Volcanic eruptions and climate change
CSIRO Atmospheric Research Greenhouse Information
A massive volcanic eruption can blast huge clouds of ash and gases into
the atmosphere. Millions of tonnes of sulfur dioxide gas may reach the
upper atmosphere (the stratosphere). There, the sulfur dioxide transforms
into tiny particles of sulfuric acid, known as aerosol. The particles
reflect energy from the sun back into space, preventing some of the sun's
rays from heating the Earth.
This information paper examines the impact of volcanic eruptions on climate
and the make-up of the atmosphere.
Volcanic eruptions that add significant quantities of sulfur dioxide
into the stratosphere lower global surface temperatures. Conversion of
the sulfur dioxide to sulfuric acid aerosol in the stratosphere takes
some months, so maximum cooling occurs up to a year after the eruption.
It may take as long as seven years before the cooling influence of the
volcanic aerosol disappears completely.
The 1991 eruption of Mt Pinatubo in the Philippines was one of the largest
in the past 100 years. The injection into the stratosphere of 14-26 million
tonnes of sulfur dioxide led to a global surface cooling of 0.5°C
a year after the eruption. The climatic impact of the Pinatubo aerosol
was stronger than the warming effects of either El Niño or human-induced
greenhouse gas changes during 1991-93.
The observation of global surface cooling caused by the Mt Pinatubo eruption
was successfully simulated by several global climate models, providing
support for the effectiveness of models in replicating the climate system.
The eruption of Toba in Sumatra 73,500 years ago may have cooled the
planet by 3-5°C, resulting in a volcanic winter.
As well as cooling the lower atmosphere (troposphere), volcanic aerosol
can absorb both thermal radiation from the ground and solar radiation,
which leads to a warming of the stratosphere above the tropics.
Volcanic eruptions need to be taken into account when using satellite
measurements of temperature to detect trends caused by human activity.
For example, once scientists had corrected the satellite temperature record
to allow for the effects of volcanic eruptions and El Niño, the
satellite measurements showed a small warming trend.
Global-mean monthly temperature for the period 1980-1999, showing the
effects of the eruption of Mt Pinatubo in 1991. The upper graph shows
the complete global-mean temperature record as context. (Source: University
of East Anglia Climatic Research Unit: http://www.cru.uea.ac.uk/cru/info/causecc/)
Probability of eruptions
Eruptions of the scale of Mt Pinatubo occur roughly every 100 years.
The impact of such eruptions needs to be taken into account when assessing
long-term climate trends.
Eruptions are sporadic, unpredictable and vary in size. Ash and gases
vary in composition. These factors present a challenge to climatologists
seeking to isolate volcanic effects from other influences on past global
There is a reasonably high probability that the temperature record will
be influenced by volcanic activity in the next decade.
Volcanoes are also sources of water vapour and carbon dioxide, but their
contribution to the global budgets of greenhouse gases is very small.
On the time-scale of decades to centuries, greenhouse gas emissions from
volcanic sources cause negligible climate change.
However, volcanic emissions of gases such as sulfur dioxide, hydrogen
sulfide and hydrogen fluoride are important compared to human-induced
sources. These gases have effects on climate (cooling), on stratospheric
ozone and possibly on global cloudiness.
Robock, A., (1999) Volcanoes and climate, Reviews of Geophysics, 38(2),