People living in the northern hemisphere have experienced extreme heat in recent months, while temperatures in the southern hemisphere have been exceptionally low. The extreme weather may be related with sunspot activity, said Yu Zhihao, an expert in meteorology from Nanjing University.

The sun was relatively dormant, with little sunspot activity from 2007 till this July, according to experts at Nanjing's Purple Mountain Observatory. The solar activity cycle, measured by the rise and fall in the number of sunspots, lasts about 11 years.

Solar activity reached a peak in 2000, when the Wolf index of sunspot activity hit 119.6. Since then sunspots decreased year by year with index values lower than 10 seen since 2007. It is the first time that the sun has passed through such a lengthy dormant phase in the past hundred years.

Climate on the Earth depends on atmospheric circulation and the sun provides the energy that powers the circulation. Climate fluctuations also follow an 11-year cycle, similar to the solar cycle. Atmospheric circulation exchanges water vapor and heat in the northern and southern hemispheres to reach a balance when solar activities are normal. Extreme weather results from unusual atmospheric circulation patterns that interfere with the normal exchange of water vapor and heat between the hemispheres, said Yu.

But, Li Hui, a researcher with the Purple Mountain Observatory, disagreed with Yu. It was abnormal for the sun to such low levels of activity in recent years, Li said, but the intensity of illumination and heat transmitted from the sun to the Earth did not change dramatically. There is no definite evidence of the link between sunspots and climate fluctuations, Li said.