Pinatubo added around 17 Mt of SO2 into the lower/middle stratosphere (34-37km); the largest since Krakatau. The aerosol cloud then took 3 weeks to spread round globe and around a year to attain global coverage.
The cloud persisted for 3 years at concentrations much larger than background levels. It affected optical properties of the atmosphere, such as unusual colouring of sunrises and sunsets and whitening of the sun.
Positives:
- The magnitude of the aerosol cloud decreased the amount of net radiation reaching the surface of Earth - cooling in the N. hemisphere was -0.5-0.7 degrees, and still -0.4 degrees in 1992-1993
- Eruption effects were strong enough to override the effects of El-Nino and anthropogenic global warming between 1991-1993.
- Can be used as a model for aerosol dispersal, circulation and decay.
Negative:
- Midlatitude ozone concs. reached lowest-ever recorded levels between 1992-1993.
- Ozone depletion rates were observed to be faster than ever recorded before.
- Half of the ejected sulfur converted to sulfuric acid 3 weeks to a month after the eruption.
- Higher than usual amounts of cirrus clouds in the upper trophosphere leading to surface warming, as the transport of aerosols from the stratosphere to the trophosphere caused sulfur-induced changes in atmospheric dynamics.
Effects on weather and climate (locally)
- The USA experienced the third coldest and wettest summer in 77 years in 1992.
- Extensive Mississippi floods in summer 1993.
- Drought in the Sahel.
- Many other areas experienced above-average warm conditions, showing cooling isn't spatially uniform.
Impacts on Ozone
Ozone is important in the upper atmosphere as it blocks harmful incoming electromagnetic radiation from the sun. Chemical reactions taking place on the injected sulfate particles cause active ozone destruction.
- In the tropical stratosphere, ozone concentrations decreased as much as 20% 3-6 months post-eruption.
- 20% ozone depletion measured over the states of Colorado and Hawaii.
- Mid-latitude ozone abundance was at the lowest-ever recorded levels between 1992-1993.
- In Antarctica 1991-1992 was huge decreases in ozone abundance and rates of destruction.
Conclusion
This natural geoengineering event proves that unfortunately there are unwanted side effects at a local and regional level, as sulfate aerosols affect atmospheric circulation and weather patterns. Other ill effects include changes in atmospheric composition in the upper stratosphere; which is really important for protecting the Earth from electromagnetic radiation. Rates of destruction of ozone reached critical levels after the eruption of Pinatubo and the depletion of the ozone layer also increased dramatically.
The eruption did cause large scale surface cooling for at least 3 years post-eruption; weather patterns causing the most dramatic cooling of -0.5-0.7 degrees in the Northern hemisphere. This is proof that sulfate injections can be used as a tool to combat global warming; especially as it managed to offset both anthropogenic warming and the El-Nino conditions. The Pinatubo eruption can be used to build in-depth models of potential climate impact, especially with the analysis of surface temperatures and weather patterns. Knowledge gained from this can then obviously be applied to artificial injection situations to determine their effects at the surface.
Although I still think injecting sulfate aerosols into the stratosphere is an excellent idea and one of the more feasible; there is unfortunately going to be a trade off between cooling the planet and causing destruction of ozone and alteration of atmospheric composition. I still believe it may be possible to find a way to bypass the problems associated with the technique, and nevertheless it is still a really important option to consider, especially as the technique can reverse global warming (as seen by cooling after Pinatubo) by blocking incoming radiation. It's also important to remember that injecting aerosols is something readily available and this may turn up to be very important in the next few years; and also the cost of the technique is comparatively low to other methods.
Self, S., J-X. Zhao, R. Holasek, R. Torres and A. King (1999) 'The atmospheric impact of the 1991 Mount Pinatubo eruption', (WWW) Washington: U.S Geological Survey (http://pubs.usgs.gov/pinatubo/self/index.html; 07/11/11).
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