Over the last few years, many possible explanations have been bandied about for the so-called pause in climate change, a plateau in global surface air temperatures that is out of step with rising greenhouse gas concentrations. But now an international research effort is laying responsibility at the feet of volcanic eruptions, whose particles it has found reflect twice as much solar radiation as previously believed, serving to temporarily cool the planet in the face of rising CO2 emissions.
It has long been known that volcanic eruptions impact the climate, spewing ash and sulfur-rich particles into the atmosphere and blocking out the warmth of the sun. These eruptions had been factored into climate modeling, though it is now the view of an international group of scientists that their influence has been majorly understated.
Scientists from Sweden's University of Lund led a team involving the NASA Langley Research Center, Germany's Karlsruhe Institute of Technology (KIT) and the Royal Netherlands Meterological Institute. Its work sought to explain the negligible rise in average surface temperatures over the northern mid-latitude continents in the first decade of the 21st century.
The scientists drew on data collected over this area in the tropopause region, the border in the Earth's atmosphere that separates the troposphere and stratosphere. They used a two-part strategy to carry out their new study.
One part involved calling on samples and measurements routinely carried out by the In-service Aircraft for a Global Observing System (IAGOS) project, a Europe-based research initiative that monitors the composition of our atmosphere. The project has been measuring trace gases and aerosol particles in the tropopause region since 1997 by flying modified Airbus A340-600s loaded with scientific instruments through the area. Samples are collected and analyzed on the ground using ion beam accelerators to measure quantities of sulfur particles.
Combining and analyzing data from the two sources led the scientists to conclude that, while the impact of volcanic eruptions was small between 1999 and 2002, between 2005 and 2012 they had a big influence. They point to three eruptions in particular: Kasatochi, USA in August of 2008, Sarychev, Russia in June of 2009 and Nabro, Eritrea in June of 2011. They say that each belched out more than one megaton of sulfur dioxide into the atmosphere, where it converts into sulfur particles.
"The cooling effect of volcanic eruptions was underestimated in the past, because the lowest part of the stratosphere was mostly not considered.," says Dr Sandra M. Andersson from the University of Lund, one of the study's lead authors.
The scientists describe a range of factors determining whether or not volcanic eruptions go onto to shape the climate globally, including the amount of sulfur dioxide that is emitted. They found that the latitude of the eruption also plays a part, because air in the stratosphere in the Northern Hemisphere flows independently of that in the Southern Hemisphere. Therefore only volcanic eruptions that occur close to the equator can disseminate particles over both hemispheres. The research also uncovered evidence that the time of year is also a factor, with eruptions taking place in the summer, where solar radiation is stronger, found to have a bigger impact.
The reaearch was published in the journal Nature Communications.