That means an area known to have experienced a massive volcanic eruption in the past, such as Yellowstone National Park, could have a large pool of magma festering beneath it and still not be close to going off as it did 600,000 years ago.

“You might expect to see a stewing magma chamber for a long period of time and it doesn’t necessarily mean an eruption is imminent,” said Sarah Gelman, a UW doctoral student in Earth and space sciences.

Recent research models have suggested that reservoirs of silica-rich magma, or molten rock, form on and survive for geologically short time scales — in the tens of thousands of years — in the Earth’s cold upper crust before they solidify. They also suggested that the magma had to be injected into the Earth’s crust at a high rate to reach a large enough volume and pressure to cause an eruption.

But Gelman and her collaborators took the models further, incorporating changes in the crystallization behavior of silica-rich magma in the upper crust and temperature-dependent heat conductivity. They found that the magma could accumulate more slowly and remain molten for a much longer period than the models previously suggested.

Gelman is the lead author of a paper explaining the research published in the July edition of Geology. Co-authors are Francisco Gutiérrez, a former UW doctoral student now with Universidad de Chile in Santiago, and Olivier Bachmann, a former UW faculty member now with the Swiss Federal Institute of Technology in Zurich.

There are two different kinds of magma and their relationship to one another is unclear. Plutonic magma freezes in the Earth’s crust and never erupts, but rather becomes a craggy granite formation like those commonly seen in Yosemite National Park. Volcanic magma is associated with eruptions, whether continuous “oozing” types of eruption such as Hawaii’s Kilauea Volcano or more explosive eruptions such as Mount Pinatubo in the Philippines or Mount St. Helens in Washington state.

Some scientists have suggested that plutonic formations are what remain in the crust after major eruptions eject volcanic material. Gelman believes it is possible that magma chambers in the Earth’s crust could consist of a core of partially molten material feeding volcanoes surrounded by more crystalline regions that ultimately turn into plutonic rock. It is also possible the two rock types develop independently, but those questions remain to be answered, she said.

The new work suggests that molten magma reservoirs in the crust can persist for far longer than some scientists believe. Silica content is a way of judging how the magma has been affected by being in the crust, Gelman said. As the magma is forced up a column from lower in the Earth to the crust, it begins to crystallize. Crystals start to drop out as the magma moves higher, leaving the remaining molten rock with higher silica content.

“These time scales are in the hundreds of thousands, even up to a million, years and these chambers can sit there for that long,” she said.

Even if the molten magma begins to solidify before it erupts, that is a long process, she added. As the magma cools, more crystals form giving the rock a kind of mushy consistency. It is still molten and capable of erupting, but it will behave differently than magma that is much hotter and has fewer crystals.

The implications are significant for volcanic “arcs,” found near subduction zones where one of Earth’s tectonic plates is diving beneath another. Arcs are found in various parts of the world, including the Andes Mountains of South America and the Cascades Range of the Pacific Northwest.

Scientists have developed techniques to detect magma pools beneath these arcs, but they cannot determine how long the reservoirs have been there. Because volcanic magma becomes more silica-rich with time, its explosive potential increases.

“If you see melt in an area, it’s important to know how long that melt has been around to determine whether there is eruptive potential or not,” Gelman said. “If you image it today, does that mean it could not have been there 300,000 years ago? Previous models have said it couldn’t have been. Our model says it could. That doesn’t mean it was there, but it could have been there.”

The work was funded by the National Science Foundation and the National Scientific and Technological Research Commission of Chile.

A new report from the Intergovernmental Panel on Climate Change states that the authors are now 95 percent to 100 percent confident that human activity is the primary influence on planetary warming.

An international panel of scientists has found with near certainty that human activity is the cause of most of the temperature increases of recent decades, and warns that sea levels could conceivably rise by more than three feet by the end of the century if emissions continue at a runaway pace.

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The scientists, whose findings are reported in a draft summary of the next big United Nations climate report, largely dismiss a recent slowdown in the pace of warming, which is often cited by climate change doubters, attributing it most likely to short-term factors.

The report emphasizes that the basic facts about future climate change are more established than ever, justifying the rise in global concern. It also reiterates that the consequences of escalating emissions are likely to be profound.

“It is extremely likely that human influence on climate caused more than half of the observed increase in global average surface temperature from 1951 to 2010,” the draft report says. “There is high confidence that this has warmed the ocean, melted snow and ice, raised global mean sea level and changed some climate extremes in the second half of the 20th century.”

The draft comes from the Intergovernmental Panel on Climate Change, a body of several hundred scientists that won the Nobel Peace Prize in 2007, along with Al Gore. Its summaries, published every five or six years, are considered the definitive assessment of the risks of climate change, and they influence the actions of governments around the world. Hundreds of billions of dollars are being spent on efforts to reduce greenhouse emissions, for instance, largely on the basis of the group’s findings.

The coming report will be the fifth major assessment from the group, created in 1988. Each report has found greater certainty that the planet is warming and greater likelihood that humans are the primary cause.

The 2007 report found “unequivocal” evidence of warming, but hedged a little on responsibility, saying the chances were at least 90 percent that human activities were the cause. The language in the new draft is stronger, saying the odds are at least 95 percent that humans are the principal cause.

On sea level, which is one of the biggest single worries about climate change, the new report goes well beyond the assessment published in 2007, which largely sidestepped the question of how much the ocean could rise this century.

The new report also reiterates a core difficulty that has plagued climate science for decades: While averages for such measures as temperature can be predicted with some confidence on a global scale, the coming changes still cannot be forecast reliably on a local scale. That leaves governments and businesses fumbling in the dark as they try to plan ahead.

On another closely watched issue, the scientists retreated slightly from their 2007 position.

Regarding the question of how much the planet could warm if carbon dioxide levels in the atmosphere doubled, the previous report largely ruled out any number below 3.6 degrees Fahrenheit. The new draft says the rise could be as low as 2.7 degrees, essentially restoring a scientific consensus that prevailed from 1979 to 2007.

But the draft says only that the low number is possible, not that it is likely. Many climate scientists see only a remote chance that the warming will be that low, with the published evidence suggesting that an increase above 5 degrees Fahrenheit is more likely if carbon dioxide doubles.

The level of carbon dioxide, the main greenhouse gas, is up 41 percent since the Industrial Revolution, and if present trends continue it could double in a matter of decades.

Warming the entire planet by 5 degrees Fahrenheit would add a stupendous amount of energy to the climate system. Scientists say the increase would be greater over land and might exceed 10 degrees at the poles.

They add that such an increase would lead to widespread melting of land ice, extreme heat waves, difficulty growing food and massive changes in plant and animal life, probably including a wave of extinctions.

 

Inside NYTimes.com

• (AP Photo/Fosphotos, Elina Liberta) Weather woes: A flash of lighting is seen in Piraeus, near Athens during a rainstorm in February. Hours of heavy rainfall in Athens caused extensive flooding. Across Europe severe weather and flooding have caused economic losses estimated at more than $22 billion so far this year.
• (AP Photo/dpa,Patrick Pleul, file) Costly: In aerial view from June this year houses stand in the floods of Elbe river in Riesa, eastern Germany. Severe flooding in central Europe has caused economic losses estimated at more than $22 billion.
• (AP Photo/The Canadian Press, Jonathan Hayward) Streets under water: This aerial photo shows a flooded downtown Calgary, Alberta, Canada, in June.

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