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. 38.________ The extra heat from the warming atmosphere will also make surface water expand and thus make it lighter still. 39.________ 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. 40.________

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

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

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

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

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

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

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