Wait a second. Volcanoes have impact on the climate?!?!
Yes, indeed they do, and they play a vital role in natural global cooling and believe it or not, but they also are important in natural global warming. Many disagree on the causes of recent climate change, whether it is natural or man-made or some combination of both, but there is widespread agreement that volcanoes play a large role in global cooling, lasting a few years, following a major volcanic eruption.
Volcanoes are constantly active around the world as there can be six going off at any given moment. Typically, these active volcanoes are small ones, that really aren’t important when it comes to climate, but when you are talking about large volcanoes, now you are talkin’!
Volcanic eruptions eject ash and gases into the atmosphere, while large volcanic eruptions eject the ash into upper troposphere and stratosphere. About 50% to 90% of the gases emitted by volcanoes is water vapor. The other major gases vary in concentration from volcano to volcano. CO2 can be 1% to 40% of the gases emitted during an eruption, while sulfur dioxide (SO2) can be 1% to 25%, H2S 1% to 10%, and HCl at 1% to 10%. Other gases are emitted, but they are minor and are irrelevant. It should be noted that H2S gets converted to SO2 fairly quickly through oxidation, which makes SO2 one of the most important gases emitted from volcanoes, as we will learn later in this section.
Furthermore, the most important gas that gets ejected is sulfur dioxide, which can reach elevations of over 80,000 feet (24.4 km). Despite many of the gases and ash falling to the Earth’s surface within a year, sulfur dioxide typically doesn’t, unless it gets confined to the troposphere, where it affects cloud development, in which it will typically fall to Earth’s surface as acid rain.
GLOBAL COOLING INDUCED BY VOLCANOES
During explosive eruptions, SO2 can be emitted into the stratosphere, where it stays for a long time, as it gets quickly converted into sulfate aerosols within weeks or a couple of months. This forms a stratospheric aerosol layer, which blocks out solar radiation, which in turn causes widespread cooling across the globe.
Global temperatures may fall 0.2°C to 1°C within three to four years following a major eruption before they recover. Such cooling events occurred after El Chichon (1982) and Pinatubo (1991). See Figure 2.
Despite any given major volcanic eruption causing global cooling over a span of three to four years, evidence has shown that the time of year that a volcano erupts affects the distribution of the aerosols around the globe within the ash cloud. Generally, the greatest cooling effects from an eruption are seen during the summer months.
Further research has shown that the location of a volcano also has effects on how much cooling occurs, while also determining whether winters are cold or mild over large swaths of North America, Europe, and Asia.
According to Robock (2003), volcanoes located near the equator, such as El Chichon and Pinatubo typically cause warmer winters due to the likelihood of a positive North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) following the eruption. When the NAO and AO are in their positive phases, low pressure and cold air are trapped in poleward regions, which causes the jet stream to send mild air into Northern Hemisphere land masses (Figure 3).
On the flip side, it has been shown that high latitude volcanoes cause very cold and snowy winters for the Northern Hemisphere land masses, as a negative NAO and negative AO tend to follow after the eruption. This causes the jet streams to plunge southward and send cold air into the continents. Such an event occurred after the 1912 eruption of Katmai in Alaska.
Major volcanic eruptions are rare events, but when they do occur, they tend to occur in groups or clusters with other eruptions. The late 1800s and early 1900s saw very high volcanic activity, which took place during a very cold era, presumably due to low solar output and high volcanic activity (Figures 4 and 5).
The 1930s and 1940s were warm, in which they took place during high solar output and positive phases of the PDO and AMO. The ’30s and ’40s also coincided with a period of low volcanic activity. Thus, the main cause for the 1930s and 1940s warmth has been debated, but it is widely accepted that it was at least 60% due to an increase in solar output and a positive PDO and AMO. However, despite these cyclical forcings, the lack of volcanic activity and stratospheric aerosols likely amplified the warmth of the 1930s and 1940s.
During the 1960s, volcanic activity started up again. Mt. Agung was the first major eruption with many others following during that decade, which sustained the stratospheric aerosol layer in the atmosphere, blocking out and scattering incoming solar radiation. The 1960s were also a decade with low sunspot activity and it was also the coldest decade in 80 years. Following the Great Pacific Climate Shift in 1977, temperatures began to rise again as the Sun returned to an active state. Global temperatures fell during the early and mid 1980s, as well as the early and mid 1990s following the eruptions of El Chichon and Pinatubo in 1982 and 1991, respectively.
While we mentioned in the previous section that El Niños are associated with global warming and La Niñas are associated with global cooling, major volcanic eruptions, such as El Chichon and Pinatubo have been shown to amplify the cooling effect from La Niñas and they can also override the warming effect from El Niño as seen during the early 1990s as well as the super El Niño of 1982-1983 (Figure 6).
GLOBAL WARMING INDUCED BY VOLCANOES
As we discussed above, the 1930s and 1940s were warm decades, similar to the past decade. The 1930s and 1940s coincided with an increase in solar activity, which likely accounted for the majority of the warmth during those years. The two decades also turned warm as the PDO and AMO went from their negative state into their positive state, which likely had a lot to do with the warmth. However, volcanic activity was also very low, which likely amplified the warmth considering the stratospheric aerosol layer would have dissipated by the early 1930s.
As the NASA GISS volcanic aerosol thickness chart shows, the past two decades have been relatively quiet in terms of volcanic activity, quieter than the 1930s and 1940s. In fact, volcanic activity is at all time historic low levels for the decade. Since the atmosphere is clear of the aerosols, the warming we saw until 2016 was likely amplified by the lack of major volcanic eruptions and the positive phase of the ocean cycles, not to mention land use changes and the urban heat island effect. Considering solar activity has been declining since the 1960s (although it has gone up since the 1980s), the oceans and the lack of major volcanic eruptions likely have been the control knob for the warming in recent decades rather than solar forcing.
It has also been shown that the stratospheric aerosols can absorb the Sun’s near infrared radiation and longwave infrared radiation (incoming and outgoing). This causes stratospheric heating, which eventually affects atmospheric circulation patterns. This generally leads to warm or mild winter for the Northern Hemisphere for one to two years after a major eruption.
Bastardi, Joe, et al. “WeatherBELL Models | Premium Weather Maps.” WeatherBELL Models, WeatherBELL Analytics L.L.C., models.weatherbell.com/climate/cfsr_monthly.php.
Christy, John R., and Roy W. Spencer. “UAH LT Global Anomaly.” The University of Alabama Huntsville Global Temperatures, The University of Alabama Huntsville, http://www.nsstc.uah.edu/data/msu/v6.0/tlt/uahncdc_lt_6.0.txt.
D’Aleo, Joseph. “HOW VOLCANISM AFFECTS CLIMATE .” ICECAP, ICECAP, icecap.us/docs/change/HOWVOLCANISMAFFECTSCLIMATE.pdf.
“Interactive 3D Globe Brings Weather to Life.” MeteoEarth.com – Interactive 3D Globe Brings Weather to Life, MeteoEarth.com, http://www.meteoearth.com/.
Javier. “The Effect of Volcanoes on Climate and Climate on Volcanoes.” Watts Up With That?, WordPress, 10 May 2018, wattsupwiththat.com/2018/05/10/the-effect-of-volcanoes-on-climate-and-climate-on-volcanoes/?cn-reloaded=1.
Maue, Ryan. “Models 3.0 JRA-55 Global Temperature Anomaly.” Model Labs, Weathermodels.com, lab.weathermodels.com/models/temperature/jra55_temperature.php.