If you follow global warming news at all, you’ve probably seen the NASA satellite images (above) many times. The images show the extent of Greenland surface ice melt on July 8 (left) and July 12 (right). In just a few days, the area of the ice sheet with surface melting increased from about 40% to 97%, including Summit Station, Greenland’s highest and coldest spot.
NASA took a drubbing from Patrick Michaels and Chip Knappenberger at World Climate Report (“Illiteracy at NASA“) for describing the ice melt as “unprecedented” in the title of the agency’s press release. The word literally means without precedent, and properly refers to events that are unique and never happened before. In reality, as one of NASA’s experts points out in the press release, over the past 10,000 years, such events have occurred about once every 150 years:
“Ice cores from Summit show that melting events of this type occur about once every 150 years on average. With the last one happening in 1889, this event is right on time,” says Lora Koenig, a Goddard glaciologist and a member of the research team analyzing the satellite data.
Equating ’rare yet periodic’ with ‘unprecedented’ is incorrect and misleading. “But apparently,” comment Michaels and Knappenberger, “when it comes to hyping anthropogenic global warming (or at least the inference thereto), redefining English words in order to garner more attention is a perfectly acceptable practice.” New York Times blogger Andrew Revkin also chided NASA for an “inaccurate headline” and the associated “hyperventilating coverage,” but for a different reason: NASA provided “fodder for those whose passion or job is largely aimed at spreading doubt about science pointing to consequential greenhouse-driven warming.”
Enough on the spin. Let’s examine the real issues: (1) Did anthropogenic global warming cause the extraordinary increase in surface melting between July 8 and July 12? (2) How worried should we be about Greenland’s potential impact on sea-level rise?
The answer to question (1) is that greenhouse warming does not appear to be the cause. Revkin links to a graph that shows similar melting events at Summit Station not only in 1889 but also in Medieval times, centuries before the advent of SUVs and coal-fired power plants.
NASA, moreover, ascribes the rapid expansion in surface ice melt to a high pressure blocking pattern, the same phenomenon that produced the recent heat wave and drought in the U.S. Midwest. NASA reports:
This extreme melt event coincided with an unusually strong ridge of warm air, or a heat dome, over Greenland. The ridge was one of a series that has dominated Greenland’s weather since the end of May. “Each successive ridge has been stronger than the previous one,” said [Thomas] Mote [a climatologist at the University of Georgia]. This latest heat dome started to move over Greenland on July 8, and then parked itself over the ice sheet about three days later.
There is no known link between such blocking patterns and global climate change. It’s also worth noting that the dramatic surface ice melt began to reverse around July 14th. Greenland did not shift into a new climate regime.
If such events start to occur more frequently than once every 80-250 years, a global warming link would be more credible. As Prof Eric Wolff of the British Antarctic Survey told BBC News: “While this is very unusual, as always we cannot attribute any individual extreme event to climate change: We will have to wait and see if more such events occur in the next few years to understand its significance for both the climate and the health of the ice sheet.”
On to question (2): How much ice is Greenland shedding, and what are the implications for global sea-level rise? A study published in Science magazine in 2006 by Scott Luthcke of NASA and colleagues used satellite gravity measurements to estimate annual net ice loss in Greenland from 2003 to 2005. The researchers estimated that the ice sheet gained 55 gigatons per year from snowfall at higher elevations and lost 155 gigatons per year at lower elevations, yielding a net annual ice loss of 101 gigatons. That translates into an annual loss of 27 cubic miles of ice per year, or 2,700 cubic miles per century. Sounds huge — until you compare it to Greenland’s total ice mass. The Greenland Ice Sheet holds 706,000 cubic miles of ice. So at the 2003-2005 ice loss rate, Greenland will lose less than 4/10th of 1% of its ice mass in the 21st century. Apocalypse not.
Pat Michaels reviews a more recent gravity measurement study (Wu. et al. 2010, published in NatureGeoscience) that estimates ice mass balances in both Greenland and Antarctica from 2002 to 2008. Similar to the Luthcke study, the Wu team finds that Greenland’s net ice loss is 104 gigatons per year. They also estimate that Antarctica is losing 87 gigatons per year. What does it mean for sea-level rise? Pat comments:
It takes about 37.4 gigatons of ice loss to raise the global sea level 0.1 millimeter—four hundredths of an inch. In other words, ice loss from Greenland is currently contributing just over one-fourth of a millimeter of sea level rise per year, or one one-hundreth of an inch. Antarctica’s contribution is just under one-fourth of a millimeter per year. So together, these two regions—which contain 99% of all the land ice on earth—are losing ice at a rate which leads to an annual sea level rise of one half of one millimeter per year. This is equivalent to a bit less than 2 hundredths of an inch per year. If this continues for the next 90 years, the total sea level rise contributed by Greenland and Antarctica by the year 2100 will amount to less than 2 inches.
Couple this with maybe 6-8 inches from the fact that the ocean rises with increasing temperatures, and 2-3 inches from melting of other land-based ice, and you get a sum total of about one foot of additional rise by century’s end.
An additional foot of sea level rise is less than a third the amount (“more than a meter“) forecast by a group of alarmist scientists calling themselves the “Copenhagan Consensus.” It is small potatoes compared to the 18-20 feet of sea-level rise Al Gore warned us about in An Inconvenient Truth. An additional foot of sea level rise is about twice the amount the world has experienced since 1880. There were surely costs associated with sea-level rise in the 20th century, but as a factor affecting public health and welfare it was so trivial most people never noticed. Our wealthier, more mobile, and more technologically advanced children’s children’s children should be able to adapt to 12 inches of sea-level rise and do just fine.