POSTED 3 MAY 2007
Has the big melt begun?
Heard the joke about the brainy primates who burned so much fuel that they cooked their home planet with greenhouse warming?
Seriously, emissions of carbon dioxide, the primary greenhouse gas, are rising every year, and many scientists say a drastic reduction is necessary to protect the only known habitable planet in the universe. In 2007, the Intergovernmental Panel on Climate Change (IPCC) noted, "Eleven of the last twelve years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature."
Continuing to fuel the greenhouse effect, says noted climatologist James Hansen, could make Earth a "different planet."
Graphics: Jonathan Overpeck, University of Arizona.
Tomorrow, the IPCC is scheduled to release its third and final report of 2007, covering strategies to reduce the greenhouse warming and prevent massive damage. The three reports of 2007 were one sign of renewed concern over global warming. Millions saw An Inconvenient Truth, history's first Oscar-worthy science lecture. Hundreds, we guess, read harrowing reports on warming from the IPCC, which were written by thousands of scientists from around the world.
But many more read headlines about those reports. The IPCC and other scientists say warming has begun -- as have the long-predicted extinctions, droughts, storms and wildfires.
L Photo courtesy Laurie J. Schmidt, NASA
See sea level
Here's a question: how much will warming raise sea level? Water expands as it warms, but warming can also destabilize or melt the vast ice caps on Greenland and Antarctica, which could cause a dangerous rise in the ocean level.
Global flooding now looms as its own category of global warming disaster.
Photo: NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team
Rising seas also threaten the existence of low-lying islands and nations. Last December, the Independent (United Kingdom) reported the first inundation of an inhabited island:
The obliteration of Lohachara island, in India's part of the Sundarbans where the Ganges and the Brahmaputra rivers empty into the Bay of Bengal, marks the moment when one of the most apocalyptic predictions of environmentalists and climate scientists has started coming true. As the seas continue to swell, they will swallow whole island nations, from the Maldives to the Marshall Islands, inundate vast areas of countries from Bangladesh to Egypt, and submerge parts of scores of coastal cities.... In all, a dozen islands, home to 70,000 people, are in danger of being submerged by the rising seas.
Images: IPCC Working Group I, 2007
Further flooding is inevitable, but matters could get a lot worse if the ice caps start to fall apart. Melting ice has bumped sea level in the past: Andrew Shepherd of the University of Edinburgh says the geologic record shows that "collapses of Earth's former ice sheets have caused [sea level] increases of up to 20 meters in less than 500 years" (see #1 in the bibliography).
A total meltdown of the Greenland ice sheet could raise sea level by seven meters, and the West Antarctic ice sheet by five, the IPCC says. Although the mammoth East Antarctic Ice Sheet could add roughly 50 meters, that sheet seems more stable than the other two.
Earth's glaciers and ice caps move in response to gravity, and sea level rises when their ice enters the ocean. (In contrast, ice shelves and sea ice float, so the furious melting of Arctic ice will not directly affect sea level.) Not that the news from the North Pole is very comforting: On April 30, researchers reported that Arctic sea ice has been melting much faster than IPCC models forecast. "While the ice is disappearing faster than the computer models indicate, both observations and the models point in the same direction: the Arctic is losing ice at an increasingly rapid pace and the impact of greenhouse gases is growing," says Marika Holland, a scientist at the National Center for Atmospheric Research, a study co-author.
Until recently, melting glaciers got more attention than ice caps. But glaciers can only add a small amount to sea level rise, while the ice caps could add up to 70 meters.
Graph: IPCC 2007
Ice: It's nice!
Still, the ice caps don't have much impact on sea level now: A new study says Greenland and Antarctica add 0.35 millimeters to sea level rise each year, about 10 percent of the annual rise of 3 millimeters (see #2 in the bibliography).
The study authors noted "renewed speculation of accelerated sea-level rise from the ice sheets under a constant rate of climate warming." Here are some of the causes for concern:
The sudden disintegration of the 200-meter-thick Larsen B ice shelf released 720 billion tons of ice from the edge of Antarctica in 2002. The breakup did not directly raise sea level, but ice shelves can restrain the giant, land-based glaciers behind them, and the glaciers behind Larsen B did lurch toward the ocean.
The same thing happened in Greenland, causing the Jakobshavn Glacier to nearly double its speed between 1997 and 2003. Because the glacier is a major discharge route for Greenland's ice, some scientists began to fret about a more general acceleration of the icecap. Even before the speedup, some called it the world's fastest glacier.
Satellite gravity measurements of Greenland showed that the sub-continent is losing 101 or 239 cubic kilometers of ice each year (see #3 and #4 in the bibliography).
A 2006 study (see #5 in the bibliography) of earthquakes produced by sliding Greenland glaciers saw a surge of activity begin in 2002. The researchers suggested that meltwater lubricating the base of the glacier could explain the express-train movement. Here we see a complication of the new glaciology: Glaciologists once thought Greenland's ice cap was stable because it would need hundreds of years to warm up. The new studies suggest the glacier may speed up without even warming up.
Even evidence that ice is accumulating in the Antarctic is not cause for celebration, despite its use by some climate skeptics to support the "what-me-worry?" approach to climate. Curt Davis, a professor of electrical and computer engineering at the University of Missouri at Columbia, measured the interior of the East Antarctic Ice Sheet with a radar satellite and saw a gain of 45 billion tons per year (see #6 in the bibliography). That finding (like a similar one in Greenland) substantiated old predictions that global warming would cause snow and ice to gather in these frozen highlands. Davis calculated that the accumulation -- by itself -- would reduce sea level by 0.12 millimeters per year. But the paper said nothing about the overall effect of the Antarctic ice sheet on sea level, Davis stresses, because the radar was not accurate around the edge of the continent: "We know coastal outlet glaciers are discharging ice very rapidly." To use measurements from the interior as evidence for Antarctica's overall impact on sea level "would open up the possibility of misrepresenting our results, by saying this is representative of the entire ice sheet.
Photo: Ian Joughin, University of Washington Applied Physics Lab.
Add it up, and you can't add it up: The world scientific community is not sure what to make of this unsettling icy news.
The 2007 IPCC reports squarely blamed humans for global warming, yet were vague about sea level. Although the international panel projected a sea level rise of 0.18 to 0.59 meters by 2090 to 2099, compared to 1980 to 1999, it did not account for the new evidence on rapid glacier movement. According to Richard Alley, an ice expert and an author of IPCC's 2007 report, "We can provide neither a best estimate nor an upper limit on sea level rise, because of a lack of understanding of future changes in ice flow in response to future warming."
Alley, a professor of earth and mineral sciences at Penn State University, continued, "After that, we ought to shut up because we don't have a basis in the scientific literature for understanding what will happen. We can't give you a number."
What can you give me?
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive