While agreeing that a tipping point has already been crossed for Arctic Sea Ice, experts still debate whether the entire planetary biopsphere has tipping points.
Tony Barnosky compares crossing a tipping point to an egg rolling off the edge of a table. A sudden change occurs for which there is no going back.
Sea ice represents a big system, but it is generally thought to be self-contained enough to follow such a tipping-point pattern. The question that has started to pop up increasingly in the last year, however, is whether that sort of phase transition, where a system shifts rapidly—in nonlinear fashion, as scientists say—from one state to another without recovery in a timescale meaningful to humanity, is possible on a truly global scale.
Part of the debate rests on how interconnected different ecosystems might be.
Barry Brook of the University of Adelaide in Australia and colleagues argues that there is no real grounding to the idea that the world could display true tipping-point characteristics. The only way such a massive shift could occur, Brook says, is if ecosystems around the world respond to human forcings in essentially identical ways. Generally, there would need to be “strong connections between continents that allow for rapid diffusion of impacts across the planet.”
Oceans and mountain ranges cut off different ecosystems from each other, and the response of a given region is likely to be strongly influenced by local circumstances. For example, burning trees in the Amazon can increase CO2 in the atmosphere and help raise temperatures worldwide, but the fate of similar rainforests in Malaysia probably depend more on what’s happening locally than by those global effects of Amazonian deforestation.
This reminds me of ongoing research to determine just how much soot from uncontrolled burning forests in several continents has darkened Arctic and Greenland ice. We already know that dust from Africa impact ocean fertility, even fertility in the Americas. To me it's naive to assume each ecosystem won't impact others separated by oceans or mountain ranges. In other words, denial of the possibility or likelihood of global tipping points comes down to tunnel vision, or even wishful thinking.
... at the bottom of the mathematical debate is a question of utility: Would the existence of a real planetary-scale tipping point change how we should confront our environmental challenges, from energy sources to land use?
A more accurate picture would not just let us prepare for rapid climate change, but might help us predict it as well. Marten Scheffer, of Wageningen University in the Netherlands, has done extensive work on ways we can see tipping points coming. On smaller scales, he says, a system can exhibit “critical slowing down”—a slowed ability to recover from perturbations—before jumping to the irreversible new state. Scheffer says, arguing for tipping points, that past global-scale, quick changes in climate appear to have exhibited a similar effect.