Reaching a substantive global agreement in Copenhagen, Denmark, at the UN-sponsored climate-change conference this December on reducing greenhouse-gas emissions is a necessary step, but it is not sufficient. In order to avoid catastrophic tipping points, we need to effectively manage key Earth system processes, and we need to do it now.
In the run-up to the climate negotiations in Copenhagen, it is easy to get the impression that as long as we reduce emissions of greenhouse gases we will be safe from dangerous, or even disastrous, outcomes for humanity.
But mounting scientific evidence strongly suggests that is very unlikely.
For decades, we have lived with the predominant belief that environmental change occurs in an incremental, linear, and predictable fashion.
But growing evidence indicates that this may be the exception, not the rule, and that long periods of gradual change can eventually push us past thresholds that result in abrupt and potentially disastrous changes.
In fact, we can no longer exclude the possibility that we are crossing hard-wired thresholds at the planetary level, threatening the self-regulating capacity of the planet to remain in the stable and favorable state in which human civilizations and societies have developed during the past 10,000 years.
Compared with the 200,000 years or so that we humans have roamed around on Earth, this Holocene state has been extraordinarily stable from an environmental perspective, providing humanity with the precondition for human development as we know it, from the rise of agriculture to the modern industrial societies of today.
In a recent article presented in the scientific journal Nature, my colleagues and I make a first attempt to identify and quantify the Earth system processes and potential biophysical thresholds that, if crossed, could generate unacceptable environmental change for humanity, such as irreversible loss of inland glaciers, a transition of rain forests to savannas, massive destruction of tropical coral reefs, desertification of current agricultural land, and the shift in the Indian and African monsoon systems.
For each of the processes we identified, we also propose planetary boundaries that should be respected in order to reduce the risk of crossing these thresholds and moving into an undesired state for humanity on Earth.
We identified nine critical Earth system processes including climate change, depletion of stratospheric ozone, land-use change, freshwater use, rate of biological diversity loss, ocean acidification, amounts of nitrogen and phosphorus inputs to the biosphere and oceans, air pollution from aerosol loading, and chemical pollution.
Our scientific proposition, based on this new concept, is that as long as we stay within the boundaries for these nine, we give ourselves a long-term safe operating space for human development on Earth. We thereby stand a good chance of keeping Earth within the stable Holocene state for at least another couple thousand years, providing ample opportunities to support long-term social and economic development in the world.
Climate change is, not surprisingly, one of the nine proposed boundary processes, and here we build on the latest climate science, which indicates that we may have to stabilize the concentration of carbon dioxide in the atmosphere to 350 parts per million (p.p.m.) in order to avoid nonlinear and potentially irreversible environmental change.
We are already at approximately 390 p.p.m., i.e., we are already in a danger zone.
But when it comes to climate change, our research shows that focusing on reducing emissions without putting in place mechanisms to maintain the integrity of current "carbon sinks" in oceans and on land will prevent us from making any significant gains in greenhouse-gas reductions.
This is because, to date, nature has been doing us a huge favor.
Land and oceans have been providing a free ecosystem service, in the form of sinks that store carbon dioxide. As much as 50 percent of today's carbon dioxide emissions are absorbed by terrestrial and marine ecosystems.
But the capacity of this ecosystem service may now be decreasing due to temperature increases, acidification of oceans, and land-use changes.
And if we continue to warm and acidify the oceans and cut down forests, we risk not only collapsing ecosystems followed by increased human starvation, but also reducing or even reversing this free service nature provides us . If the planet turns from friend to foe, i.e., from sink to source, when eroding the resilience of the biosphere, we will enter a potentially disastrous domain of runaway climate change. To avoid such outcomes will require radical action not only on emission reductions but also on active stewardship of the world's ecosystems.
The boundaries are, in other words, tightly coupled; transgressing the boundaries for nitrogen, land, water, oceans, ozone, and biodiversity will all threaten the ability to stay within the safe space of the climate system. An example of this is that fresh water determines the amount of biomass growth, i.e., the amount of carbon captured from the atmosphere in trees and plants, which in turn determines the amount of organic matter, i.e., carbon, in the world's soils. The world's soils hold some 1,500 billion tons of carbon, compared with the annual global emissions of some 9 billion tons of carbon.
Whether or not humanity will be able to stabilize climate within safe levels depends upon our ability to reduce emissions and constructively manage a number of critical natural systems on the planet.
This profoundly changes the agenda on solving the problem of anthropogenic climate change, as it indicates the need for an Earth systems approach to climate mitigation.
This is a rather depressing conclusion given the worrying state of the current climate negotiations, where the rift continues to widen between what science shows is needed to solve the human-induced climate problem and what is considered politically possible to do.
But if the best available science is telling us that the Earth system is in serious jeopardy of tipping into an unfavorable state for human development, should today's political realities dictate how we define success?
Unfortunately, in this drama there are no second chances. Nature does not do bailouts.
This is why reaching a substantive global agreement in Copenhagen on reducing greenhouse-gas emissions is necessary, but not sufficient for steering clear of catastrophic, irreversible tipping points in the Earth system.
To reduce the risk of tipping into the unknown, we need a new global deal for sustainable development.
Copenhagen should be viewed as a first, necessary step toward this new deal.
Johan Rockström is the director of the Stockholm Resilience Centre at Stockholm University, and lead author, joined by Nobel laureate Paul Crutzen and others, of a new article in the scientific journal Nature on how to cope with climate change.