The ocean's role in global warming stems principally from its huge capacity to absorb carbon dioxide and to store and transport heat.
In the sea, photosynthesis by marine plants and algae, especially phytoplankton, removes great quantities of carbon dioxide from the atmosphere.
Hence, the greater the growth (productivity) of phytoplankton in the sea, the greater the removal of carbon dioxide.
But what controls the ocean's productivity?
There are several limiting factors, but results from a recent experiment suggest that in areas of the ocean where other nutrients are plentiful, iron may be one of the most important and, until recently, unrecognized variables controlling phytoplankton production.
Some have proposed a radical, highly controversial, and uncertain means to counteract global warming: adding iron to the oceans to induce phytoplankton blooms.
Perhaps increased phytoplankton growth would use up a significant amount of carbon dioxide in the atmosphere, but perhaps not, and there might well be side effects that could be detrimental to the ocean ecosystem.
summary
The paragraph states that oceans influence global warming by absorbing carbon dioxide and heat.
Phytoplankton help reduce atmospheric carbon dioxide through photosynthesis, and their growth depends partly on iron in nutrient-rich waters.
This insight has inspired a controversial proposal to add iron to the ocean to trigger phytoplankton blooms, though the ecological risks remain unclear.
摘要
本段指出海洋因能大量吸收二氧化碳並儲存熱量，而在全球暖化中扮演重要角色。
浮游植物透過光合作用可移除大氣中的二氧化碳，因此其生產力越高，去除的量也越大。
最新研究顯示，在其他養分充足的海域，鐵可能是控制浮游植物生長的關鍵因子。
基於此，有人提出往海洋添加鐵以促進浮游植物大量繁殖、進而減少大氣二氧化碳的做法，但其效果與可能造成的生態危害仍具高度爭議與不確定性。
6. According to paragraph 2, how might increasing phytoplankton growth help lower global temperatures？
O By cooling the oceans
O By decreasing carbon dioxide levels in the ocean
O By reducing the amount of carbon dioxide in the atmosphere
O By transporting heat from the ocean's surface to deeper levels
根據段落內容，關鍵句為：
“the greater the removal of carbon dioxide from the atmosphere.”
浮游植物光合作用會「從大氣中移除大量二氧化碳」，而二氧化碳是主要溫室氣體，因此降低大氣中的 CO₂ 有助於降低全球暖化程度。
答案：By reducing the amount of carbon dioxide in the atmosphere （藉由減少大氣中的二氧化碳）