单选题
Dr Corell heads a team of some 300 scientists who
have spent the past four years investigating the matter in a process known as
the Arctic Climate Impact Assessment (ACIA). The group, drawn from the eight
countries with territories inside the Arctic Circle, has just issued a report
called "Impacts of a Warming Arctic", a lengthy summary of the principal
scientific findings. Scientists have long suspected that
several factors lead to greater temperature swings at the poles than elsewhere
on the planet. One is albedo (反照率)-the posh scientific name for how much
sunlight is reflected by a planet's surface, and how much is reflected. Most of
the polar regions are covered in snow and ice, which are much more reflective
than soil or ocean. If that snow melts, the exposure of dark earth (which
absorbs heat) acts as a feedback loop that accelerates warming. A second factor
that makes the poles special is that the atmosphere is thinner there than at the
equator, and so less energy is required to warm it up. A third factor is that
less solar energy is lost in evaporation at the frigid poles than in the steamy
tropics. Arctic warming may influence the global climate in
several ways. One is that huge amounts of methane, a particularly potent
greenhouse gas, are stored in the permafrost of the tundra. Although a thaw
would allow forests to invade the tundra, which would tend to ameliorate any
global warming that is going on (since trees capture carbon dioxide, the
greenhouse gas most talked about in the context of climate change), a melting of
the permafrost might also lead to a lot of trapped methane being released into
the atmosphere, more than offsetting the cooling effects of the new
forests. Another worry is that Arctic warming will influence
ocean circulation in ways that are not fully understood. One link in the chain
is the salinity of sea water, which is decreasing in the north Atlantic thanks
to an increase in glacial meltwaters. Because fresh water and salt water have
different densities, this "freshening" of the ocean could change circulation
patterns. The most celebrated risk is to the mid-Atlantic Conveyor Belt, a
current which brings warm water from the tropics to north-western Europe, and
which is responsible for that region's unusually mild winters. Some of the
ACIA's experts are fretting over evidence of reduced density and salinity in
waters near the Arctic that could adversely affect this current.
The biggest popular worry, though, is that melting Arctic ice could lead
to a dramatic rise in sea level. Here, a few caveats are needed. For a start,
much of the ice in the Arctic is floating in the sea already. Archimedes's
principle shows that the melting of this ice will make no immediate difference
to the sea's level, although it would change its albedo. Second, if land ice,
such as that covering Greenland, does melt in large quantities, the process will
take centuries. And third, although the experts are indeed worried that global
warming might cause the oceans to rise, the main way they believe this will
happen is by thermal expansion of the water itself.
Nevertheless, there is some cause for nervousness. As the ACIA researchers
document, there are signs that the massive Greenland ice sheet might be melting
more rapidly than was thought a few years ago. Cracks in the sheet appear to be
allowing melt water to trickle to its base, explains Michael Oppenheimer, a
climatologist at Princeton University who was not one of the report's authors.
That water may act as a lubricant, speeding up the sheet's movement into the
sea. If the entire sheet melted, the sea might rise by 6-7 metres. While
acknowledging that disintegration this century is still an unlikely outcome, Dr
Oppenheimer argues that the evidence of the past few years suggests it is more
likely to happen over the next few centuries if the world does not reduce
emissions of greenhouse gases. He worries that an accelerating Arctic warming
trend may yet push the ice melt beyond an "irreversible on/off
switch". Not everybody wants to hear a story like that. But
what people truly believe is happening can be seen in their actions better than
in their words. One of the report's most confident predictions is that the
break-up of Arctic ice will open the region to long-distance shipping and,
ironically, to drilling for oil and gas. It is surely no coincidence, then, that
the Danish government, which controls Greenland, has just declared its intention
to claim the mineral rights under the North Pole. It, at least, clearly believes
that the Arctic ocean may soon be ice-free.
单选题
Which of the following factors may lead to greater temperature changes
at the poles than elsewhere on the planet?
A. The albedo of the poles will be larger if snow melts.
B. the albedo of snow is larger than that of the exposed dark earth if snow
melts.
C. More energy is needed to warm up the Arctic since the atmosphere there is
thinner.
D. Less solar energy is lost in volatilization at the poles than at the
equator.
单选题
What does the author imply in the last paragraph by the sentence "But
what people truly believe is happening can be seen in their actions better than
in their words"?
A. Not everybody wants to hear a story like that.
B. One of the report's most confident predictions is that the break-up of
Arctic ice will open the region to long-distance shipping and, ironically, to
drilling for oil and gas.
C. The Danish government, which controls Greenland, has just declared its
intention to claim the mineral rights under the North Pole.