题型：阅读理解 题类：常考题 难易度：普通
上海市行知高中2020-2021学年高一下学期英语4月第一次月考英语试卷（含完整音频）
In the past decade ocean oxygen levels have taken a dive — an alarming trend that is linked to climate change, says Andreas Oschlies, an oceanographer at the Helmholtz Center for Ocean Research in Germany, whose team tracks ocean oxygen levels worldwide. "We were surprised by the intensity of the changes we saw, how rapidly oxygen is going down in the ocean and how large the effects on marine ecosystems are," he says. It is no surprise to scientists that warming oceans are losing oxygen, but the scale of the drop calls for urgent attention. Oxygen levels in some tropical regions have dropped by an astonishing 40 percent in the last 50 years, some recent studies reveal. Levels have dropped less significantly elsewhere, with an average loss of 2 percent globally.
A warming ocean loses oxygen for two reasons; First, the warmer a liquid becomes, the less gas it can hold. That is why carbonated drinks(碳酸饮料) go flat faster when left in the sun. Second, as polar sea ice melts, it forms a layer of water above colder, more salty sea waters. This process creates a sort of lid that can keep currents from mixing surface water down to deeper depths. And because all oxygen enters the surface, less mixing means less of it at depth.
Ocean animals large and small, however, respond to even slight changes in oxygen by seeking refuge(避难所) in higher oxygen zones or by adjusting behavior, Oschlies and others in his field have found. These adjustments can expose animals to new predators or force them into food-scarce regions. Climate change already poses serious problems for marine life, such as ocean acidification, but deoxygenation is the most pressing issue facing sea animals today, Oschlies says. After all, he says, "they all have to breathe."
Aside from food web problems, animals face various other physiological challenges as their bodies adjust to lower oxygen levels. Chinese shrimp(虾) move their tails less vigorously to preserve energy in lower oxygen environments. Some creatures, such as jellyfishes, are more tolerant of low oxygen than others are. But all animals will feel the impact of deoxygenation because they all have evolved their oxygen capacity for a reason, says Oschlies. "Any drop in oxygen is going to damage survivability and performance," he says.
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