Small volcanic eruptions could be slowing global warming by spewing sulfur aerosols that reach the upper atmosphere and reflect sunlight away from the Earth, US scientists said Tuesday.

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 latest findings in the journal Geophysical Research Letters 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, sulfuric acid particles from these recent eruptions could be responsible for decreasing global temperatures by 0.05 to 0.12 degrees Celsius (0.09 to 0.22 degrees Fahrenheit) 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 90s has leveled off.

A variety 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.

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 20 million metric tons (44 billion pounds) of sulfur, are believed to have impacted global climate.

David Ridley, an atmospheric scientist at the Massachusetts Institute of Technology 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 10 and 15 kilometers (six to nine miles) above the Earth, and are below the reach of most satellites.

"The satellite data does a great job of monitoring the particles above 15 kilometers, which is fine in the tropics," 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 10 kilometers."

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.