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Volcanic eruptions may be behind the slowdown in global warming over the past 15 years, a study claims.

US scientists believe small volcanoes are spewing sulphur aerosols into the upper atmosphere that reflect sunlight away from the Earth.

Researchers have long known that volcanoes can protect against global warming, but they did not think that minor eruptions did much to the atmosphere.

The Sarychev Peak Volcano, on Matua Island, erupted on June 12, 2009. New research shows that eruptions of this size may contribute more to the recent lull in global temperature increases than previously thought

The latest findings, by researchers at Massachusetts Institute of Technology, show that small volcanic eruptions have deflected almost twice the amount of solar radiation previously estimated.

'By knocking incoming solar energy back out into space, sulphuric acid particles from these recent eruptions could be responsible for decreasing global temperatures by 0.05 to 0.12°C (0.09 to 0.22°F) since 2000,' said the study.

'These new data could help to explain why increases in global temperatures have slowed over the past 15 years, a period dubbed the "global warming hiatus".'

The warmest year on record was in 1998, and although recent years have been warmer than the 20th century average, the steep climb seen in the 1990s has levelled off.

In April, the IPCC said it is '95 per cent' certain that climate change is man made, but still could not explain why the world has barely got any hotter in the last 15 years. Now researchers claim its due to volcanic eruptions. Pictured is the change in average sea surface temperatures from 1901 - 2012

A number of theories exist on why the globe is experiencing a warming hiatus, including changes in the way heat is absorbed by the ocean or a period of weak solar activity.

But most climate projections do not factor in volcanic eruptions because they are so hard to predict.

However, large ones like the 1991 Mount Pinatubo eruption in the Philippines, which emitted some 44 billion lbs (20 million metric tons) of sulphur, are believed to have impacted global climate.

David Ridley, an atmospheric scientist at MIT in Cambridge, felt that a piece of the climate puzzle was missing.

According to the study, he located it in the intersection of the stratosphere and the troposphere - the bottom layer of the atmosphere- where all weather takes place.

The two layers meet between six to nine miles (10 and 15km) above the Earth, and are below the reach of most satellites.

'The satellite data does a great job of monitoring the particles above nine miles (15km), which is fine in the tropics,' Professor Ridley said.

'However, towards the poles we are missing more and more of the particles residing in the lower stratosphere that can reach down to six miles (10km).'

The study combined observations from ground, air and space-based instruments to better observe aerosols in the lower portion of the stratosphere, and found that there are more aerosols that previously thought.

Experts say future climate models need to incorporate better aerosol data, which can only be obtained with a more robust monitoring system for stratospheric aerosols.

The latest findings, by researchers at Massachusetts Institute of Technology, show that small volcanic eruptions have deflected almost twice the amount of solar radiation previously estimated. Pictured the Tavurvur Volcano at Papua New Guinea