19．"The world's oceans are slowly getting more acidic．"say scientists．The researchers from California report that the change is taking place in response to higher levels of carbon dioxide in the atmosphere．
The lowering of the waters'PH value is not great at the moment but could cause a serious threat to current ocean life if it continues，they warn．Ken Caldeira and Michael Wickett，from the Lawrence Livermore National Laboratory，report their concerns in the journal Nature．Increasing use of oil fuels means more carbon dioxide is going into the air，and most of it will eventually be absorbed by seawater．Once in the water，it reacts to form carbonic acid．Scientists believe that the oceans have already become slightly more acidic over the last century．
These researchers have tried to predict what will happen in the future by combining what we know about the history of the oceans with computer models of climate change．"This level of acidity will get much more extreme in the future if we continue releasing CO2 into the atmosphere，"said Dr Caldeira．And we predict the amount of future acidity will exceed（超過） anything we have seen over the last several hundred million years，let alone perhaps after rare disastrous events such as asteroid（小行星） impacts
However，it is not absolutely clear what that means for ocean life．Most organisms live near the surface，where the greatest PH change would be expected to occur，but deep-ocean life forms may be more sensitive to PH changes．Coral reefs（珊瑚礁） and other organisms whose shells contain calcium carbonate（碳酸鈣） may be particularly affected if the water's acidity levels keep going up，the team predict．They could find it much more difficult to build these structures in water with a lower PH．
In recent years some people have suggested storing carbon dioxide from power station in the deep ocean as a way of dealing with global warming．But Dr Caldeira said that such a strategy should now be re-considered．"Previously，most experts had looked at ocean absorption of carbon dioxide as a good thing--because in releasing CO2 into the atmosphere we warm the planet，and when CO2 is absorbed by the ocean，it reduces the amount of greenhouse warming．"
57．The ocean is becoming more acidic due toC．
A．the lower water PH value
B．the warming atmosphere
C．the higher level of CO2 in the air
D．the increasing use of oil fuels
58．According to Dr Caldeira，D．
A．ocean absorption of carbon dioxide is a good thing
B．more oil fuels will be used in the near future
C．scientists may predict climate changes with computer models
D．the future situation of the amount of acidity is extremely serious
59．If the water's acidity level keeps rising，A
A．ocean life whose structures contain calcium carbonate may be affected
B．the water's PH value will become higher and higher
C．organisms living near the surface are more sensitive to PH changes
D．some disastrous events will occur more often than before
60．Most experts once believed storing carbon dioxide in the ocean would reduceB．
A．the CO2 absorbed by the ocean
B．the amount of greenhouse warming
C．the acidity of the ocean
D．the gradual release of CO2．
