The 24 'solar terms', a set of timings reflecting the seasonal cycle, were laid down 2,000 years ago as a supplement to the traditional Chinese calendar. They are marked on the calendar under specific dates — two terms per month — and refer to agricultural activities.
The terms, which mark seasonal transitions and indicate the stages crop growth, include 'great cold', 'great heat', 'rain water', 'waking of insects' and 'grain in ear'.
They are used alongside modern agro-meteorological data to plan agricultural activities. In rural areas, many proverbs and songs associated with the terms are also used.
But now, Chinese meteorologists say the traditional divisions no longer apply because they do not take climate change into account.
The researchers collected air temperature data from 549 meteorological stations across the country from 1960 to 2008 to determine how the mean temperatures indicating seasonal transitions have changed.
They found that the mean temperature in China has risen substantially, with spring and summer starting earlier, and autumn and winter having a later onset. Summer has lengthened by 15 days, while winter has shortened by the same amount.
Such a dramatic change means the farming practices associated with the solar terms need to be adjusted, they say. The number of cold days for 'great cold' has reduced by 56.8 per cent over the past ten years compared with the 1960s, for example, whereas the number of hot days for 'great heat' has increased by 81.4 per cent.
The researchers conclude that this warming tendency has affected solar terms in spring and summer more than those in autumn and winter. 'Rain water' has the clearest warming trend, with a rise of 2.43 degrees Celsius from 1961 to 2007.
They suggest bringing forward the agricultural activities associated with the spring and summer solar terms, especially for 'waking of insects', 'pure brightness', 'grain full' and 'grain in ear'. In some semi-arid places in northern China, these four terms occur up to 16 days earlier now than in the 1960s.
But Qian Cheng, a meteorologist at the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing, the study's principal investigator, told SciDev.Net the research "only provides a reference for farming from the perspective of climate change".
Luo Weihong, an agricultural meteorologist at Nanjing Agricultural University, said the research has practical significance in guiding farming activities in China. However, the recommended planning method is based solely on temperatures, she said.
A more scientific approach, she says, is to observe plant phenology, such as flowering and fruiting, rather than simply following the 24 solar terms.
The research was published in the Chinese Science Bulletin in January.
Chinese Science Bulletin doi: 10.1007/s11434-011-4724-4 (2012)