Volcano eruptions found to have cooled global temperatures since 2000

Eruptions of volcanoes like Tavurvur in Papa New Guinea in 2006 had a greater impact on the climate in the past 15 years than had previously been appreciated and may require climate models to be revised

Small volcanic eruptions over the past 20 years have been protecting the Earth from global warming, according to a new study.

Scientists have confirmed that droplets of sulphur-rich aerosols spewed into the upper atmosphere by volcanoes have been reflecting sunlight away from the Earth.

Until recently it was thought that only particularly large eruptions had any noticeable affect on the climate.

However, the new study has confirmed results from the end of last year that showed these small eruptions can have an accumulative impact on global temperature.

This could have helped decrease the global temperatures by between 0.05°C to 0.12°C over the past 15 years.

Since 1998, the warmest year on record, the steep increase in global temperatures seen during the 1990s has levelled off, failing to match computer model predictions for climate change.

This pause, or hiatus, has been blamed on weak solar activity and increased uptake of heat by the world's oceans.

This graph shows average land and ocean temperature anomalies between 1961 and 1990 around the world

The UN's Intergovernmental Panel on Climate Change last year concluded that the deep oceans had been responsible for absorbing an increasing amount of heat, but warned that this could not continue indefinitely.

However, in a paper published in November last year, atmospheric scientists at the Massachusetts Institute of Technology found that small volcanic eruptions in the early 21st century, which had been largely overlooked, were responsible for up to a third of the hiatus in warming.

Small eruptions from volcanoes like Japan's Mount Ontake throw sulphuric acid and aerosols into the air but many of these may have been accumulating unnoticed as a layer between the troposphere and stratosphere

Now researchers at the Lawrence Livermore National Laboratory, in California, have found signs of the effects from eruptions from the late 20th century and early 21st century in the atmospheric temperature, moisture and amount of sunlight reflected from the atmosphere.

They also found that the eruption of Pinatubo, a volcano on the island of Luzon in the Philippines, which last erupted in 1991, also caused a drop in tropical rainfall.

'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, a climate scientist at Lawrence Livermore and one of the authors of the new study.

The apparent mismatch between observed temperature increases and predictions from climate change models has led to some to claim that global warming has stalled, as seen by the black line in the graph above

'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.'

The findings confirm the work of Dr David Ridley, a climate scientist at MIT who was the lead author on the November study on volcanos.

He found that droplets of sulphuric acid and aerosols were accumulating at the intersection between the stratosphere and troposphere layers of the atmosphere.

Using ground, air and space based instruments, Dr Ridley and his colleagues were able to get a better estimate of the aerosols accumulating in these layers, around six to nine miles above the Earth's surface.

Globally, there are thousands of volcanoes, like this one in Russia, throwing aerosols into the atmosphere

Previously they had been missed as most satellite measurements of volcanic aerosols are restricted to the parts of the stratosphere higher than nine miles as cloud cover can interfere with measurements below that.

This means, particularly around the poles where the stratosphere extends down to six miles above the surface, a significant chunk of volcanic aerosols were being missed.

He estimated that that since 2000 volcanoes may have caused cooling of between 0.05 degrees C and 0.12 degrees C.

Dr Benjamin Santer, who led the new study, said: '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.'

Professor Ross McKitrick, an environmental economist from the University of Guelph in Canada, said the results could mean that climate models that are used to inform policy decisions are inaccurate.

If volcanic eruptions are masking some of the impacts of greenhouse gases like carbon dioxide, then the climate may be more sensitive than previously thought, or changes in aerosols could be responsible for some of the temperature changes attributed to carbon dioxide.

He said: 'If small variations in volcanic activity turn out to have larger and more persistent climatic effects than previously thought, this should have important implications for how climate models are parameterized and how greenhouse gas attribution studies are done.

'The study seems to me to raise the possibility that commonly-used historical reconstructions of volcanic dust forcing may not have been all that reliable.

'Climate modelers rely on dust veil indexes that use measures of known historical volcanic activity to estimate optical effects in the atmosphere.

'These are then used to estimate natural forcings over the 20th century, which are then used in greenhouse gas signal detection (attribution) studies.

'If the historical reconstructions weren't capturing the full magnitude of volcanic effects on the climate, as this study suggests, that presumably means those attribution studies need re-examination as well.'

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