Hi-- the problem with the 30yr moving average is that it doesn't keep up with changes in the last ten years or so.  In the historical record, we eventually center the 30yr average, but you can only use the last 30yrs in real time, which means it is a lagging indicator. We are currently noticing 0.5C+ differences between our current method of computing ONI and Relative ONI.  This is quite an offset and points to one limitation of the moving 30yr method to deal with trends.