Initial data from GRACE has already sparked debates and some concern. “We’re seeing very quick responses to changes in temperature, responses that people used to think would take hundreds or thousands of years,” says Zwally’s boss, glaciologist Waleed Abdalati, head of the Cryospheric Sciences Branch at the Goddard Space Flight Center. “It’s not a ‘run for the hills’ kind of story, but it’s ‘Wow, this is serious.’ Things seem to be stirring in a way that is going to contribute to [an increase in] sea level.”
But there is no agreement among scientists on how quickly the sea level will rise, how quickly the global climate will heat up, or even how warm Earth might get. According to a panel of scientists who looked into the question for the Bush administration in 2001, the temperature could rise an average of 2.5 to 10.4 degrees Fahrenheit in the next 90 years.
What is certain, even among scientists who have criticized their colleagues for over-hyping global warming, is that at least some of Earth’s warming is due to a buildup of carbon dioxide from the burning of fossil fuels. These gases trap heat from the sun, resulting in the greenhouse effect. The main disagreement is over the urgency of the problem.
“This issue is not going to come and kill us all, despite the apocalyptic renditions you see in newspapers,” says University of Virginia climatologist Pat Michaels, whose book Meltdown criticizes the methods used by most climate scientists. Michaels calculates the pace of human-caused warming to be slow enough for correction: “We have time to invest in future [energy] technologies.”
More information is needed to gauge the seriousness of the problem. If ICESat corroborates the GRACE findings, Earth’s coastlines could look dramatically different within 100 years.
GRACE is the first satellite system of its kind. It consists of two satellites that follow the same orbit about 120 miles apart, says Mike Watkins, the GRACE project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “This is not like the 13th mission to make the same measurement a little bit better,” Watkins says. “It’s really a revolutionary thing.”
When the lead GRACE satellite reaches a point in its orbital path above an area of increased mass, such as a mountain or ice sheet, the corresponding increased gravity tug slows it down and changes ever so slightly the distance between it and its twin. Microwave signals that are continuously passed between the two satellites now arrive at slightly different points along their wavelengths than they did before the slight disruption. GRACE’s electronics detect the change, and software measures the change in distance between the satellites to an accuracy of one micron, or a millionth of a meter.
A team of researchers at the University of Texas’ Center for Space Research in Austin converts that data into monthly maps that are color-coded to show changes in the distribution of Earth’s mass—primarily the movement of water. Other researchers use the data to explore the rate of the planet’s ice loss. Isabella Velicogna and John Wahr, satellite researchers at the University of Colorado in Boulder, wrote what is perhaps the most stunning GRACE-based report. The two examined the data and reported in the March 2, 2006 issue of Science that Antarctica is losing 36 cubic miles of ice a year. That’s enough to fill Lake Erie in three years.
The job facing Velicogna and Wahr was not quite as simple as placing the entire Antarctic ice sheet on the equivalent of a bathroom scale. Earth’s crust is still rising in some places and subsiding in others in reaction to the weight lifted by the melting of the glaciers at the end of the last ice age, more than 10,000 years ago. The scientists had to factor out the post-glacial rebound. “We use some models to remove this signal, and then what’s left is the mass change of the ice,” Velicogna explains.
Zwally doesn’t doubt that Antarctica’s ice cover is shrinking, and the effect over the centuries could be devastating, but he thinks Velicogna and Wahr have the scale wrong. “Too big a number for Antarctica,” he says tersely over the phone, while preparing to leave for Greenland. He later adds that a true accounting of ice being lost will be calibrated by ICESat, aiding scientists’ calculations of the resultant rise in sea levels.