There is no question that global warming hasn’t increased at the expected pace. To date, however, no one has been able to say why. Now MIT’s David Ridley and colleagues believe they have an answer.

Researchers have known for some time that volcanic activity has a cooling effect on the Earth. During an eruption sulfuric acid combines with oxygen in the upper atmosphere and reflects sunlight away from the earth. This effect can last for several months. Low level volcanic activity and small eruptions, however, are too unpredictable and frequent to be added to climate models.

Ridley and his colleagues used air and satellite readings to measure the impact of small eruptions which occurred between 2000 and 2013. They demonstrated that the accumulated impact of these eruptions double the amount of solar radiation deflected back into space.

This extra shielding could be responsible for a temperature decrease of 0.09 to 0.22 degrees Fahrenheit since 2000. Large volcanos, like the 1991 Mount Pinatabu eruption in the Philippines, which ejected 20 million metric tons of sulfur, have been included in climate models but those models did not account for the hiatus.

“The prediction of global temperature from the [latest] models indicated continuing strong warming post-2000, when in reality the rate of warming has slowed,” said Ridley, in a statement.

The additional sulfuric acid was found at the intersection of the stratosphere and the troposphere, 6 to 9 miles above the Earth. The troposphere is the lowest level of the atmosphere where weather takes place. This region is difficult to measure because ordinary water vapor can obscure measurements. The researchers got around this by combining ground, air and space based instruments.

“The satellite data does a great job of monitoring the particles above 15 km, which is fine in the tropics. However, towards the poles we are missing more and more of the particles residing in the lower stratosphere that can reach down to 10 km,” said Ridley.

If the researchers, whose work is published in Geophysics Research Letters, are correct it could mean good news for global warming but bad news from other areas of the environment. While volcanic activity may reduce the pace at which the Earth’s climate is warming, it increases air pollution and causes acid rain.

However, research published in January 2013 showed that an increase in global temperatures and rising sea levels can lead to increased volcanic activity, which in turn cools the atmosphere.

Ultimately, volcanoes are not a good solution to climate change. Too much volcanic activity is thought to have caused the end-Permian mass extinction, known as the Great Dying, when the temperature became too cole and the air and water toxic.