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.
So what makes one year warmer or colder than its predecessor? The answer to this depends on a number of factors. Large volcanic eruptions can cause a temporary dip in temperatures for a few years due to reflection of sunlight by the resulting atmospheric particles. This happened following the eruption of Mt Pinatubo in 1991, for example. Fluctuations in the ocean can also cause temporary ups and downs, and the massive heat release during an El Nino event in the tropics can temporarily raise global temperatures, such as occurred in 1998. This year, the tropical ocean has indeed warmed up in both the Pacific and the Atlantic but both of these ocean basins have also been very warm in northern latitudes, compounding the effect on global temperature.