Cosmic Journeys – Fate of Antarctica

, , Leave a comment

It�s a land piled high with ice. Whipped by fierce winds. And powerful ocean currents. In a world buffeted by rising temperatures,
rising sea levels, and changing climate patterns, what is the fate of Antarctica? If you could go back to a time 250 million
years ago. What�s now the frozen continent of Antarctica,
was a land of forests and flowing water. In a warmer, wetter world, it harbored a wide
diversity of plants and animals. In this period, known as the Permian, Antarctica
was at the southern end of a vast single continent that spanned the globe, called Pangaea. Within the sweeping evolution of this grand
continent, a single event would reverberate down through history. An asteroid headed toward the southern hemisphere. At up to fifty kilometers in diameter, it
would have been four to five times larger than the one thought to have killed off the
dinosaurs. It crashed into a region that�s now part
of East Antarctica, but its impact was global in scale. One theory is that it sent powerful seismic
waves to the exact opposite, or antipodal, side of the Earth. On what�s now Siberian Russia, a plume of
magma pushed up through Earth�s crust. For tens of thousands of years, lava flooded the
landscape. A release of toxic elements, a rush of carbon
dioxide into the atmosphere, acid rain, all led to one of the worst mass extinctions in
history. 70% of all species on the planet vanished. On Earth, traces of most major impacts are
obscured by erosion or geological activity. A new type of observational tool is now allowing
scientists to see them. In 2002, they launched the twin spacecraft
of the GRACE mission, short for Gravity Recovery and Interior Laboratory. Its goal was to map subsurface features by
measuring variations in Earth�s gravity. A gravitational surge or a dip causes the
distance between the two craft to change, which GRACE measures down to the width of
a human hair. Down along the Eastern part of Antarctica,
GRACE detected an immense subsurface presence. Scientists matched it up with subtle rings
etched on the wider landscape, and with chemical traces found in nearby mountains. They concluded
that a large meteor had struck at the end of the Permian. It would have a lasting impact on Antarctica�s
long and tumultuous journey. That journey began in the break up of an immense
continent known as Rodinia, beginning around 750 million years ago. In those days, Antarctica basked in the tropical
sun. Some 300 million years later, as Earth�s
landmasses reshuffled into the supercontinent Pangaea. When Pangaea finally fragmented, Antarctica
began a steady drive toward the southern pole as part of the great southern continent of
Gondwana. At around 130 million years ago, a series
of rifts developed in Earth�s crust, tearing South America and Africa away from Gondwana. Australia finally split off at about 85 million
years ago, leaving Antarctica on its own. Because the rift between the two cuts through
the Permian crater, one theory holds that the force of the impact may have actually
initiated it. Now separated from its sister landmasses,
Antarctica carried a rich biological legacy, evident in fossils recently pulled from the
frozen ground. The Cryolophosaurus, from 190 million years
ago, was in a class of meat eaters called theropods. These bipedal creatures gave rise to some
of the fiercest predators ever, including TRex, as well as modern-day birds. Out on the West Antarctic Peninsula, scientists
turned up a smaller theropod from 70 million years ago. This swift predator would have
grown to about two meters in length. From the same period, another group found
a young plesiosaur, a marine reptile that plied warm Southern Oceans. If it had reached adulthood, it would have
grown to around 10 meters in length. From a continent that once hosted dinosaurs,
Antarctica grew steadily more hostile to life. Wind the clock forward, to 50 million years
ago. All around the world, warm conditions were
giving way to cooler, drier times. The new era saw a decline in the heat-trapping
atmospheric gas, carbon dioxide. Around Antarctica, conifers and other cold-tolerant
plants took hold. In some areas, they gradually turned to tundra. Ice remained year round, gradually forming
a thick sheet. At the same time, powerful wind currents circling
the pole from west to east drove the circumpolar ocean current. Known as the mightiest current in the world,
it helped shield the continent from tropical waters to the north. A combination of factors came together: declining
CO2, the isolation of Antarctica, and the tendency of permanent ice stores to reflect
more solar energy back to space. At around two and a half million years ago,
Earth entered the last great ice age, the one we live in, called the Quaternary. Year after year, as storms rolled off the
oceans, they deposited layer upon layer of snow and ice across Antarctica. Today, fully 70 percent of all the fresh water
on the planet is here, in ice that averages nearly two kilometers in thickness. Antarctica today is known as the windiest,
driest, and coldest place on Earth. There are no permanent human populations,
only a few thousand scientists and support workers living at scattered research stations. At the Russian Vostok station, in July 1983,
scientists documented the lowest natural temperature on record: -89.2 degrees Celsius. A recent calculation based on satellite data
went even lower, to -93.2 degrees Celsius. But as cold and isolated as it is, Antarctica
is not immune to changes in the larger environment. Take, for example, this reconstruction of
the continent during the last glacial maximum, 20 thousand years ago. Compare it to this image of the current interglacial
period. It shows how much ice the continent has shed. In recent years, the rate of ice loss has
picked up speed. Scientists have turned to satellites to find
out where and how quickly this trend is playing out. In these images, from the GRACE mission, areas
that lost ice are shown in blue, while orange and red gained ice. European scientists backed up this finding
with radar data from the Cryosat-2 spacecraft. By tracking changes in the elevation of Antarctic
ice, they found that the continent lost on average 159 billion tons of ice each year
from 2010 to 2013. That�s an increase of 31% per year over
the previous five years. Like GRACE, Cryosat-2 found that the greatest
losses have occurred in West Antarctica, especially where a series of fast-flowing glaciers empty
into the Amundsen Sea. A small number of scientists were warning
about this as far back as the 1960s. The reason, they pointed out, is the intensification
of winds that encircle the continent. As the temperature difference with northern
regions has increased, these winds have picked up speed. To sailors brave enough to venture into them,
the southern ocean once offered the quickest route around the world. Nowadays, stronger winds have had the effect
of drawing to the surface relatively warm water that occurs naturally in the depths
of the ocean. These warmer waters have begun to undermine
a series of vast floating ice shelves that extend out from inland glaciers. Scientists, using data from the IceSat spacecraft,
documented this effect by correlating areas of greatest ice loss with the location of
submarine troughs that can funnel the warm water up. These satellite images from over a decade
ago show the effect this can have. Out on the West Antarctic Peninsula, the Larsen ice
shelf extended out over the ocean. This image, from early February of 2002, shows
Larsen beginning to splinter. By March 7th of that year, this ice shelf,
hundreds of meters thick, broke apart. Countless icebergs tumbled into the sea. Without the shelf�s buttressing effect,
the glaciers behind it picked up speed, dumping an additional 27 cubic kilometers of ice into
the ocean each year. That has led scientists to monitor the other
great ice shelves of west Antarctica. One team set up its base on the Pine Island
Glacier, where it juts out into the Amundsen Sea. From the surface, they drilled down through
500 meters of ice to track changes in temperature, salinity, currents, and ice volume. They found that warmer waters had been eroding
the underside of the ice shelf, with melt rates of about 6 centimeters per day, or about
22 meters per year. Another group has been using satellite and
airborne radar to track the changes on a regional scale. Red shows where the glaciers are traveling
at their highest speeds, at the intersection of ice and ocean. The flow speed is steadily increasing. The
darker the red, the faster the ice is moving. Some of the most dramatic changes have been
observed on the Smith glacier, one of the smallest in this group. Back in 1996, this is where water, ice, and
land met beneath Smith. By 2011, that so-called �grounding line�
had moved 35 kilometers farther back, a retreat of more than two kilometers per year. Now peel off the ice from the continent. The
red arrows show the highest flow rates. You can see that these fast moving glaciers
sit within valleys, some of which are below sea level. The more the grounding lines retreat, the
more seawater can creep into the sub-glacial basins. For most of the glaciers, there are no major
barriers such as hills or mountains that would slow them down once they get going. The danger is the more the glaciers speed
up, the more likely the ice sheets behind them will collapse. If that happens, it may not be completely
due to climate change. Scientists have found that the flow of water
beneath some glaciers is too high to come just from seawater. The Transantarctic Mountains, dividing East
and West Antarctica, are the product of a rift that is occurring in the underlying crustal
plate. Scientists have detected seismic activity
they say is consistent with magma moving within the crust over 25 kilometers down. They have now found there is a significant
amount of geothermal activity beneath one of the largest glaciers, the Thwaites, to
account for the extra melting. To date, most studies of future sea level
rise have focused on the ice sheets of Greenland and Western Antarctica. Together, they would
account for up to 12 meters of sea level rise. The massive East Antarctic ice sheet has been
largely ignored. Because of its sheer size and unwavering cold, it has seemed impervious
to warming trends in the north. New research has given scientists reason to
wonder how stable it really is. One new study takes us back to a period called
the Pliocene, from about 5 to 2.5 million years ago. Back then, global temperatures
and atmospheric CO2 concentrations were about where they are projected to be at the end
of this century. Sea levels are thought to have been at least
20 meters higher than today. If any part of East Antarctica had melted,
it would have been here in a region called Wilkes Land. This is where the GRACE satellites detected
signs of the massive Permian impact. If the
glaciers of Wilkes Land are prone to melting
in warmer times, then erosion would have washed its distinctive volcanic soils into the sea. By drilling down into ocean sediments, scientists
were able to find them just off shore. They concluded that in the Pliocene, seawater
was able to erode rocks about 160 kilometers inland in an area called the Wilkes Sub-glacial
Basin. How stable is the Wilkes ice sheet today? Currently, it is said to be in balance. This
means the amount of ice that melts or falls into the sea is replaced by new ice that forms
inland. Scientists used a computer simulation to find
out what it would take to throw Wilkes out of balance. A crucial factor in their calculation is a
zone in which the ice shelf is wedged against a series of ridges on the sea bottom. These
ridges act as a stopper, preventing the glacier, and the ice sheet behind it, from moving forward. If warmer waters were to undermine the ice
that rests on these ridges, then the ice sheet could begin to move out to sea. The effect has been described as that of a
bottle of water tilted downward. Remove the stopper, and gravity empties it out. For a glacier, once the ice starts flowing,
there would be no stopping it. How plausible is this scenario? On the much smaller Pine Island glacier, scientists
have shown that the ice shelf has already broken free of an undersea ridge. The disintegration of coastal Antarctic glaciers
is a process that would take centuries to run its course. But once it gets going, it
would mean a steady rise of sea levels for the foreseeable future. The changes we experience in our everyday
lives are shaped mostly by near-term developments, in technology, politics, and culture. Some of the bedrocks of modern life, cities
like New York, Hong Kong, London, and seaports around the world, have grown and evolved on
the scale of centuries. Within the long arc of their histories, rising
seas could threaten their existence in a relatively short time frame. The emerging story of ice sheet melting and
sea level rise is often couched in uncertainties, with the drama of what�s happening damped
by the rational language of science. Critics focus on the margins of error inherent
in the scientific process, or downplay the predictive power of computer simulations. Politicians debate, while conspiracy theories
abound. One of the lessons of Antarctica is that our
choices are beginning to narrow. The trends we see now will increasingly shape the fate
of Antarctica, and that of the world we know. 3


Leave a Reply