Recently, I was examining various weather records to see if I could calculate the average length of the frost-free period on Denman Island. Between Environment Canada’s historical records and my own recent observations, I was able to get intermittent data going back to 1960.

When I examined the data, I was startled to notice that there was a huge difference between the frost free period in the 1960s and now. In the early 1960s, it averaged about 165 days, compared to about 230 days in recent years.

Before jumping to conclusions, I noted that there were major gaps in the records. Perhaps there was some statistical thing happening due to the small sample size and lack of continuity. Furthermore, the records were made at different locations. Environment Canada’s records appear to have been made at locations on Lacon and East Roads, and mine are at Pickles Road. We all know about how the effects of micro-climate can give different locations completely different weather. Perhaps that was skewing the results.

To get a better handle on whether I was seeing a real effect, I looked up the records for the Comox airport, which are continuous back to 1944. Being an official weather station, the Comox records have few gaps, and all were made at the same location.

The result confirmed what I had suspected. The frost-free period at Comox has increased steadily from an average of about 170 days in 1945 to 220 days in 2003, the last year for which I have the Comox data. What I was seeing was neither a statistical blip nor an effect of micro-climate. It was a real trend.

The frost-free period is an extremely sensitive indicator of temperature changes. So, a consistent trend in the length of the frost-free period is an indication of a trend in temperature. Similar trends have been observed in other parts of the world.

It may be too soon to plant banana trees outdoors, but we are already experiencing the effects of climate change here on Denman Island.