The cherry blossom has come into bloom in Japan, marking the start of a huge national festival celebrated with outdoor parties and with special blossom tours. This is a serious business and the cherry trees, together with daily temperatures, are closely monitored to predict exactly when the blossoms will emerge.

In fact, the festival is an ancient tradition going back at least a thousand years. During that time, the dates of the first cherry blossoms have been accurately noted, and this has given a wonderful insight into how the climate of Japan has changed.

Scientists have scoured historic court records and diaries and found that the dates of the first blossom can extend anytime from late March to early May, depending on temperatures in the springtime. Many of the dates in the 1100s and 1200s often show early blossoms. This was during a warm period over much of the globe known as the medieval optimum.

The climate later took a downturn, and from the mid1600s to the early 1800s the blossoms tended to be later, during a cooler period called the Little Ice Age. Since about 1830 the dates have been growing earlier and in recent decades they have been the earliest since the records began.