Human activity has been the primary cause of global ocean warming during the past 50 years, a team of U.S. and international researchers says. Scientists say the observed ocean warming in the last 50 years is consistent with climate models only if the models include the effects of observed increases in greenhouse gas during the 20th century. \"We have taken a closer look at factors that influence these results,\" lead author Peter Gleckler at the Lawrence Livermore National Laboratory in California said. \"The bottom line is that this study substantially strengthens the conclusion that most of the observed global ocean warming over the past 50 years is attributable to human activities.\" The scientists looked at the average temperature, or heat content, in the upper layers of the ocean and found the global average ocean warming, from the surface down to 2,300 feet, has been around 0.025 degrees Celsius per decade, a LLNL release reported Monday. \"What we are trying to do is determine if the observed warming pattern can be explained by natural variability alone,\" Gleckler said. \"Although we performed a series of tests to account for the impact of various uncertainties, we found no evidence that simultaneous warming of the upper layers of all seven seas can be explained by natural climate variability alone. Humans have played a dominant role.\"