The genesis of two category-five hurricanes (Katrina and Rita) in a row over the Gulf of Mexico is an unprecedented and troubling occurrence. But for most tropical meteorologists the truly astonishing “storm of the decade” took place in March 2004. Hurricane Catarina–so named because it made landfall in the southern Brazilian state of Santa Catarina–was the first recorded South Atlantic hurricane in history.
Textbook orthodoxy had long excluded the possibility of such an event; sea temperatures, experts claimed, were too low and wind shear too powerful to allow tropical depressions to evolve into cyclones south of the Atlantic equator. Indeed, forecasters rubbed their eyes in disbelief as weather satellites downlinked the first images of a classical whirling disc with a well-formed eye in these forbidden latitudes.
In a series of recent meetings and publications, researchers have debated the origin and significance of Catarina. A crucial question is this: Was Catarina simply a rare event at the outlying edge of the normal bell curve of South Atlantic weather, just as, for example, Joe DiMaggio’s incredible fifty-six-game hitting streak in 1941 represented an extreme probability in baseball (an analogy made famous by Stephen Jay Gould)? Or was Catarina a “threshold” event, signaling some fundamental and abrupt change of state in the planet’s climate system?
Scientific discussions of environmental change and global warming have long been haunted by the specter of nonlinearity. Climate models, like econometric models, are easiest to build and understand when they are simple linear extrapolations of well-quantified past behavior–that is, when causes maintain a consistent proportionality to their effects.
But all the major components of global climate–air, water, ice and vegetation–are actually nonlinear: At certain thresholds they can switch from one state of organization to another, with catastrophic consequences for species too finely tuned to the old norms. Until the early 1990s, however, it was generally believed that these major climate transitions took centuries, if not millennia, to accomplish. Now, thanks to the decoding of subtle signatures in ice cores and sea-bottom sediments, we know that global temperatures and ocean circulation can, under the right circumstances, change abruptly–in a decade or even less.
The paradigmatic example is the so-called “Younger Dryas” event, 12,800 years ago, when an ice dam collapsed, releasing an immense volume of meltwater from the shrinking Laurentian ice sheet into the Atlantic Ocean via the instantly created St. Lawrence River. This “freshening” of the North Atlantic suppressed the northward conveyance of warm water by the Gulf Stream and plunged Europe back into a thousand-year ice age. Abrupt switching mechanisms in the climate system–such as relatively small changes in ocean salinity–are augmented by causal loops that act as amplifiers. Perhaps the most famous example is sea-ice albedo: The vast expanses of white, frozen Arctic Ocean ice reflect heat back into space, thus providing positive feedback for cooling trends. Alternatively, shrinking sea-ice levels increase heat absorption, accelerating both further melting and planetary warming.