The BBC reports:
The report’s authors ultimately conclude that 15 years is still not a long enough timescale to draw firm conclusions about the pause. Scientific studies on the slow-down have cited uptake of heat by the upper oceans as a possible cause, along with the properties of particulate matter in the atmosphere which can reflect solar energy back into space. But published research is still relatively sparse.
A favourite of climate “sceptics”, the Medieval Warm Period, is also referenced in the report. In some places this period, from 950-1250 AD, was comparable in its warming with the late 20th Century, but the report says this did not occur as coherently across seasons and regions as the current phase of warming.