I have been in the field in Iceland and unable to answer the question raised by Andrew Lockley and others “why frequent volcanic eruptions cause a warming effect?” Many of the details are in my paper (www.tetontectonics.org/Climate/SO2InitiatesClimateChange.pdf) but much of what I am going to write is in another paper in preparation.
The Basic Observation: The highest rate of volcanism in the past 25,000 years (actually over a lot longer, but the data become less clear with increasing age) is between 15,000 and 9,500 BP with a strong lull in volcanism during the Younger Dryas. Huybers and Langmuir (2009, EPSL 286(3-4):479-491) argue, from fairly limited data from that time period, that the rate of volcanism showed a factor of 2 to 6 increase. Sigvaldason (2002, Bull. Volcanol. 64:192-205) argues that volcanism in Iceland was 30 times higher than present. Volcanic sulfate per century measured in GISP2 peaks during the deglaciation are at values 12 times the average between 5,000 and 150 BP and more than 30 times the lowest values during the Younger Dryas. The highest rates of volcanism are contemporaneous with the highest rates of global warming. This very clear increase in volcanism must be explained. Huybers and Langmuir and others show that melting of glacial ice in places such as Iceland is likely to lead to an increase in volcanism. This is probably a very important feedback mechanism helping to explain the very rapid warming, but only after significant amounts of ice have melted. Furthermore, a significant number of volcanoes were not located under major ice sheets at the time. Also melting ice increases sea level, increasing loading in some volcanic areas in a less well specified manner and thus decreasing volcanism. Similarly numerous models propose that a sudden influx of fresh water into the North Atlantic could lead to a variety of processes that could trigger rapid warming. Again this may be important, but what melted the fresh water? Most calculations of Milancovic cycles show a rapid warming around this time but the phasing is very sensitive to model assumptions and cannot be independently determined as far as I know. As I show in my paper, there is good correlation between temperature and Milankovic cycles over the past 130,000 years at long periods. At shorter periods, however, something else significant is going on. What could this be? Global cooling of ~0.5oC for ~3 years after large volcanic eruptions is obvious and has been well observed after all reported large eruptions throughout written history. Most people are convinced that this is the whole story: volcanoes cool. Full stop. But much more is going on. The effects of the Pinatubo eruption in 1991 were well observed especially by satellites. I will be describing all the different effects in detail in the paper in preparation. A few important observations: 1. Pinatubo erupted up to 921 Mt water, 234 Mt CO2, 19 Mt SO2 and 16 Mt chlorine. 2. Much of the water and chlorine fell back to earth in the eruption column. Much of the water reaching the lower stratosphere was used to form the aerosol. Some of the water remained in the troposphere and possibly the stratosphere. We do not know for sure how much. Global rainfall actually decreased because of the effects of the aerosol. Water is an important greenhouse gas. 3. The rate of increase of CO2 measured at Mauna Loa actually decreased following Pinatubo probably because of the increase in photosynthesis caused by scattering of sunlight by the aerosol and by cooling of the ocean. But CO2 was added to the atmosphere and its effects would last much longer that the 3-year effects of the aerosol. 4. CH4 and CO increased following Pinatubo and OH decreased. OH is known as the “tropospheric vacuum cleaner.” OH and H2O2¬ are the primary oxidants, the way the atmosphere cleanses itself of SO2¬, CH4, CO, etc. SO2 appears to be chemically more active and to grab OH most effectively at least in the stratosphere where most OH is formed. Formation of the aerosol appears to have seriously depleted the OH available causing CH4, CO, etc to accumulate. A great paper on this issue is Ehhalt (1999, Phys. Chem. Chem. Phys., 1:5401-5408). These increases in greenhouse gases will last in the atmosphere much longer than the 3-year effects of the aerosol. 5. OH and H2O2 are formed primarily in the stratosphere by the effects of ultraviolet light on ozone. Ozone is formed by the effects of ultraviolet light on oxygen. Ozone concentration fell 5% after Pinatubo, the largest drop observed since satellite measurements began. It is not an accident that the aerosol layer formed near the base of the ozone layer. The diameter of the ozone hole increased 17%. A potential cause still being looked at is that the surfaces of the aerosol provide an excellent place for heterogeneous process to cause destruction of ozone influenced by erupted chlorine and other related chemicals emitted by man. 6. The stratosphere was clearly observed to have warmed 3oC (6 times the surface cooling). I am still not sure of all of the effects of this but many involve warming. 7. Robock (2002, Science 295:1242-1244) and others have shown clearly that while global temperatures fell for ~3 years, winter temperatures over land in the Northern Hemisphere actually rose. Stenchikov et al. (2004, JGR 109, D03112 and 2006, JGR, 111:D07107) and others have done some interesting modeling of this, but I would argue that this is evidence of an increase in greenhouse gases. In the summer, reflection of insolation by the aerosol is dominant, masking other effects, but when the sun goes into the southern hemisphere, this effect is not as strong in northern latitudes and other effects can be seen. All of these effects and more provide evidence that while the reflective aerosol is dominant, cooling occurs and increases in greenhouse gases are masked. But greenhouse gases last much longer than 3 years. Models and observations suggest it takes the atmosphere at least a decade to fully recover from a Pinatubo sized eruption and the ocean at least a century. Some greenhouse gases may last for many decades. A large volcanic eruption sets a whole lot of processes in motion in the atmosphere with a variety of time constants. If such large eruptions start occurring frequently, the atmosphere does not have time to recover and I argue that greenhouse warming can then become dominant, at least after the aerosols dissipate. Climate has been unusually stable for the last 5,000 years and Pinatubo-sized and larger eruptions have averaged during most of this time ~1 per century. At the end of the last ice age, large eruptions such as this appear to have occurred at least once per decade and probably once every few years and possibly in a few cases every few months. These are the times when rapid warming did occur and when the observations above suggest it is likely to occur. What appears to be most important for warming to occur, is that large volcanic eruptions occur relatively continuously, much more frequently than it takes the atmosphere to recover from a single eruption. Furthermore active volcanoes often emit even larger amounts of CO2 between eruptions. The number of active volcanoes was apparently very high during the end of the last ice age. Basaltic fissure eruptions emit considerably more gases than typical andesitic or rhyolitic volcanoes. Laki in Iceland in 1783 erupted 122 Mt SO2 primarily into the troposphere because the explosions were much smaller than Pinatubo. A “dry fog” hung over much of Europe for many months and temperatures were very high. This dry fog appears to have been SO2. Leaves were burned by sulfuric acid from Iceland to Finland, to Italy. This is the type of circumstance where the localized greenhouse effects of SO2 may be seen. While SO2 in the troposphere is normally oxidized within days to weeks, it will last much longer when the oxidizing capacity is overwhelmed. Ultimately we need to capture the above issues and more in climate models to demonstrate whether I am right. The paper I am currently writing is designed to communicate to climate modelers the primary observations after Pinatubo, Laki, and Tambora to help them build the appropriate processes into models. As describe in my paper, from 1930 to 1980, the amount of “volcanic” sulfate deposited in Greenland was similar to the largest amounts deposited at the end of the last ice age. The rate of volcanic eruptions did not increase significantly during this time, but known amounts of sulfur emitted by man burning fossil fuels did. (“Volcanic” sulfate was calculated by Mayewski et al. (1997, JGR 102:26345–26366) using an EOF analysis as total measured sulfate minus sulfate due to continental dust and oceanic winds.) When sulfur emissions by man were reduced 18% between 1980 and 2000 in an effort to decrease acid rain, the rates of growth in methane and in temperature (HadCRUT3 global) both leveled off to nearly zero by 2000 while CO2 concentrations continue to climb unabated. The leveling off of methane concentrations is expected if the highest rates of anthropogenic sulfur had decreased oxidizing capacity. Human sulfur emissions are now on the rise again, especially in China. Volcanoes have many more effects on climate than just cooling for ~3 years. We need to study, understand, and model these effects in order to be sure we really understand man’s effects on climate and to approach geoengineering knowledgeably. Peter Peter L Ward U.S. Geological Survey, retired Teton Tectonics Box 4875 Jackson, WY 83001 307-733-3664 [email protected] www.tetontectonics.org --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
