My name is Anna Latham. I am a first year geography student at Bristol university. Following lectures at the end of last term, I have chosen 'Volcanoes and Climate Change' as the subject for the web page that I am required to produce as part of the Computer Methods course that I take alongside geography in my first year. If anyone would like to e-mail me with any comments about my first web page - or highlight any spelling mistakes that may have crept into it along the way (!!) please CLICK HERE! This page probably doesn't contain any new information about volcanoes and climate change than you can already find on most other web pages on this subject. What I hope it will do is provide an overview of the main topics within the subject and easy access to relevant links to read about certain aspects of the page in more detail if you wish to do so.

Introduction

The scientific understanding of the environment of the Earth is increasing all the time. In climatology this increase has been most dramatic over the last 25 years. However, much still remains unknown, especially the record of past climatic variations and their causes. At the moment, global warming is at the top of the political agenda. The temperature increase of 0.3-0.6^oC since the mid 19th century is taken as evidence that anthroprogenic increases in greenhouse gases are affecting the climate. This report focuses on a process counter to this namely; global cooling. In particular, the role that volcanic eruptions are thought to play in this 'negative forcing' (global cooling). Volcanoes have long been suspected of having an important effect on the climate. However, eruptions are rarely well documented, so it is very difficult to know what the climatic effects may be. The page is split up into the following sections:

• The effects that volcanoes have on the atmosphere
• The example of Mount Pinatubo
• How can the climatic effects of volcanoes be measured?
• What affects the extent of the climatic impact an eruption has?
• Other factors contributing to global cooling
• Conclusions
• Pictures!
• Other useful links

Click here to see the full image

These aerosols are responsible for the radiative forcing of the atmosphere. The following diagram shows the solar and terrestrial radiation balance (CALMET '95):

Click here to see the full image,

Radiative forcing can be either:

• positive (keep in long wave radiation, therefore warming the Earth's temperature). or
• negative (keep out short wave radiation, therefore cooling the Earth's temperature).

Whether aerosols are warming or cooling depends on the size of the particles. If the particle is <2mm in diameter it has a cooling effect. If it is >2mm in diameter, it has a warming effect. Tropospheric aerosols found in the atmosphere due to pollution have a warming effect. The ones produced during volcanic eruptions reflect back incoming solar radiation, preventing it from reaching the earth’s surface, therefore volcanic aerosols have a cooling effect.

Mount Pinatubo

Mount Pinatubo erupted in the Philippines in 1991. The eruption cloud was 18km wide and 30km high. It contained 30x10² grams of aerosol, very rich in both sulphur and sulphur-dioxide. It had a very big atmospheric effect. After the eruption in 1991, 1992 was the coolest year on Earth for 30 years. Although the aerosols were emitted from a very small point i.e. the vent of the volcano, they are quickly distributed globally. They interfere with the energy input of the sun resulting in a cooling effect all over the globe.

How can the effect of an eruption on the climate be measured?

There are two main indeces used to assess the probable climatic impact of volcanic eruptions:

• Dust Veil Index (DVI): This is based on estimations of the amount of material dispersed into the atmosphere and the temperatures at and amount of sunlight reaching the surface of the Earth. This index is, however, very subjective and is dependant on observations made at mid-latitudes. It therefore, is only really representative of the effects of eruptions on the mid-latitude climate, and is not entirely representative of the whole global scenario.
• Volcanic Explosivity index (VEI): This system ranks eruptions using only volcanological criteria to assess the magnitude, intensity, dispersion and destructiveness of an event. It does not rely on temperature and solar radiation measurements at the surface, therefore is not weighted purely to eruption effects in mid-latitudes. Eruptions are ranked from 1 to 8. Those thought to have injected material into the stratosphere have a VEI of >4. The amount of SO₂ and H₂S emitted is very important as it is these which form the sulphuric acid aerosols responsible for global cooling. CLICK HERE to see a table of H₂SO₄ yields from major eruptions over the last 500 years.
• Other Methods:Other indications of the climatic impact of volcanic eruptions can be found within glaciological records, relating to the changes in the acidity of past precipitation coinciding with volcanic eruptions. Tree ring records may indicate periods of stunted tree growth due to reduced sunlight receipts at the earth’s surface after a volcanic eruption. Finally, instrumental records of decreasing solar radiation with increased concentration of dust particles in the atmosphere may indicate the impact of an eruption, however it must be made certain that the dust found in the atmosphere is due to an eruption and not to some other anthroprogenic cause.

The impact an eruption has on the climate is more likely to be affected by the latitude of the volcano with high latitude eruptions having less impact than eruptions at lower latitudes where circulation is greater, there is more dispersal of the dust cloud and therefore a more global effect on the climate. Other influences the extent of an eruptions climatic impact include the height of the emission and the chemical composition of the emission (especially how much sulphur dioxide is emitted).

Are there any other things that could contribute to negative forcing in the atmosphere, other than volcanic eruptions?

There are two other possible negative forcing factors. They are:

• Solar radiation changes: The reduction in the number of sun spots by 51 between 1880 and 1965 is thought to have reduced global mean temperatures by 0.12^oC. However, this is fairly small given that global mean temperatures have increased by 0.45^oC since 1880 and is unlikely to be a major contributor to global cooling.
• The change in frequency of El Nino/Southern Oscillation (ENSO) events: It has only recently been shown that the changes in frequency of the ENSO event has global effects. Large scale temperature and precipitation anomalies are often related to the phase and magnitude of El Nino events. Little is known about the contribution of ENSO events to global cooling but it is thought to be very important in future research to consider the impact of ENSO events when they coincide with volcanic eruptions.

In addition to these other negative forcing factors the case for global warming must be remembered. The IPCC Report suggested that the increase in anthroprogenic aerosols would counteract the warming occurring due to the increase in greenhouse gases. However, others have suggested that this 'cancelling out' effect would only be in the short term. Aerosols only survive for about one week in the atmosphere, greenhouse gases can remain in the troposphere for centuries. After the eruption of Mount Pinatubo in 1991, there was a marked global cooling the following year, but by 1993/4 the cooling was reversed as the aerosols fell out of the atmosphere. 1994 was recorded as being the fourth warmest year on record.

The diagram below compares the different radiative forcing mechanisms. Houghton, Jenkins, Ephraums eds, 1990:1990 IPCC, Cambridge University Press, fig2.6.

Click here to see the full image.

I have hopefully outlined how a volcano can contribute to global cooling and what factors affect the extent of an eruptions impact, as well as introducing the case for global warming and suggesting that there may be other processes operating that are also contributing to global cooling.
It must be remembered that there is a great deal of uncertainty surrounding the subject of climate change in general. Although some evidence can be found for the negative forcing effect of volcanic eruptions on the atmosphere. The record of explosive eruptions is still quite incomplete, especially relating to the chemical characteristics of the plumes produced during volcanic events. A complete chronology of eruption events is essential for a better understanding of the variability of the climate over the last, and the next, few centuries.

Other useful links

• Volcanoes and Global Climate Change
  http://spso2.gsfc.nasa.gov/NASA_FACTS/volcanoes/volcano.html
• Volcanoes and Climate Change
  http://tgl.geology.muohio.edu/focus/VolcanoesandClimateChange.html
• Volcanoes and Global Climate Change
  http://ww.circles.org/cdroms/CIESIN/nasafact/volc/volcano.htm
• Images on Global Energy Balance, Radiative Forcing
  http://www.physics.iastate.edu//gcp/forcing/images.html