题目内容：

    As warming continues, scientists warn the oxygen content of oceans across the planet could be more and more reduced, with serious consequences for the future of fish and other sea life.

1.One is the simple fact that as water gets warmer, it can hold less dissolved (溶解的) oxygen. The other reason is less obvious. The entire ocean gets its oxygen from the surface — either from the atmosphere, or from photosynthesizing algae floating at the top of the sea. 2.

Global warming is expected to reduce the mixing of the ocean by making surface seawater lighter. That’s because in a warmer world we can expect more rainfall and more melting(融化) of glaciers, icebergs, and ice sheets. 3.The extra heat from the warming atmosphere will also make surface water expand and thus make it lighter still. 4.Instead, more of the oxygen will remain near the surface, where it will be used up by oxygen-breathing organisms.

A low-oxygen ocean may become an inescapable feature of our planet. A team of Danish researchers wondered how long oxygen levels would drop if we could somehow reduce our carbon dioxide emissions to zero by 2100. They determined that over the next few thousand years oxygen levels would continue to fall, until they declined by 30 percent. The oxygen would slowly return to the oceans, but even 100,000 years from now they will not have fully recovered. 5.

A. It’s not known why the oxygen level of oceans has reduced.

B. Scientists point to two reasons to expect a drop in ocean oxygen.

C. Fresh water’s inpouring will make the water at the ocean’s surface lighter.

D. The oxygen then spreads to the deep ocean as the surface water slowly sinks.

E. Global warming has caused the reduction of the oxygen content of oceans worldwide.

F. The light surface water will be less likely to sink so the deep ocean will get less oxygen.

G. If they are right, we have every reason to worry about the major effect it has on sea life.