Ancient Ice Yields Forecast of Sharp Shifts in Climate
Evidence frozen in a core of ice from Greenland suggests that global warming, rather than a slow, steady phenomenon, could trigger wild climate fluctuations in the next 100 years.
Global weather patterns have been remarkably consistent since the last Ice Age ended 10,000 years ago, so scientists came to believe that a stable climate was the norm for interglacial periods.
But evidence from a slender ice core pulled out of the Greenland ice sheet last summer indicates that this was not always the case, and scientists fear the climate may not be stable in the future if pollution causes enough change in the atmosphere.
Dust and gas trapped in the ice suggest that the last interglacial period more likely was a chaotic jumble of climatic swings apparently caused by radical changes in atmospheric gases and Atlantic Ocean circulation patterns.
The 40 scientists on the Greenland Ice Core Project who make the report in today’s issue of the journal Nature said they were troubled by the news because the last interglacial period, the Eemian period about 120,000 years ago, is often considered much like what today’s world would be if it were slightly warmer.
They speculate that if the global-warming theory proves true and air pollution causes the Earth to gain several degrees in average temperature, the planet could experience a similar period of wild climatic swings between today’s temperate weather and near-Ice Age conditions. Each period could last a few decades to a few millennia.
Confirmation of this hypothesis could come from a second ice core that is being taken near the first, the team of European and American scientists said. But in a separate Nature article, two Princeton University scientists suggest that there may already be reason for concern, based on the rate at which atmospheric carbon dioxide is increasing.
Assuming the accepted annual carbon dioxide growth rate of 1%, Syukuro Manage and Ronald J. Stouffer calculated that the level of the greenhouse gas would double in 70 years and quadruple in 140 years.
Using a computer model to simulate the effect of the resulting increased temperature, they predicted an eventual collapse of the Gulf Stream, a conveyor belt of warm ocean water from the Gulf of Mexico to the north Atlantic Ocean.
Scientists generally believe that ocean-driven heat exchange is the reason northern Europe enjoys temperate weather despite being as close to the North Pole as the Yukon and Siberia.
Andrew J. Weaver of the University of Victoria in Canada said the dramatic and rapid global climate change hypothesized in the articles is plausible. The Earth has had many “equilibrium” climates that were different from today’s temperate climate, he said, such as the hotter, wetter Cretaceous Period, when dinosaurs roamed the planet.
“We can affect our climate so that it shifts from one equilibrium to another,” he said, “and the shift, the transition period, will be filled with violent extremes until it sets into a new equilibrium.”
Earth’s climate system is so vast and complex that human activity alone is unlikely to trigger a short-term catastrophic change, he said, but it could be enough to speed up or exaggerate natural cycles in ways that are hard to predict.
New evidence of warm-weather climatic oscillations adds to such concerns, scientists said.
“Previously we just assumed the last warm period was very similar to the present period and just stable like it is now,” Eric Wolff of the British Antarctic Survey told Reuters news service. “Finding that it wasn’t raises fears about what could happen if it gets warmer.”
Wolff and his colleagues in the Greenland Ice Core Project retrieved the ice sample on which they based their conclusions by drilling nearly 10,000 feet into the ice sheet covering the island. Their sample is the most detailed one yet, extending back through the warm period before the last Ice Age, which started 100,000 years ago.
The scientists deduced the ancient air temperatures by examining different isotopes of oxygen and other elements in the ice samples.
Concern about a modern warming trend grows out of evidence from the ice core indicating that when temperatures in the interglacial period rose an average of 3.8 degrees above current levels they started fluctuating by as much as 18 degrees in as little as one or two decades.
In the past, scientists have predicted that the release of carbon dioxide and other pollutants--most often from the burning of gasoline and other fossil fuels--could raise global temperatures by 3 to 9 degrees over the next 100 years. But some scientists have challenged that figure as being too high, and a small minority question whether pollution is having any measurable effect on climate.
Even an increase of only a few degrees in the average temperature of the planet could have significant effects, scientists have estimated. Warm water expands, so oceans could rise by several feet. Melting ice from glaciers and the polar ice cap could increase the rise of the oceans, inundating coastal areas, where the majority of the world’s population is located.
Climate change also has been predicted to alter weather patterns, bringing moist tropical weather to parts of the United States and other higher-latitude countries, Weaver said.
“If we fail to curb greenhouse gas emissions . . . our descendants several centuries from now could face an average temperature rise of (13 degrees) and sea levels at least (six feet) higher than today,” he said.
