A huge amount of CO2 is absorbed by the ocean and stored in the ocean, and methane is stored as clathrates. For every mega-meteorite or asteroid that impacts on land, two hit the sea. In a range of sizes between Barringer and Chicxulub, how big would a mega-meteorite have to be to release climate changing amounts of CO2 and methane from the sea? The multi-megaton fusion device that destroyed Elugulab Island in the Eniwetok atoll, Marshal Islands, in i952 was considerably more powerful than the Barringer meteorite, but does not appear to have released significant amounts of greenhouse gases. Obviously a lot depends on where in the ocean the mega-meteorite lands, so let's say it lands in a place with higher than average quantities of CO2 and methane.