To demonstrate this, consider the output of one of the 24 climate models on ClimateData.ca, the number of days with maximum temperature greater than 30°C.
Looking at the whole graph, from 1950 to 2100, the trend is increasing.
Similarly, looking at a thirty-year period (2041 to 2070), the trend is increasing.
But what if a shorter period was considered? There are a couple of 10-year periods in this time series (blue), where the trend is decreasing.
These 10 year-periods are not representative of the longer-term climate change trend.
Decisions (e.g. risk assessments or adaptation actions) based on the trend of a shorter time period may lead to insufficient actions, inappropriate for the longer-term climate trends.