Volcanic eruptions played role in global warming hiatus

Chuck Bednar for redOrbit.com – Your Universe Online

Small volcanic eruptions could have played a role in the so-called “warming hiatus” that has taken place over the past 15 years, Lawrence Livermore National Laboratory researchers report in a recent edition of the journal Geophysical Research Letters.

Scientists have long known that volcanoes can cool the atmosphere by releasing sulfur dioxide during eruptions, the study authors explained. When that gas combines with oxygen in the upper atmosphere, it can cause droplets of sulfuric acid to form and persist for months at a time.

As a result, sunlight is reflected away from the Earth, lowering temperatures both in the lower atmosphere and at the planet’s surface. Previous studies have suggested that eruptions that took place in the early 21st century could explain up to one-third of the recent warming hiatus.

The newly published research, the authors noted, further identifies observational climate signals caused by recent volcanic activity. The new paper complements one published in November that used a combination of ground, air and satellite measurements to demonstrate that several small 21st-century eruptions deflected significantly more solar radiation than was expected.

“This new work shows that the climate signals of late 20th- and early 21st-century volcanic activity can be detected in a variety of different observational data sets,” said Benjamin Santer, lead author of the study and a scientist at the Lawrence Livermore National Laboratory.

The warmest year ever recorded was 1998, and in the years that followed, the trend of rising global surface temperatures observed throughout the 20th century appeared to start leveling off, the researchers explained. This hiatus has received a tremendous amount of attention, they said, even though the full observational record shows instances of varying warming rates.

Scientists had previously suggested that weak solar activity and increased heat uptake by oceans could be at least partially responsible for the recent slowdown in temperature increases. Research published in 2011, however, first suggested that increasing volcanic activity could also play a role, and that it very large eruptions weren’t the only ones contributing to the phenomenon.

The authors of that study found that the intersection of two atmospheric layers, the stratosphere and the troposphere, hold amounts of sulfuric acid often missed by satellite measurements. Since this region of the atmosphere is where whether occurs, that means that data used in simulations that were obtained from space could be missing a significant amount of this volcanic material.

By combining ground-, air- and space-based instruments, the authors of the 2011 study were able to get a clearer look at volcanic aerosols in the lower portion of the atmosphere. They used this information to improve estimates of volcanic aerosols in a simple climate model, and estimated that eruptions may have caused cooling of 0.05 degrees to 0.12 degrees Celsius since 2000.

The more recent study indicates that the signals of these late 20th and early 21st eruptions could be positively identified in atmospheric temperature, moisture and the reflected solar radiation at the top of the atmosphere. One of the primary reasons they were better able to detect these signals was the removal of “climate noise” caused by phenomena such as El Niños and La Niñas.

“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,” explained study author Mark Zelinka, also of the Livermore Lab. “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.”

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