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Unexpected Ice Unexpected ice “I always thought, and as far as by Natasha Vizcarra sunlight, and food—they all come and go with I can tell everyone else thought, the seasons in the Southern Ocean. On the prowl for food, Adelie penguins scan the that the biggest changes must ice ceiling. They peck at silverfish and hunt for Paul Holland, a climate modeler with the British be in autumn.” polynyas, gaping holes in the sea ice where shoals Antarctic Survey, has spent the last ten years of krill and bug-like copepods graze on clouds of studying Antarctica’s sea ice and the Southern Paul Holland algae. When spring comes, the huge plates of sea Ocean. Lately, he has been scrutinizing the British Antarctic Survey ice start to melt and later in the brief Antarctic seasons of Antarctica and how fast the ice comes summer all but disappear. Then, algae blooms and goes. Holland thinks these seasons may be a unfurl: a bacchanalian feast for krill and critters key to a conundrum: If Earth’s temperatures are all the way up the Antarctic food chain. Sea ice, getting warmer and sea ice in the Arctic has been A pressure ridge forms on the sea ice near Scott Base in Antarctica. These form when separate ice floes collide and pile up on each other. Lenticular clouds are seen above. (Courtesy M. Studinger/NASA) 28 shrinking fast, why then is sea ice in the Antarc- “The Antarctic case is as interesting as the Arctic tic slowly increasing? case,” Holland said. “You can’t understand one without understanding the other.” Opposite poles Minding the models Sea ice is simply frozen seawater. Although found only in the Arctic and the Antarctic, it To Holland, the discrepancy calls parts of the influences Earth’s climate in big ways. Its bright climate models into question. Modeling groups surface reflects sunlight back into space. Icy areas from around the world collaborate on the Cou- absorb less solar energy and remain relatively pled Model Intercomparison Project Phase 5 cool. When temperatures warm over time and (CMIP5), which simulates Earth’s climate and more sea ice melts, fewer bright surfaces reflect predicts how it will change in the near future. sunlight back into space. The ice and exposed World leaders and policy makers rely on it to seawater absorb more solar energy and this causes decide how much countries should limit car- more melting and more warming. bon emissions, known to cause some aspects of climate change. This scene shows a mixture of sea ice types commonly Scientists have been watching this feedback loop seen in the Southern Ocean. The different thicknesses of warming and melting in the Arctic. To them, “Almost all of the CMIP5 models produce a of sea ice form a spectrum of colors and shapes ranging decrease in Antarctic sea ice,” Holland said. from dark black open water, a thin grease-like covering Arctic sea ice is a reliable indicator of a chang- called grease ice, and thicker grey ice. Older sea ice ing global climate. They pay the most attention “There is a problem in the bit that reproduces the has a bright white covering of snow and many chaotic in September when Arctic sea ice shrinks to its last 30 years of sea ice variability.” Holland was deformation features, visible as ridges and rubble fields smallest extent each year. Measured by satel- searching for data to improve and verify his own and caused by the continuous motion of the ice pack. lites since 1979, this minimum extent has been modeling of trends in Antarctic ice when he no- (Courtesy M. Studinger/ NASA) decreasing by as much as 13.7 percent per dec- ticed that other researchers were finding that the ade. Antarctic sea ice, on the other hand, has trends varied in strength in the different seasons. day, which he called intensification; and the total not been considered a climate change indicator. ice area rate of growth, which he called expansion. Whereas Arctic sea ice mostly sits in the middle Most studies on Antarctic sea ice trends focus on “I did that for all thirty years of data and plotted of land-locked ocean—which is more sensitive changes in ice extent. For Holland, it was more the trends,” he said. Holland’s plots showed that to sunlight and warming air—Antarctic sea ice important to look at how fast the ice was grow- the different regions in the Southern Ocean con- surrounds land and is constantly exposed to high ing or shrinking from season to season. “Changes tributed to the overall increase, but they had very winds and waves. in climate forcing directly affect the rate of ice diverse trends in sea ice growth. This suggested growth,” he said, “not the amount of ice.” Year to that geography and different wind patterns played According to climate models, rising global year cooling in autumn, for example, may cause a role. So to gain more insight Holland looked at temperatures should cause sea ice in both faster ice growth during autumn, but not neces- seasonal wind trends for the different regions. regions to shrink. But observations show that sarily an increase in the amount of autumn ice. ice extent in the Arctic has shrunk faster than Holland found that winds were spreading sea ice models predicted, and in the Antarctic it has Spring surprise out in some regions and compressing or keeping been growing slightly. Researchers are looking Holland used data from the NASA National it intact in others and that these effects began much closer at Antarctica, saying, “Wait, what Snow and Ice Data Center Distributed Active in the spring. It contradicted a previous study is going on down there?” Holland is one of Archive Center (NSIDC DAAC) to calculate the in which, using ice drift data, Holland and Ron those intrigued. ice concentration rate of growth for each single Kwok from the NASA Jet Propulsion Laboratory 29 15 40 responding,” said researcher Sharon Stammer- a b john, referring to the measures of intensification and expansion. The new metrics help assess how /y) 20 2 the system is responding as opposed to simply ) 2 10 km monitoring the state of the system. “Say your 6 0 km temperature is at 99.2 degrees Fahrenheit,” 6 Stammerjohn said. “You don’t have any insight Circumpolar -20 Indian to that temperature unless you take it again an 5 West Pacific hour later and you see that it changed to 101 Area (10 Area Ross degrees. Then you can say, okay, my system is -40 Expansion (10 A-B responding to something.” Weddell 0 -60 Partial explanations D J F M A M J J A S O N D D J F M A M J J A S O N D Holland continues to study the Antarctic spring 30 150 to better understand why Antarctic sea ice is c ) d 2 changing. While Holland’s work helps research- /y 2 100 ers begin to see the problem in more detail, /y) 20 2 scientists continue to develop ideas about why km 3 50 the ice is expanding. km 3 10 0 One study paradoxically suggests that ocean 0 warming and enhanced melting of the Antarctic -50 ice sheet is causing the small but statistically sig- nificant sea ice expansion in the region. Another -10 Area trend (10 trend Area -100 study suggests that rain caused by a warmer Expansion trend (10 Expansion trend climate has been causing an influx of fresh water summer autumn winter spring -20 -150 into the Southern Ocean, making it less dense D J F M A M J J A S O N D D J F M A M J J A S O N D and inhibiting oceanic heat from reaching sea ice in the Antarctic. To date, there is no consensus The panels above show seasonal variations of sea ice quantities for each region and the whole Southern Ocean. The mean monthly total ice expansion (b) peaks in autumn, and mean monthly total ice area (a) peaks in the winter. Inter- on the reason for the expansion. annual trends in monthly total ice area (c) show that for the last thirty years, Antarctic sea ice has tended to expand during the autumn. Interannual trends in monthly total ice expansion (d) show that changes in ice growth in the spring “Partial explanations have been offered, but produced the change in the ice the following summer and autumn. (Courtesy P. R. Holland) we don’t have the complete picture,” said Ted Scambos, a scientist at the NSIDC DAAC. “This found that increasing northward winds during for me now is we need to look at spring. The may just be a case of ‘we don’t know yet.’” the autumn caused the variations. trend is bigger in the autumn, but it seems to be created in spring.” To access this article online, please visit http://earthdata.nasa “I always thought, and as far as I can tell every- .gov/sensing-our-planet/unexpected-ice one else thought, that the biggest changes must “Paul has created two more sea ice metrics that be in autumn,” Holland said. “But the big result we can use to assess how Antarctic sea ice is 30 References About the remote sensing data Bintanja, R., G. J. Van Oldenborgh, S. S. Drijfhout, B. Wouters, and C. A. Katsman. 2013. Important role Satellites Nimbus 7 and Defense Meteorological Satellite Program (DMSP) F8, F11, F13, F17 for ocean warming and increased ice-shelf melt in Sensors Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager Antarctic sea-ice expansion.
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