There are two opposing effects the latest threatening volcano, Barðarbunga in Iceland, could have on the planet earth. If it ejects an ash particles and sulfuric gases column, within weeks a cloud could block the sun resulting in global cooling. However, the carbon dioxide released could increase the greenhouse effect trapping more radiated surface heat adding to global warming. Total volcanic carbon dioxide is still only about one percent of that released by burning fossil fuels.
A prediction cannot be made by scientists as to whether the volcano will cause lava or flooding which will restrict ground travel or clouds of ash which halt air travel. Iceland's National Crisis Coordination Center (NCCC) was activated on August 20, 2014 to evacuate the Barðarbunga volcano area and roads were closed where it was feared glacial melting could trigger much flooding. It is located under the ice cap of a glacier in the center of the island, not in the same range as the Eyjafjallajökull volcano of 2010 fame.
Officials added surveillance and extra seismic monitors to help give earlier warning of imminent eruptions. On August 23rd, a red aviation alert was raised warning that an ash-emitting eruption could be imminent. A no-fly zone was imposed for 115 miles by 162 miles around the volcano, but the next day the alert was lowered to orange indicating "heightened or escalating unrest with increased potential of eruption" and the no-fly lifted. Hit with thousands of earthquakes over the past week and not slowing, an eruption remains a possibility.
The last Icelandic 2010 eruption of the Eyjafjallajökull volcano stopped air travel over Europe's air space for five days with over 100,000 cancelled flights. Visibility is not the issue. In explosive eruptions, large amounts of dust and gaseous material like sulfur dioxide are ejected into the stratosphere, the upper atmosphere, where sulfur dioxide coming in contact with water converts into sulfuric acid droplets called aerosols. Aircraft can stall when particles in the air damage engines and clog ventilation.
The billions of microscopic aerosols remain in the stratosphere for a few years, eventually spreading to cover much of the earth. They absorb radiation from the sun keeping the earth's surface from that heat and cool the planet. In the 1900s three large eruptions cooled the earth by 1°C, El Chichon in Mexico in 1982, Mt. Pinatubo in the Philippines on June 15, 1991, and Mt. Hudson in southern Chile in July 1991. But the aerosols fall from the stratosphere in 2 to 3 years and are dispersed by wind and rain.
Water vapor is typically the most abundant volcanic gas erupted, up to as high as 97 percent. It has little climate impact since rain and gravity remove it from the lower atmosphere fairly quickly. Second most common is carbon dioxide (CO2), from 1 to 50 percent depending on the eruption type. Heavier than air, CO2 tends to collect in low areas where it can kill animals breathing it.
Individual volcanic eruptions have a short-lived effect on significant long-term climate changes because of ocean heat storage. There have been too few major eruptions in more recent times for scientists to have a well-documented history of long-term climate change effects. Ice cores drilled from glaciers have been measured for presence of sulfuric acid aerosols as indicators of past eruptions over hundreds of thousands of years. During the period called the Little Ice Age, episodes of high volcanic activity (from 1250 to 1500 and from 1500 to 1700) were found. Less evidence of volcanic activity was found during the Medieval Warm Period from 1100 to 1250.
Greenhouse gases carbon dioxide and methane concentrations are clearly very significantly correlated with temperatures. Current off the charts CO2 levels are measurably influencing temperatures. Possible climate change side effects in addition temperature changes are wildfires, droughts and tropical storms.