How volcanoes affect the atmosphere and climate
The main effect on weather right near a volcano is that there is often a lot of rain, lightning, and thunder during an eruption. This is because all the ash particles that are thrown up into the atmosphere are good at attracting/collecting water droplets. We don't quite know how the lightning is caused but it probably involves the particles moving through the air and separating positively and negatively charged particles.
Another problem that we are having here in Hawai'i involves the formation of vog, or volcanic fog. The ongoing eruption is very quiet, with lava flowing through lava tubes and then into the ocean. Up at the vent is an almost constant plume of volcanic fume that contains a lot of sulfur dioxide. This SO2 combines with water in the atmosphere to form sulfuric acid droplets that get carried in the trade winds around to the leeward side of the Big Island. The air quality there has been really poor since the eruption started in 1983 and they are getting pretty tired of it.
As for the world-wide affects of volcanic eruptions this only happens when there are large explosive eruptions that throw material into the stratosphere. If it only gets into the troposphere it gets flushed out by rain. The effects on the climate haven't been completely figured out. It seems to depend on the size of the particles (again mostly droplets of sulfuric acid). If they are big then they let sunlight in but don't let heat radiated from the Earth's surface out, and the net result is a warmer Earth (the famous Greenhouse effect). If the particles are smaller than about 2 microns then they block some of the incoming energy from the Sun and the Earth cools off a little. That seems to have been the effect of the Pinatubo eruption where about a degree of cooling was noticed around the world. Of course that doesn't just mean that things are cooler, but there are all kinds of effects on the wind circulation and where storms occur. Some folks think that large eruptions can cause the weather phenomena called "El Nino" to start. This is a huge disruption of the Earth's atmospheric circulation. The connection hasn't been accepted by everybody though.
An even more controversial connection involves whether or not volcanic activity on the East
Pacific Rise (a mid-ocean spreading center) can cause warmer water at the surface of the East
Pacific, and in that way generate an El Nino. Dr. Dan Walker here at the University of Hawai'i
has noticed a strong correlation between seismic activity on the East Pacific Rise (which he
presumes indicates an eruption) and El Nino cycles over the past ~25 years.
As a long-term average, volcanism produces about 5X10^11 kg of CO2 per year; that
production, along with oceanic and terrestrial biomass cycling maintained a carbon dioxide
reservoir in the atmosphere of about 2.2X10^15 kg. Current fossil fuel and land use practices
now introduce about a (net) 17.6X10^12 kg of CO2 into the atmosphere and has resulted in a
progressively increasing atmospheric reservoir of 2.69X10^15 kg of CO2. Hence, volcanism
produces about 3% of the total CO2 with the other 97% coming from anthropogenic sources.
Volcanoes affect people in many ways, some are good, some are not. Some of the bad ways are that houses, buildings, roads, and fields can get covered with ash. As long as you can get the ash off (especially if it is wet), your house may not collapse, but often the people leave because of the ash and are not around to continually clean off their roofs. If the ashfall is really heavy it can make it impossible to breathe.
Lava flows are almost always too slow to run over people, but they can certainly run over houses, roads, and any other structures.
Pyroclastic flows are mixtures of hot gas and ash, and they travel very quickly down the slopes of volcanoes. They are so hot and choking that if you are caught in one it will kill you. They are also so fast (100-200 km/hour) that you cannot out-run them. If a volcano that is known for producing pyroclastic flows is looking like it may erupt soon, the best thing is for you to leave before it does.
Some of the good ways that volcanoes affect people include producing spectacular scenery, and producing very rich soils for farming.
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