Smaller volcanic eruptions may be cooling Earth more than expected, says study

Monday, November 24, 2014, 3:21 PM - It's well known that big volcanic eruptions help to cool the Earth, but an international team of researchers is reporting that recent smaller eruptions - like that of the Sarychev Peak Volcano, pictured above - have been having a much more substantial effect on global warming, slowing it by nearly twice as much as previously estimated.

The Earth has maintained a fairly careful balance for millennia - absorbing short-wave radiation (UV and visible light) from the Sun, heating up, and then radiating that heat away as long-wave (infrared) radiation. Rather than losing heat just as quickly as it's absorbed, though, the planet keeps a bit of it in storage, so to speak, partly due to the small amount of naturally-occurring greenhouse gases (carbon dioxide, methane, etc) in the lowest level of the atmosphere - the troposphere. This has kept a fairly comfortable surface temperature (overall) for the various forms of life that exist here.

Courtesy: Wikimedia Commons

Other gases in the atmosphere are capable of substantially cooling the planet, though. One of these is sulphur dioxide, which is released by volcanic eruptions. This gas, when it's blasted out into the atmosphere, combines with water and oxygen to produce sulphur aerosols - tiny particles that are highly-reflective to short-wave radiation from the Sun. It's been shown time and again that big volcanic eruptions, like El Chichon in the 1980s and Mount Pinatubo in 1991, are followed by a period of cooling for the planet. This is directly due to these sulphur particles forming in the stratosphere, the layer of the atmosphere just above the troposphere. Basically, by blocking more short-wave radiation from reaching the surface, there is less long-wave radiation produced for the greenhouse gases to absorb. The effect of this has even been strong enough to slow down the progress of global warming that we're seeing due to the increased amount of greenhouse gases in the atmosphere due to human activity.

The effects of smaller eruptions have mostly been ignored up until now, since it was thought that they wouldn't pack enough of a punch for their sulphur emissions to reach the stratosphere. Compounding the problem is the fact that it's difficult to get good measurements of the lower stratosphere the further away you are from the equator, due to two factors - how satellites take measurements as they orbit and the overall 'shape' of Earth's atmosphere.

CALIPSO and other satellites scan the atmosphere. Credit: CNES

Satellites used to detect sulphur aerosols are designed to focus their measurements so that they don't extend deeper into the atmosphere than about 15 kilometres above the ground - at the level of the tropopause, where the troposphere ends and the stratosphere begins. This avoids the problem of data contamination, since extending readings lower than that would also pick up the tiny, reflective water droplets in clouds too. Since the satellite orbits at about the same height around the planet, maintaining this constant focus is easy. 

However, the depth of the atmosphere, and thus the height of the tropopause, isn't uniform all over the world. Since warmer air takes up more space than cooler air, the atmosphere is deeper near the equator than at the poles.

Thus, while the tropopause near the equator is at that 15 km height, but it's located at around 10 kilometres above the ground near the poles. So, by keeping measurements above 15 kilometres, to avoid all the clouds in the tropics, the satellites are also missing any aerosols that may be between 10 and 15 km above the surface at higher latitudes.

To solve this problem and fill in the gaps, MIT climate scientist David Ridley and his colleagues turned to data from other sources.

Fortunately, since these aerosols are just as reflective to other forms of short-wave radiation as they are to sunlight, this allowed them to use ground-based LIDAR (Light Detection and Ranging, like RADAR with lasers) to detect their concentrations in the lower levels of the stratosphere that were being missed, and then verify their readings using satellites, instruments carried aloft by balloons, and AERONET (AErosol RObotic NETwork) - a series of instruments positioned all around the world to read the amount of aerosols in the troposphere.

As reported in an article in Science, at one site above Tsukuba, Japan, satellite measurements were apparently only seeing about two-thirds of the total amount of aerosols present, while at another site near Tomsk, Russia, only around half the aerosols were being spotted by satellites. These 'recovered' aerosols may go a long way towards helping us to understand their effect on our weather and climate system, and possibly help explain the apparent 'slow down' in global warming over the past 15 years.

According to a press release from the American Geophysical Union (AGU):

Rutgers University climatologist Alan Robock, who was not involved in the study, cautioned that a system specifically designed to read aerosols in the stratosphere would be needed for better results (as AERONET is designed to read aerosols below the stratosphere), but he still had high praise for the study and its results:

"This is part of the story about what has been driving climate change for the past 15 years," he told the AGU. "It's the best analysis we've had of the effects of a lot of small volcanic eruptions on climate."