Washington: The "warming hiatus" that has occurred over the last 15 years has been caused in part by small volcanic eruptions, according to a new study.
Scientists have long known that volcanoes cool the atmosphere because of the sulphur dioxide that is expelled during eruptions.
Droplets of sulphuric acid that form when the gas combines with oxygen in the upper atmosphere can persist for many months, reflecting sunlight away from Earth and lowering temperatures at the surface and in the lower atmosphere.
Previous research suggested that early 21st-century eruptions might explain up to a third of the recent warming hiatus.
New research further identifies observational climate signals caused by recent volcanic activity.
"This new work shows that the climate signals of late 20th- and early 21st-century volcanic activity can be detected in a variety of different observational data sets," said Benjamin Santer, a Lawrence Livermore National Laboratory scientist and lead author of the study.
The warmest year on record is 1998. After that, the steep climb in global surface temperatures observed over the 20th century appeared to level off, researchers said.
This "hiatus" received considerable attention, despite the fact that the full observational surface temperature record shows many instances of slowing and acceleration in warming rates.
Scientists had previously suggested that factors such as weak solar activity and increased heat uptake by the oceans could be responsible for the recent lull in temperature increases.
David Ridley, an atmospheric scientist at Massachusetts Institute of Technology (MIT) and colleagues found the missing piece of the puzzle at the intersection of two atmospheric layers, the stratosphere and the troposphere - the lowest layer of the atmosphere, where all weather takes place. Those layers meet between 10 and 15 kilometres above Earth.
Satellite measurements of the sulphuric acid droplets and aerosols produced by erupting volcanoes are generally restricted to above 15 km. Below 15 km, cirrus clouds can interfere with satellite aerosol measurements.
This means that towards the poles, where the lower stratosphere can reach down to 10 km, the satellite measurements miss a significant chunk of the total volcanic aerosol loading.
To get around this problem, the study by Ridley and colleagues combined observations from ground-, air- and space-based instruments to better observe aerosols in the lower portion of the stratosphere.
They used these improved estimates of total volcanic aerosols in a simple climate model, and estimated that volcanoes may have caused cooling of 0.05 degrees to 0.12 degrees Celsius since 2000.
The research was published in the journal Geophysical Research Letters (GRL).