Is the current warm spell, now lasting almost two years, due to variability of the weather, or is it evidence of climate change? Many in the Pacific Northwest have been asking this question for months.

On Thursday, Aug. 20, well-known University of Washington atmospheric scientist and author Cliff Mass will try to answer it in a timely Orcas Currents lecture on “Why is the Northwest so warm?” Accompanied by his usual plethora of weather maps and ocean-temperature charts, his presentation will begin at 5:30 p.m. in the Orcas Island Community Church on Madrona Street in Eastsound.

Mass favors the weather interpretation, saying that the warm spell can be traced to a high-pressure ridge that set up over the Pacific coast during the fall of 2013 and has remained stubbornly in place ever since, warming offshore waters.

But others dispute this explanation, claiming that recent Arctic warming has weakened the jet stream, accentuating its loops and folds — and that this is causing both the warming of the Northwest and the frigid air impacting North America east of the Rockies.

An accomplished public speaker with a weekly radio program on KPLU–FM, Mass writes a popular weather blog that people often consult before planning outdoor activities.  A month ago, he issued a blog on the same subject at his Aug. 20 lecture, which interested attendees can consult beforehand: http://cliffmass.blogspot.com/2015/07/why-is-northwest-so-warm.html.

This Orcas Currents event is cosponsored by Coates Vineyards, Eastsound Water Users Association and Orcas Island Public Library.

Afterward, there will be a reception with light refreshments. As always, admission is free.