The Case for a Shifting Climate is Heating Up

By J. MADELEINE NASH

For well in excess of a thousand years, Peru's Quelccaya ice cap has sparkled like a trillion-carat diamond. But satellites reveal that a flaw has appeared on its frozen face. That, Ohio State University glaciologist Lonnie Thompson believes, signifies the incipient birth of an Andean lake, meaning that the ice may be melting faster than he had feared. During the past two decades, Thompson has watched Quelccaya's surface shrink nearly a fifth. Now that the pace appears to be accelerating, he says worriedly, "it could be completely gone in another 20 years."

Is the earth warming? Few questions have provoked more noisy wrangling or more impassioned debate--or, during hot spells and droughts like those now affecting large swaths of the U.S., more overblown rhetoric. In the verbal heat, all too often the scientific case for warming gets lost. While one side overstates the certainty of cataclysmic change, reading doom into every rise of temperature, the other categorically dismisses any threat. In fact, the science is more finely nuanced--and more firmly grounded--than either side will concede.

For starters, it seems indisputable that the earth's mean temperature has advanced about 0.6[degrees]C (1[degree]F) over the past 120 years. The question scientists continue to debate is why. One possibility is that the worldwide warming so far is due entirely to natural long-term shifts, like changes in the sun's luminosity. The other possibility is that the warming bears the fingerprints of greenhouse gases.

The problem, observes British climatologist Phil Jones of the University of East Anglia, is that it's not a simple case of either-or. That's because purely natural swings of climate can operate in both directions, enhancing or masking the greenhouse effect. To complicate matters further, scientists expect the buildup of heat-trapping gases to manifest itself in ways that mimic the natural variability of climate. Some places, in other words, may even grow colder.

Take, for example, the large-scale atmospheric feature known as the North Atlantic Oscillation, whose swings of pressure strongly affect the weather of northern latitudes. Over the past several decades, the phenomenon has reliably sent warm air shooting across Northern Europe and Siberia during wintertime and simultaneously enveloped Labrador in frigid cold. And although this shift could be the result of natural fluctuations, some scientists are beginning to suspect that it occurs because the same greenhouse gases that warm the lower reaches of the atmosphere cool the stratosphere, and the latter somehow affects the direction and speed of Arctic winds.

If it's difficult to disentangle what is natural from what is not, that's because the earth's climate machine is almost unimaginably intricate, filled with myriad interlocking gears and shock absorbers. But the lack of clarity about where we're headed will not last forever. By the end of the next century, calculates climatologist Tom Wigley of the National Center for Atmospheric Research, the earth's mean temperature is likely to have risen somewhere between 1.3[degrees]C and 4.0[degrees]C (2.3[degrees]F and 7.2[degrees]F), with consequences, such as rising sea levels, that will be hard to mistake.

Will future generations regard the vanishing Quelccaya ice cap as a false prophet or as a climatological Cassandra whose warnings were not heeded? Ohio State's Thompson, for one, believes nature has already answered this question. An ice cap has no political opinions, he observes. "Quelccaya is melting because the earth's temperature is going up," he says, "and it's foolish to argue that it's not."

--By J. Madeleine Nash