Searching for Clues to Calamity

SO far 2012 is on pace to be the hottest year on record. But does this mean that we’ve reached a threshold — a tipping point that signals a climate disaster?

For those warning of global warming, it would be tempting to say so. The problem is, no one knows if there is a point at which a climate system shifts abruptly. But some scientists are now bringing mathematical rigor to the tipping-point argument. Their findings give us fresh cause to worry that sudden changes are in our future.

One of them is Marten Scheffer, a biologist at Wageningen University in the Netherlands, who grew up swimming in clear lowland ponds. In the 1980s, many of these ponds turned turbid. The plants would die, algae would cover the surface, and only bottom-feeding fish remained. The cause — fertilizer runoff from nearby farms — was well known, but even after you stopped the runoff, replanted the lilies and restocked the trout, the ponds would stay dark and scummy.

Mr. Scheffer solved this problem with a key insight: the ponds behaved according to a branch of mathematics called “dynamical systems,” which deals with sudden changes. Once you reach a tipping point, it’s very difficult to return things to how they used to be. It’s easy to roll a boulder off a cliff, for instance, but much harder to roll it back. Once the ponds turned turbid, it wasn’t enough to just replant and restock. You had get them back to their original, clear state. More