NICE, France—As world leaders prepare to attend the third U.N. Ocean Conference here next week, scientists are urging them to take action to combat a dual climate and biodiversity crisis plaguing marine ecosystems, and base their decisions on the best available science.

To guide them, the International Scientific Committee of the One Ocean Science Congress unveiled a list of comprehensive recommendations for ocean recovery its members hope can be transformed into actionable policy.

“We say, act where science is clear,” said François Houllier, a member of the committee and president and managing director of Ifremar, a French ocean research institute that co-organized the scientific summit.

The 18-member committee spent the last year developing the list of recommendations and included ins

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