Mid-latitude storm tracks are major weather patterns that account for the majority of precipitation in the globe’s middle latitudes, which includes most of the heavily populated areas of North America, Eurasia, and Australia. Due to atmospheric circulation and the dynamics of weather systems, these bands of low pressure form repeatedly in the same locations. Apart from being meteorologically important, they’re also major players on the climate scene—clouds in these regions are responsible for reflecting much of the incoming solar radiation that is bounced back to space before penetrating Earth’s atmosphere.
Many climate models have predicted that the positions of these storm tracks would slowly migrate toward the poles, but so far this trend had not been detected. However, analysis of 25 years worth of data from the International Satellite Cloud Climatology Project now indicates that this shift is probably already taking place.
The International Satellite Cloud Climatology Project (or ISCCP [ http://isccp.giss.nasa.gov/ ]) operates a network of geostationary and polar orbiting satellites that have been collecting data on clouds since 1983. A team of researchers carefully analyzed data for Northern and Southern Hemisphere storm tracks in the Atlantic and Pacific Oceans to look for trends in storm track positions. (The Indian Ocean could not be included because of issues with satellite coverage.) The results indicated a slight poleward shift of the storm tracks.
These satellites have known data issues: measurement changes when new satellites came online, lower data quality at the “seams” between coverage from different satellites, etc. So the authors tried several different analysis techniques to test the robustness of the observed trend. Each technique decreased the rate of the observed poleward movement somewhat, but the general trend remained.
That’s mainly interesting because it had been predicted by many climate models. But the data also shows something that may be much more important, though there are some considerable uncertainties involved. The satellite observations also show a roughly two-to-three percent reduction in total cloud cover since 1983. This occurred through a large decrease in low-level cloudiness, and it came despite a slight increase in high-level cloudiness.
Both of these changes act as positive feedbacks to warming, and, as we recently covered [ http://arstechnica.com/science/news/2011/07/how-hot-will-earth-be-by-2010.ars ], cloud feedbacks are among the largest sources of uncertainty in temperature projections. High-level cirrus clouds aren’t thick enough to reflect much incoming solar radiation, but the increase in water vapor means more trapping of outgoing infrared radiation (the greenhouse effect). Most reflective action is in the low-level clouds, so a decrease there means more incoming solar radiation penetrating to the Earth’s surface.
Although the same models that predict the poleward movement of storm tracks also predict reductions in total cloud cover, the paper is heavy on caveats here. The most interesting data comes at the limits of detection for these satellites, making it unclear how robust the signal is. Like the storm track positions, the trend is consistent among the regions studied, though. In addition, satellite observations of atmospheric radiation fluxes (from the Earth Radiation Budget Experiment [ http://asd-www.larc.nasa.gov/erbe/ASDerbe.html ]) corroborate the changes in cloud behavior.
Studies like this underscore the importance of Earth-monitoring satellites operated by NASA and the ESA. Global climate data isn’t easy to come by, and the analysis is often difficult under the best of circumstances. Increasingly accurate projections require the kind of data only these satellites can provide.
Put the arguments over how fast Antarctic ice is melting to one side for the moment. The latest study [ http://www.sciencemag.org/content/early/2011/08/17/science.1208336 ] of the southern continent, by a group of scientists led by Eric Rignot of the University of California, Irvine, shows how fast the ice rivers are moving and where they are going.
The map of ice in motion, which traces parts of the eastern Antarctic region that have previously been hard to see, offers a new and powerful tool for the study of the dynamics of ice melting into the southern seas.
The data used in the map was obtained from satellites in polar orbit. Dr. Rignot said in an interview that 3,000 different orbital tracks were studied, then combined into a mosaic of the continent.
The study was published on Thursday in Science Express. The work was done in conjunction with NASA, which said in a press release that the map, showing glaciers moving from the deep interior to its coast, “will be critical for tracking future sea-level increases from climate change.”
Dr. Rignot said that one important finding that emerged during the mapping effort was that inland glaciers do not degrade solely because the ice beneath is being crushed by the ice above.
It is impossible to explain the extent of motion revealed in the study, he said, without assuming that the deformation process is supplemented by melting at the base of the ice. “We have to account for a significant component of sliding on ice,” he said. Dr. Rignot is also affilated with NASA’s Jet Propulsion Laboratory.
The scientists’ mapping technique involves using radar pulses to identify unique formations like geological fingerprints several meters below the surface and then tracking these as they move. Some moved as much as 800 feet each year.
The researchers were able to use billions of individual data points sent by satellites that belong not just to the United States but also to the European, Canadian and Japanese space agencies, the Alaska Satellite Facility at the University of Alaska at Fairbanks and MacDonald, Dettwiler and Associates, a Canadian aerospace company.
News of the “unofficial” 2011 record for minimum Arctic sea ice voume follows acceptance of an article in the Journal of Geophysical Research [ http://psc.apl.washington.edu/wordpress/schweiger-2011/ ] which demonstrates the 2010 minimum sea ice volume was lower than 2007, the previous recordholder.
The sea ice volume metric is important because, unlike extent, it provides information on ice thickness and, thus, offers a more complete picture of what’s happening to the ice.
As for sea ice extent, the University of Bremen (in Germany) reports it reached a new historic minimum of 4.24 million square kilometers on September 8. In a press release [ http://www.iup.uni-bremen.de/seaice/amsr/minimum2011-en.pdf ] it stated:
It seems to be clear that this is a further consequence of the man-made global warming with global consequences.
An alternative estimate from the National Snow and Ice Data Center (NSIDC) indicates [ http://nsidc.org/arcticseaicenews/ ] that Arctic sea ice extent is currently at the second-lowest levels in the satellite record - very slightly higher than 2007.
NSIDC provides the following reasoning for the difference between its ranking and University of Bremen’s:
... data from the University of Bremen indicate that sea ice extent from their algorithm fell below the 2007 minimum. They employ an algorithm that uses high resolution information from the JAXA AMSR-E sensor on the NASA Aqua satellite. This resolution allows small ice and open water features to be detected that are not observed by other products. This year the ice cover is more dispersed than 2007 with many of these small open water areas within the ice pack.
Irrespective of whether the extent this year is lower than 2007 or not, NSIDC states: “...all of the data agree that Arctic sea ice is continuing its long-term decline.”
While the minimum sea ice extent and sea ice volume in August and September is an interesting indicator of climate change, the blog Real Climate recently posted a commentary [ http://www.realclimate.org/index.php/archives/2011/09/the-unnoticed-melt/ ] emphasizing the importance of sea ice trends earlier in the melt season:
This importance of sea-ice evolution during the early summer months is directly related to the role of sea ice as an efficient cooling machine: Because of its high albedo (reflectivity), sea ice reflects most of the incoming sunlight and helps to keep the Arctic cold throughout summer. The relative importance of this cooling is largest when days are long and the input of solar radiation is at its maximum, which happens at the beginning of summer. If, like this year, sea-ice extent becomes very low already at that time, solar radiation is efficiently absorbed throughout all summer by the unusually large areas of open water within the Arctic Ocean.
Receding sea-ice cover can disrupt indigenous people’s way of life and threaten animals like polar bears and walruses. The loss of the “refrigerator” on top of the world can alter weather patterns elsewhere in the world. And once ice is lost it becomes more difficult to replace because light can then reach the ocean, which absorbs it and warms.
A well-documented potential beneficial impact of melting ice is the opening of new navigation routes throug the Arctic.
Ice thickness is just as important or more so in helping understand what is happening in the far north Photo: AP/John McConnico
Sea-ice coverage across the Arctic Ocean has dwindled to its second-lowest level since satellite records started in 1979, the National Snow and Ice Data Centre said, days after another study said ice melt was at its worst levels ever.
8:00AM BST 14 Sep 2011
Areas of the Arctic with at least 15 per cent sea-ice as of Saturday totalled 1.68 million square miles, slightly above the record-low of 1.61 million square miles recorded in 2007.
Yet to be determined is whether the sea-ice cover will be the lowest for the year. Annual minimums are usually reached around mid-September.
"We're getting close, but there's still the potential for further loss of ice," said Walt Meier, a research scientist at the Boulder, Colorado-based National Snow and Ice Data Centre.
Ice coverage could diminish either through more melt or from winds or both, Mr Meier said. However, some areas, including those near the North Pole, were showing signs of ice growth, he said.
"Probably there's a little bit of both going on - there's melting and refreezing," he said.
At least one other institution has reported that this year's Arctic ice coverage was the lowest on record. A report issued last week by the University of Bremen in Germany said sea-ice coverage on Sept. 8 fell below the 2007 minimum.
The University of Bremen researchers use finer-resolution measurements that can better distinguish smaller areas of ice and open water, Meier said. But that university's methodology also has some drawbacks, he said.
Under either measurement, however, Arctic ice cover has diminished dramatically over recent decades. Saturday's coverage, as measured by the National Snow and Ice Data Centre, was only about two-thirds the average coverage measured from 1979 to 2000.
Reduced sea ice is believed to have cascading impacts on climate in the circumpolar north and even lower latitudes.
According to an academic study released on Tuesday by the US Geological Survey, Yupik Eskimo residents in southwestern Alaska are living with some of those affects.
The study, published in the current edition of the journal Human Organisation, examined observations of elders and longtime hunters in two Lower Yukon River villages.
The residents detailed dramatic changes over the years in river-ice thickness, a public-safety risk because no roads connect villages in that part of Alaska, and residents in winter travel over river ice.
The residents also testified to changing ranges for several animals, particularly moose and beavers, changes in vegetation and concerns about reduced availability of driftwood that used to be pushed downstream by powerful currents of spring meltwater.
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