查了一下Longman词典,关于single有一条解释：
the single biggest/greatest etc：used to emphasize that you are talking about the one thing that is the biggest,greatest etc.
single的这种用法,是表示强调,用来加强语气.
类似例子还有：
1.Cigarette smoking is the single most important cause of lung cancer.
2.Tourism is the country's single biggest earner.