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Abrupt Climate Changeby Richard B Abrupt Climate ChangeBy Richard B. Alley n the Hollywood disaster thriller Inevitable, too, are the potential chal- The Day after Tomorrow, a climate lenges to humanity. Unexpected warm Winter temperatures catastrophe of ice age proportions spells may make certain regions more catches the world unprepared. Mil- hospitable, but they could magnify swel- plummeting six degrees lions of North Americans fl ee to tering conditions elsewhere. Cold snaps sunny Mexico as wolves stalk the could make winters numbingly harsh Celsius and sudden last few people huddled in freeze- and clog key navigation routes with ice. dried New York City. Tornadoes rav- Severe droughts could render once fertile droughts scorching I age California. Giant hailstones pound land agriculturally barren. These conse- farmland around the Tokyo. quences would be particularly tough to Are overwhelmingly abrupt climate bear because climate changes that oc- globe are not just the changes likely to happen anytime soon, cur suddenly often persist for centuries or did Fox Studios exaggerate wildly? or even thousands of years. Indeed, the stuff of scary movies. The answer to both questions appears collapses of some ancient societies—once to be yes. Most climate experts agree attributed to social, economic and politi- Such striking climate that we need not fear a full-fl edged ice cal forces—are now being blamed largely age in the coming decades. But sudden, on rapid shifts in climate. jumps have happened dramatic climate changes have struck The specter of abrupt climate change many times in the past, and they could has attracted serious scientifi c investiga- before—sometimes happen again. In fact, they are probably tion for more than a decade, but it has inevitable. only recently captured the interest of within a matter of years SLIM FILMS FILMS SLIM 62 SCIENTIFIC AMERICAN NOVEMBER 2004 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. SUDDEN FLIP-FLOPS are an unavoidable element of earthly climate. COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. fi lmmakers, economists and policymak- other hand it achieved roughly half of Ice layers that trapped dust from Asia ers. Along with more attention comes the heating sustained since the peak of indicated the source of prevailing winds, increasing confusion about what trig- the last ice age—more than 10 degrees for instance. Investigators concluded gers such change and what the outcomes C—in a mere decade. That jump, which that the winds must have been calmer will be. Casual observers might suppose occurred about 11,500 years ago, is the during warm times because less wind- that quick switches would dwarf any ef- equivalent of Minneapolis or Moscow blown sea salt and ash from faraway fects of human-induced global warming, acquiring the relatively sultry conditions volcanoes accumulated in the ice. And which has been occurring gradually. But of Atlanta or Madrid. the list of clues goes on [see “Greenland new evidence indicates that global warm- Not only did the ice cores reveal what Ice Cores: Frozen in Time,” by Richard ing should be more of a worry than ever: happened in Greenland, but they also B. Alley and Michael L. Bender; Scien- it could actually be pushing the earth’s hinted at the situation in the rest of the tifi c American, February 1998]. climate faster toward sudden shifts. world. Researchers had hypothesized that Intense, abrupt warming episodes the 10-degree warming in the north was appeared more than 20 times in the Jumping Back and Forth part of a warming episode across a broad Greenland ice records. Within several scientists might never have swath of the Northern Hemisphere and hundreds or thousands of years after fully appreciated the climate’s ability to that this episode enhanced precipitation the start of a typical warm period, the lurch into a radically different state if not in that region and far beyond. In Green- climate reverted to slow cooling fol- for ice cores extracted from Greenland’s land itself, the thickness of the annual lowed by quick cooling over as short a Global warming should be more of a worry than ever: it could be pushing the earth’s climate faster toward sudden shifts. massive ice sheet in the early 1990s. ice layers showed that, indeed, snowfall time as a century. Then the pattern be- These colossal rods of ice—some three had doubled in a single year. Analyses of gan again with another warming that kilometers long—entomb a remarkably old air bubbles caught in the ice corrobo- might take only a few years. During the clear set of climate records spanning the rated the prediction of increased wetness most extreme cold conditions, icebergs past 110,000 years. Investigators can dis- in other areas. In particular, measure- strayed as far south as the coast of Por- tinguish annual layers in the ice cores and ments of methane in the bubbles indi- tugal. Lesser challenges probably drove date them using a variety of methods; the cated that this swamp gas was entering the Vikings out of Greenland during the composition of the ice itself reveals the the atmosphere 50 percent faster during most recent cold snap, called the Little temperature at which it formed. the intense warming than it had previ- Ice Age, which started around A.D. 1400 Such work has revealed a long his- ously. The methane most likely entered and lasted 500 years. tory of wild fl uctuations in climate— the atmosphere as wetlands fl ooded in Sharp warm-ups and cool-downs in long deep freezes alternating with brief the tropics and thawed up north. the north unfolded differently elsewhere warm spells. Central Greenland experi- The cores also contained evidence in the world, even though they may have enced cold snaps as extreme as six de- that helped scientists fi ll in other details shared a common cause. Cold, wet times grees Celsius in just a few years. On the about the environment at various times. in Greenland correlate with particularly cold, dry, windy conditions in Europe and North America; they also coincide Overview/Inevitable Surprises with anomalously warm weather in the ■ Most climate change policy discussions and research efforts center on South Atlantic and Antarctica. Investi- global warming. But another problem looms as well: climate has suddenly gators pieced together these regional his- fl ip-fl opped in the past and will surely do so again. tories from additional clues they found ■ A regional drought could, for instance, arrive one summer and stay for in the ice of high mountain glaciers, the decades, wiping out rich agricultural lands across Asia and North America; thickness of tree rings, and the types of weather patterns in Europe could shift in a matter of years, making that area’s pollen and shells preserved in ancient climate more like Siberia’s. mud at the bottoms of lakes and oceans, ■ Scientists cannot yet predict when such abrupt changes will occur, but most among other sources. climate experts warn that global warming and human activities may be The evidence also revealed that abrupt propelling the world faster toward sudden, long-lasting climate changes. shifts in rainfall have offered up chal- lenges rivaling those produced by tem- 64 SCIENTIFIC AMERICAN NOVEMBER 2004 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. PAST AS PROLOGUE? Abrupt climate change has marked the earth’s history for from Mexico’s Yucatán Peninsula record sudden and severe eons. Ice cores from Greenland, for instance, reveal that droughts (bottom left) because a diagnostic ratio of oxygen wild temperature swings (top left) punctuated the gradual isotopes in the shells shoots up when more water evaporates warming that brought the planet out of the last ice age starting from the lake than falls as rain. Societies have often suffered about 18,000 years ago. Fossil shells in lake sediments as a result of these rapid shifts (photographs). Medieval Warm Period –30 –35 8,200-Year Event Little Ice Age –40 End of Younger Dryas (degrees Celsius) (degrees –45 Start of ; Temperature in Central Greenland Greenland in Central Temperature Younger Dryas Viking settlement, now in ruins, was among those in Greenland abandoned during an –50 abrupt cold spell called the Little Ice Age. 15 10 5 0 Time (thousands of years ago) Drier Collapse of Mayan National Geophysical Data Center Data Geophysical National civilization ) bottom graph bottom ) bottom photo bottom Climate and Precipitation and Climate ) AND D. HODELL ET AL. ( Inferred from Oxygen Isotope Composition Isotope Oxygen from Inferred Mayan rain god (statue in foreground) top graph was apparently no match for the drought Wetter now widely blamed for the collapse of Mayan 7 6 5 4 3 2 1 0 civilization about 1,100 years ago. Time (thousands of years ago) perature swings. Cold times in the north cifi c occasionally have steered weather key driver of climate toward an invisible typically brought drought to Saharan patterns far enough to trigger surprise threshold. At the point that threshold Africa and India. About 5,000 years ago droughts, such as the one responsible for was crossed, the climate driver—and thus a sudden drying converted the Sahara the U.S. dust bowl of the 1930s. the climate as well—fl ipped to a new and );FREELANCE CONSULTING SERVICES PTY LTD/CORBIS ( PTY LTD/CORBIS SERVICES CONSULTING );FREELANCE from a green landscape dotted with lakes different state and usually stayed there to the scorching, sandy desert it is today. Point of No Return for a long time [see box on next page]. top photo Two centuries of dryness about 1,100 be they warm spells, cold snaps Crossing a climate threshold is simi- years ago apparently contributed to the or prolonged droughts, the precipitous lar to fl ipping a canoe.
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