But, as a recent study by scientists has shown, the growing lack of ozone is also due to the emissions produced by all the private cars.

study 是名词，意思是"研究、调查"，指对某个问题进行系统性的科学考察和分析。 这里 study 特指科学家们进行的关于臭氧层减少原因的研究项目。