Lava near Bárðarbunga on September 1st. PHOTOGRAPH BY EGGERT JOHANNESSON/AP

In the tenth century, a Norseman named Bárður Bjarnason settled in northern Iceland with his nine sons. Conditions were rough, and Bjarnason decided to follow his sons south in search of more fertile land. His route, the story goes, stretched across Europe’s largest ice cap, Vatnajökull, and passed by several volcanoes, including one that he decided to name after himself. Blanketed in white, it probably looked much like it does today—a massive, radiant hump. Bjarnason, or one of his descendants, called the volcano Bárðarbunga. The name means “Bárður's bulge.”

Over the weekend, Bárður's bulge started to erupt. A glowing wall of molten rock is now leaping and streaming from a fissure north of the volcano’s ice cap. It’s a “nicely behaved” eruption, as one scientist put it—a “nice fire curtain with lava squirting out”—following a few nerve-wracking weeks of “seismic crisis,” marked by thousands of subglacial earthquakes and a threatening intrusion of magma. An eruption could still explode through the volcano’s glacier. Major floods of ice melt would follow, as well as a giant ash cloud, perhaps similar to the one in 2010 from Eyjafjallajökull, which shut down European air space for a week and left millions of travellers stranded around the world. Earthquakes continue to rattle Bárðarbunga’s caldera. Lava is flowing north of the glacier at about a hundred cubic metres per second, and a dramatic white plume of steam and gas rises to four and a half kilometers above the sea.

Meanwhile, the attention on Bárður's bulge has prompted a discussion of the relationship between volcanoes, melting glaciers, and climate change. As a glacier recedes, its enormous mass is removed from the land. Relieved of that load, the land rebounds slightly and the pressure underground is reduced, enabling more magma to accumulate; eventually, some of that magma will rise and erupt through the Earth’s surface. In other words, global warming could alter the shape of the planet. “If you deglaciate Iceland, volcanism in Iceland should increase,” Jerry Mitrovica, a professor of geophysics at Harvard, said. “We’re moving the hand from the door and allowing the door to swing open.”

No one is suggesting that climate change is causing Bárðarbunga, in particular, to erupt; the ways in which atmospheric changes might affect geophysical events of this kind are hard to isolate on anything close to a human time frame. “It can be very difficult to decide if changes in the rate of earthquakes or volcanoes have actually happened,” Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography, wrote in an e-mail. “It’s a statistical question, so one event isn’t enough—you need to have a long prior history to compare it with.”

Iceland’s geological record offers some insights. The nation’s volcanoes sit directly over a hot spot on the Mid-Atlantic Ridge, and have been depositing new layers of rock for millions of years. Scientists who have analyzed these layers have found that, beginning about ten thousand years ago, after the last ice age’s glaciers began to melt, Iceland’s volcanoes started erupting as much as fifty times more frequently. Lava flowed freely and ash fell abundantly for the next two thousand years.

Still, in some cases, changes in the atmosphere can drive geophysical events over the short term. In a paper published recently in the journal Science, Agnew and his colleagues revealed that the ongoing, severe drought in the western United States has made a wide swath of the region, particularly the mountains in California, a tiny bit taller. The scientists analyzed more than a decade’s worth of data from a network of G.P.S. stations west of Wyoming and New Mexico; they found that, in the past year, coinciding with the onset of the drought, the mountains rose by half an inch, while lower elevations rose by slightly more than a tenth of an inch. For the land to rise so quickly, they calculated, it would need to have lost two hundred and forty gigatons of water. That’s roughly equivalent to the loss of a four-inch layer of water over the entire region, and equal to the amount lost annually from the Greenland ice sheet.

The melting of glaciers will not only affect volcanoes on land. Eighty per cent of Earth’s volcanoes are under the ocean. As melting causes the sea level to rise, the added weight will increase pressure on the ocean floor, which may suppress eruptions from deep-sea volcanoes. This theory is under investigation by two Harvard scientists—Charlie Langmuir, a geochemist, and Peter Huybers, a climatologist—working from a ship a few hundred miles off the coast of Washington State. If they can prove their hypothesis, Langmuir told me, it will establish a connection “between big changes in climate at Earth’s surface, ridge volcanism, the carbon cycle, and the entire topographic fabric of the sea floor.” Huybers added that melting ice sheets may also change the earth’s gravitational field, affecting where and how much the sea level will rise. “Earth is more interconnected than is often realized,” he said.

People have been slow to accept responsibility for climate change and the reality of its consequences: droughts, floods, melting poles, rising seas. The Earth beneath us, at least, should be impervious to our presence. But it may be that our footprint is far heavier than we ever imagined, and its impact much longer-lasting.