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OUR planet is balancing on a knife edge. Even small, random events such as
surging glaciers or sudden floods can trigger fundamental changes in the Earth鈥檚
climate鈥� provided they happen at the right moment.

Several times during the last ice age our planet warmed by up to 10 掳C in
little more than a decade. Typically these sudden warmings, known as
Dansgaard-Oeschger events, happen every 1500 years, but sometimes they miss one
or two beats, reappearing after 3000 or 4500 years.

Andrey Ganopolski and Stefan Rahmstorf of the Potsdam Institute for Climate
Impact Research in Germany developed a computer model of the warmings to try to
explain this mysterious pattern, and have come up with what climatologists are
this week hailing as the most likely answer鈥攔andom events coinciding with
a subtle and otherwise benign climatic pulse.

In Dansgaard-Oeschger warmings, ocean circulation flips abruptly between two
stable states. Its state at a given time depends on the salinity of the far
northern Atlantic Ocean near Greenland. When that water is salty and dense it
sinks to the ocean floor, driving a vigorous ocean circulation that keeps the
planet cool. But if salinity falls, the circulation slows or even halts and our
world warms.

The Earth鈥檚 climate system seems to have a natural pulse every 1500 years,
say Rahmstorf and Ganopolski, probably caused by regular fluctuations in the
energy output of the Sun. By itself this weak rhythm would have little effect on
our climate. But if random geological events on Earth happen to coincide with
the peak of the cycle, the cumulative effect can be enough to tip the balance
and cause a flip from one state to the other. This effect is called stochastic
resonance, and it comes into play in a variety of systems, from radio reception
to the stock market.

Climatologist Richard Alley of Penn State University says the finding is a
breakthrough. It鈥檚 not proof鈥攏o climate model can provide that, he says.
鈥淏ut this is right at the front of important ideas driving this research. It
gives us hope of understanding the instability of climate.鈥�

Since the last ice age ended, the planet has been fixed in a relatively
stable cool phase. But, says Ganopolski, 鈥渢he ocean circulation is not
unconditionally stable in present conditions. It simply means that our modern
climate needs stronger perturbations for it to be disturbed.鈥�

And global warming could do it. Changing rainfall patterns and melting ice in
Greenland mean that the water in the north Atlantic is already becoming fresher.
Ganopolski believes that a flip in the oceanic system could boost temperatures
still further as early as the beginning of the 22nd century.

鈥淭hat鈥檚 not a prediction. But it is something more than pure speculation,鈥� he
says. Alley agrees: 鈥淭he more the climate is forced to change, the more likely
it is to hit some unforeseen threshold that can trigger quite fast, surprising
and perhaps unpleasant changes.鈥�