In 2015, Hong Kong recorded a summer (June – August) mean temperature of 29.4 degrees, the highest since 1884 and breaking the previous record set only last year in 2014. Why do the summer temperature records get broken so easily?

The summer mean temperature in Hong Kong exhibited a significant rising trend in the last hundred years or so. In the early 20^th century (1901-1930), summer mean temperature ranged between 26.8 and 28.4 degrees (blue bars in Figure 1). However, summer mean temperatures have risen to a range between 27.8 and 29.4 degrees (red bars in Figure 1) in the past three decades (1986-2015). In other words, the distribution of summer mean temperatures has apparently shifted towards the high side by about one degree. While summer mean temperatures never exceeded 29.0 degrees in the early 20^th century, it has become quite common to see such high summer mean temperatures nowadays. Naturally, record-breaking temperatures would occur more easily against the backdrop of a hotter summer as a result of a warming climate. The situation is like rolling a loaded dice in which the probability of getting a '6' has been deliberately increased.

The world is moving towards the trajectory of a high greenhouse gas concentration scenario. If no effective mitigation measures are taken to reduce greenhouse gas emissions, we can expect more record-breaking summer temperatures in the future.