The Science News

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Climate change may act more suddenly than we have expected till now.

The genesis of two Category 5 hurricanes in a row (Katrina and Rita) 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, are 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. Was Catarina simply a rare event at the far outlier of the normal bell curve of South Atlantic weather, or was Catarina a 'threshold' event, signalling some fundamental and abrupt change of state in the climate system?

Scientific discussions of environmental change and global warming have long been haunted by the spectre of nonlinearity. Climate models are easiest to build and understand when they are simple linear extrapolations of well quantified past behaviour - 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 switch from one state of organisation 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 we know that global temperature and ocean circulation can 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. The freshening of the North Atlantic suppressed the northward conveyance of warm water by the Gulf current and plunged Europe back into a thousand-year ice age. Thresholds, switches, amplifiers, chaos - contemporary geophysics assumes that earth history is inherently revolutionary. This is why many prominent researchers - especially those who study topics like ice-sheet stability and North Atlantic circulation - have always had qualms with the consensus projections of the Intergovernmental Panel on Climate Change (IPCC), the world authority on global warming.

In contrast to Bushite flat-earthers and accomplices of the oil industry, their scepticism has been founded on the fear that the IPCC models fail to adequately allow for catastrophic nonlinearities like the Younger Dryas or Hurricane Catarina. Where other researchers model late 21st century climate upon the precedents of the Altithermal (the hottest phase of the current Holocene period, 8,000 years ago) or the Eemian (the previous and warmer interglacial episode 120,000 years ago), they toy with the possibilities of runaway warming returning the earth to the torrid chaos of the Paleocene-Eocene Thermal Maximum (PETM, 55 million years ago) when extreme and rapid heating of the oceans led to massive extinctions.

Dramatic new evidence has emerged recently that we may be headed, if not back to the dread and almost inconceivable PETM, then at least towards a much harder landing than envisioned by the IPCC.

Recently I was pole-axed by an article entitled 'Arctic System on Trajectory to New, Seasonally Ice-Free State', co-authored by 21 scientists from almost as many universities and research institutes.

The article begins with a recounting of trends familiar to many: for almost 30 years Arctic sea ice has been thinning and shrinking so dramatically that 'a summer ice-free Arctic Ocean within a century is a real possibility.' It adds, however, the new observation that this process is probably irreversible: 'Surprisingly, it is difficult to identify a single feedback mechanism within the Arctic that has the potency or speed to alter the system's present course.'

An ice-free Arctic Ocean has not existed for at least one million years, and the authors warn that the earth is inexorably headed towards a 'super-interglacial' state 'outside the envelope of glacial-interglacial fluctuations that prevailed during recent earth history'. They emphasise that within a century global warming will probably exceed the Eemian temperature maximum and thus obviate all the models that have used this as a likely scenario. They also suggest the total or partial collapse of the Greenland ice sheet is a real possibility - an event that would definitely throw a Younger Dryas wrench into the Gulf Current.

'Out of the envelope' means that we are not only leaving behind the serendipitous climatic parameters of the Holocene - the last 10,000 years of mild, warm weather that has favoured the explosive growth of agriculture and urban civilisation - but also of the late Pleistocene that fostered the evolution of homo sapiens in eastern Africa.

Other researchers undoubtedly will contest the extraordinary conclusions of the EOS article and - we must hope - suggest countervailing forces to this scenario of an Arctic albedo catastrophe. But for the time being, at least, research on global change is pointing towards worst-case scenarios.

All of this, of course, is a perverse tribute to industrial capitalism and extractive imperialism as geological forces so formidable that they have succeeded in scarcely more than two centuries - mainly in the last 50 years - in knocking the earth off its climatic pedestal.

The demon in me wants to say party and make merry. No need now to worry about Kyoto, recycling your aluminium cans or using too much toilet paper, when we'll soon be debating how many hunter-gatherers can survive in the scorching deserts of New England or the tropical forests of the Yukon.

The good parent in me, however, screams, 'How is it possible that we can now contemplate with scientific seriousness whether our children's children will themselves have children?' Let Exxon answer that in one of their sanctimonious ads.