"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 COZ into the atmosphere," said Dr Caldeira.
"And we predict the amount of future acidity will exceed(超过)anything we have seen over the last
several hundred million years, let alone perhaps after rare disastrous events such as asteroid(小1j-%+)
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 and other organisms whose shells contain calcium carbonate(小行星) 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 stations 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一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."

A.   the lower water pH value            
B．the warming atmosphere
C．the higher level of COZ in the air    
D．the increasing use of oil fuels