I was reading an essay about the environment and how we can save the forests. But actually while reading I felt that the following sentence is not correct, it used the plural are instead of is as I think it should. Look the sentence:
...Is this a problem? Yes it certainly is.
First, rising CO2 levels are causing global warming.
I think the (rising) in CO2 levels is the cause of the global warming, then the sentence must use the singular is. Why did it use the plural? How can you explain this?
Sorry if the question is too basic.