It is well established that major volcanic eruptions like Mount Pinatubo in the Philippines in 1991 emit millions of tons of sulphur dioxide that effectively cool the earth slightly for several years. It has also been theorised that the stagnation in global warming in the mid twentieth century was caused by the atmospheric nuclear testing carried out between 1945 and 1980. It seems that nuclear winter actually occurred, although on just a small scale. A small nuclear war would also likely cause global cooling through the amount of particulate matter thrown up into the stratosphere. But really this is not something to even contemplate happening due to the amount of death and devastation involved.
Scientists have theorised that other reasons for the slowing in the rate of atmospheric global warming over the last decade is that a greater proportion of heat is being taken up by the world's oceans; and secondly that Asia, particularly India and China, have increased their sulfur dioxide emissions from coal fired power by 60% between 2000 and 2010 which has increased the stratospheric aerosol layer of the atmosphere changing the reflectivity.
Andrew Glikson, an earth and paleo-climate scientist with the Australian National University stated in a recent article on the Conversation website: Fact check: has global warming paused?
Warming was in part mitigated by emitted sulphur aerosols (direct effects -0.5 Watt/m2; cloud albedo effects -0.7 Watt/m2) and by land clearing (-0.2 Watt/m2).
Glickson describes the impact of oceans on overall warming of the planet:
"As some 90% of the global heat rise is trapped in the oceans (since 1950, more than 20x1022 joules), the ocean heat level reflects global warming more accurately than land and atmosphere warming. The heat content of the ocean has risen since about 2000 by about 4x1022 joules."
Stratospheric Aerosols mitigated 25 per cent of atmospheric global warming
Ryan Neely from the University of Colorado Boulder said previous observations suggest that increases in stratospheric aerosols since 2000 have counterbalanced as much as 25 percent of the warming scientists blame on human greenhouse gas emissions. "This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet," said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration.
There has been scientific debate on the origins of sulfur dioxide in the stratosphere. A 2009 paper by David Hoffman from NOAA suggested aerosol increases in the stratosphere may have come from rising emissions of sulfur dioxide from India and China. From the abstract of Increase in background stratospheric aerosol observed with lidar at Mauna Loa Observatory and Boulder, Colorado:
Since about 2000, an increase of 4-7% per year in the aerosol backscatter in the altitude range 20-30 km has been detected at both Mauna Loa and Boulder. This increase is superimposed on a seasonal cycle with a winter maximum that is modulated by the quasi-biennial oscillation (QBO) in tropical winds. Of the three major causes for a stratospheric aerosol increase: volcanic emissions to the stratosphere, increased tropical upwelling, and an increase in anthropogenic sulfur gas emissions in the troposphere, it appears that a large increase in coal burning since 2002, mainly in China, is the likely source of sulfur dioxide that ultimately ends up as the sulfate aerosol responsible for the increased backscatter from the stratospheric aerosol layer. The results are consistent with 0.6-0.8% of tropospheric sulfur entering the stratosphere.
A 2011 paper by J.P. Vernier et al - Major influence of tropical volcanic eruptions on the stratospheric aerosol layer during the last decade (abstract) - showed moderate volcanic eruptions play a role in increasing particulates in the stratosphere.
A 2011 study by Solomon - The Persistently Variable "Background" Stratospheric Aerosol Layer and Global Climate Change (abstract) - showed stratospheric aerosols offset about a quarter of the greenhouse effect warming on Earth during the past decade.
This new study lead by Neely used long-term measurements of changes in the stratospheric aerosol layer's "optical depth,". According to Neely, since 2000, the optical depth in the stratospheric aerosol layer has increased by about 4 to 7 percent, meaning it is slightly more opaque now than in previous years.
"The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth's climate," said Brian Toon of CU-Boulder's Department of Atmospheric and Oceanic Sciences. "But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up."
The computer modelling simulated 10 years of atmospheric activity tied to both coal-burning activities in Asia and to emissions by volcanoes around the world. Each run took about a week of computer time using 192 processors, allowing the team to separate coal-burning pollution in Asia from aerosol contributions from moderate, global volcanic eruptions.
While the 10 year climate data is not long enough to determine long term climate change trends, it provides some answers regarding natural and human caused sulfur dioxide emissions and their impact on global warming.
"This paper addresses a question of immediate relevance to our understanding of the human impact on climate," said Neely. "It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes."
The paper's full abstract:
Observations suggest that the optical depth of the stratospheric aerosol layer between 20 and 30km has increased 4-10% per year since 2000, which is significant for Earth's climate. Contributions to this increase both from moderate volcanic eruptions and from enhanced coal burning in Asia have been suggested. Current observations are insufficient to attribute the contribution of the different sources. Here we use a global climate model coupled to an aerosol microphysical model to partition the contribution of each. We employ model runs that include the increases in anthropogenic sulfur dioxide (SO2) over Asia and the moderate volcanic explosive injections of SO2 observed from 2000 to 2010. Comparison of the model results to observations reveals that moderate volcanic eruptions, rather than anthropogenic influences, are the primary source of the observed increases in stratospheric aerosol.
This is not a reason for complacency. We need to reduce our use of fossil fuels and drastically reduce carbon emissions from this decade if we are to manage the problems and impacts of climate change in the coming decades.
I came across this extraordinary timelapse photography video on vimeo by Sean Stiegemeier of Icelandic volcano Eyjafjallaj\F6kull in May 2010. This eruption had a big impact on Air travel disruption due to the danger of silica particles in the ash plume ejected into the jetstream potentially damaging aircraft engines. Much of European airspace remained closed for weeks. The irony is that the volcano caused a net fall in carbon emissions due to the grounding of flights saving more in emissions than the volcano emitted. It was estimated that 150,000 to 300,000 tonnes were emitted each day by the volcano, while the European aviation industry saved some 344,000 tonnes per day while being grounded.
- University of Colorado Boulder media release, 1 March 2013 - Volcanic aerosols, not pollutants, tamped down recent Earth warming, says CU study
- R.R. Neely et al (2013), Geophysical Research Letters, Recent anthropogenic increases in SO2 from Asia have minimal impact on stratospheric aerosol (abstract)
- Andrew Glikson, The Conversation, 26 February 2013 - Fact check: has global warming paused?
- Lead image Eyjafjallaj\F6kull eruption from the south by S\E6var Helgi Bragason / Flickr Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic (CC BY-NC-ND 2.0)