent to some of the great forces of nature - just by going about their daily lives.

This force has given rise to a new era - Anthropocene - in which human actions have become the main driver of global environmental change.

"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," says co-author Sander van der Leeuw who directs the School of Human Evolution and Social Change at Arizona State University. Van der Leeuw is an archaeologist and anthropologist specializing in the long term impacts of human activity on the landscape. He also co-directs ASU's Complex Adaptive Systems Initiative that focuses ASU's interdisciplinary strength on large-scale problems where an integrated effort is essential to finding solutions.

Defining planetary boundaries

It started with a fairly simple question: How much pressure can the Earth system take before it begins to crash?

"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. We are sending these ideas out through the Nature article to be vetted by the scientific community at large," explains van der Leeuw, whose experience includes leading interdisciplinary initiatives in ASU's College of Liberal Arts and Sciences.

"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. There are many other systems and they all interact, so that crossing one boundary may make others even more destabilized," he warns.

Nine boundaries were identified, including climate change, stratospheric ozone, land use change, freshwater use, biological diversity, ocean acidification, nitrogen and phosphorus inputs to the biosphere and oceans, aerosol loading and chemical pollution. The study suggests that three of these boundaries -climate change, biological diversity and nitrogen input to the biosphere - may already have been transgressed.

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

"A safe operating space for humanity"

Using an interdisciplinary approach, the researchers looked at the data for each of the nine vital processes in the Earth system and identified a critical control variable. Take biodiversity loss, for example, the control variable is the species extinction rate, which is expressed in extinctions per million species per year.

They then explored how the boundaries interact. Here, loss of biodiversity impacts carbon storage (climate change), freshwater, nitrogen and phosphorous cycles, and land systems. (click link above for complete article)