| Unit 4: EarthComm | ||||
Digging Deeper (continued) How do volcanoes affect climate? If the Earth system were simple, the task of answering that question might be easy. Suppose that volcanic activity is the independent variable. This is the variable that, when changed, causes a change in something else (the dependent variable). In a simple model, climate would be the dependent variable. You could plot volcanic activity over time and compare it to temperature (an aspect of climate that can be measured) over time. Temperature changes that follow volcanic events would allow you to make inferences about the effects of eruptions on climate. However, the Earth system is complex. Records of climate and volcanic activity are imperfect. Some volcanic products should warm the atmosphere (carbon dioxide, a greenhouse gas). Others should cool the climate (dust, which reduces sunlight). The task of understanding climate change is obviously very complicated. The evidence at hand, however, suggests that major volcanic eruptions can lower the average temperature of the Earth’s surface by a few tenths of a degree Celsius for as long as a few years.It is often thought that volcanic eruptions increase or cause rainfall near or downwind of the eruption. Volcanoes put dust into the air. Water droplets in clouds form around small dust particles. Eruptions can also heat the local atmosphere. This should increase convection, or circulation, of the atmosphere. Finally, some volcanic eruptions release great quantities of water vapor needed to form clouds and rain. However, a number of studies show that an increase in rainfall is rare after an eruption. The major eruption of Krakatoa in 1883 did not increase rainfall, and it occurred during the wet (monsoon) season. It seems that conditions in the atmosphere near a volcanic eruption have to be just right for rainfall to increase just because of the eruption. Enormous quantities of sulfur dioxide gas from a volcanic eruption can be put all the way into the stratosphere (the upper layer of the atmosphere, above the weather). It then slowly reacts with water to form tiny droplets of sulfuric acid, less than a thousandth of a millimeter in diameter. Unlike in the troposphere (the lowest layer of the atmosphere), these sulfur dioxide droplets are not affected by the water cycle. They stay suspended in the stratosphere for as long as a few years. The sulfur dioxide droplets, as well as the large quantities of very fine volcanic ash particles that also reach the stratosphere during major volcanic eruptions, reflect sunlight and are thought to cause the global cooling that is often observed for a few years after a major volcanic eruption. For example, following the eruption of Tambora in Indonesia in 1815, many areas in the United States and Canada had unusually cold summer weather. In New England, 1815 was called the “year without a summer.” |
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Check Your Understanding 1. What gases escape from volcanoes?2. Why does the emission of carbon dioxide pose a threat near volcanic eruptions? 3. How are volcanoes connected to the water cycle? 4.
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