Aerosols in the STRATOSPHERE reflect incoming sunlight, producing a cooling effect on global temperatures. The cooling effect lasts until the aerosols are cycled out of the atmosphere by natural physical and chemical processes. In the case of large eruptions or a succession of eruptions such as in the early 1800s, the cooling effect can last several decades. A reduction in solar output is thought to be primarily responsible for the large-scale cooling experienced during the LITTLE ICE AGE in the late 1600s, but strong volcanic activity at that time may have also contributed (see Solar variations for a simulation of the relative contributions of volcanic activity, changes in solar output, and other factors to climate forcing over the past thousand years).
In recent decades, observations following the major volcanic eruptions of Mount Agung in 1963, El Chichón in 1982 and Mount Pinatubo in 1991 illustrate the cooling influence of these events, as illustrated in the figure below.
Major volcanic eruptions and temperature anomalies since 1940
Recent calculations of the reduction in global mean temperature following major volcanic eruptions.
Source: Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003, Figure 19, p. 13.
Volcanic aerosols may also have indirect influences on the climate that are less well understood. For example, the presence of aerosols in parts of the stratosphere can produce temperature gradients both vertically and horizontally, which could alter the air circulation patterns in the stratosphere, and in turn affect TROPOSPHERIC air circulation. Aerosols may also interact with chemical processes occurring in the stratosphere, which could affect distribution and concentration of ozone and other trace gases. There is evidence to suggest that volcanic aerosols in the stratosphere have contributed to the depletion of the ozone layer and changes in the dominant Arctic atmospheric circulation pattern. Feedbacks with the BIOSPHERE have also been observed, with the vegetation CO2 sink increasing after volcanic eruptions. This is thought to occur because ecosystems tend to release less carbon dioxide through soil respiration due to the lower temperatures after volcanic eruptions. At the same time, uptake of carbon dioxide through photosynthesis is relatively enhanced due to increased scattering of sunlight in the atmosphere by aerosols, which increases the amount of diffuse light that plants can utilise more efficiently. Considerable uncertainties remain over the magnitude and mechanisms of these indirect influences of stratospheric aerosols on the climate system.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing.
Bureau of Meteorology, The greenhouse effect and climate change, Bureau of Meteorology, 2003.