Gas hydrates contains a large quantity of organic carbon which exceeds the total organic carbon from other sources. It is an important carbon reservoir in shallow geosphere. A slight variation of temperature/pressure would cause the formation/decomposition of gas hydrates,and subsequent consumption/release of methane. This exerts significant influence on global carbon cycle and greenhouse effect. The Polar regions differ from the mediate low latitudes on the feedback of gas hydrates to global climate change and galcial/interglacial alternation. Even in mediate low latitudes there exist two processes of opposite directions. The total effect of gas hydrates on global changes needs to be estimated in further researches. In China seas, there exist T P conditions suitable for the occurrence of gas hydrates. The BSR as an indication of gas hydrates has been reported in southern South China Sea. The total reserves of gas hydrates and their feedback to sea level changes in China seas may be assessed based on careful re interpretation of seismic profiles and computer simulations. This may provide an example for the marginal seas.