Justin Gillis

As many people know, human civilization arose during a particularly stable period in the history of the earth – the Holocene, encompassing the 12,000 years since the end of the last ice age. It may have been no accident that civilization burst forth during an era when ice had retreated from some of the most favorable land and when climate, sea level and other conditions had settled down after the turbulence of the ice age.

But they have not entirely settled down. The Holocene does feature some large-scale climate oscillations, including two recent ones — the Medieval Warm Period and the Little Ice Age – that have been the focus of a great deal of discussion and research within mainstream climate science. The Medieval Warm Period, from the 10th to the 13th centuries, is also beloved by climate contrarians. They would like to claim that conditions worldwide were warmer than they are today and that this somehow proves that humans cannot be influencing modern climate.

But evidence has been accumulating for several years that these events and other examples of Holocene climate variability were generally not global in scope. Now, a group of researchers led by Aaron E. Putnam of the Lamont-Doherty Earth Observatory at Columbia University may have come pretty close to nailing the case.

In a tour de force of glacial geology, Dr. Putnam and his collaborators reconstructed much of the Holocene history of a group of mountain glaciers in New Zealand. That allowed them in turn to determine the snowlines for each glacier at various times. A snowline, or equilibrium line, is a level above which a glacier is gaining ice and below which it is losing ice. Snowlines are sensitive to air temperature, and thus are capable of tracking regional climate changes.



Before we get to the details of the study, a word about the locale. For those who have not been lucky enough to visit New Zealand’s South Island and its mountain range, the Southern Alps, simply recall the scenery from “The Lord of the Rings” movies – the Southern Alps became the Misty Mountains in that trilogy. In real life, they may not be inhabited by orcs and dwarves, but they are indeed magical. The South Island is relatively small, and the steep mountain range that dominates it creates some of the world’s most spectacular scenery.

These mountains wring remarkably heavy snows out of the warm, moist air that flows across New Zealand from the Tasman Sea — in some places, enough snow every year to equal 45 feet of rain. Those snows feed a series of steep, fast-moving glaciers. I took the photograph above in 2005 while visiting the névé, or accumulation basin, of the Fox Glacier on New Zealand’s western coast. According to the helicopter pilot who landed me there, I was standing on 2,000 feet of snow at the time.

Dr. Putnam and his collaborators have been working in New Zealand for several years. Their goal is to analyze the notion that Holocene climate changes are global in scope. “The idea that the modern global warming is part of a natural cycle is a valid, testable hypothesis, and merits attention,” he said. “Thus, we approached the idea as a testable hypothesis.”

In a paper in 2009, the group offered preliminary evidence that the New Zealand glaciers probably behaved out of synchrony with those in the European Alps over most of the Holocene. The importance of that assertion cannot be overstated. These glaciers are among the best indicators of past climate in the Southern Hemisphere. If they were behaving differently from those of the Northern Hemisphere through most of the past 12,000 years, that says something pretty significant about how the world works.

In the new paper, published online on Sunday by the journal Nature Geoscience, Dr. Putnam and his collaborators offer much stronger evidence for their case. Applying a high-precision technique that dates rock deposits left behind by retreating glaciers, they compiled a definitive new history of the Cameron Glacier and improved histories of three other glaciers. They were finally able to meet a long-sought goal of reconstructing the snow lines for these glaciers over most of the Holocene.

The results show that climate in the region around New Zealand did indeed behave quite differently than the climate of Europe during much of the Holocene – evidence that under natural conditions, the earth’s two hemispheres are out of step climatologically. For example, the paper suggests that climate in the early Holocene, which was notably warm in the Northern Hemisphere, was far colder in New Zealand. And during the Medieval Warm Period, when European glaciers were retreating, those in New Zealand advanced, and vice versa during the Little Ice Age.

The details of why are not entirely clear, but the reconstruction does make sense given what scientists think they understand about how the earth’s climate works. Because of wobbles in the planetary orbit, the amount of sunlight tends to strengthen in one hemisphere as it weakens in the other, apparently driving opposite shifts in rainfall, temperature and other patterns.

Scientists say the earth can certainly undergo big climatic changes that are global in scope. The ice ages, for instance, had effects from pole to pole, and even regional warming or cooling can alter enough land ice to produce global changes in sea level. But the new paper adds to the growing body of research suggesting that natural climate variability in the Holocene was basically driven by regional factors.

Over all, the results undermine the idea that today’s rapid planetary warming, which is decidedly global, is part of some natural cycle encompassing the whole planet. The bulk of scientific evidence in fact suggests the opposite.

As Dr. Putnam and his collaborators point out in their paper, mountain ice has begun to retreat worldwide, as have the ice caps in Greenland and parts of Antarctica, with much of that retreat occurring in the late 20th century, as the climate began to warm substantially. Some of the New Zealand glaciers were advancing even into the last decade, but lately nearly all of them have begun a rapid retreat.

The new paper thus offers evidence, albeit circumstantial, that we have departed from the climate regime that characterized most of the Holocene, the climate that allowed our civilization to arise and flourish.

“The modern warming is not consistent with the detected pattern of Holocene climate change, and implies a global forcing,” Dr. Putnam told me. “We think the simplest explanation is that greenhouse gases have warmed the planet.”