Extreme hot weather is currently being experienced throughout the northern hemisphere in countries such as China, Japan, the United States and Russia. But climate experts can't be sure if global warming is responsible.

Scientists say the present hot weather is the result of several natural factors and not simply evidence of global warming. They say the heat waves neither prove nor deny the existence of global warming because meteorology is a statistical science requiring substantial and coherent data.

They point out the only credible signal of a warming planet is for such hot weather to repeat annually, or at least every five years, for three decades.