Scientists have long understood the key role that oceans play in regulating the Earth’s climate. Oceans cover 70 percent of the globe and store a thousand times more heat than the atmosphere does. What’s newer is the understanding of how this key component of our climate system responds to global warming.
A brake on global warming — for now
One of oceans’most important climate functions is absorbing heat and carbon dioxide (CO₂), one of the gases that cause global warming. Acting as a heat sponge (海绵), oceans have absorbed huge amounts of heat and CO₂ in the last forty years.
Fujita explains that "oceans are saving us from faster climate change — they are a big flywheel that delays rapid overheating of the Earth, putting a brake on the climate system."
"That’s the good news," he adds. "The bad news is that oceans only slow the atmospheric warming. Once oceans come to balance with a greenhouse gas warmed earth, the extreme heat will remain in the atmosphere and things will get much hotter." But where and how oceans release this accumulated (积累的) heat is uncertain. And as oceans store heat, fragile underwater ecosystems are struggling.
The most recent scientific report from the Intergovernmental Panel on Climate Change (IPCC) also notes with concern that oceans are acidifying because of increased absorption of atmospheric CO₂, and thus causing a threat for shell-forming species. Sharp increases in carbon dioxide levels will cause further acidification of oceans.
Currents distributing heat
Another important role oceans play is that of distributor. Oceans deliver heat and life-sustaining nutrients around the globe. Just as blood vessels bring oxygen and nutrients to cells in the human body, oceans’ currents carry oxygen, nutrients and heat throughout the Earth. Oceans distribute 25 percent to 50 percent of the energy the planet receives from the sun. For example, the Gulf Stream carries heat across the Atlantic. This warm current gives northwestern Europe a milder climate than it would normally have so far north. A change to oceans’ circulation (循环) patterns could throw Europe into a colder period, even as the rest of world experiences warmer temperatures.
This passage mainly talks about     .
A.the roles of oceans B.global warming
C.ocean currents      D.carbon dioxide

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