1. Volcanoes and Climate

2. The Earth’s Energy (Radiation) Budget
Huge explosive volcanic eruptions in the Tropics, energetic enough to push sulphur gases up into the relatively stable stratosphere where they condense into aerosol (small particles), can have a cooling effect on climate by increasing the albedo of the atmosphere. More of the suns light is reflected before it reaches the ground. The eruption of Pinatubo in 1992 resulted in a global cooling of up to half a degree for a couple of years. Other recent energetic eruptions include El Chichon (1982) and Agung (1963) which were preceded by half a century of little volcanic activity. The combined eruptions of La Soufrière (1812), Mayon (1814) and Tambora (1815) had catastrophic global effects, leading to a ‘year with no summer’ in 1816.

3. 1963 Agung Santa Maria 1902 1982 El Chichon 1992 Pinatubo
Santa Maria 1902
1982 El Chichon
1992 Pinatubo
In this climate graph of the past 150 years, you can clearly see the cooling effects of the major volcanic eruptions

4. Atmospheric Stability
Many things can cause air to rise
As it rises, the pressure falls and the air cools
If the air is then warmer than the surrounding air, it carries on rising – unstable.
If the air is colder than the surrounding air, it sinks back to where is came from – stable.
The stratosphere is always very stable – but explosive volcanic eruptions can blast material up into the stratosphere.
Once in the stratosphere, the stability means that the volcanic materials tends to stay there for several years.
In the troposphere, the temperature mainly falls with height. Air rising through the troposphere cools (at the adiabatic lapse rate) and as it cools, can be either warmer or colder than the air around it.
On the other hand, in the stratosphere, the temperature increases with height. This is because the ozone in the ozone layer absorbs the Sun’s UV and reemits the energy as heat, warming the stratosphere. This means that any air trying to rise from the troposphere, cooling as it rises, will ALWAYS be colder than the surrounding air, and will sink back to the troposphere.
You can see from the shape of the cumulonimbus cloud at the top, that air rising in the unstable troposphere reaches the top of the troposphere, the tropopause, and then cant rise into the stratosphere, so it has to spread out sideways.
Only very explosive volcanoes can blast material past the tropopause.
If the volcanic material remains in the troposphere, it is fairly rapidly removed – eg by rain. If it can get up into the stratosphere, it has an impact for much longer.
Eruptions that are mainly restricted to the troposphere are much less likely to alter climate, for two main reasons:
Firstly, the SO2 will not all be oxidised to sulphuric acid aerosol, as gas deposition processes operate on similar timescales to oxidation.
Secondly, the aerosol generated will be efficiently removed by deposition processes (e.g. rain!), and it will have a tropospheric lifetime of only days to weeks, compared with months to years in the stratosphere.

5. Explosive volcanoes and latitude
If material makes it into the stratosphere, it gets caught up in the Brewer-Dobson circulation
Major eruptions in lower latitudes are more climatically effective as the veil is capable of reaching the higher latitudes of both hemispheres, because of the direction of the atmospheric circulation. Material from major eruptions in the middle-to-high latitudes of each hemisphere tends to remain poleward of the eruption latitude. Major Icelandic or Alaskan/Aleutian/Kamchatkan eruptions, therefore, only influence the higher latitudes of the Northern Hemisphere.
Tropospheric circulation
Stratospheric circulation

6. For maximum effect a volcano should be…
Explosive
Low latitude
Effects are greater over land than over sea
Northern hemisphere summer season
Lots of Sulphur dioxide emitted.
Explosive – to get material into the stratosphere
Low latitude – material is spread through both hemispheres.
Cooling is most pronounced over land regions because the thermal inertia is much smaller than over the oceans i.e. it’s quicker for them to heat up or cool down. The effects in the Southern Hemisphere are less, because there is less land, and tend to spread out over a longer period.
The effects in the Northern Hemisphere are greatest in the summer season because, then, the Sun’s radiation levels are at their maximum (and so increasing the reflectivity of the atmosphere will have the biggest effect). The crucial factor in determining the climate impact is not necessarily the magnitude of the eruption, but is thought to be the quantity of sulphur dioxide (SO2) injected into the stratosphere.
Ash is removed from the atmosphere far faster than SO2.

7. Volcanic forcing of climate over the past 1500 years: An improved ice core-based index for climate models 2008, Chaochao Gao, Alan Robock, and Caspar Ammann
The amount of sulphate aerosol in the stratosphere in the northern hemisphere (top), southern hemisphere(middle) and whole stratosphere (bottom) for the last 1500 years. There was a major volcanic event in the 1200s which may have triggered the start of the Little Ice Age. Together with subsequent volcanoes, by allowing more Arctic sea-ice to form.

8. Volcanoes and their effect on climate David Viner & Phil Jones
In the upper four panels, Figure 1 shows the effects on global average temperatures of four low-latitude eruptions between the 1880s and 1980s. Whilst the individual eruptions show a lot of variability in the timing of the cooling (partly because the eruptions occur at different times of the year) the average of the four, labelled "composite" in the fifth panel, shows significant cooling for many of the months in the subsequent three years (particularly the boreal summers).

9. Effusive or small volcanoes - e.g. Bárðabunga, 2014-15
Explosive volcanoes
Super volcanoes or intense volcanic activity
Snowball Earth (pre-Cambrian)
Super volcanoes - any volcano capable of producing a volcanic eruption with an ejecta mass greater than 1015 kg
Toba, Sumatra (74,000 years ago), preceded major glaciation
Yellowstone (640,000 years ago), 5°C global cooling
A more dramatic change in albedo is associated with the ‘snowball Earth’ hypothesis. It has been suggested that during the Proterozoic ( million years ago) the positive albedo feedback associated with ice accumulation (as the Earth cools, more light coloured ice forms, which reflects more of the sun’s light, leading to further cooling) led to ice covering the whole Earth. In this scenario, volcanoes and the huge amounts of greenhouse gases they can emit, would be necessary to break out of the ice-climate feedback

10. Laki & Grímsvötn,
Lasted for eight months during 1783 to 1784, and produced one of the largest basaltic lava flows in historic times
The release of sulphur gases during fountaining produced an acid haze (aerosol) which spread widely and had a considerable environmental, and possibly climatic, impact on the Northern Hemisphere.
Troposphere, not stratosphere
Fire-fountains of magma reached 1.4km, ash reached 9-13km
The quantity of SO2 released was comparable to the total annual present-day anthropogenic input to the atmosphere
Benjamin Franklin, in 1783, first postulated that major volcanic eruptions affect climate, after the eruption of the Laki volcano in Iceland. Ironically, most of the ejected material from this eruption remained in the lower parts of the atmosphere, so Franklin had the right idea but the wrong volcano.

11. Laki & Grímsvötn,
During the explosive phases, the atmosphere over Iceland became loaded with fine ash and sulphuric acid droplets.
Grass growth was stunted, 50% of grazing livestock died and 22% of Icelanders died.
Haze or dry fog was reported over much of the Northern Hemisphere, blood red sunsets – affected vegetation, animals and people
Summer of 1783 was warm, was colder than usual, but was it due to the volcano?
Linked to famine and plague in middle east, virtual dying out of Inuit in NW Alaska
Did the aerosol reach the stratosphere?
Food poverty was a major factor in the build-up to the French revolution of 1789
Benjamin Franklin, in 1783, first postulated that major volcanic eruptions affect climate, after the eruption of the Laki volcano in Iceland. Ironically, most of the ejected material from this eruption remained in the lower parts of the atmosphere, so Franklin had the right idea but the wrong volcano.
this
study has shown that large and long duration eruptions such
as Laki, producing aerosol mainly in the troposphere, can
have a significant effect on climate on at least hemispheric
and seasonal scales through the formation of large-scale tropospheric
aerosol clouds that cause a large and widespread
negative radiative forcing.
El nino and monsoon weirdness also going on,
Anticyclone over Iceland affected summer temperatures and the dispersal of the sulphuric acid and ash
Realclimate: There can be some exceptions to the tropics-only rule, and at least one high latitude volcano appears to have had significant climate effects; Laki (Iceland, ). The crucial factor was that the eruption was almost continuous for over 8 months which lead to significantly elevated sulphate concentrations for that whole time over much of the Atlantic and European regions, even though stratospheric concentrations were likely not particularly exceptional.

12. Eyjafjallajökull, 2010
Eyjafjallajokull was not explosive enough to put material into the stratosphere, however it affected the climate in other ways. Firstly, the cancelled flights (because of the ash cloud) saved more carbon dioxide, a greenhouse gas, than the volcano itself emitted.
Secondly, the cancelled flights also meant there were fewer contrails (high level clouds) in the atmosphere. There is some evidence that contrails act a little bit like greenhouse gases – although they cool the climate, by reflecting the Sun’s light during the day, they absorb outgoing heat during the night, keeping the Earth warmer.

13. Volcanoes and Climate Change?
Less Ice
Less Pressure
More magma generation

14. Additional Sources: https://www.wunderground.com/climate/volcanoes.asp