It involves a degree of variation and improvisation in both vocal parts but always ends with two performers singing in unison or an octave apart.

UNESCO: Culture

"It is also my view that there is too great a degree of variation of performance and that, were we able to reduce this, the effect on children, teachers and schools would be profound, " he said.

BBC: School action urged on behaviour

The research shows that a one-degree temperature variation that took 11 millennia to occur since the end of the last major ice age has now been replicated in 150 years, since the early days of the Industrial Revolution.

WSJ: Earth Hotter Now Than Most of Past 11,000 Years

It's also worth chucking into the mix that the global average temperature variation between even the strongest El Nino event and the next ordinary year is a fraction of a degree Celsius - certainly less than the rise of about 0.6C that we've seen since the beginning of the study period, in 1950.

BBC: Warm, wet - and warring?