|
Eruptions and Climate Change
Fire and Ice: Volcanic Eruptions and Global
Climate Change
The greatest immediate dangers and possible changes to the global
climate arise from volcanoes that erupt on time scales of hundreds
to thousands of years. Several 'cataclysmic' eruptions of the past
few centuries have resulted in a volcanic-forcing of climate and
a marked cooling of Earth's surface temperatures, with wide-ranging
consequences.
The detrimental impacts of volcanism may extend beyond regional
effects on populations and the evironment to influence the planet
as a whole and specifically its climate. Our present understanding
how volcanic eruptions affect climate derives from a much improved
understanding of eruptive processes, revised interpretations of
written accounts of famous historic eruptions, and new geologic
and climate-proxy evidence of the effects of volcanism in pre-historic
and historic times. These all lend support to the concept of `volcano-forcing'
as a mechanism of climate change.
Any sustained explosive eruption which generates a high-altitude
(stratospheric) eruption plume has the potential to influence global
climate. Such eruptions are termed 'Plinian' and are characteristic
of those at subduction-related volcanoes. Tephra, and hot gases
released from the magma, are forcefully ejected into the atmosphere,
forming a low-level eruption cloud above the vent. Initially, the
momentum of this cloud allows it to propagate upward to heights
of many hundreds of metres. However, in order for the cloud to reach
higher stratospheric altitudes, its density must be reduced below
that of the surrounding atmosphere. This will occur if sufficient
atmospheric air is entrained into the cloud and heated. Buoyancy
forces will then dominate and strong vertical convection will allow
the cloud to continue to rise tens of kilometres until it reaches
an altitude where its density becomes equal to that of the surrounding
atmosphere. Here, the plume will begin to spread laterally forming
a characteristic stratospheric 'umbrella' cloud.
Volcanic ash carried to high altitudes in the convecting plume rains
out from the base of the umbrella cloud over a period of several
days to months, blanketing the landscape below it. Due to the short
residence time of ash in the stratosphere, the effect of the particulate
loading on Earth's global radiation budget is negligible. Rather,
it is the gas content of the plume, specifically the sulphur dioxide
loading, that is of importance. Once injected into the stratosphere,
sulphur dioxide gradually reacts to form sulphuric acid aerosols,
clouds of fine liquid droplets dispersed in air. These aerosols
may persist for several years during which time they reduce the
receipt of solar radiation at the Earth's surface by scattering
and absorbing solar radiation. Their effect is to reduce tropospheric
temperatures. The degree of cooling is a function of both the optical
density (the extent to which aerosols scatter and absorb solar radiation)
of the aerosol and its latitudinal distribution as it spreads with
the atmospheric circulation. Eruptions at equatorial latitudes are
nicely positioned to disperse aerosols into both hemispheres and
so have far greater potential to affect global climate.
In recent years, eruptions such as the 1982 El Chichón and
1991 Pinatubo eruptions have had small but measurable effects on
global climate. Sulphuric acid aerosols from these eruptions encircled
the Earth in less than three months, reducing mid-latitude northern
hemisphere temperatures by 0.3-0.5EC.
The overall tendency of upper-atmospheric winds is to spiral towards
the poles where downward movement of air masses aids sedimentation
into the ice caps. As a result, sulphate concentrations in polar
ice cores provide a direct measure of the deposition of volcanically
produced sulphuric acid. Ice layers showing high sulphate concentration
correlate to well-documented recent eruptions, and such `acid spikes'
can be used to identify probable climate-forcing volcanic events
in early historic and pre-historic times. Identified are the eruptions
of Taupo (New Zealand) in 181, Huaynaputina (Peru) in 1600, Tambora
(Indonesia) in 1815, Krakatau (Indonesia) in 1883 and Gunung Agung
(Indonesia) in 1963, amongst many others.
Historical accounts of some of these events describe 'volcanic winters',
characterised by deteriorating climatic conditions and markedly
cooler summer temperatures for up to three years following the eruption.
The effects of a volcanic winter can be devastating, resulting in
widespread crop and livestock losses, famine and deaths of thousands
of people.
Some eruptions produce acid spikes at both poles, indicating widespread
circulation of sulphur-rich aerosols. These eruptions are assumed
to be of global importance and may be intimately linked with major
climatic global perturbations. One such eruption occurred 74,000
years ago at Mt Toba in Indonesia, during a period of climate change
associated with the last glacial cycle. The eruption produced a
stratospheric sulphuric acid loading in excess of one billion tonnes,
resulting in an estimated 3-5EC decrease in global surface temperatures
for up to 3 years, but more importantly, a lowering of summer temperatures
of >10EC in areas adjacent to those of established ice-cover.
This temperature change would have been sufficient to tip the balance
toward annual snow accumulation and the establishment of a snow-albedo
feedback mechanism. Dramatic increases in the planetary surface
albedo would thus have triggered rapid glacierization and an acceleration
of the global cooling already in progress.
The Toba eruption may itself have been triggered by a lowering of
sea levels associated with the onset of glaciation. Throughout the
Quaternary (the last 2 million years), periods of increased global
volcanic activity, especially at volcanoes along coastlines, appear
coincident with periods of rapidly fluctuating sea levels. During
low sea levels, hydrostatic pressures acting to confine magma at
depth are reduced, so allowing magma to ascend through new and reactivated
crustal fractures caused by increased crustal tension. During periods
of high sea level, the undermining of a volcanic edifice and periodic
flank collapses may trigger decompression related explosive eruptions.
Indeed, studies at Mt Etna, on the coast of Sicily, suggest that
past explosive eruptions are linked to rapidly changing sea levels
in the Mediterranean area. Given that more than 90% of all active
volcanoes are located within 20 km of the sea, the predicted rise
in global sea levels attributable to global warming may have important
implications for future eruptive activity, both in terms of eruption
periodicity and impact on global climate.
References:
Hammer CU, Clausen HB, Langway CC (1997) 50,000 Years of Recorded
Global Volcanism. Climatic Change 35: 1-15.
McGuire WJ (1992) Changing sea levels and erupting volcanoes: cause
and effect? Geology Today (July-August): 141-144.
Rampino MR, Self S (1993) Climate -Volcanism Feedback and the Toba
Eruption of ~74,000 years Ago. Quaternary Research 40:
269-280.
Self S, Zhao J-X, Holasek RE, Torres RC, King AJ (1995) The atmospheric
impact of the 1991 Mount Pinatubo Eruption, p. 1089-1115, in G.
Newhall and R. Punongbayan (Eds). Fire and Mud, University
of Washington Press
Sparks RSJ, Bursik MI, Carey SN, Gilbert JS, Glaze LS, Sigurdsson
H, Woods AW (1997) Volcanic Plumes. Wiley: Chichester.
Zielinski GA, Mayewski PA, Meeker LD, Whitlow S, Twickler MS, Morrison
M, Meese DA, Gow AJ, Alley RB (1994) Record of Volcanism Since 7000
B.C. from the GISP2 Greenland Ice Core and Implications for the
Volcano-Climate System. Science, 264: 948-952.
|
|