How small volcanic eruptions may have slowed surface temperature rise

Yesterday, the Daily Mail reported on new research that suggests small volcanic eruptions have a bigger effect on the climate system than scientists previously thought.

The  new study, just published in Geophysical Research Letters, shows the impact of 20th and 21st century volcanic eruptions can be detected in a number of climate variables, including sea surface temperature, rainfall, tropospheric temperatures and atmospheric water vapour.

This reinforces the findings of a  paper Carbon Brief  covered in November last year. Both studies could help explain why recent warming at the Earth’s surface has been slower than in previous decades. Here’s a repost of the original article we wrote back in November.

A cataclysmic event

In June 1991 Mount Pinatubo in the Philippines erupted, sending a cloud of ash, dust and sulphur dioxide 35 kilometers into the atmosphere.

That sulphur dioxide combined with oxygen and water to form sulphuric acid  aerosols. These particles reflected sunlight and encouraged clouds to form, cooling parts of the world by up to 0.4°C for two years after the eruption.

Volcanic eruptions are rated from zero to eight on a scale of  explosivity, measured by the amount of ash and debris they produce. The Pinatubo eruption was rated as a six, or ‘colossal’.

While the world hasn’t seen such a huge volcanic eruption since, on average there is one small eruption somewhere in the world every week. A new study, published in  Geophysical Research Letters, finds that these smaller eruptions may together have a bigger impact on global climate than previously thought.

Mount Pinatubo eruption in June 1991. US Geological Survey.

Are volcanoes slowing surface warming?

Over the last 15 years or so, surface temperatures have risen more slowly than in previous decades, despite greenhouse gases emissions rising faster than ever.

Scientists think there are probably a number of reasons for this, the dominant one being more heat finding its way to the deep ocean instead of staying at the surface where it can warm the atmosphere.

But studies also suggest that aerosols from volcanic eruptions and human activities, along with a drop in solar activity, are contributing to the slowdown.

The new research suggests that the effect of volcanoes may be larger than previously thought, because scientists tend to overlook small volcanic eruptions that may be adding aerosols to the atmosphere.

New measurements

The researchers collected readings of atmospheric aerosol levels from 2000 to 2013, from instruments on the ground as well as weather balloons and satellites.

Clouds can prevent satellites from measuring aerosols accurately, the researchers say, particularly over mid-latitude and polar regions. This means satellites often underestimate the amount of aerosols in the lower layers of the atmosphere.

By supplementing satellite measurements with other records, the researchers found aerosols that the satellites had missed.

They conclude that atmospheric aerosols from minor eruptions since 2000 are reflecting almost double the amount of the sun’s energy than earlier estimates. That would mean aerosols have a bigger cooling effect on the climate than scientists previously thought.

Improving climate models

This is important because this knowledge could be added to climate models to improve how realistically they represent the climate system, which makes for more reliable projections.

The climate models behind the projections in the latest Intergovernmental Panel on Climate Change report don’t include recent small volcanic eruptions in their calculations, for example.

As a test, the researchers added their improved aerosol measurements to a climate model to see the difference it would have made over the past 14 years. Their simulations of the climate show global temperatures of between 0.05 and 0.12°C lower than without the aerosols.

The new findings could be another piece of the puzzle in explaining the slower rise in surface temperatures in recent years. And this can help scientists’ understand how the climate system behaves over short timescales, and improve model projections of temperature change over the longer term.

Main image: Volcano erupting in Iceland.