Half of the world's coral reefs have died in the last 30 years. Now scientists are racing to ensure that the rest will survive.
"The threat to coral reefs isn't something that's going to happen 100 years from now. We're losing them right now," Julia Baum told the Associated Press. She is a marine biologist at the University of Victoria, in British Columbia, Canada. "We're losing them really quickly, much more quickly than I think any of us ever could have imagined."
Even if global warming were in stop right now, scientists predict that more than 90% of corals will die by 2050. If no major steps are taken to address the issue, the reefs may be headed for total extinction.
The planet's health depends on the survival of coral reefs. They are often described as underwater rainforests, because they are ecosystems that provide habitats for one in four of all marine species. In addition, the reefs serve as barriers that protect coastlines from the full force of powerful storms. Corals are used in medical research for cures to diseases. They are key to local economies, as well, since the reefs attract tourists, the fishing industry, and other businesses, bringing in billions of dollars of revenue（收入）.
"To lose coral reefs is to fundamentally damage the health of the human race," Ruth Gates, director of the Hawaii Institute of Marine Biology, said.
Corals are animals that lack a backbone. They live mostly in tropical waters. The corals release a substance called calcium carbonate（碳酸钙）， which forms protective skeletons around them. The skeletons grow and take on vivid colors. This is the result of the corals' mutually beneficial relationship with algae（藻类） that live in their tissues（组织） and give them energy.
But corals are particularly sensitive to changes in temperature. A rise of just 1 to 2 degrees Celsius can force the corals to drive out the algae. Then the corals' skeletons turn white in a process called "bleaching". Corals can survive in these higher temperatures only for a few months.



What can we know about the world's coral reefs？ ______

A. The number of the world's coral reefs is on the decrease.
B. 45% of the world's coral reefs have died in the last 30 years.
C. Global warming has no influence on the world's coral reefs.
D. Scientists have taken major steps to address the issue of world's coral reefs.

Paragraph 4 is mainly about ______ .

A. economic value of coral reefs
B. the challenges faced by coral reefs
C. the medical functions of coral reefs
D. the importance of coral reefs.

What makes corals take on different colors？ ______

A. Changes in temperature.
B. Ocean waters.
C. Algae living inside corals.
D. Protective skeletons.

The corals' skeletons turn white when ______ .

A. there is not enough oxygen.
B. there are no algae in the corals' skeletons.
C. water pollution is worsening.
D. the ocean temperature is too low.