How do volcanoes affect weather and climate?

Before tropical storm Iselle’s landfall on Aug. 7, conventional wisdom among many residents was that the Island of Hawaii 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 Hawaii’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 Kilauea 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 Kilauea 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.

Kilauea activity update

The summit lava lake within Halema‘uma‘u Crater produced nighttime glow that was visible via HVO’s webcam over the past week. The lava lake level fell slightly, in response to gradual summit deflation and was roughly 45 meters (150 feet) below the rim of the Overlook crater on Thursday.

On the East Rift Zone of Kilauea Volcano, the June 27 flow from Pu’u ‘O’o advanced toward the northeast and was 9.4 kilometers (5.8 miles) from the vent on Aug. 18. Within the Pu’u ‘O’o’s crater, glow was visible above several outgassing openings in the crater floor.

Visit the HVO website (http://hvo.wr.usgs.gov) for past Volcano Watch articles and current Kilauea, Mauna Loa, and Hualalai activity updates, recent volcano photos, recent earthquake data, and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

No earthquakes were reported felt during the past week on the Island of Hawaii.

HVO recorded a magnitude-4.2 earthquake located southwest of the island of Molokai at 2:37 a.m. on Friday. The Pacific Tsunami Warning Center determined that no damaging tsunami was generated.

The earthquake was located 55 kilometers (34 miles) west-southwest of Maunaloa, Molokai. A map showing the location of the earthquake is posted on the HVO website at http://hvo.wr.usgs.gov/seismic/volcweb/earthquakes/.

The depth of the earthquake was calculated to be shallow — about 6 kilometers (4 miles). However, determining accurate earthquake locations and depths in this region can be difficult because of the sparse coverage of seismometers on Molokai and adjacent islands. Earthquake locations are most accurate when seismometers surround the earthquake source, which is nearly impossible given the geography of the islands in the area. Often, the depths are the most uncertain in these situations. Therefore, subsequent and more sophisticated analysis will likely result in locating this earthquake at a greater depth in the mantle.

HVO’s seismic records show that three earthquakes of magnitude-3 or larger have occurred in the same area in the past 25 years.

According to Wes Thelen, HVO Seismic Network manager, earthquakes outside of the Island of Hawaii are not uncommon and are typically caused by a structural adjustment of the mantle in response to the weight of the Hawaiian Islands. “This event is a good reminder that the Island of Hawaii is not the only Hawaiian island prone to earthquakes,” he said.

Earthquake activity generally decreases northwest up the Hawaiian Island chain, but the potential for larger earthquakes still exists. Earthquakes with magnitudes greater than 6.0 occurred in the Molokai region in 1870, 1871, 1895 and 1894.

“No aftershocks have been recorded, and aftershocks typically do not follow earthquakes at these depths in the mantle,” Thelen added.

Friday’s earthquake was felt on several Hawaiian islands, from the Island of Hawaii to Oahu. The USGS “Did you feel it?” Web site (http://earthquake.usgs.gov/eqcenter/dyfi/) received more than 70 felt reports by 8 a.m., ranging from weak to light shaking. Thus far, there have been no reports of damage, nor do we expect any given the location and size of the earthquake.

“The earthquake today caused no detectable changes on the active volcanoes on the Island of Hawai‘i,” said Jim Kauahikaua, HVO’s scientist-in-charge.

The magnitude was initially estimated at 4.0, but more detailed seismic analyses resulted in a final magnitude of 4.2.

The second annual Great Hawaii Shakeout, scheduled for 10:16 a.m. on Oct. 16, is an opportunity for all Hawaii residents to practice “Drop! Cover! Hold On!”—actions that are proven to reduce injury in an earthquake. For details, please visit shakeout.org/hawaii/.

For information on recent earthquakes in Hawaii and eruption updates, visit the Hawaiian Volcano Observatory website at hvo.wr.usgs.gov.

Volcano Watch (http://hvo.wr.usgs.gov/volcanowatch/) is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory. The USGS provides science for a changing world. For more information, visit www.usgs.gov.