Cherry blossom season officially began in Tokyo this year on March 21 -- five days ahead of schedule and a full week earlier than the average for the last 30 years of the 20th century.

Far from being a freak occurrence, the phenomenon of early blossoming has been happening for several years, according to the Japan Meteorological Agency (JMA).

Traditionally, the first sakura -- cherry tree -- flowers appear in the second half of March on the southern islands of the Japanese archipelago and advance slowly up the central island of Honshu towards the far north.

However, according to the JMA, the "blossoming line" -- the latitude where trees start to flower on a given day -- on April 1, which 40 years ago was in the south of Honshu, is now about 200 kilometres (125 miles) further north.

This change, according to JMA climate expert Takashi Yoshida, is caused "by a warming climate and urbanisation."

City temperatures are noticeably higher than those in the countryside, say experts.

They point to the warming effects of cars, heaters and air-conditioners, coupled with the absence of open spaces and the concentration of materials that absorb the sun's heat, such as tar on roads.

Nobuyuki Asada, a member of the Japan Cherry Blossom Association, says meteorological changes mean the future for the trees does not look good.

"With the change in temperatures and a more erratic rainy season, I am not sure that we will still have cherry trees in 50 or 100 years," he said, adding that many trees were "not blossoming as well as they used to."

Every year the JMA tries to predict the exact dates that the trees will come into flower across Japan.

These predictions are crucial in a country where hanami -- flower-viewing parties at which lavish picnics are consumed, accompanied by beer or sake -- are planned weeks in advance among friends or by businesses looking to boost employee morale.

The blooms, which have been a source of inspiration for poets through the centuries, are a sign for most Japanese that winter is over and spring has arrived.

"Since childhood, I have felt a sense of well-being by contemplating the sakura," said 76-year-old Sumiko as she walked along an avenue of cherry trees in Tokyo's Naka-meguro district.

"When I came here more than 40 years ago, the sakura flowered around April 10, not March 20 or 25."

Japan, despite hosting the most famous conference on climate change in 1997, has struggled to set an example to the world.

According to the Kyoto Protocol, by 2012, the country is supposed to reduce its greenhouse gas emissions by six percent from 1990 levels.

But the latest figures show Asia's largest economy is still producing 9.2 percent more greenhouse gases than it was in 1990.

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