This year looks set to be the warmest on record, and continues a continuous trend of increasing temperatures over the last 50 or 60 years.

However, while there is often focus on the long term trend, we don't often stop to think about what causes year-to-year variations in temperature - why might one year bit a bit cooler and the next particularly warm?

This article on the BBC website, written by researchers at the UK Met Office Hadley Centre introduces some of the mechanisms that are responsible for year-to-year variations in global mean temperature - ENSO and volcanic eruptions are two of key phenomenon.