“To be honest, when others have said that a planetary critical transition is possible [or] likely, they’ve done so without any underlying model,” Brook says. “It’s just speculation…. No one has found the opposite of what we suggested—they’ve just proposed it.” In their analysis Brook’s group concluded that the diversity of local responses to global forcings like increasing temperature means we cannot identify any particular point of no return.
Tim Lenton, an expert on tipping points at the University of Exeter in England, says there is no convincing evidence of global shift yet, but he doesn’t rule out the possibility. “It’s not obvious how you can get a change in Siberia then causing a synchronized change in Canada or Alaska,” he says, referring to a commonly cited climate feedback loop of permafrost melting at northern latitudes. “That doesn’t seem likely. It’s more that different parts of the Arctic are going to reach the thawing threshold at the same time just because they’re getting to the same kind of temperature.” This is a fine distinction: Are we looking at multiple systems tipping as one, or just a coincidental amalgam of unconnected systems falling off a ledge at similar time points?
Lenton says that there is a chance that ice-free Arctic summers could start a cascade effect—for example, elevated temperatures on nearby land that eventually find their way down into the permafrost and cause rapid melting. The carbon released by the permafrost could in turn initiate further warming, and maybe tip another disconnected system and so forth. “It’s a bit like having some dominos lined up,” he says. “I’m not sure yet whether we have a scenario like that for future climate change, but it’s worth consideration.”
Such a domino effect could end up looking more like a “smooth” response than a nonlinear one, but NASA’s Hansen says this doesn’t suggest we should ignore it. “Most tipping points are ‘smoothed out,’ but that does not decrease their importance,” he says. Even Arctic sea ice shows a smoothed response as it rolls past the point of no return. “Once you have passed a certain point, it takes only little additional forcing to lose all the sea ice.” And he echoes Lenton on the idea of dominos and hugely important sub-global systems: “[It’s] hard to see how the Greenland ice sheet would survive if we have sea ice-free summers.” In other words, melted sea ice could beget massive sea level rise, thanks to a supposedly unconnected system.
And further, that non-connectivity is not necessarily a given. Barnosky argues that the fundamental assumption that systems around the world are not strongly connected is no longer true. “What used to be isolated parts of the Earth really are very connected now, and we’re the connectors,” he says. Further, his group’s paper based the possibility of a global tipping point largely on comparisons to planetary history: Earth has exhibited rapid phase shifts in the past, and we are blowing those types of changes out of the water now. For example, the shift from the last glacial period into the current interglacial, which took only a few millennia ending around 11,000 years ago, featured abrupt land-use change: about 30 percent of the land surface went from ice-covered to ice-free over those few thousand years. In just a few hundred years, humans have converted about 43 percent of the world’s land to agricultural or urban landscapes.
Whether such rapid changes portend a new global shift is, to some extent, an esoteric, academic question. The answer depends on whether the world can really follow the classic mathematical definition of tipping points that relies on “bifurcation theory.” That theory holds that a system follows a smooth curve until a certain threshold—the egg rolls at similar speed until it hits the edge of the table—when it jumps to a new state with no obvious change in external pressures. And importantly, once that jump is made there is essentially no going back; you can’t “unbreak” the egg.