Volcanoes may be partly responsible for a lack of noticeable global warming over the last few years.
Researchers at Lawrence Livermore National Laboratory believe several eruptions since the year 2000 have served to cool down the planet. These events could offset warming caused by greenhouse gases, according to the study.
Levels of greenhouse gases in the atmosphere have risen since 1998. Despite this, average temperatures of the land surface and the heat content of the oceans have remained relatively unchanged. The lowest level of the atmosphere, the troposphere, has also maintained a relatively constant temperature.
Fairly stable temperatures for the last 15 years has attracted significant interest from the scientific community, along with the press and general public. Between 1970 and 1998, average temperatures rose by nearly one-third of a degree Fahrenheit per decade. Since then, temperatures have changed by a miniscule 0.072 degrees every ten years.
When volcanoes erupt, they can push large quantities of sulfur dioxide, the gas that gives rotten eggs their distinctive smell, into the stratosphere. This is the layer of the atmosphere directly above the troposphere, where the highest-flying jets cruise. While there, the gas converts into small pellets of sulfuric acid, called volcanic aerosols. The years between 1998 and 2012 saw 17 medium-sized volcanic eruptions.
"In the last decade, the amount of volcanic aerosol in the stratosphere has increased, so more sunlight is being reflected back into space. This has created a natural cooling of the planet and has partly offset the increase in surface and atmospheric temperatures due to human influence," Benjamin Santer, lead author of the study, said.
Normally, increased emissions of greenhouse gases at the Earth's surface cause temperatures in the troposphere to rise, while cooling the stratosphere. Volcanic eruptions are bringing lower temperatures near the ground, while warming air ten miles above the surface.
Several other factors also come into play when determining average temperatures over the last decade and-a-half. These include an unusually low number of sunspots. The number of these features recorded on our home star over the last decade is the lowest in a century. Because of this, the Sun is not warming our planet quite as much as it would under normal conditions. Factories in China are also emitting ever-greater quantities of sulfur dioxide, contributing to cooling.
"This is the most comprehensive observational evaluation of the role of volcanic activity on climate in the early part of the 21st century. We assess the contributions of volcanoes on temperatures in the troposphere... and find they've certainly played some role in keeping the Earth cooler," Susan Solomon, MIT professor and co-author of the study, said.
The study was funded by the Department of Energy.
