Global Warming 'pause' May Last Until 2025
  
The hiatus in the rise in global temperatures could last for another 10 years, according to new research.

Scientists have struggled to explain the so-called pause that began in 1999, despite ever increasing levels of CO2 in the atmosphere.

The latest theory says that a naturally occurring 30-year cycle in the Atlantic Ocean is behind the slowdown.

The researchers says this slow-moving current could continue to divert heat into the deep seas for another decade.

However, they caution that global temperatures are likely to increase rapidly when the cycle flips to a warmer phase.

According to the Intergovernmental Panel on Climate Change (IPCC), global average temperatures have increased by around 0.05C per decade in the period between 1998 and 2012.

This compares with a decadal average of 0.12 between 1951 and 2012.

More than a dozen theories have been put forward on the cause of this pause in temperature growth that occurred while emissions of carbon dioxide were at record highs.

These ideas include the impact of pollution such as soot particles that have reflected back some of the Sun's heat into space.

Increased volcanic activity since 2000 has also been blamed, as have variations in solar activity. 

The researchers say that there was another hiatus between 1945 and 1975 due to this current taking down the heat, that led to fears of a new ice age.

From 1976 though, the cycle flipped and contributed to the warming of the world, as more heat stayed on the surface.

But since the year 2000, the heat has been going deeper, and the world's overall temperatures haven't risen beyond the record set in 1998.

"The floats have been very revealing to us," said Prof Tung.

"I think the consensus at this point is that below 700 metres in the Atlantic and Southern oceans [they are] storing heat and not the Pacific."

A key element in this new understanding is the saltiness of the water. The waters in the Atlantic current coming up from the tropics are saltier because of evaporation. This sinks more quickly and takes the heat down with it. 

Eventually though, the salty water melts enough ice in Arctic waters to lower the saline level, slowing down the current and keeping the heat near the surface.

"Before 2006 the saltiness was increasing, this indicated that the current was speeding up," said Prof Tung.

"After 2006, this saltiness is diminishing but it's still above the long-term average. Now it is slowly slowing down.

"Once it gets below the long-term average, then it is the next period of rapid warming."

BBC