Passage Four
In 1542, Francisco de Orellana led the first European voyage down the Amazon River. During this voyage the explorers met a lot of resistance (抵抗) from the local Indians. In one particular tribe the women fighters were so fierce that they drove their male fighters in front of them with spears. Thus the river was named after the famous women fighters of the ancient Greek stories, the Amazons.
This voyage also started our wonder of the greatest river and the largest area of rainforest in the world. About 20% of all fresh water carried to the oceans is from the Amazon River. The Amazon basin is the world's largest, about the size of Europe. The river is a product of the rainy season , which brings huge rains every summer. This also produces a large area of rainforest, which supports the largest number of diverse plants and animals of any area in the world.
The Amazon rainforest is also important when it comes to the future of global warming, as it is a huge natural store of carbon. Up until recently it was thought that the Amazon had fully grown and thus could not take up any more CO2. Experiments have shown this could be wrong and that the Amazon rainforest might be sucking up an additional five tons of CO2 from the atmosphere per hectare (公顷)per year. This is because plants react favorably to increased CO2; because it is the raw material for photosynthesis (光合作用) ， the more of it the better. So having more CO2 in the atmosphere acts like a fertilizer, stimulating plant growth. Because of the size of the Amazon rainforest, it seems that presently it is taking up a large percentage of our CO2 pollution in the atmosphere, about 75% of the world's car pollution.

50.Why is CO2 compared to a fertilizer in Para. 3?