Volcanic Eruptions Contribute to a Slowdown in Global Warming, Study

Researchers say that gases from erupting volcanoes have contributed to the recent cooling of the earth's surface and most climate models have ignored the effects of this volcanic activity.

During the 1990s scientists noticed over two decades of rapid global warming and predicted no change in the trend. But later, researchers were surprised to see that despite the rise in the emission of greenhouse gases, the surface temperature of the earth has remained stable for the last 15 years. In order to solve this mystery, the researchers focused on the heat entering into the ocean, alterations in the wind patterns and various other factors. 

A team of scientists from the Massachusetts Institute of Technology and the Lawrence Livermore National Laboratory discovered that this temporary respite was offered by the volcanic eruptions that occurred during the early part of the 21^st century. This warming 'hiatus' (also called as slow-down) was confirmed by the International Panel on Climate Change.

"This is the most comprehensive observational evaluation of the role of volcanic activity on climate in the early part of the 21st century," co-author Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT, said in a news statement. "We assess the contributions of volcanoes on temperatures in the troposphere - the lowest layer of the atmosphere - and find they've certainly played some role in keeping the Earth cooler."

The big explosions are known to emit sulphur dioxide into earth's atmosphere. If the explosions are really large, they are capable of placing sulphur dioxide into the stratosphere and the gas gets converted into small droplets of sulphuric acid called volcanic aerosols. These droplets return a part of the sunlight back to space, thereby cooling the temperatures of the earth's atmosphere as well as the lower atmosphere.

"In the last decade, the amount of volcanic aerosol in the stratosphere has increased, so more sunlight is being reflected back into space," said Lawrence Livermore climate scientist, Benjamin Santer, lead author of the study. "This has created a natural cooling of the planet and has partly offset the increase in surface and atmospheric temperatures due to human influence."

Since the industrial revolution there has been a rapid increase in the amount of greenhouse gases emitted into the atmosphere. This change triggers the troposphere to warm up, cooling the stratosphere. The reverse of this is noticed during large volcanic eruptions; they cause the troposphere to cool and the stratosphere to warm.

"The recent slow-down in observed surface and tropospheric warming is a fascinating detective story," Santer said. "There is not a single culprit, as some scientists have claimed. Multiple factors are implicated. One is the temporary cooling effect of internal climate noise. Other factors are the external cooling influences of 21st century volcanic activity, an unusually low and long minimum in the last solar cycle, and an uptick in Chinese emissions of sulfur dioxide. The real scientific challenge is to obtain hard quantitative estimates of the contributions of each of these factors to the slow-down."

In order to confirm whether these 21^st century volcanic activities led to the cooling of the surface, the researchers conducted various statistical tests, differentiating their effect from the intrinsic variability of the climate. The team spotted a significant link between volcanic aerosol observations and the estimations made using satellite data of the lower troposphere temperature and the sunlight sent back to space.

The scientists noticed that volcanoes play a crucial role in keeping the earth cooler.

The finding was documented in Nature Geoscience.