新托福阅读复习材料：美国科学文摘精选(一)

文章作者 100test 发表时间 2008:05:15 09:10:43
来源 100Test.Com百考试题网

The Planet Earth

The 2000 Antarctic Ozone Hole Was Largest Ever

The 2000 Antarctic Ozone Hole was the largest ever observed. Earths wetter
upper atmosphere may delay global ozone recovery.

NASA researchers have found that an increase in water vapor in the
stratosphere, stemming partially from greenhouse gases, may delay ozone
recovery and increase the rate of climate change.

To check on the long-term stratospheric cooling and ozone depletion, NASA put
data from satellites and other remote sensors into its GISS global climate
model. It was the first study to link greenhouse gases to increased ozone
depletion over populated areas.

Water and ozone. Climate models show cooler stratospheric temperatures happen
when there is more water vapor present. Water vapor also leads to the
breakdown of ozone molecules.

The stratosphere is the dry layer of the atmosphere above the troposphere,
where temperatures increase with height.

According to satellite data, upper atmospheric temperatures around the world -
at altitudes of 20 to 35 miles high -- have cooled between 5.4 and 10.8
degrees Fahrenheit over recent decades.

Driving forces. NASA found two driving forces behind the change in
stratospheric moisture:
Increased emissions of the greenhouse gas methane are transformed into water
in the stratosphere, accounting for about a third of the observed increase
in moisture there.


More water is transported from the lower atmosphere. Warmer air holds more
water vapor than colder air, so the amount of water vapor in the lower
atmosphere increases as it is warmed by the greenhouse effect. Greenhouse
gases, such as carbon dioxide and methane, may enhance the transport of
water into the stratosphere.
The increased transport of water vapor to the stratosphere seems likely to
have been induced by human activities.

Ozone destruction. Rising greenhouse gas emissions account for all or part of
the water vapor increase, which causes stratospheric ozone destruction.

When more water vapor works its way into the stratosphere, the water molecules
can be broken down, releasing reactive molecules that can destroy ozone. If
the trend of increasing stratospheric water vapor continues, it could increase
future global warming and impede ozone stratospheric recovery.

The impact on global warming comes about because both water vapor and ozone
are greenhouse gases, which trap heat leaving the Earth. When they change, the
Earths energy balance changes too, altering the surface climate.

Warmer ground. Increased water vapor in the stratosphere makes it warmer on
the ground by trapping heat, while the ozone loss makes it colder on the
ground.

Water vapor has a much larger effect, so that overall the changes increase
global warming.

Although ozone depletion cools the Earths surface, repairing stratospheric
ozone is important to block harmful ultraviolet radiation. Other greenhouse
gas emissions also need to be reduced.

UARS satellite. NASA combined seven years of data from the Upper Atmosphere
Research Satellite (UARS) Halogen Occultation Experiment (HALOE) with data
collected on the ground to paint a complete picture of the upper atmosphere.

NASAs HALOE was aboard the UARS spacecraft when it was launched September 12,
1991 as part of the Earth Science Enterprise Program. The spacecrafts mission
at launch was to improve understanding of stratospheric ozone depletion by
analyzing vertical profiles of ozone, hydrogen chloride, hydrogen fluoride,
methane, water vapor, nitric oxide, nitrogen dioxide, and aerosols.

Fourteen years of lower stratospheric measurements have revealed large
increases in water vapor. Though some older studies conflict with lower
stratospheric observations of water vapor trends, new studies agree with the
increases, showing they have been taking place for more than four decades.

What Is An Ozone Hole?

Ozone molecules are made up of three atoms of oxygen. They comprise a thin
layer of the atmosphere that absorbs harmful ultraviolet radiation from the
Sun.

Most atmospheric ozone is found between approximately six miles and 18 miles
above the Earths surface.

An ozone "hole" is what scientists call an "ozone depletion area" of in that
region of Earths atmosphere.

Really big hole. The largest-ever ozone hole was detected on September 6,
2000, by the Total Ozone Mapping Spectrometer (TOMS) aboard a NASA satellite
known as Earth Probe (TOMS-EP).

The Antarctic ozone hole is three times larger than the entire land mass of
the United States, making it the largest such area ever observed.

The hole had expanded to a record size of 11 million square miles. The
previous record was 10.5 million square miles in September 1998.

Scientists were surprised by its enormous size. The lowest readings in the
Antarctic ozone hole usually are observed in late September or early October
each year.

Frail layer. The year 2000 observations reinforced concerns about the frailty
of Earths ozone layer. Although production of ozone-destroying gases had been
curtailed under international agreements, concentrations of the gases in the
stratosphere have been reaching their peak.

Due to their long persistence in the atmosphere, it will be many decades
before the ozone hole is no longer an annual occurrence.

Antarctic vortex. The year 2000 saw an extremely intense Antarctic vortex --
an upper-altitude stratospheric air current that sweeps around the Antarctic
continent, confining the Antarctic ozone hole.

Variations in the size of the ozone hole and of ozone depletion accompanying
it from one year to the next are not unexpected.

NASA instruments have been measuring Antarctic ozone levels since the early
1970s. Since the discovery of the ozone hole in 1985, TOMS has been a key tool
for monitoring ozone levels above Earth.

TOMS-EP and other ozone-measurement programs are important parts of a global
environmental effort of NASAs Earth Science enterprise, a long-term research
program designed to study Earths land, oceans, atmosphere, ice and life as a
total integrated system.

To learn more:
Goddard Institute for Space Studies
Upper Atmosphere Research Satellite
Halogen Occultation Experiment
Largest ozone hole detected by TOMS
TOMS ozone data and pictures
NASA Says Wet Upper Atmopsphere Delays Ozone Recovery
NASA Press Release: Wetter Atmopsphere May Delay Ozone Recovery

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