Small volcanic eruptions partly explain 'warming hiatus'

Deep ocean water has continued to get warmer during this time. The "hiatus" refers to surface air temperatures.

http://www.eurekalert.org/pub_releases/2015-01/dlnl-sve010915.php

Public Release: 9-Jan-2015
DOE/Lawrence Livermore National Laboratory

The "warming hiatus" that has occurred over the last 15 years has been partly caused by small volcanic eruptions.

Scientists have long known that volcanoes cool the atmosphere because of the sulfur dioxide that is expelled during eruptions. Droplets of sulfuric 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 available online in the journal Geophysical Research Letters (GRL) further identifies observational climate signals caused by recent volcanic activity. This new research complements an earlier GRL paper published in November, which relied on a combination of ground, air and satellite measurements, indicated that a series of small 21st century volcanic eruptions deflected substantially more solar radiation than previously estimated.

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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. 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. After publication of a 2011 paper in the journal Science by Susan Solomon of the Massachusetts Institute of Technology (MIT), it was recognized that an uptick in volcanic activity might also be implicated in the "warming hiatus."

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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.

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"The fact that these volcanic signatures are apparent in multiple independently measured climate variables really supports the idea that they are influencing climate in spite of their moderate size," said Mark Zelinka, another Livermore author. "If we wish to accurately simulate recent climate change in models, we cannot neglect the ability of these smaller eruptions to reflect sunlight away from Earth."