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Huge iceberg breaks free

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An massive iceberg, about the size of the state of Delaware, has split off from Antarctica’s Larsen C ice shelf.

The calving of the massive new iceberg was captured by the Moderate Resolution Imaging Spectroradiometer on NASA’s Aqua satellite, and confirmed by the Visible Infrared Imaging Radiometer Suite instrument on the joint NASA/NOAA Suomi National Polar-orbiting Partnership (Suomi-NPP) satellite. The final breakage was first reported by Project Midas, an Antarctic research project based in the UK.

Larsen C, a floating platform of glacial ice on the east side of the Antarctic Peninsula, is the fourth largest ice shelf ringing Earth’s southernmost continent.

In 2014, a crack that had been slowly growing into the ice shelf for decades suddenly started to spread northwards, creating the nascent iceberg. Now that the close to 5 800 square kilometres chunk of ice has broken away, the Larsen C shelf area has shrunk by approximately 10%.

“The interesting thing is what happens next, how the remaining ice shelf responds,” says Kelly Brunt, a glaciologist with NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, and the University of Maryland in College Park. “Will the ice shelf weaken? Or possibly collapse, like its neighbours Larsen A and B? Will the glaciers behind the ice shelf accelerate and have a direct contribution to sea level rise? Or is this just a normal calving event?”

Ice shelves fringe 75% of the Antarctic ice sheet. One way to assess the health of ice sheets is to look at their balance: when an ice sheet is in balance, the ice gained through snowfall equals the ice lost through melting and iceberg calving.

Even relatively large calving events, where tabular ice chunks the size of Manhattan or bigger calve from the seaward front of the shelf, can be considered normal if the ice sheet is in overall balance.

But sometimes ice sheets destabilise, either through the loss of a particularly big iceberg or through disintegration of an ice shelf, such as that of the Larsen A Ice Shelf in 1995 and the Larsen B Ice Shelf in 2002.

When floating ice shelves disintegrate, they reduce the resistance to glacial flow and thus allow the grounded glaciers they were buttressing to significantly dump more ice into the ocean, raising sea levels.

Scientists have monitored the progression of the rift throughout the last year was using data from the European Space Agency Sentinel-1 satellites and thermal imagery from NASA’s Landsat 8 spacecraft.

Over the next months and years, researchers will monitor the response of Larsen C, and the glaciers that flow into it, through the use of satellite imagery, airborne surveys, automated geophysical instruments and associated field work.

In the case of this rift, scientists were worried about the possible loss of a pinning point that helped keep Larsen C stable. In a shallow part of the sea floor underneath the ice shelf, a bedrock protrusion, named the Bawden Ice Rise, has served as an anchor point for the floating shelf for many decades.

Ultimately, the rift stopped short of separating from the protrusion.

“The remaining 90% of the ice shelf continues to be held in place by two pinning points: the Bawden Ice Rise to the north of the rift and the Gipps Ice Rise to the south,” says Chris Shuman, a glaciologist with Goddard and the University of Maryland at Baltimore County. “So I just don’t see any near-term signs that this calving event is going to lead to the collapse of the Larsen C ice shelf.

“But we will be watching closely for signs of further changes across the area.”

The first available images of Larsen C are airborne photographs from the 1960s and an image from a US satellite captured in 1963. The rift that has produced the new iceberg was already identifiable in those pictures, along with a dozen other fractures.

The crack remained dormant for decades, stuck in a section of the ice shelf called a suture zone, an area where glaciers flowing into the ice shelf come together.

Suture zones are complex and more heterogeneous than the rest of the ice shelf, containing ice with different properties and mechanical strengths, and therefore play an important role in controlling the rate at which rifts grow. In 2014, however, this particular crack started to rapidly grow and traverse the suture zones, leaving scientists perplexed.

“We don’t currently know what changed in 2014 that allowed this rift to push through the suture zone and propagate into the main body of the ice shelf,” says Dan McGrath, a glaciologist at Colorado State University who has been studying the Larsen C ice shelf since 2008.

McGrath says the growth of the crack, given our current understanding, is not directly linked to climate change.

“The Antarctic Peninsula has been one of the fastest warming places on the planet throughout the latter half of the 20th century. This warming has driven really profound environmental changes, including the collapse of Larsen A and B,” McGrath says.

“But with the rift on Larsen C, we haven’t made a direct connection with the warming climate. Still, there are definitely mechanisms by which this rift could be linked to climate change, most notably through warmer ocean waters eating away at the base of the shelf.”

While the crack was growing, scientists had a hard time predicting when the nascent iceberg would break away. It’s difficult because there are not enough measurements available on either the forces acting on the rift or the composition of the ice shelf.

Further, other poorly observed external factors, such as temperatures, winds, waves and ocean currents, might play an important role in rift growth. Still, this event has provided an important opportunity for researchers to study how ice shelves fracture, with important implications for other ice shelves.

The US National Ice Centre will monitor the trajectory of the new iceberg, which is likely to be named A-68. The currents around Antarctica generally dictate the path that the icebergs follow. In this case, the new berg is likely to follow a similar path to the icebergs produced by the collapse of Larsen B: north along the coast of the Peninsula, then northeast into the South Atlantic.

“It’s very unlikely it will cause any trouble for navigation,” Brunt says.

Arctic winter warming events – winter days where temperatures peak above minus 10 degrees Celsius – are a normal part of the climate over the ice-covered Arctic Ocean. But new research by an international team that includes NASA scientists finds these events are becoming more frequent and lasting longer than they did three decades ago.

Because fall and winter is when Arctic sea ice grows and thickens, warmer winter air temperatures will further impede ice growth and expansion, accelerating the effects of global warming in the Arctic.

A new study, published in Geophysical Research Letters on 10 July, shows that, since 1980, an additional six warming events are occurring each winter in the North Pole region. The study also shows the average length of each event has grown from fewer than two days to nearly two and a half days.

The researchers arrived at the results by gathering and analysing data from field campaigns, drifting weather stations and buoys across the Arctic Ocean from 1893 to 2017, as well as the ERA-Interim record, a global atmospheric reanalysis provided by the European Centre for Medium-Range Weather Forecasts in Reading, UK, from 1979 to 2016.

The findings build on other recent evidence of Arctic winter warming. The winter of 2015-2016, for example, saw temperatures nearly 2 degrees Celsius warmer than the previous record high monthly winter temperature. At the end of December 2015, scientists recorded a temperature of 2.2 degrees Celsius in the Central Arctic, the warmest temperature ever recorded in this region from December through March.

In the most recent years of the study, each warming event was associated with a major storm entering the region. During these storms, strong winds from the south blow warm, moist air from the Atlantic into the Arctic.

“The warming events and storms are in effect one and the same,” says Robert Graham, a climate scientist at the Norwegian Polar Institute in Tromsø, Norway, and lead author of the new study. “The more storms we have, the more warming events, the more days with temperatures less than minus 10 degrees Celsius rather than below minus 30 degrees Celsius, and the warmer the mean winter temperature is.”

Storms that bring warm air to the Arctic not only prevent new ice from forming, but can also break up ice cover that is already present, Graham said. He added that the snowfall from storms also insulates current ice from the cold atmosphere that returns to the Arctic after the cyclones, which can further reduce ice growth.

Two of the study’s authors, Alek Petty and Linette Boisvert of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, previously researched one such storm that took place in the Arctic during the winter of 2015-2016.

“That particular cyclone, which lasted several days and raised temperatures in the region close to the melting point, hindered sea ice growth while its associated strong winds pushed the sea ice edge back, leading to a record low spring sea ice pack in 2016,” say Petty and Boisvert. “This new study provides the long-term context we were missing, using direct observations going back the end of the 19th century. It shows that these warm events have occurred in the past, but they were not as long-lasting or frequent as we’re seeing now. That, combined with the weakened sea ice pack, means that winter storms in the Arctic are having a larger impact on the Arctic climate system.”

Yet the frequency and duration of these warming events varies by region. On average, the Atlantic side of the North Pole now has 10 warming events each winter, while the Pacific Central Arctic has five such events, according to the study. More storms come in to the Arctic from the Atlantic Ocean during winter, which results in more warming events on the Atlantic side of the North Pole.

The next step for Graham and his colleagues is to understand what is fueling the increase of these storms and how they might change. Recent research shows that reduced ice cover and shifting weather patterns due to climate change may increase storms’ frequency and impact, Graham says.

“It is difficult to say how much this pattern will amplify in the future.”

Pictured: A polar bear wandering on the thinning sea ice in the spring of 2015.

Credits: Marcos Porcires / Norwegian Polar Institute