BBC NEWS | Science & Environment | What happened to global warming?

"The oceans, he says, have a cycle in which they warm and cool cyclically. The most important one is the Pacific decadal oscillation (PDO).

For much of the 1980s and 1990s, it was in a positive cycle, that means warmer than average. And observations have revealed that global temperatures were warm too.

But in the last few years it has been losing its warmth and has recently started to cool down.

These cycles in the past have lasted for nearly 30 years."