www.theinsider.org/new...sp?id=1541
Global warming has passed the point of no return
A record loss of sea ice in the Arctic this summer has convinced scientists
that the northern hemisphere may have crossed a critical threshold beyond
which the climate may never recover. Scientists fear that the Arctic has
now entered an irreversible phase of warming which will accelerate the loss
of the polar sea ice that has helped to keep the climate stable for
thousands of years.
They believe global warming is melting Arctic ice so rapidly that the
region is beginning to absorb more heat from the sun, causing the ice to
melt still further and so reinforcing a vicious cycle of melting and heating.
The greatest fear is that the Arctic has reached a "tipping point" beyond
which nothing can reverse the continual loss of sea ice and with it the
massive land glaciers of Greenland, which will raise sea levels dramatically.
Satellites monitoring the Arctic have found that the extent of the sea ice
this August has reached its lowest monthly point on record, dipping an
unprecedented 18.2 per cent below the long-term average.
Experts believe that such a loss of Arctic sea ice in summer has not
occurred in hundreds and possibly thousands of years. It is the fourth year
in a row that the sea ice in August has fallen below the monthly downward
trend - a clear sign that melting has accelerated.
Scientists are now preparing to report a record loss of Arctic sea ice for
September, when the surface area covered by the ice traditionally reaches
its minimum extent at the end of the summer melting period.
Sea ice naturally melts in summer and reforms in winter but for the first
time on record this annual rebound did not occur last winter when the ice
of the Arctic failed to recover significantly.
Arctic specialists at the US National Snow and Ice Data Centre at Colorado
University, who have documented the gradual loss of polar sea ice since
1978, believe that a more dramatic melt began about four years ago.
In September 2002 the sea ice coverage of the Arctic reached its lowest
level in recorded history. Such lows have normally been followed the next
year by a rebound to more normal levels, but this did not occur in the
summers of either 2003 or 2004. This summer has been even worse. The
surface area covered by sea ice was at a record monthly minimum for each of
the summer months - June, July and now August.
Scientists analysing the latest satellite data for September - the
traditional minimum extent for each summer - are preparing to announce a
significant shift in the stability of the Arctic sea ice, the northern
hemisphere's major "heat sink" that moderates climatic extremes.
"The changes we've seen in the Arctic over the past few decades are nothing
short of remarkable," said Mark Serreze, one of the scientists at the Snow
and Ice Data Centre who monitor Arctic sea ice.
Scientists at the data centre are bracing themselves for the 2005 annual
minimum, which is expected to be reached in mid-September, when another
record loss is forecast. A major announcement is scheduled for 20
September. "It looks like we're going to exceed it or be real close one way
or the other. It is probably going to be at least as comparable to
September 2002," Dr Serreze said.
"This will be four Septembers in a row that we've seen a downward trend.
The feeling is we are reaching a tipping point or threshold beyond which
sea ice will not recover."
The extent of the sea ice in September is the most valuable indicator of
its health. This year's record melt means that more of the long-term ice
formed over many winters - so called multi-year ice - has disappeared than
at any time in recorded history.
Sea ice floats on the surface of the Arctic Ocean and its neighbouring seas
and normally covers an area of some 7 million square kilometres (2.4
million square miles) during September - about the size of Australia.
However, in September 2002, this dwindled to about 2 million square miles -
16 per cent below average.
Sea ice data for August closely mirrors that for September and last month's
record low - 18.2 per cent below the monthly average - strongly suggests
that this September will see the smallest coverage of Arctic sea ice ever
recorded.
As more and more sea ice is lost during the summer, greater expanses of
open ocean are exposed to the sun which increases the rate at which heat is
absorbed in the Arctic region, Dr Serreze said.
Sea ice reflects up to 80 per cent of sunlight hitting it but this "albedo
effect" is mostly lost when the sea is uncovered. "We've exposed all this
dark ocean to the sun's heat so that the overall heat content increases,"
he explained.
Current computer models suggest that the Arctic will be entirely ice-free
during summer by the year 2070 but some scientists now believe that even
this dire prediction may be over-optimistic, said Professor Peter Wadhams,
an Arctic ice specialist at Cambridge University.
"When the ice becomes so thin it breaks up mechanically rather than
thermodynamically. So these predictions may well be on the over-optimistic
side," he said.
As the sea ice melts, and more of the sun's energy is absorbed by the
exposed ocean, a positive feedback is created leading to the loss of yet
more ice, Professor Wadhams said.
"If anything we may be underestimating the dangers. The computer models may
not take into account collaborative positive feedback," he said.
Sea ice keeps a cap on frigid water, keeping it cold and protecting it from
heating up. Losing the sea ice of the Arctic is likely to have major
repercussions for the climate, he said. "There could be dramatic changes to
the climate of the northern region due to the creation of a vast expanse of
open water where there was once effectively land," Professor Wadhams said.
"You're essentially changing land into ocean and the creation of a huge
area of open ocean where there was once land will have a very big impact on
other climate parameters," he said.
SOURCE
The Independent, "Global warming 'past the point of no return'", 16
September 2005.
news.independent.co.uk...312997.ece
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Severe weather danger increased by global warming
Global warming linked to increase of hurricanes
HURRICANES of the intensity of Katrina have become almost twice as common
over the past 35 years, according to research suggesting that global
warming could be worsening severe storms.
The overall frequency of tropical storms worldwide has remained broadly
static since 1970, but the number of extreme Category 4 and 5 events has
risen sharply, satellite measurements have shown.
Since 1990 an average of 18 Category 4 and 5 storms, of similar strength to
Hurricane Katrina, have occurred every year, compared with an average of 10
in the 1970s, US scientists have found.
Ocean surface temperatures — one of the key drivers of hurricane intensity
— have increased by an average of 0.5C (0.9F) over the same period,
indicating a potential connection to global warming.
Researchers said that it was too early to be certain that climate change is
fuelling stronger hurricanes, but such a link would be consistent with the
best predictions of the likely effects of warmer seas.
Tropical storms, which always form over water, are known as hurricanes when
they occur over the Atlantic, and as typhoons or cyclones in the Pacific
and Indian Oceans. The storms are heat engines that build intensity by
sucking up more and more water vapour, generating winds of more than 100mph
(161km/h).
Category 4 hurricanes sustain winds of between 131mph and 155mph, and the
biggest Category 5 storms blow at 156mph or more. Hurricane Katrina reached
Category 5 at its peak over the Gulf of Mexico and stood at Category 4 when
it devastated New Orleans and Mississippi.
In the latest study, published today in the journal Science, a team led by
Peter Webster, of the Georgia Institute of Technology, analysed all the
satellite records of hurricanes and typhoons since 1970.
“What we found was rather astonishing,” he said. “In the 1970s, there was
an average of about 10 Category 4 and 5 hurricanes per year. Since 1990,
the number of Category 4 and 5 hurricanes has almost doubled, averaging 18
per year.”
Judith Curry, his colleague, said: “Category 4 and 5 storms are also making
up a larger share of the total number of hurricanes. Category 4 and 5
hurricanes made up about 20 per cent of all hurricanes in the 1970s, but
over the past decade they have accounted for about 35 per cent of these
storms.”
The findings, Dr Webster said, could be a result of global warming, though
he said much more research was needed.
“Our work is consistent with the concept that there is a relationship
between increasing sea surface temperature and hurricane intensity,” he
said. “However, it’s not a simple relationship.” Several recent studies
indicate that intense storms can be expected to become more common with
climate change. In August, research by Kerry Emmanuel, of the Massachusetts
Institute of Technology, published in Nature, found that the destructive
energy of hurricanes had increased in line with rising ocean temperatures.
“We are clearly seeing the same signal in the data,” Dr Emmanuel said
yesterday.
Another Science paper, by Kevin Trenberth, of the US National Centre for
Atmospheric Research, found in July that there is 2 per cent more water
vapour above the oceans today than there was in 1988. This suggests that
more water will be drawn into swirling tropical storms, generating higher
wind speeds and greater rainfall.
SOURCE
The Times, "Global warming linked to increase of hurricanes", 16 September
2005.
www.timesonline.co.uk/...29,00.html
