The gas absorbed by ocean water which can moderate a potential greenhouse effect is carbon dioxide.
Explanation;
Greenhouse effect is is a process that warms the Earth's surface. This is as a result of radiation and absorption of sun's energy by greenhouse gases.
The ocean decrease the rate of accumulation of carbon dioxide in the atmosphere by absorbing carbon dioxide gas which is a green house gas.
Oceans absorbs carbon dioxide from the atmosphere where the air meets the water. The turbulence and waves caused by wind makes water to absorb the carbon dioxide.
