On August 5 2014, the Visible Infrared Imaging Radiometer Suite (VIIRS) sensor on Suomi-NPP captured natural-color images of both Iselle and Hurricane Julio en route to Hawaii. The image above is a composite of three satellite passes over the tropical Pacific Ocean in the early afternoon. Note that Iselle’s eyewall had grown less distinct; the storm had descreased to category 2 intensity. The bright shading toward the center-left of the image is sunglint, the reflection of sunlight off the water and directly back at the satellite sensor. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Before tropical storm Iselle’s landfall on August 7, conventional wisdom among many residents was that the Island of Hawai‘i is immune to hurricanes because its large volcanic mountains obstruct approaching storms, diverting them around the island.

Topography indisputably influences the weather—that’s why precipitation is so much greater on the windward side of the island. But how much did Hawai‘i’s topography influence Iselle? Hurricane Iselle weakened to a tropical storm just as it reached the island, but still managed to make landfall. As it did, the bulk of the storm stalled on the east flank of Mauna Loa, but its weakened upper parts continued moving westward.

Aside from Mauna Loa’s arguable topographic effects on Iselle, there is another potential impact of volcanoes to consider. Active volcanoes can sometimes affect weather—and climate—by discharging gases and particles into the atmosphere.

The three dominant gases emitted by volcanoes are water vapor (about 90%), carbon dioxide, and sulfur dioxide. Both water vapor and carbon dioxide are important greenhouse gases. “Greenhouse” refers to the fact that these gases trap solar radiation.

In a simplified way, here’s how it works: Visible and ultraviolet radiation from the sun heats the earth’s surface. The surface then re-radiates some of this energy back up through the atmosphere as infrared radiation, which selectively heats greenhouse gases in the atmosphere—and the greater the concentration of greenhouse gases, the greater the atmospheric heating. Without greenhouse gases, the infrared radiation would just escape into space. The greenhouse gases, however, re-radiate the heat in all directions, including back to the surface.

Carbon dioxide released into the atmosphere remains there for a long time, so increasing concentrations of this gas result in long-term global warming. The residence time of water vapor in the atmosphere is normally much less than that of carbon dioxide. However, the concentration of water vapor in the atmosphere does increase with temperature. So, heating of the atmosphere by carbon dioxide buildup increases the amount of atmospheric water vapor, creating a positive feedback mechanism that further increases the temperature.

The scientific community generally accepts that the buildup of carbon dioxide in the atmosphere is the principal contributor to global warming. But, it’s noteworthy that volcanoes contribute less than one percent to this buildup. The bulk of increased atmospheric carbon dioxide comes, instead, from human activity (http://www.pnas.org/content/early/2014/07/23/1409659111.full.pdf+html).

For example, the largest volcanic eruption during the past 100 years occurred in 1991 at Mount Pinatubo in the Philippines. It would take 700 Pinatubo-like eruptions each year to equal the annual carbon dioxide emissions from human activities. Closer to home, it would take more than 11,000 simultaneous KÄ«lauea eruptions to equal that amount.

Large volcanic eruptions have been observed to affect Earth’s climate, but through global cooling rather than warming. This cooling is the work of sulfur dioxide, the third common volcanic gas.

Sulfur dioxide injected into the stratosphere by powerful eruptions reacts chemically, producing sulfur acids, which in turn form the same sulfate aerosols commonly found in vog (volcanic smog). These tiny stratospheric aerosol particles reflect sunlight (heat) energy back into space, causing cooling of the lower atmospheric layers.

The 1991 Mount Pinatubo eruption (http://pubs.usgs.gov/pinatubo/hoblitt2/index.html) created what is thought to be the largest stratospheric sulfur dioxide injection of the 20th century. For three years following the eruption, the earth’s surface cooled by as much as 1.3 degrees Celsius (2.3 degrees Fahrenheit).

Sulfate aerosols also act as nuclei for condensation in clouds, which, in turn, can affect weather dynamics. In a recently published scientific paper (http://onlinelibrary.wiley.com/doi/10.1002/2014GL060033/full), investigators suggest that sulfate aerosols in vog (from KÄ«lauea gas emissions) were ingested by Tropical Storm Flossie as it passed by the Hawaiian Islands in July 2013. It appears that this vog ingestion triggered lightning, which was previously absent. This effect is thought to be a consequence of vog-borne sulfate aerosols that enhanced the condensation of small cloud droplets. According to the investigators, this enhanced condensation triggered a chain of events that produced ice pellets in the higher, colder part of Flossie. Collisions between pellets caused a buildup of static electricity that was then discharged as lightning.

So, it appears that Mauna Loa did have an impact on Tropical Storm Iselle, but only as a large mountain and not as an active, degassing volcano. Past eruptions elsewhere, however, have shown that volcanic gas emissions can cause changes in local weather, as well as global cooling.

For additional information on the effects of volcanic gases on climate, please visit: http://volcanoes.usgs.gov/hazards/gas/climate.php.