Legitimate science is the process of ruling out options. In order to say the warming observations are caused by human activity, you need to start with statements like the warming changes can not have been caused by the sun and the warming changes can not have been caused ocean circulation. Skipping those steps is essentially abandoning the scientific method.
NASA Sees Arctic Ocean Circulation Do an About-Face: A team of NASA and university scientists has detected an ongoing reversal in Arctic Ocean circulation triggered by atmospheric circulation changes that vary on decade-long time scales. The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.