Global warming that has slowed down over the last 15 years is partly the result of many small volcanic eruptions.

At top August 2014 eruption of Tavurvur in Papua New Guinea. LLNL

Scientists have long known that volcanoes cool the atmosphere due to volcanic gases especially sulfur dioxide that is expelled during eruptions and iron carried in volcanic ash that nourishes ocean plankton pastures downwind. A hazy cloud of sulfuric acid that forms when the volcanic 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.

Likewise, volcanic ash carries iron that is missing in the sea, feeding ocean pastures making them rich and green. That lush green plankton, like grass of pastures or forests on land, is made of CO2 scrubbed out of the air by photosynthesis and made into ocean plant life, carbon-rich biomass. Previous research suggested that early 21st century eruptions might explain up to a third of recent slowing global warming effects but this new research shows the effect is even greater.

Research just published in Geophysical Research Letters (GRL) further illuminates observed climate signals caused by recent volcanic activity. This new research complements and follows 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.

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

The warmest year on record is 1998. After that, the steep climb in global surface temperatures observed over the 20th century appeared to slow down and level off. This “hiatus” received considerable attention, despite the fact that the full observational surface temperature record shows many instances of both slowing and acceleration in warming rates. Scientists had previously postulated that factors such as weak solar activity and increased heat uptake by the oceans might be responsible for the recent lull in temperature increases. After the publication of a 2011 paper in the journal Science by Susan Solomon of the Massachusetts Institute of Technology (MIT), it was recognized that the uptick in volcanic activity might also be implicated in the “warming hiatus.”

Prior to the 2011 Science paper, the prevailing scientific misinterpretation was that only very large eruptions – on the scale of the cataclysmic 1991 Mount Pinatubo eruption in the Philippines. Pinatubo, classed as a colossal volcano ejected 10 billion tonnes of ash and rock and 20 million metric tons of sulfur – were capable of impacting global climate.

The former orthodox dogma was largely based on flawed climate model simulations of the sulfur effects according to David Ridley, an atmospheric scientist at MIT and lead author of the November GRL paper. There remains more work to do to upgrade the model to include the reduction of global warming CO2 due to enhanced ocean pasture productivity.

Ridley and colleagues found the missing piece of the sulfur puzzle at the intersection of two atmospheric layers, the stratosphere and the troposphere – the lowest layer of the atmosphere, where all weather takes place. Those layers meet between 10 and 15 kilometers (six to nine miles) above the Earth.

Satellite measurements of the sulfuric acid droplets and aerosols produced by erupting volcanoes are generally restricted to above 15 km. Below 15 km, cirrus clouds can interfere with satellite aerosol measurements. This means that toward the poles, where the lower stratosphere can reach down to 10 km, the satellite measurements miss a significant chunk of the total volcanic aerosol loading.

To get around this problem, the study by Ridley and colleagues combined observations from ground-, air- and space-based instruments to better observe aerosols in the lower portion of the stratosphere. 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.

The second Livermore-led study shows that the sulfur signals of these late 20th and early 21st eruptions can be positively identified in atmospheric temperature, moisture and the reflected solar radiation at the top of the atmosphere. A vital step in detecting these volcanic sulfur signals is the removal of the “climate noise” caused by 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,” 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.”

It is hoped that the LLNL scientists will speed up their work to include iron driven photosynthesis feedback mechanisms that directly and dramatically reduce atmospheric CO2 into their models.