The effects of Arctic climate change include a marked decrease in Arctic sea ice; thawing permafrost, leading to the release of methane, a potent greenhouse gas;[43] the release of methane from clathrates, leading to longer time-scale methane release;[44] the observed increase in melt on the Greenland Ice Sheet in recent years; and potential changes in patterns of ocean circulation. Scientists worry that some of these effects may cause positive feedbacks which could accelerate the rate of global warming.