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Advanced Review
Volcanoes and climate
Jihong Cole-Dai
Of the natural forcings causing short-term climatic variations, volcanism, along
with its climatic impact, is perhaps the best understood. The primary net result
of the impact is the reduced receipt of solar energy at Earth’s surface due to
the scattering of incoming solar radiation by secondary sulfate aerosols formed
from volcanic sulfur. The quantitative effects can be measured in energy-balancebased climate models, which require validation using high-quality paleoclimatic
and paleovolcanic data. An important advancement in the effort to understand
the role of volcanism in climate change in the recent decade is the significant
improvement in paleovolcanic records from polar ice cores, represented by
long records with unprecedented temporal accuracy and precision, and by the
potential to identify climate-impacting stratospheric eruptions in the records.
Other improvements include (1) the investigation of long-term relationship
between eruptions (including super-eruptions) and climate variations, beyond
an eruption’s radiative impact of up to a few years; (2) a better understanding
of the response to volcanic perturbation of feedback mechanisms in the climate
system; and (3) the limited role of volcanic eruptions in the era of human-induced
greenhouse warming. Urgent research/investigation is needed to evaluate the
geoengineering proposition to counteract greenhouse warming by injecting sulfur
dioxide into the stratosphere, which is based on the significant cooling effects of
stratospheric volcanic eruptions, and its serious unintended consequences.  2010
John Wiley & Sons, Ltd. WIREs Clim Change
INTRODUCTION
E
arth’s climate is primarily determined by the
energy in the climate system. A stable climate
is maintained by an energy steady state between
incoming and outgoing radiation on Earth and by
balanced energy distribution and exchange among the
system components (atmosphere, ocean, and land).1
Climate change occurs when the energy steady state
is altered or perturbed by changes in external or
internal factors, called climate forcings. The most
important climate forcings are: (1) the reception of
insolation and its latitudinal distribution determined
by the cyclic orbital relationship (Milankovitch cycles)
between Earth and Sun2 ; (2) occasional volcanic
eruptions altering the atmospheric albedo and
therefore the energy budget; (3) fluctuations in solar
irradiance upsetting the Earth’s energy steady state;
and (4) changes in minor atmospheric components
∗ Correspondence
to: [email protected]
Department of Chemistry and Biochemistry, South Dakota State
University, Brookings, SD 57007, USA
DOI: 10.1002/wcc.76
controlling the greenhouse effect—absorption and
subsequent emission of outgoing thermal radiation.
The substantial and growing human greenhouse
gas emissions since the nineteenth century3 have
caused, or at least contributed to, the significant
temperature rise over the last century.4 Forecasting
future climate with well-constrained uncertainties
requires the use of climate models,5 which need to be
continuously evaluated and improved.6 Improvement
and validation of sophisticated climate models benefit
from reliable paleoclimatic records and accurate
knowledge of past variations of climate forcings
including volcanism.7
The climatic impact of volcanic eruptions has
long been a topic of intense interest and of inquiry
and considerable research effort. Early work on volcanism focused on documenting active volcanoes and
past volcanic activities. Scientific efforts to explore
the connection between volcanoes and climate began
in the nineteenth century and were greatly aided by
systematic documentation of volcanic eruptions that
began in the mid-twentieth century. Since then, the
development and improvement of our knowledge on
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Advanced Review
the volcano–climate connection have been no less
spectacular than the eruptions themselves. In particular, the advance in volcano–climate research over the
last two decades has essentially elucidated the primary physical processes linking climatic response to
volcanic emissions and contributed to the effort to
quantify natural climate variability and the role of
volcanism in recent climate variations. This gives us
the capability and confidence to predict the probable
climatic response to a given level of volcanic activities.
This remarkable advance is, in part, due to the heightened interests in climate change brought about by the
concern of the human-induced greenhouse warming
and also aided by the advent of tools that greatly
increased our capabilities to study all aspects of the
volcano–climate system. Those tools include satellite
and other modern observational platforms, a global
volcano monitoring system, paleovolcanic data from
a variety of proxy measurements (developed as part
of paleoclimatic data), and sophisticated climate models able to simulate accurately the physical processes
relating atmospheric properties to climate conditions.
For a thorough understanding of the volcano–
climate topic and past research achievement, the
reader is referred to an excellent comprehensive and
well-illustrated review by Alan Robock.8 At the time
of its publication (2000), the state of the art for the
discipline was characterized by a clear understanding
of the global radiative effects of volcanic aerosols and
of regional differences in climatic response attributed
to atmospheric dynamics, a recognition of the potential impact on stratospheric ozone, and a qualitative
description of the role of volcanism in past episodes of
climate changes. Robock concluded that although no
index of past volcanism was perfect, polar ice cores
held the most promise to provide volcanic records in
the assessment of the climatic impact of past volcanic
eruptions. This review summarizes what is known
about the volcano–climate connection with an emphasis on the most significant advancements, particularly
those in ice-core volcanic records, since 2000 and
offers a brief outlook on future developments.
THE PHYSICAL CONNECTION
Unlike orbital, solar and greenhouse climate forcings,
volcanic eruptions are highly visual to an observer
and, therefore, can be, in concept, readily connected
to their impact on climate. For a long time, people
knew through direct experience and by instinct
that explosive volcanic eruptions changed weather
patterns. The instances and extent of climate change
caused by volcanic eruptions can be examined in
the records of past volcanic eruptions and climate
data. However, a complete understanding of the
physical processes connecting a volcanic eruption to
its climatic effects is necessary to the efforts to model
the climate system accurately to forecast future climate
change. This section offers a close look at the physical
connection between volcanic eruptions and climate.
Volcanic Emissions
Volcanic eruptions are a strong and visible
manifestation of the internal dynamics of the solid
Earth. A strong characteristic is the emission into
the atmosphere of large quantities of solid particles
(ash) and gaseous substances in a short period of
time. Ash clouds can block sunlight and darken
the skies visibly, resulting in reduced solar heating.
However, such effects are typically short-lived and
geographically limited, as the ash settles by gravity out
of the atmosphere quickly and locally.9 The gaseous
emissions consist primarily of water vapor, carbon
dioxide and reduced sulfur compounds (mainly SO2 ),
nitrogen,10 and halogen compounds.11 They become
constituents of the atmosphere and their atmospheric
residence times are subject to the biogeochemical
cycles of the elements (O, C, and S). The amounts of
water vapor and carbon dioxide emitted by volcanoes
are negligibly small, compared with the atmospheric
reservoir size of these gases, and therefore their climate
impact is insignificant.11
Aerosols and Climate
In the oxidizing atmosphere, SO2 from volcanic and
other sources is converted to the chemically stable
sulfuric acid (H2 SO4 ) or sulfate. The conversion is
quite rapid. In the troposphere, SO2 is converted
to H2 SO4 in a matter of a few days,12 while
the conversion in the stratosphere may take up to
several weeks and months.12,13 Sulfuric acid is highly
hygroscopic and consequently exists as H2 SO4 ·H2 O
or sulfuric acid aerosols. These sulfuric acid or sulfate
aerosols scatter efficiently the visible part of the solar
spectrum; their presence increases the optical depth
of the atmosphere and therefore the atmospheric
albedo.12 In addition, they also interact with both
incoming and outgoing infrared radiation. Because
the net effect of aerosols is the reduction of energy
receipt near the surface,8 the most significant climatic
impact of aerosols is the cooling at the surface and in
the lower troposphere.
Stratospheric Eruptions
Small or moderate eruptions and continuous noneruptive degassing by quiescent volcanoes emit relatively small amounts of volcanic substances into the
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Volcanoes and climate
troposphere.11,14 More importantly, the average residence time of sulfate aerosols in the troposphere
is short (a few days), due to efficient removal via
precipitation.15,16 Therefore most tropospheric eruptions and degassing possess no significant potential
for climate impact beyond the short-lived effect of ash
clouds on local weather.9,14 Exceptionally large tropospheric eruptions may have some climate impact,
as the result of the brief presence of sufficiently
large quantities of volcanic aerosols.14,17 This brief
impact of tropospheric eruptions needs to be distinguished from the climatic effect of tropospheric
sulfate aerosols from anthropogenic emissions of
sulfur. The anthropogenic sulfate aerosols, primarily resulting from coal combustion and industrial
smelting operations,18,19 play a significant role in
human-induced climate forcing (the ‘aerosol forcing’
effect) because the aerosols are sustained by continuous emissions.20–22
In contrast, stratospheric eruptions are much
more likely to have significant and lasting climatic
impact, due to (1) the usually large amount of SO2
emitted, (2) the much longer aerosol residence times in
the stratosphere than in the troposphere, and (3) the
hemispheric or global distribution of the volcanic
aerosols via stratospheric circulation. Extraordinarily large eruptions injecting massive amounts of SO2
directly into the stratosphere can cause significant surface and tropospheric cooling. Such eruptions in the
tropics are capable of global impact, for their aerosols
are usually distributed globally through rapid longitudinal circulation and poleward transport in both
directions.8
That our understanding of the main physical
processes linking volcanic eruptions to climate has
not changed significantly since 2000 testifies to the
excellent state of the discipline from the prior research
reviewed by Robock.8 One of the significant developments since then may be the possible application of
the knowledge on volcano–climate to solving a pressing problem. The global cooling effect of stratospheric
volcanic eruptions provides the physical basis for the
geoengineering proposition (albedo enhancement) to
counteract the anthropogenic greenhouse warming by
putting SO2 into the stratosphere. A brief discussion of the anticipated benefits and potential unintended consequences is presented in Section Future
Developments.
PAST VOLCANISM
Due to the complexity of the climate system, understanding and disentangling the effects of individual
forcings require careful analysis of time series of both
climate and forcing data. Not surprisingly, records of
volcanic eruptions are needed to assess the role of volcanism in climate variations. In this section, the types
of volcanic records and their strengths and weaknesses
are discussed from the perspective of robust records
for climate modeling. The volcanic impact on climate
can be assessed by comparing these records with paleoclimatic data, which is discussed in detail in Section
Volcanic Impact on Climate.
Documentary History of Volcanic Eruptions
Devastating volcanic eruptions were documented in
written history since the early times. An example is the
eruption of Mount Vesuvius that buried the Roman
city Pompeii in 79 AD.23,24 Historical documentation
was limited in several ways. First, only those eruptions
that actually caused immediate and significant societal and economic damages were worthy of entry in
the history books. Second, documentation often relied
on and therefore required eyewitness accounts and
reliable record-keeping; as a consequence, eruptions
in remote locations with sparse human presence were
very unlikely to be documented for lack of eyewitnesses or of systematic information gathering. Third,
in many regions and countries of the world, written
histories did not begin until the most recent centuries.
Finally, historical documentation is often only descriptive, neither scientific nor quantitative.25 In a few
cases, historical documentation may be supplemented
by private papers such as diaries.26
Early volcanic records relied on occasional
reports and anecdotes in personal or news accounts
and historical/political documents. More reliable compilation of active volcanoes and past volcanic eruptions began in the mid-nineteenth century.27 These
efforts were expanded in the early twentieth century
with the addition of volcanological data from geological investigations. A major advance occurred in
the 1950s with the establishment of a global monitoring and reporting network and the publication
of Catalog of Active Volcanoes of the World by the
International Association of Volcanology and Chemistry of the Earth’s Interior.28 In addition to relying on
records of eyewitness description, the efforts have also
tapped into documentation of indirect observations.
One of the spectacular effects of a volcanic eruption
is the variation in visible atmospheric characteristics
(color and turbidity) and temperature anomalies due
to the presence of volcanic dust or aerosols in the
atmosphere. The dust veil index (DVI) by Lamb29 is a
comprehensive compilation of both historical records
and the observed atmospheric aftermath of volcanic
eruptions since 1500 AD.
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Volcanological studies document geological
evidence and consequences of eruptions. The extent of
volcanic eruptions is estimated with the size of craters
and ashfall coverage (distance from the volcano,
and depth and area of ash layer), and the type of
eruption and the immediate environmental impact
can be studied using the geochemistry of volcanic
materials. This type of measurement has led to a
semiquantitative volcanic explositivity index (VEI)
introduced by Newhall and Self.25
Researchers at the U.S. National Museum of
Natural History of the Smithsonian Institution collected all available volcanic records and published the
chronology Volcanoes of the World28 in the early
1980s. This comprehensive compilation incorporated
the content of Catalog of Active Volcanoes of the
World and its successor Bulletin of Volcanic Eruptions, and the DVI and VEI. The original chronology
was updated30 in the 1990s. Since then, the chronology has been continuously updated and the current
catalog can be found on the website of the museum.31
An apparent and significant shortcoming of
volcanic records from historical and geological documentation of direct and indirect observations is that
volcanic eruptions may not be included due to the
lack of documented observations. The much more
comprehensive proxy records from ice cores, discussed next, have shown that many eruptions in
the past, even very large eruptions, do not appear
in the observation-based records. Since the establishment of the global volcano monitoring network in the
1950s, it has become much less likely that an explosive
eruption anywhere in the world is not observed and
recorded. The advent of global satellite measurement
of atmosphere properties has significantly reduced the
remaining possibility that any eruptions are unnoticed, although small eruptions in very remote regions
may still be missed.
Proxy Records
To quantify the role of volcanism in natural climatic
variations and to assess the human influence on climate, longer and more comprehensive records than the
documentary history of volcanic eruptions are needed.
These records can come from geological and biological
systems that archive environmental variables including the effects of volcanic eruptions. The two major
types of the proxy records use polar ice cores and tree
rings, which are described below, respectively.
Ice Cores
Volcanic sulfuric acid or sulfate aerosols deposit from
the atmosphere onto the Earth’s surface. On the polar
ice sheets of Antarctica and Greenland, the volcanic
sulfuric acid fallout is preserved in the snow layers
accumulated over hundreds and thousands of years.
Measurement of acidity and/or sulfate concentration
in polar ice cores can be used to detect and quantify
signals of past volcanic eruptions in the snow strata.
The foundation for ice-core volcanic records was
established by Claus U. Hammer and colleagues, in
a landmark study of volcanic acid in Greenland ice
cores.23 Measurement of electric conductivity on solid
ice (ECM) and of meltwater conductivity performed
on early Greenland ice cores32,33 showed the presence
of large acidity signals corresponding to well-known
historic volcanic eruptions as well as eruptions with no
entry in the documentary records. The most successful
application of the ECM measurement yielded a
50,000-year record of volcanism from a deep ice
core from Byrd Station, Antarctica.34 At about the
same time when the initial conductivity measurement
was performed on Greenland ice cores, Delmas
and Boutron35 made the first sulfate concentration
measurement in Antarctica ice cores in search of
volcanic sulfuric acid signals.
The conductivity techniques, consisting of the
ECM method and the similar dielectric property (DEP)
method, allow fast measurement of the ionic content
(including volcanic sulfuric acid) of ice.36–38 The volcanic sulfuric acid in polar snow is superimposed
on a relatively stable background of sulfuric acid
and other ionic compounds from continuous, nonvolcanic sources such as marine biogenic emissions.39
Detection of volcanic signals in ice cores using conductivity measurement assumes that the acid content
of the ice is the most important contributor to the
ice electric conductivity and any brief elevation of
the acidity is caused by the input of volcanic sulfuric
acid.33 However, several factors complicate the detection of volcanic signals: the presence of other acids
(e.g., nitric acid) and the variation of their concentrations in ice40 ; the neutralization of volcanic acid by
alkaline dust; and the influence of physical structure
and temperature on the conductivity of the solid ice.36
These complications limit the quality of the volcanic
records derived from conductivity measurements.
In constructing ice-core volcanic records, sulfate
measurement is preferred over acidity measurement
because sulfate is a volcanic species and its sources are
few. The introduction of ion chromatography to ice
core research41 significantly improved the accuracy
and speed of sulfate measurement. Because the ion
chromatographic sulfate measurement is not sensitive
to interference by non-sulfur substances in ice and
is generally not affected by non-chemical factors, the
technique allows the detection and quantification of
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Volcanoes and climate
volcanic signals with significantly reduced analytical
noise and errors, as compared with conductivity
measurement,42 and therefore improved the quality
of ice-core volcanic records. At the time (2000) of
the review by Robock,8 only a few ice-core volcanic
records were available and the quality of the records
was quite variable.42–44 Since 2000, a relatively large
number of new ice-core volcanic records have been
published. In general, these new records are of
higher quality than previous ice core records, due
to remarkable improvements in ice core chemical
measurement, and in increased accuracy and precision
of ice core dating. The EPICA Dome C core45,46
provides the longest continuous volcanic record
(45,000 years) from Antarctica ice cores. Detailed
sulfate measurement in a Siple Dome, West Antarctica
ice core is used by Kurbatov et al.47 to construct
a 12,000-year volcanic record with annual dating
resolution. These long records from Antarctica ice
cores complement previously developed long records
from Greenland ice cores.44,48,49 There are numerous
ice core records of shorter temporal coverage.50–56
Legrand and Delmas43 first proposed to establish
a volcanic index (the glaciological volcanic index)
based on ice core measurements (concentration and/or
flux of volcanic sulfate) to represent the atmospheric
impact (aerosol mass loading) of volcanic eruptions.
However, the presence and magnitude of volcanic
signals in a single ice core may be influenced by
local glaciology factors such as deposition mechanism,
snow accumulation rate, and the preservation of
volcanic materials in snow. Ice core records from
multiple locations need to be combined statistically to
yield composite ice core indexes more representative
of volcanic forcing of regional or global climate.
Robock and Free57 used a composite of several icecore volcanic records available at the time (1995) to
construct a more robust ice-core volcanic index (IVI)
for climate forcing. Recently (2008), the IVI has been
improved by Gao et al.58 based on acidity and sulfate
data from a large number (54) of bipolar ice cores.
Characteristics of Ice-Core Volcanic Records
It has been pointed out59 that the most important factor of the climate impact is the sulfur or aerosol mass
loading, not the explositivity, of a volcanic eruption.
Therefore, the ice core-based volcanic index enjoys
an important advantage over other indexes such as
DVI and VEI in that the ice-core volcanic parameter
(i.e., sulfate or acidity) is a direct measure of, but not
affected by, the climate forcing by a volcanic eruption.
Ice core records are quantitative. The experimentally measured concentration or flux of volcanic
sulfate in ice can be extrapolated to aerosol mass
loading by eruptions,33,60,61 and the aerosol mass
loading can in turn be converted to atmospheric optical depth61 representing volcanic forcing. However,
such conversion must be done with caution, for the
relationship between aerosol mass loading and optical
depth may not be linear over the range of mass loading
by eruptions of varying magnitude, as optical depth
also depends on the size distribution of the aerosol
particles.62
A notable limitation of ice-core volcanic records
has been the inability to distinguish signals of a relatively few large, climate-impacting stratospheric eruptions from those of numerous tropospheric eruptions
of much lesser significance. In general, stratospheric
eruptions are large in magnitude (SO2 emission) and
appear as large sulfate or acidity signals in ice cores.
However, sulfate signals of some small or moderate tropospheric eruptions may also appear large
in certain ice cores, due to the close proximity of
the erupting volcanoes to the ice core sites. Mistaking these as signals by large eruptions may lead to
overestimate of aerosol mass loadings of the small
or moderate eruptions and of their climate impact.
Savarino et al.63,64 first pointed out that the isotopic
composition of volcanic sulfate of stratospheric eruptions is likely different from that of tropospheric
eruptions, resulting from the unique chemical processes in the stratosphere causing mass independent
fractionation of the isotopes. The isotopic distinction
in sulfate of stratospheric eruptions has been recently
confirmed65,66 in a large number of volcanic sulfate
signals in Antarctic ice cores. Cole-Dai et al.67 used the
isotope technique to confirm that a prominent sulfate
signal in bipolar ice core records is from a low-latitude,
stratospheric eruption in 1809 AD., rather than two
small or moderate eruptions in the high latitudes of
Northern and Southern Hemispheres, respectively.
Significant work remains in ice-core volcanic
records. Local factors that contribute to the
quantitative characteristics of ice cores from a specific
location need to be identified and minimized when
composite indexes are produced from multilocation
ice core records.58 Although several long ice-core
volcanic records exist, the quality of the records
beyond the last 1000 years suffers from low temporal
resolution of the sulfate or acidity measurement and
from large dating uncertainties.45,49,68 Detailed ice
core chemical analysis and high-precision chronology,
requiring sub-annual analytical measurement and
dating, are needed to capture the volcanic signals of
very short duration (less than a year). The challenges
of such high-quality analytical measurement and
chronology development are formidable. Fortunately,
recent advances in ice core chemical analysis69,70
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and dating techniques make it likely that significant
improvements can be expected in future ice-core
volcanic records.
Tree Rings
The tropospheric cooling by volcanic eruptions
reduces the growth of annual rings of trees near
the upper elevation limit of vegetation and, in cases
where freezing occurs during the growing season,
results in frost rings—rings with visibly damaged cells.
Therefore, the identification of frost rings and the
accurate measurement of the width and/or density of
tree rings can yield records of volcanic eruptions.
LaMarche and Hirschboeck71 used frost rings in
bristlecone pines to investigate climatically effective
volcanic eruptions since 3435 BC. Jones et al.72 found
evidence of widespread effects of volcanic eruptions
in annual ring width and maximum density data
of a large number of trees . Salzer and Hughes73
used tree ring data to compile a list of explosive
volcanic eruptions in the last 5000 years. These
data sets complement ice core records by providing
confirmation from an alternative form of proxy record
and suggesting more precise dates for older volcanic
events. However, an important difference between
volcanic records from ice cores and those from tree
rings is that the sulfate and acidity records are a direct
measure of the volcanic forcing, whereas the tree
growth parameters reflect the climatic response to the
volcanic forcing. As a result, tree ring volcanic records
are less valuable in representing volcanic forcing in
climate modeling, as they are subject to the influence
of other climate forcings. Perhaps for this reason,
Robock in the 2000 review8 did not consider tree ring
data in discussing the construction of volcanic indexes.
The climatic effect of volcanic eruptions may be
underestimated from tree ring data. Volcanic aerosols
scatter, rather than reflect, solar radiation. This not
only causes a net reduction of solar receipt at the
Earth’s surface, but also results in an increase in the
amount of diffuse sunlight near the surface which
stimulates photosynthesis,74 with the consequent
effect of growth reduction of trees/vegetation less than
what may be expected from the tropospheric cooling.
The influence of indirect effects such as the increased
diffuse sunlight can in theory be quantified75 using
climate models.
Other Records
Volcanic records could be derived using a few other
methods, with varying degrees of coverage and
efficacy. For example, ground-based measurement
of solar radiation using pyrheliometers since the
1880s recorded the stratospheric effect of periodic
volcanic eruptions.76 In addition, the presence of
volcanic aerosols changes the refractive properties of
the stratosphere and alters the visual appearance of a
totally eclipsed Moon. Stothers77,78 used descriptions
of total lunar eclipse episodes in historical documents
to compile a list of stratospheric volcanic eruptions
in the period 1671–1881. A three-century chronology
of stratospheric aerosol optical depth influenced by
episodic volcanic inputs has been established79 using
these methods.
Advancements Since 2000
In the 2000 review, Robock8 evaluated volcanic
records derived from the various sources of
measurement and observation discussed above, except
for tree ring records, and concluded that ice cores
provided superior records. Despite the fact that, at the
time, only a few relatively short (up to 2000 years)
volcanic records were derived from ice cores and most
were not based on sulfate measurement,57,80 Robock
expected that future polar ice cores would yield much
better records. Numerous ice-core volcanic records
published in the last decade have perhaps exceeded
this expectation: they are mainly sulfate-based, are
much more robust in quantitative aspects (eruption
dates and volcanic signal strength), and have much
longer time coverage. Furthermore, the development
of the isotopic technique to identify signals of
climatically important stratospheric eruptions in ice
cores has the potential to yield very high-quality, if
not perfect, records valuable to understanding the
volcano–climate connection.
VOLCANIC IMPACT ON CLIMATE
The relatively brief residence time of volcanic aerosols
in the stratosphere limits the duration of apparent
volcanic impact on climate to a few years immediately
following an eruption. This aspect of the volcanic
impact is well understood, as discussed by Robock.8
However, longer term impact, which was not
extensively covered in the 2000 review,8 is possible
through feedback mechanisms in the climate system.
The magnitude and duration of volcanic impact can
be investigated by examining volcanic and climate
records (this section) and using climate models (see
Section Modeling the Volcanic Impact).
Ice core studies have produced long volcanic
records over the glacial–interglacial timescale. Two of
the longest of such records, the 110,000-year GISP2,
Greenland sulfate record48 and the dust data81 from
a Siple Dome, Antarctica ice core borehole suggest
that connections may exist between active volcanism
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Volcanoes and climate
and millennial scale climate change. On the other
hand, an analysis of a 45,000-year record from the
EPICA Dome C ice core45 indicated no clear relationship between volcanic eruption frequency and climate
change during that time period. It is apparent that, at
the present, the number of long volcanic records and
the quality of these records are insufficient for a credible evaluation of the volcanic-climate connection on
such timescales. Our knowledge based on the currently
available data indicates that the long-term connections
between volcanism and climate change are complex and that evaluation of the connections requires
detailed information such as magnitude of eruptions,
location of volcanoes, and eruption frequency.
Several Antarctic ice core records46,47 suggest
no particular pattern in the frequency of large
volcanic eruptions during the Holocene, except for
an apparent increase in eruption frequency in the
last 2000 years.46,47,68 This is in contrast to the
GISP2 record,44,48 showing a large number of volcanic
eruptions in the early part of the Holocene. The
discrepancy between the Antarctica and Greenland
records may very well be the result of ice core
signals of local volcanic eruptions, as the Greenland
record contains numerous signals of small and
moderate eruptions in Iceland, Alaska, and the JapanKamchatka Arc. The higher eruption frequency in
the last 2000 years, if verified by additional ice core
records, could be a factor in the gradual cooling trend
in late Holocene prior to the onset of the greenhouse
warming.
The discussion below focuses on the volcanic
records for the last two millennia and the role of
volcanism in the climate variations of the time period,
because natural climate variations during this period
provide the most relevant backdrop for the current
climate.
Prior to 1850 AD
Available climatic and volcanic records indicate that
significant climate variations before 1850 AD were
primarily caused by fluctuations in solar irradiance
and the frequency of large, stratospheric volcanic
eruptions.82,83 Although a volcanic eruption can cause
significant cooling only for a few years immediately
following the eruption, sustained active volcanism
over an extended period of time could cause longer
term impact.8 In a recent modeling exercise, Schneider
et al.84 show that feedback mechanisms can propagate
and extend the impact of a single eruption to over a
decade for regions such as the Arctic. There has been
strong interest in the role of volcanism during the
climatic episodes of Medieval Warm Period (MWP,
800–1200 AD) and Little Ice Age (LIA, 1400–1900
AD), when direct human influence on the climate
was negligible. Several studies attempted to determine
the influence of solar forcing and volcanic forcing
and came to different conclusions: Crowley and
colleagues82,85,86 suggested that increased frequency
of stratospheric eruptions in the seventeenth century
and again in the early nineteenth century was
responsible in large part for LIA. Shindell et al.83,87
concluded that LIA is the result of reduced solar
irradiance, as seen in the Maunder Minimum of
sunspots, during the time period. Ice core records show
that the number of large volcanic eruptions between
800 and 1100 AD is possibly small (Figure 1), when
compared with the eruption frequency during LIA.58
Several researchers have proposed8,53,88 that more
frequent large eruptions during the thirteenth century
(Figure 1) contributed to the climatic transition from
MWP to LIA, perhaps as a part of the global shift
from a warmer to a colder climate regime.46 This
suggests that the volcanic impact may be particularly
significant during periods of climatic transitions.
FIGURE 1 | Weighted annual average concentration of volcanic sulfate for the period of 176–2005 AD in a South Pole, Antarctica ice core
(Cole-Dai, manuscript in preparation).
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Regardless of the extent of volcanic and solar
forcing, the natural variations were very small and the
climate appeared to be relatively stable in the last two
millennia up to 1850.7
chlorine and the return to pre-anthropogenic levels by
2050,91 as a result of implementing the Montreal
Protocol on Substances that Deplete the Ozone
Layer.
Volcanoes and El Niño/Southern Oscillation
After 1850 AD
The climate since mid-nineteenth century has been
dominated by an apparent increase in global average
temperature.4 However, the temperature did not
rise continuously. The first of the two temperature
increases occurred in the first half of the twentieth
century (Figure 2). This period was characterized
by a remarkable volcanic quiescence, with only
two relatively large eruptions in the early part of
the period (Santa Maria, 1904 and Katmai, 1912).
It is possible that reduced volcanism may have
contributed to the warming in 1910–1940 (Figure 2).
In contrast, several large eruptions occurred in the
second half of the century; yet global temperature
increased substantially during this period (Figure 2).
This suggests that the greenhouse warming caused by
the continued accumulation of fossil fuel CO2 and
other anthropogenic greenhouse gases86 has begun to
dominate over climate variations caused by natural
factors such as volcanic eruptions. The large volcanic
eruptions, Agung in 1963, El Chichón in 1982, and
Pinatubo in 1991, only caused brief interruptions to
the warming trend (Figure 2).
At the time (2000) of the review by Robock,8
serious concerns had been expressed89 on the
impact of volcanic eruptions on stratospheric ozone,
stemming from the observed ozone effects by the 1991
Pinatubo eruption.90 These concerns have since eased
considerably by the expected decline of stratospheric
It was suggested that another mechanism of volcanic
impact on climate is that El Niño events are triggered
in part by volcanic radiative forcing.92 Handler and
Andsager93 found that an unusually large number of
El Niño/Southern Oscillation (ENSO) episodes in the
last 150 years were preceded by volcanic eruptions
and proposed that the reduction of shortwave solar
radiation at the surface following an eruption causes
ENSO events. However, Self et al.94 found that
such volcano–ENSO connection is not statistically
significant over the past 150 years. On the other hand,
Adams et al.95 found that the ENSO system may
become more active in response to volcanic eruptions.
Emile-Geay et al.,96 after analyzing millennium-long
climate and volcanic records, concluded that most
explosive eruptions are too small to affect ENSO
statistics.
Super-eruptions
Rare, extremely large eruptions were suspected to
be capable of starting glaciation and ushering in ice
ages.97 The only known eruption of this kind in
the Quaternary is the Toba (2.5◦ N, 99◦ E) eruption
approximately 74,000 years ago. The probably severe
climatic impact of the eruption was suggested to have
caused a ‘bottleneck’ in the late Pleistocene prehistoric
human population growth.98 Evidence of the Toba
super-eruption has been found in its caldera,99,100 ash
FIGURE 2 | Global annual mean surface temperature departure, from the 1951 to 1980 average, from 1880 to 2009. Data source: NASA Goddard
Institute for Space Studies (http://data.giss.nasa.gov/gistemp). Up arrows mark the timing of the five large volcanic eruptions in the twentieth century.
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deposit in the ocean101,102 and sulfate in a Greenland
ice core.103
Initial assessment,97 based on extrapolation
from the volcanological evidence of less powerful eruptions (e.g., Tambora, 1815 AD), indicated
that the global average atmospheric cooling was at
least 3–5◦ C, with a 1◦ C decrease in tropical ocean
temperature,101 approaching glacial–interglacial temperature variations.104 Oppenheimer,105 after examining available evidence, suggested that these initial
estimates of cooling were probably too high, given
the large uncertainties of the volcanological data. The
more problematic issues are the duration of the impact
of the Toba eruption and its role in millennium scale
climate change (stadials) in the last Ice Age. Zielinski
et al.103 found the possible Toba signal in the GISP2,
Greenland ice core at the beginning of a 1000-year
long stadial event and suggested that such supereruptions are capable of initiating cold periods of
century-to-millennium duration. Jones et al.106 used a
simple climate model to quantify the Toba impact and
found that, although the initial cooling was substantial, the duration of the impact was brief—the climate
recovered after only a decade or so. Robock et al.107
used a full ensemble of climate variables and feedbacks in two climate models to simulate the response
to Toba and concluded that it is very unlikely that the
Toba super-eruption would have initiated a stadial or
an ice age.
MODELING THE VOLCANIC IMPACT
Significant advance has been made in the last decade
in assessing the volcanic impact by quantifying the
climatic response in climate models. This success has
come in part from the improvements of climate models to evaluate natural climate variability and provide
a quantitative basis for hindcasting the climate variations in the last several hundred years and forecasting
future extent of the greenhouse warming. On the
other hand, the unprecedented observation and measurement of the atmospheric impact of the Pinatubo
eruption in 1991 provided extensive data valuable to
the modeling efforts.13,108
Energy balance climate models measure forcing
factors in energy flux (W m−2 ). Atmospheric parameters affected by volcanic eruptions are albedo and optical depth, which are related to volcanic aerosol mass
loadings. Lacis et al.109 modeled and calculated the
temperature effect of a stratospheric aerosol layer and
computed a conversion factor between optical depth
and radiative forcing. Sato et al.110 expanded on the
model to establish a history of volcanic forcing based
on a stratospheric aerosol optical depth record derived
from atmospheric extinction measurement and simple extrapolation of volcanological data. When ice
cores began to emerge as the most promising source
of volcanic records, a number of researchers57,61,80
developed methods to convert ice core measurements
(volcanic sulfate concentration and deposition flux) to
stratospheric optical depth and to volcanic radiative
forcing (W m−2 ). The most recent and comprehensive update of the volcanic records allows volcanic forcing to be incorporated directly into climate
models.58,111
Once the volcanic forcing data from ice cores are
incorporated, climate models can be used to evaluate
the full impact of volcanic eruptions. In addition,
comparison with climate records can assist with
calibrating the climate models.84 Modeling can assess
not only the immediate atmospheric radiative impact,
but also the secondary effects, such as the connection
to ENSO96 and the response of the ocean112 as a
feedback mechanism. Much modeling effort has also
explored the regional characteristics of the volcanic
impact.113
FUTURE DEVELOPMENTS
Despite the previous achievements and the impressive
advancements over the last decade in our understanding of the volcano–climate relationship, significant
challenges remain. Below is a brief summary of some
of the important issues and how they may be addressed
in ongoing and future research.
Ice-Core Volcanic Records
Ice cores have provided valuable records of past volcanism over long time periods. The development of
new, mainly sulfate-based ice core records over the last
decade demonstrates the potential for highly robust
and quantitative paleovolcanic information. However,
high-quality records are presently limited to only the
last 1000–1500 years. Although several individual ice
core records cover longer time periods, there are significant discrepancies among these records in older
time periods, usually resulting from low temporal resolution of chemical analysis and poor dating accuracy
and precision.34,45–47,68 Improved chronology or dating and high resolution measurement for long/deep ice
cores will be needed to enhance the quality and value
of ice-core volcanic records.
Of the volcanic signals detected in an ice core,
a large number may be small or moderate, tropospheric eruptions with no significant climate implication. Two approaches have traditionally been used
to identify and eliminate the insignificant signals to
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Advanced Review
produce records of climatically important eruptions:
(1) matching the established volcanic chronology
(i.e., Volcanoes of the World) with VEI ratings30
and (2) comparison of signals in bipolar (Greenland
and Antarctica) ice cores. These approaches become
much less effective when volcanic signals older than
1000 years are considered, as the accuracy of both the
volcanic chronology and ice core dating deteriorates
with the length of time covered. The use of sulfur isotope composition of volcanic sulfate appears to be a
promising technique to generate records of only stratospheric eruptions, although the ice sampling and measurement of sulfur isotopes are very challenging.65,66
Climate Modeling
Climate models evaluate the climatic impact of
volcanic eruptions by computing the temperature
response to the presence of volcanic aerosols in
the atmosphere. In most cases, the modeled temperature response compares well with instrumental
and proxy temperature records.58,82,83,113 However,
in some instances, the models appear to overestimate the climate system’s response, particularly in
cases of unusually explosive eruptions with extremely
large aerosol mass loadings.58,82,114 While it is possible that the tree-ring-dominated climate records may
underrepresent the actual temperature response,75 several researchers84,96,115 have suggested that converting
aerosol mass loading linearly to climate forcing58
for model use may be too simplistic, for increasing aerosol mass loading above a certain level may
have diminishing radiative impact as aerosol particles may be enhanced more in size than in number.
Only one recent study by Timmreck et al.62 has investigated the role of aerosol particle size in volcanic
impact.
The issue of climate sensitivity to volcanic forcing has been previously investigated.112 However, the
greenhouse warming could alter or trigger feedback
processes that affect the climate response to volcanic
forcing, as demonstrated by the important role of
snow/ice cover in the Arctic.84 A key question for modeling the volcano–climate connection in the changing
climate may be: What will be the level of climate
response in the current climate system (greenhouse
warming, reduced stratospheric ozone, tropospheric
sulfate aerosols, diminished Arctic sea–ice cover, etc.)
that is significantly different from the pre-industrial
system?
Geoengineering via Stratospheric Aerosols
The cooling effect on climate by large volcanic eruptions has attracted much attention in the search for
effective and appropriate solutions to the problem
of greenhouse warming by anthropogenic CO2 emissions. (A comprehensive review on the topic of climate engineering116 can be found in a forthcoming
issue of Wiley Interdisciplinary Reviews: Climate
Change.) One of the potential geoengineering solutions is entirely based on our understanding of the
radiative effects of volcanic eruptions. It has been
proposed117,118 that large quantities of sulfur compounds, most likely SO2 , be injected directly into
the stratosphere, where the subsequently formed sulfate aerosols will reduce the amount of shortwave
(solar) radiation reaching Earth surface. Blackstock
et al.119 examine the details of this albedo enhancement or solar radiation management solution. While
attractive as a short-term countermeasure to greenhouse warming, this approach has been criticized for
its potential side effects on global-warming-related
issues. First, albedo enhancement does not affect the
levels of greenhouse gases, and therefore will not
mitigate other effects of growing atmospheric CO2 ,
such as ocean acidification. Second, it is known that
stratospheric aerosols exacerbate the loss of stratospheric ozone120 ; the artificial enhancement of the
aerosol layer would delay the recovery of,121 or even
worsen,122 the polar ozone decline. Also impacted
will be the amount and pattern of precipitation globally and regionally.123,124 Finally, because the cooling
effect is short-lived due to the 1–2 year residence
time of stratospheric aerosols125 while the greenhouse
warming is long-lasting, SO2 injection will need to
be continuous or repeated frequently,118 which will
result in prolonged acid deposition.
Despite the concerns, geoengineering via
stratospheric aerosol enhancement appears to be an
attractive tool to counteract the serious effects of
global warming. However, as seen in the proxy
records and modeling results, there is a significant
level of uncertainty in the climate response to an
enhanced stratospheric aerosol layer. In addition,
our knowledge is rather limited on the effect of a
significantly enhanced level of stratospheric aerosols
for an extended period of time. Furthermore, all
geoengineering proposals, in addition to the scientific
and technical challenges, are fraught with societal,
ethical, and governance questions that will require
evaluation and possible revision of existing legal
framework and/or international agreements.126 In
the current debate on the albedo enhancement
approach of geoengineering, a consensus appears to
be building117–119,123,124 that continued and targeted
research is urgently needed to determine its efficacy,
feasibility, and the scope and consequences of
unintended side effects.
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Volcanoes and climate
CONCLUSION
insignificant. Third, individual super-eruptions such
as Toba, despite their extraordinary magnitude, are
unlikely to influence the climate beyond the immediate radiative impact. Fourth, the role of volcanic
eruptions in the anthropogenic era (since the nineteenth century) has been limited—eruptions in the
twentieth century have not significantly affected the
trend of the greenhouse warming and their potential
to impact stratospheric ozone will diminish as stratospheric chlorine returns gradually to natural levels.
An area of tremendous improvement is the quantitative knowledge of past volcanism. The number and
quality of paleovolcanic records from polar ice cores
have increased substantially. Long (>10,000 years)
and well-dated records are now available from ice
cores of both polar regions. Detailed sulfate measurement and the ability to identify stratospheric eruptions
make it possible to obtain volcanic records that truly
represent volcanic forcing in the climate system. With
the addition of new ice cores and the advance in ice
core chemical analysis and chronology, the quality of
future records will be even more robust, which will
enhance the ability of climate models to provide more
accurate forecast with reduced uncertainties.
Geoengineering via stratospheric aerosols
appears at the present to be potentially the most
effective tool to counteract the worst effects of anthropogenic global warming, in the absence of effective
mitigation to stabilize atmospheric greenhouse gas
concentrations. This is largely owing to our current knowledge of the radiative effects of volcanic
aerosols. The same knowledge also indicates that substantial side effects of the aerosol/albedo enhancement
approach may outweigh the benefits of the temperature decrease and therefore must be seriously
considered.
Climate change caused by the emission of greenhouse
gases from human activities is likely the most serious
environmental concern for the next few decades and
centuries. Accurate prediction of the change requires
thorough understanding of the quantitative effects
of the various forcing factors in the climate system.
Volcanism is one of the important natural forcings
causing short-term climatic variations and its impact
on climate has been the subject of past and ongoing research. Previous research, as summarized by
Robock,8 has shown that only large stratospheric
eruptions are capable of affecting the climate on a
global or hemispheric scale. When such an eruption
occurs, a significant cooling at the surface and the
lower troposphere can be expected, usually for a few
years immediately following the eruption, although
regional differences are common. The magnitude of
the cooling is clearly demonstrated by the 1991
Pinatubo eruption—a global temperature decrease
of about 0.5◦ C for a stratospheric SO2 loading of
20 million tons. The climate usually recovers from the
volcanic impact after a brief time period (1–3 years).
Over the last decade, there has been notable
improvement in our knowledge of the relationship
between volcanic eruptions and climate. First, long
records of volcanic eruptions from ice cores have
offered the opportunity to evaluate the long-term
relationship between eruptions and climate variations
(e.g., frequent eruptions could be an important factor in century-to-millennium-scale climate variations).
Second, modeling efforts have demonstrated that the
volcanic impact can be propagated through the climate
system through feedback mechanisms, although the
volcanism–ENSO connection has been shown to be
ACKNOWLEDGEMENTS
Funding for this work and for the underlying ice core research is provided by U.S. National Science Foundation
(Awards 0337933, 0538553, 0612461, and 0839066). Contributions to field work and laboratory analysis
from South Dakota State University staff and students are gratefully acknowledged. I thank the reviewers for
their constructive comments and suggestions.
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