It is generally believed that impro

It is generally believed that improper human activities contribute to global warming. But this belief is facing an inconvenient challenge after the release of new temperature data showing the planet has not warmed for the past 15 years. Based on statistics from more than 30,000 measuring stations, the data confirms that the rising trend in world temperatures ended in 1997. The figures also suggest that we could even be heading for a mini ice age comparable to the 70-year temperature drop in the 17th century.Meanwhile, climate scientists said that, after releasing unusually high levels of energy throughout the 20th century, the sun is now heading toward a "grand minimum" in its output (输出). This is likely to cause cold summers, bitter winters and a shortening of the season available for growing food.Solar output goes through 11-year cycles, with high numbers of sunspots (太阳黑子) seen at their peak. We are now at what should be the peak of what scientists call "Cycle 24". But sunspot numbers are running at less than half those seen during cycle peaks in the 20th century. Analysis by experts suggests that Cycle 25, whose peak is due in 2022, will be a great deal weaker still.According to a paper published last week, there is a 92 percent chance that both Cycle 25 and those taking place in the following decades will be as weak as, or weaker than the period of 1790 to 1830. In that period, average temperatures in parts of Europe fell by 2℃. It is also possible that the new solar energy drop could be as deep as what happened between 1645 and 1715 when the canals of Holland froze solid.