By analysing stalagmites found in a cave in central–eastern China, US and Chinese researchers have discovered a link between the duration and intensity of the annual monsoon and small variations in the Earth’s orbit.

In a paper published in Nature, lead author Hai Cheng of Nanjing Normal University and colleagues present a 224,000-year history of the East Asian monsoon, created using oxygen isotope ratios from stalagmites from Sanbao Cave in Hubei Province, where the regional climate is dominated by the annual monsoon.

They found that the intensity of the monsoon fluctuates every 23,000 years, in line with climate scientist André L Berger’s calculations on orbital insolation data conducted in 1978, which found that changes in the Earth’s orbit – the angle of tilt and the axis of rotation – cause the amount of incoming solar radiation to vary. The researchers suggest that these changes in insolation lead to long-term changes in monsoon duration and strength.

‘In this paper, we confirm the idea that tropical and subtropical monsoons respond dominantly and directly to changes in Northern Hemisphere summer insolation on orbital timescales,’ said Cheng. ‘One of the implications of this finding is that the present Asian monsoon is relatively weak in comparison to a few thousand years ago and will stay at this level for centuries.’