There are a number of natural forcing factors at work on the earth’s climate, causing short-term changes to weather patterns and longer-term climate change. It is against this backdrop of natural variability that the effects of human-induced climate change must be measured. Of these, volcanoes are a key forcing mechanism, which have had large impacts on human civilization in the recent past. For example, in northern Europe, the medieval Little Ice Age corresponded with a period of high volcanic activity, whilst the Medieval Warm Period occurred at a time of low volcanic activity. Solar variation was also a key factor in these climate changes.
Volcanic forcing can have both short-lived and longer-term impacts on global climate.
- Sulphuric aerosols injected up into the stratosphere reflect back nearly all the sun’s energy that they encounter, sending it back into space and preventing it from reaching the planet’s surface. This is a short-lived effect that can be measured in terms of years.
- Volcanic ash, a form of black carbon, can be deposited on snow fields, glaciers and other reflective bodies, reducing their albedo. Hence they reflect less of the sun’s energy and absorb more of it. This would result in a short-term warming, until the deposits are covered by further snowfall.
- The volcanoes inject carbon dioxide and other long-lasting blanket gases into the atmosphere, and their effect can be measured in centuries. This have a warming effect on global climate.
The effect of volcanoes on climate in the short and long term has been used by some climate change sceptics to attempt to undermine the credibility of climate science. However, as the British Geological Survey puts it:
“The contribution to the present day atmospheric CO2 loading from volcanic emissions is … relatively insignificant.”
…especially when compared with the effects of anthropogenic carbon dioxide pollution. Volcanoes contribute c.100–300 million tonnes of CO2 each year; which is only 1% of the annual anthropogenic carbon dioxide load.