Examining the global temperature record of the past few decades the effect of large volcanic eruptions is obvious. Click on image to enlarge.
There are two prominent dips in global temperature due to large eruptions. They are coincident with the 1982 El Chichon (curiously meaning “The Bump”) and the much larger 1991 Pinatubo eruption. The global cooling caused by these events was due to aerosols deposited in the stratosphere that reflected sunlight back into space, as much as 10% of it. Click on images to enlarge.
Recently there has been a growing appreciation of the importance of the less powerful, but more frequent, volcanic eruptions that may have a lower, but longer-term influence on the “background” aerosol component of the stratosphere.
Vernier et al comment that the recent trend of increasing aerosol content of the stratosphere has been tentatively attributed to an increase in sulphur aerosols entering the stratosphere associated with coal burning in Southeast Asia. However, Vernier et al demonstrates with satellite measurements that the observed trend is not anthropogenic, but is chiefly driven by a series of moderate but increasingly intense volcanic eruptions primarily at tropical latitudes. For the first time they show that moderate volcanic events located in the tropics can be an important source of aerosols for the stratosphere.
Some fifty years ago Junge et al Meteorol. 18, 81 (1961) carried out observations of the stratosphere when there had been no major eruptions for a while and speculated on what might be the background level of stratospheric aerosols.
These days measurements are made from many regions and from balloons and satellites. Using them Hoffman et al noted that the background aerosol level of the stratosphere increased by 5-9% a year between 1960s to 1980s. But in the 90s the stratospheric aerosols decreased by similar magnitudes. Such observations have led to speculation of what influence the low frequency, low amplitude variations in stratospheric aerosols can have on global warming.
Solomon at al recently noted that the increase in the amount of aerosols in the stratosphere over the past decade or so may help explain why global warming has occurred at a slower pace recently. The aerosols in the stratosphere are probably not anthropogenic.
Combining ground-based and satellite measurements, Solomon and colleagues modeled the amount of radiative forcing (the imbalance that occurs when there is more solar energy radiating down on Earth than there is infrared energy radiating back out to space) stratospheric aerosols have produced over the past 50 years. Using the radiative forcing data in a climate model, they observed that an increase in stratospheric aerosols decreased the global warming that would have otherwise occurred by 20 percent since 1998.
Soloman’s fig 3 shows there were periods when stratospheric aerosols went down slightly (1988 – 1998) making some contribution to warming, but the over all volcanic ups and down in the 80s and 90s would have gone both ways. Click on image to enlarge.
It is not possible at present to determine the effect that changes in stratospheric aerosol cover may have contributed to the global temperature increase between 1980 and 2000. The effect of the Pinatubo eruption and the unknown character of oceanic temperature cycles and its time lag make any estimates uncertain.
Fig 4 shows aerosols may have contributed to a global cooling of 0.07 deg C in the past decade. Also suggested is that the increase in the background from the very low levels seen in 1960 may have reduced the global warming that might have occurred by 0.05 deg C.Click on image to enlarge.
The stratosphere seems to be the source of many lines of interesting science pertinent to global warming. A short while ago Soloman analysed water vapour in the stratosphere and concluded that changes seen in the past decade had acted in a way to slow down expected temperature rises seen in the past decade by 25%.
It will be interesting to see how the global temperature changes in the forthcoming years and if there is any relationship to stratospheric water vapour and aerosols.