In 2013, the world pumped 36 billion metric tons (40 billion US tons) of CO2 into the atmosphere through the burning of fossil fuels. Such emissions form a carbon dioxide blanket that allows the sun to penetrate, but prevents much of the surface heat from reflecting back into space. As a result, the oceans are rapidly warming, arctic sea ice is diminishing, and glaciers and the Greenland Ice Sheet are melting at a record rate.
The world’s surface temperature has steadily increased for the past 150 years, and it was assumed the curve would keep climbing at the same rate. But unexpectedly, global surface temperature peaked to its highest historical level in 1998, then flattened out and has remained about the same since, raising questions in the scientific community.
A Lawrence Livermore National Laboratories study that appeared in the Feb. 23, 2014, issue of the journal Nature Geoscience suggests one reason for this unexpected development is the higher than normal rate of volcanic activity over the past 15 years. Eruptions during that period included 17 ranked VEI 4 on the Volcanic Explosivity Index. A VEI 4 is termed cataclysmic and sends an ash plume 10 to 25km (6 to 15 mi) into the air, sufficient to penetrate the stratosphere with sulfur dioxide aerosols that remain there for months, even years.
“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 on 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.” The paper states the research team found evidence for significant correlations between volcanic aerosol observations and satellite-based estimates of lower temperatures, as well as sunlight reflected back into space by the aerosol particles.
Santer’s conclusions seem to be supported by an earlier study by the University of Saskatchewan. In this study, the researchers found that sulfur dioxide aerosols from a very small African eruption had “hitchhiked” their way into the stratosphere. Warm air rising from the seasonal Asian Monsoon lifted the volcano’s aerosols from the lower atmosphere into the stratosphere, where it was detected by the Canadian Space Agency’s satellite OSIRIS, an instrument specifically designed to measure atmospheric aerosols. Even though coming from a small eruption, the concentration of particles was the largest load of SO2 aerosol ever recorded by OSIRIS in its 10 years of operation.
The Lawrence Livermore paper suggests that one other possible contributor to the temporary cooling effect is the unusually long and low minimum in the solar cycle. Don’t be surprised to see surface temperatures start climbing again when volcanic activity subsides and the cooler phase of the solar cycle concludes.