D
In
the atmosphere,carbon dioxide actd rather like a one-way mirror or the glass in
the roof of a greenhouse which allows the sun's rays to enter but prevents the
heat from excaping.
According
to a weather expert's prediction,the atmosphere will be 3 warmer in the year
2050 than it is today, if man continues to burn luels at the present rate. If
this warming up took place,the ice caps in the poles would begin to melt,thus
raising sea level several meters and severely flooding coastal cities.Also，the increase in atmospheric
temperature would lead to great changes in the climate of the northern
hemisphere(半球),psooicbly
resulting in an alteration of earth's chief food growing zones.
In
the past,concern about a man - made warming of the earth has concentrated on
the Arctic because the Antarctic is much colder and has a much thicker ice
sheet.But the weather experts are now paying more attention to West
Antarctic,which may be affected by a warming on the scale that will possibly
take place in the next fifty years from on burning of fuels.
Satellite
pictures showthat large areas of Antarctic ice are already disappearing.The
evidence available suggests that a warming has taken place.This fits the theory
that carbon dioxide warms the earth.
However,most
of the fuel is burnt in the northern hemisphere,where temperatures seem to be
falling.Scientists conclude,therefore,that up to now natural influences on the
weather have exceeded those caused by man.The question is:Which natural cause
has most effect on the weather?
One
possibility is the variable behavior of the sun.Astronomers at one research
station have studied the hot spots and cold spots(that is,the relatively less
hot spots) on the sun. As the sun rotates(旋轉(zhuǎn)),every 27.5 days,it presents hotter or colder faces to the
earth,and different aspects to different parts of the earth.This seems to have
a considerable effect on the distribution of the earth's atmospheric
pressure,and consequently on wind circulation.The sun is also variable over a
long term:its heat output goes up and down in cycles,the latest trend being
downward.
Scientists
are now finding mutual relations between models of solar weather interactions
and the actual climate over many thousands of years ,including the last Ice
Age.The problem is that the models are predicting that the world should be
entering a new Ice Age and it is not.
One
way of solving this theoretical difficulty is to assume a delay of thousands of
years while the solar effects overcome the inertia(不活動，惰性) of the earth's
climate.If this is right,the warming effect of carbon dioxide might thus be
serving as a useful counter balance to the sun's diminishing heat.
68.It
can be concluded that a concentration of carbon dioxide in the atmosphere would
___.
A.mean
a warming - up im the Arctic
B.raise
the temperature of the earth's surface
C.prevent
the sun's rays from reaching the earth's surface
D.account
for great changes in the climate in the northern hemiphere
69.Although
the fuel consumption is greater in the northern hemisphere,temperatures there
seem to be falling.This is _______.
A.mainly
because the levels of carbon dioxide are rising
B.partly
due to variations in the output of solar energy
C.possibly
because the ice caps in the poles are melting
D.exclusively
due to the effect of the inertia of the earth's climate
70.On
the basis of their models,scientists are of the opinion that ________.
A.the
climate of the world should be becoming cooler
B.the
new Ice Age will be delayed by the greenhouse effect
C.the
man - made warming effect helps to increase the solar effects
D.it
will take thousands of years for the inertia of the earth's climate to take
effect
