\documentclass[acp]{egu}
\begin{document}

\title{Atmospheric impact of the 1783--1784 Laki Eruption: Part II Climatic effect of sulphate aerosol}
\runningtitle{Climatic impact of the 1783--1784 Laki eruption}

\runningauthor{E.~J.~Highwood and D.~S.~Stevenson}
\correspondence{E.~J.~Highwood (e.j.highwood@reading.ac.uk)}

\author[1]{E.~J.~Highwood}
\affil[1]{Department of Meteorology, University of Reading, UK}
\author[2]{D.~S.~Stevenson}
\affil[2]{Institute for Meteorology, University of Edinburgh, UK}

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\received{22 January 2003}
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\accepted{25 February 2003}
\firstpage{1599}

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\begin{abstract}
The long 1783--1784 eruption of Laki in southern Iceland, was one of
the first eruptions to have been linked to an observed climate
anomaly, having been held responsible for cold temperatures over much
of the Northern Hemisphere in the period 1783--1785. Results from the
first climate model simulation of the impact of a similar eruption to
that of 1783--1784 are presented. Using sulphate aerosol fields
produced in a companion chemical transport model simulation by
\cite{Stevenson03}, the radiative forcing and climate response due to
the aerosol are calculated here using the Reading Intermediate General
Circulation Model (IGCM). The peak Northern Hemisphere mean direct
radiative forcing is $-$5.5\,Wm$^{-2}$ in August 1783. The radiative
forcing dies away quickly as the emissions from the volcano decrease;
however, a small forcing remains over the Mediterranean until March
1784. There is little forcing in the Southern Hemisphere. There is
shown to be an uncertainty of at least 50\% in the direct radiative
forcing due to assumptions concerning relative humidity and the
sophistication of the radiative transfer code used. The indirect
effects of the Laki aerosol are potentially large but essentially
unquantifiable at the present time. In the IGCM at least, the aerosol
from the eruption produces a climate response that is spatially very
variable. The Northern Hemisphere mean temperature anomaly averaged
over the whole of the calendar year containing most of the eruption is
$-0.21$\,K, statistically significant at the 95\% level and in
reasonable agreement with the available observations of the
temperature during 1783.
\end{abstract}

\introduction\label{sec:intro}

% enter introduction here

Volcanic eruptions are a major natural source of sulphate aerosols.
SO$_2$ emitted by an eruption is rapidly converted to sulphuric acid
aerosol. Both \cite{Stevenson03} and \cite{Grainger03} describe the
chemical reactions involved in considerable detail. Some eruptions
also emit H$_2$S but the emission data is scarce and in any case it is
rapidly oxidised to SO$_2$. Stratospheric aerosol reflects solar
radiation back to space causing a negative radiative forcing. It also
absorbs some solar radiation, heating the stratosphere where the
aerosol is itself present. Finally, aerosol of sufficient diameter can
act to some extent like a greenhouse gas, preventing terrestrial
radiation from reaching space. This long wave direct radiative forcing
is largest when the aerosol is in the region of the tropopause, and is
generally smaller than the effect of the aerosol on solar (including
both visible and near-IR) radiation on timescales of longer than a few
days \citep[e.g.][]{Stenchikov98}. Thus the predominant effect of
volcanic aerosol is an increase in the planetary albedo except for
regions in polar night. If a large explosive eruption increases
stratospheric aerosol levels, it can have a considerable influence on
climate for a period of several years \citep{Minnis93}.  The explosive
eruption of Mt.  Pinatubo in 1991 produced a global cooling of the
surface of a few 10ths of a degree which persisted for about two years
after the eruption \citep{IPCC95}. This type of relatively long
climate perturbation is possible since stratospheric aerosol is
generally removed slowly, and perturbations can persist for a year or
more. Effusive (or less explosive) eruptions that result only in
enhanced tropospheric aerosol are generally thought to be much less
likely to impact upon climate, because aerosol here has a lifetime of
only a few days, being effectively removed by deposition processes.
These emissions are an important part of the natural sulphur budget
\citep{Graf97} and in some ways their effect on tropospheric chemistry
can be considered analagous to that of slow, continuous anthropogenic
releases.  However, if such an effusive volcanic SO$_2$ source is
long-lived and sufficiently large, and if the emissions are
efficiently oxidised to aerosol (i.e. the emissions occur in the
summer hemisphere or the tropics), then the aerosol could perturb the
Earth's radiation budget at least while the emissions continue.

Usually, the releases from an individual effusive eruption are too
small to allow a specific case study. However, in June 1783, the
Icelandic volcano Laki began erupting basalt and releasing
SO$_2$. Over the following eight months, an estimated 15\,km$^3$ of
magma and 122\,Tg\,SO$_2$ were emitted, with about 60\% of this
released over the first six weeks of the eruption
\citep{Thordarson02}. A dry fog was recorded over much of Europe
during the second half of the year \citep{Stothers96}, eventually
reaching eastwards across Asia \citep{Renovantz88}. Ice core data
suggests that there was strong fall-out of aerosols from this eruption
over Greenland \citep[e.g.][]{Hammer77,Fiacco94}, the north-eastern
Canadian Arctic \citep{Fisher81} and Spitzbergen \citep{Fujii90}
although estimates of how long the aerosol remained in the atmosphere
before being deposited in the ice cores are somewhat contradictory.

There has been much debate over the climatic impact attributable to
this eruption. Documentary evidence suggests that the summer of 1783
was warm in Europe at least \citep{Stothers99} with frequent
thunderstorms. The July mean temperature as recorded in the Central
England Temperature record was 18.8$^\circ$C, the highest in the
dataset prior to 1981. However, 1976 and 1852 were only 0.1$^\circ$C
cooler, suggesting that a volcanic eruption is not the only possible
cause of such peaks in the record. \cite{Wood92} shows warm
temperatures across Europe during July 1783 and there are also
suggestions of unusually warm temperatures in north America during
this time \citep{Sigurdsson82}. A dry fog and haze persisted
throughout much of the summer across the Northern
Hemisphere. \cite{Franklin85} was the first to suggest that the
extremely cold winter of 1783/84 may have been connected to the
aftermath of the Laki eruption. In fact, the period 1784--1786 was
unusually cold in many places \citep{Wood92}. In a study of
temperature records after 6 major eruptions in the last 100 years,
\cite{Angell85} attributed a cooling of $-$0.14$\,^\circ$\,C in the
Northern Hemisphere annual mean temperature in 1783 (compared to the
mean of the 5 years before), to the combined eruption of Laki and
Asama, a Japanese eruption also occuring in 1783. \cite{Briffa98}
found changes in tree rings which indicated a Northern Hemisphere mean
temperature anomaly of $-0.27^\circ$\,C ($\pm 0.3^\circ$\,C being the
95\% confidence limits of this change). However, \cite{Zielinski94}
assert that Asama probably had little climatic impact based on
previous eruptions; perhaps this was because Asama eruptions generally
do not release much sulphur \citep{Kohno93}. This eruption may,
however, account for reports of a cold wet summer in Japan during 1783
\citep{Mikami87}. \cite{Thordarson02} analysed temperature records
from Europe and the eastern United States, obtaining a larger cold
anomaly (from the 1768--98 mean) of 1.3 to 1.4\,K during the three
years following Laki. This anomaly is considerably larger than those
from other sources but this may have been due to the relatively small
geographic distribution of data used.

\cite{Stothers99} analysed documentary and temperature evidence for
several large eruptions and concluded that the appearance of the warm
summer in 1783/84, cold winter in 1783 and cool temperatures over the
following two to three years were due to both the immediate effect of
aerosol from the eruption and a perturbation of the atmospheric
circulation. He also suggested that the cold temperatures led to a
famine and plague pandemic across the Middle East. \cite{Briffa94}
found that large anomalies in the density of tree rings, and the
implied cold temperatures during summer 1783 were restricted to the
Northern part of Alaska and Yukon. They also pointed out that low tree
ring density in Scandinavian and possibly Canadian forests may have
been influenced by the transport and deposition of acidic debris
rather than indicating significantly cold temperatures. Perhaps
consistent with reduced summer temperatures is the study of
\cite{Jacoby99} which attributed the virtual dying out of the Inuit in
Northwest Alaska to climatic impacts of the Laki eruption.

The magnitude of some of the effects attributed to the Laki eruption
are large, considering that it is not uniformly agreed that there was
much stratospheric injection of aerosol precursor. In particular, it
is unclear in many earlier studies whether the phrase ``dry fog''
applies to stratospheric or tropospheric aerosol. Although
\cite{Thordarson02} suggest that the haze extended to substantial
heights (5--16\,km), \cite{Grattan99} consider that the documentary
descriptions of the haze are more consistent with an abundance of gas
and aerosol in the lowest layers of the atmosphere. They also
concluded that the temperature anomalies experienced in 1783 and 1784
were not significantly cold, taking into account the natural
variability of the region. \cite{Durand99} demonstrated that the haze
had serious health consequences for many European
communities. Extensive details of the observations of the haze and
coincident temperature anomalies can be found in
\cite{Thordarson02}. Previous studies have generally used scarce
direct temperature measurements and indirect evidence of temperatures
such as ice cores and tree rings to infer the climatic impact of
Laki. Apart from problems of calibrating these sources, it is likely
that the geographical pattern of any response to the volcanic aerosols
would be highly inhomogeneous \citep{Robock00}. Data from one region
may well be unrepresentative of a wider area, and this may be one
reason for the differing views in previous studies.

This study offers a new approach for this eruption, in that the
climatic impact of a Laki eruption like that which occurred in 1783-84
is simulated using a climate model rather than relying on
interpretation of scarce data. The impact upon the chemical
composition of the troposphere resulting from the Laki eruption has
been simulated in a companion paper using a 3-D chemistry-transport
model \cite{Stevenson03} (hereafter STE03). The sulphate aerosol
distributions from the chemical modelling are used here with the
Reading Intermediate General Circulation Model (IGCM) to directly
simulate the radiative forcing and climate response due to the direct
effect of the sulphate aerosol on radiation. Section~2 describes the
distribution of sulphate aerosol produced by the eruption and
simulated in STE03, while Sect.~3 describes the climate model used and
the experimental design. The radiative forcing and resulting surface
temperature changes are presented in Sect.~4. Section~5 discusses the
uncertainties associated with this simulation and quantifies some of
them using sensitivity tests.


\section{Sulphate aerosol distributions}
\label{sec:sul}

The simulation of sulphur dioxide and sulphate aerosol due to the Laki
eruption are described in detail in STE03. For the purposes of this
paper we use 3 different timeseries of monthly mean sulphate aerosol
distributions in the year immediately following the eruption,
beginning in June 1783. These are the pre-industrial background
distribution and two Laki simulations, one resulting from injection of
the Laki sulphur dioxide evenly between the surface and approximately
9\,km (``Lo") and the other (``Hi") resulting from the injection of
25\% of the Laki sulphur dioxide between the surface and 3\,km and the
remaining 75\% around the tropopause (9--13\,km). The latter case is
based on preliminary modelling (S.Self, Pers. Comm., 2002) while the
former is designed to give a low bound to the altitude of SO$_2$
emission since most gases are like to be released at the top of the
cloud. Sulphate aerosol concentrations showed a dramatic increase over
much of the Northern Hemisphere. Peak sulphate concentrations of
1--2\,ppbv occurred over much of the summer troposphere polewards of
around 50$^\circ$\,N, roughly 4 times the 1990 value. Mean sulphate
lifetimes for the Laki cases were up to 9.1 days, longer than in a
1990 simulation by the same model, due to a larger proportion of the
Laki aerosol residing in the upper troposphere and lower stratosphere
where loss processes are slower. The eruption impact was essentially
limited to the Northern Hemisphere, with a small amount of sulphate
transport into the Southern Hemisphere in the lower stratosphere.

The peak sulphate impact occurs in the second and third month of the
eruption (July / August). In the lower troposphere the anomaly ceases
as soon as the emissions cease in February. At higher levels a small
perturbation persists for a few months. Since the stratospheric aerosol
is still perturbed after the end of the first year following the start
of the eruption, in order to calculate radiative forcing and climate
impact in subsequent years without the prohibitive expense of running
the CTM simulations for further years, an assumption has been made
about the removal of the volcanic aerosol from the stratosphere. In
the ``Lo" scenario, all the extra aerosol is assumed to have left the
atmosphere by the end of May 1784 in accordance with the findings in
STE03. In the ``Hi" scenario, the data sets have been extended for a
further two years using different e-folding times for the removal of
stratospheric aerosol. For the second and third years following the
start of the eruption, in one case (``Hi/Short") the remaining
perturbation is assumed to decay with an e-folding time of 3.6 months
as was observed after the Mt. St. Helens eruption in 1980
\citep[another extratropical eruption injecting matter into the upper
troposphere and lower stratosphere,][]{Jager87} while in the other
(``Hi/Long") the decay time is 10 months as was observed after
Pinatubo \citep{Kent97}.

It is important to remember that the chemical model which produced
these aerosol fields was driven in each case by meteorology
representative of the 1990s and no attempt was made there or here to
recreate the conditions of 1783 as this is currently beyond the
capability of the models. The aerosol distributions are therefore 
likely to be more representative at larger time and space scales
(i.e. monthly means and zonal averages) rather than the day to day
variations at a local scale immediately after the eruption.

% enter main text here

\section{Climate model details and experimental design}
\label{sec:mod}

The Reading Intermediate General Circulation Model (IGCM) is based on
the spectral primitive equation model of \cite{Hoskins75}. A more
detailed description of the model is given by
\cite{Forster00}. Deliberately simple parameterisations for physical
processes such as surface transfer, vertical diffusion, convection and
stratiform precipitation have been used to allow the many runs needed
to explore a large range of climate forcing scenarios. In order to be
able to include the effect of aerosol on both short and longwave
radiation, a version of the model using the \cite{Morcrette90}
radiation scheme has been used here. The radiation code (run once a
day) uses interpolated monthly averaged climatological ozone
\citep{Li95} and prognostic water vapour.

In this study, the direct effect of aerosols on radiation has been
included by interpolating between monthly mean 3-D distributions of
sulphate mixing ratios from the STOCHEM-Ed chemical transport model as
described by STE03 and the previous section. The possible impact
of the sulphate aerosol on clouds (the indirect forcing) is outside
the capability of this model but is considered briefly in Sect.~5.2.
The aerosol mixing ratios have been interpolated onto the same
vertical grid as the IGCM. Aerosol properties have been derived using
Mie scattering for the 2 shortwave regions used in the IGCM (0.25 to
0.68\,$\mu$m and 0.68 to 4.0\,$\mu$m). Properties were similarly
calculated for the longwave region of the spectrum where the aerosol
is assumed to be ``grey'', i.e. the properties are an average over the
longwave spectrum. We assume the aerosol is sulphuric acid and the
refractive indices for sulphuric acid are taken from \cite{WCP86}. The
size distribution depends on whether the aerosol is in the troposphere
or stratosphere. In both cases we use a log-normal size distribution;
in the troposphere this has mean radius 0.05$\mu$m and standard
deviation of 2.0 \citep{Kiehl83}, while in the stratosphere the
distribution has mean radius 0.15\,$\mu$m with standard deviation 1.25
\citep{Stenchikov98}. For simplicity the aerosols are assumed to be
dry. This will be likely to result in an underestimate of the
radiative forcing since sulphate aerosols grow and become more
effective at scattering solar radiation as the humidity rises (see
Sect.~5.1 for further discussion). The aerosol is assigned
tropospheric or stratospheric optical properties at each call to the
radiation scheme depending on its altitude compared to the
climatological tropopause height at that latitude in the IGCM.

In this study the IGCM has been used with 22 levels in the vertical (7
levels in the stratosphere, the top being at 1\,mb), T21 resolution
(approximately 6$^\circ$ by 6$^\circ$, similar to the resolution used
in STE03), and a 100\,m deep mixed layer ocean. The IGCM was spun up
using a ``clean'' or pre-industrial aerosol distribution. The
pre-industrial aerosol is produced from emissions of SO${_2}$ from
natural sources and some anthropogenic biomass burning at 20\% of the
1990 value (see STE03 for more details). A 50 year long control run
with the pre-industrial aerosol distribution was then completed to
establish a base climatology and ensure no drift. An ensemble of 3
year transient runs was then performed, starting each ensemble member
from one of 10 different May 1 states (i.e. 10 different years) from
the control simulation. The 3-D Laki aerosol distributions are used
for 1 year (June 1783 to May 1784). Subsequently the aerosol used in
the IGCM reverts back to the clean pre-industrial distribution for the
``Lo" simulation or is scaled acccording to an estimate of the removal
rate in the stratosphere for the ``Hi/Short" and ``Hi/Long"
scenarios. In the results section of this paper we describe the
ensemble mean difference calculated from the 10 sets of transient
runs, each Laki scenario being compared with one with pre-industrial
aerosol.

As in the chemical transport model, no attempt is made here to
recreate the meteorological conditions of 1783. Indeed the
meteorological conditions used in the IGCM are different to those used
to produce the aerosol climatology. However, by imposing the Laki
sulphate distributions on a generic representation of climate we can
generalise the results of this study to describing how the climate
might respond to a Laki-style eruption today, as well as comparing
results with the observations from the 1783--1784 eruption.

\section{Results}
\label{sec:res}

\subsection{Radiative forcing}

Radiative forcing is defined here as the change in net radiation at
the top of the atmosphere on adding the aerosol from the Laki eruption
after stratospheric temperature adjustment. It is calculated off-line
using the same radiative transfer scheme and grid as the IGCM. Monthly
climatologies of all meteorological parameters are produced from the
clean atmosphere runs and these are then used as input to the
radiation scheme, since everything but the aerosol must remain fixed
to calculate the radiative forcing. The fixed dynamical heating
approximation was used to bring the stratosphere back into equilibrium
when calculating radiative forcing, although in fact for this type of
forcing the adjustment should, and does, have very little impact. We
consider here the direct component of shortwave radiative forcing,
i.e. that which is due to the scattering of solar radiation by the
sulphate particles, and the longwave radiative forcing due to the
higher altitude and larger magnitude aerosol which can behave
similarly to a greenhouse gas.


Figure~\ref{fig:forcing1} shows the temporal variation of global and
hemispheric mean radiative forcing for all 3 ``Laki" aerosol
distribution scenarios. Both the global mean and the Northern
Hemisphere mean show large radiative forcings during June, July and
August 1783, while the Southern Hemisphere mean forcing is much
smaller throughout the eruption.

A clear difference can be seen between the ``Hi" and ``Lo" scenarios in
all three means. The peak global mean value was around $-3$\,Wm$^{-2}$
under the ``Hi" scenario and $-1.5$\,Wm$^{-2}$ under the 'Lo'
scenario. Comparatively, the global mean forcing due to the explosive
eruption of Mt. Pinatubo reached around $-4$\,Wm$^{-2}$ but
persisted for around two years. The simulations therefore give quite a
large magnitude global mean forcing for an eruption that only
affected the lower atmosphere (lower than 15\,km), but this forcing
lasted for only a short time. The Northern Hemisphere mean forcing
peaks at almost $-6$\,Wm$^{-2}$ in August under the ``Hi" Scenario,
roughly twice that in the ``Lo" scenario. Under the ``Lo" scenario there
is negligible forcing in the Southern Hemisphere, while under the ``Hi"
scenario there is a small peak in January 1784. This indicates that
there is some long range transport of aerosol under the ``Hi" scenario
which reaches the southern hemisphere and then produces a maximum in
forcing when the hemispheric insolation is a maximum. This is
consistent with both the evidence that the forcing due to Laki was
predominantly a Northern Hemisphere phenomenon (no signal being
observed in Southern Hemisphere ice cores) and the expectation that
aerosol from high latitude eruptions is not transported significantly
into the opposite hemisphere \citep{Robock00}. The two ``Hi"
scenarios differ slightly in years two and three reflecting the
different residence time of the aerosol, however, in all cases the
forcing dies away rapidly once the eruption has finished.

The latitudinal extent of radiative forcing is shown in
Fig.~\ref{fig:forcing2}, confirming that the radiative forcing is
largely confined to the northern hemisphere in all three cases. In the
``Hi" scenarios the maximum occurring between 55 and 70$^\circ$\,N
extends into August and September 1783, while in the ``Lo" scenario the
peak zonal mean forcing is more restricted to July and August. The
relative maximum in the tropics during the winter following the
eruption is also more clearly seen in this figure. In both ``Hi"
scenarios small negative forcing amounts (of the order $-0.5$\,Wm$^{-2}$)
extend in the midlatitudes of both hemispheres well into 1785.

Figure~\ref{fig:forcing3} shows the geographical distribution of the
ensemble mean direct radiative forcing due to Laki aerosol relative to
a ``clean'' pre-industrial atmosphere calculated for the peak forcing
month of July 1783, for ``Hi" (Fig.~3a) and ``Lo" (Fig.~3b)
scenarios.  We present only plots for the Northern Hemisphere since
Figs.~1 and 2 show the Southern Hemisphere forcing to be small. The
largest forcings are not surprisingly in regions close to and downwind
of the eruption over North-western Europe and Greenland. The peak
radiative forcing reaches in excess of $-17.5$\,Wm$^{-2}$ (in the ``Hi''
scenario). Considerable negative forcings of greater magnitude than
2.5\,Wm$^{-2}$ cover extensive parts of the Northern Hemisphere
throughout June--September 1783. Most of the direct radiative forcing
of aerosols shown here is due to the impact of aerosols on shortwave
radiation. The peak magnitude of the longwave effect was only a few
percent of the total direct forcing and of the opposite sign.

In summary, the sulphate aerosol from the simulations of the Laki
eruption produced a large negative radiative forcing in the Northern
Hemisphere through the scattering of solar radiation to space which
peaked in July to September, depending on the emission scenario
considered. The forcing was considerably stronger for the ``Hi" cases due
to both an increase in total column burden in this simulation and the
higher altitude of the aerosol. For the ``Hi" scenarios, the forcing
persisted, albeit much reduced, into 1785 in the mid-latitudes of both
hemispheres.

\subsection{Surface temperature changes}

We now calculate the climate response to the direct forcing caused by
the Laki sulphate aerosol. We concentrate mainly on changes in surface
temperature since this is the only climatic variable of which we have
observations at this time. Additionally, we concentrate mainly on
hemispheric and global means since the IGCM has been run at relatively
low horizontal resolution and is therefore not expected to capture
variations on small spatial scales.

Figure~\ref{fig:mean_delt} shows the ensemble mean, global and
hemispheric mean temperature anomalies due to the Laki aerosol for
each simulation, along with an indication of the months for which a
non-directional student t-test shows significance at the 95\% level.
In all three quantities, the ``Lo" simulation produces little
consistency in temperature anomaly even during the times when peak
forcing was occuring. The ``Hi" simulations however do show significant
cold anomalies in the Northern Hemisphere and global mean temperatures
throughout 1783 and 1784. Peak Northern Hemisphere mean cold anomalies
reach $-0.35$\,K during August to November 1784 and the global mean
anomaly reaches $-0.2$\,K during the same period. These results
demonstrate that the temperature effect due to direct radiative
forcing from the sulphate aerosol from Laki would mainly occur in the
Northern Hemisphere, consistent with the lack of a temperature signal
for Laki in Southern Hemisphere ice cores. Assuming negligible
difference between the pre-industrial and Laki runs in the months
before the eruption in 1783, the Northern Hemisphere mean temperature
anomaly for 1783 as a whole is $-0.21$\,K, while the global mean
temperature anomaly is $-0.13$\,K for the same period. The Northern
Hemisphere ensemble and geographic mean anomalies for each simulation
and each year are shown in Table~\ref{tab:means}.



The Northern Hemisphere mean calculated using the Laki ``Hi" simulation
for 1783 is in reasonable agreement with observations of
$-0.14$\,K \citep{Angell85} and $-0.27$\,K ($\pm$0.3\,K)
\citep{Briffa98}. Our simulations suggest that this cold
anomaly could have persisted for at least two years after the
eruption.

Although we are aware of the limitations of the IGCM, and climate
models in general, in reproducing regional scale climate anomalies,
the temperature response due to the Laki aerosols is highly spatially
variable. Figure~\ref{fig:delt_map} shows seasonally averaged
temperature anomalies in the ``Hi/Long" simulation in order to compare
with reports of ``warm summers'' and ``cold winters''. The coldest
temperature anomalies occur over Greenland and other high latitudes
during September-October-November (SON) 1783. Greenland and Northern
Canada remain cold throughout the winter but there is evidence of a
1.5\,K warming over Scandinavia in later seasons. The Middle East
persistently suffers cold anomalies until March--April--May (MAM)
1785, perhaps consistent with reports of famine there
\citep{Stothers96}. Alaska appears to generally experience anomalies
of up to $-1$\,K during JJA 1783 which may be consistent with
\cite{Jacoby99} although there is no indication of the $-4$\,K
anomalies reported by that study. Interestingly there is no indication
of positive temperature anomalies over Western Europe during the
summer of 1783. The observations of warm summers are generally
believed to be a result of anomalous southerly flow bringing western
Europe under the influence of warm tropical airmasses
\citep{Thordarson02}. This simulation does not produce such a pattern
in response to aerosol direct forcing, suggesting that the existence
of such a pattern may have been either due to one of the volcanic
influences neglected in this simulation (see the following section for
more discussion), independent of the volcanic forcing, or a limitation
of the relatively low horizontal resolution of the models employed.

It is important to note however, that the statistical significance of
these spatially variable differences is generally small since the
Northern Hemisphere climate is very variable during Northern
Hemisphere winter. Only a few regions in some seasons reach 95\%
significance.  Due to the relatively coarse horizontal resolution of
the IGCM and the use of 1980 meteorology rather than the meteorology
of 1783/84 in the chemical transport model, any discussion of
potential circulation changes or mechanisms which could bring about
the observed and modelled temperature changes is somewhat
premature. There was no significant change in hemispheric or global
mean cloud cover or precipitation. Statistically significant (95\%
level) increases in the Northern Hemisphere surface albedo were
produced during the 1st year of the ``Hi" simulations, while the mean
snowdepth was increased in these simulations during
December-January-February (DJF) 1783/84. There was no significant
change in mean sea level pressure across the Northern Hemisphere.

The results of these simulations should be viewed as a first attempt
to simulate the climate response to such an eruption. Ideally a
consistent ensemble of simulations of the chemical transport model and
climate model would test the robustness of our results, particularly
the spatial pattern of forcing and response. It may also be possible
to use some observations of surface pressure over Europe and the USA
to constrain the meteorology used in STOCHEM-Ed and the IGCM, or to
use a fully coupled chemistry-climate model. However, this is beyond
the capabilities of the present study.

\section{Uncertainties}
\label{sec:unc}

The uncertainties in the magnitude of SO$_2$ emission, eruption height
and chemistry have been extensively discussed in STE03 (Sect.~4.7).
In this study we restrict our discussion of uncertainties to
those arising from our treatment of the direct radiative forcing,
our neglect of additional possible forcings and the limitations of climate
modelling in general.

\subsection{Direct effect}

The direct radiative forcing has been calculated using the radiative
transfer scheme of the IGCM for consistency with the climate response
results. Necessarily, this scheme is simplified. The number of
radiation bands used is restricted and only a 2-stream calculation is
used. In addition we have made assumptions about the aerosol
itself. To quantify the likely effect of these assumptions on our
radiative forcing results, we have used a more sophisticated and
flexible radiation code than is present in the IGCM. The radiative
transfer model used is a discrete-ordinate model
\citep{Stamnes88}. Here we use it in configurations with a varying
number of spectral bands, the absorption data for ozone and water
vapour coming from exponential sum fit techniques similar to those
used in \cite{Edwards96}. It is used to calculate forcing in clear
skies. The forcing in cloudy skies due to sulphates is assumed
negligible \citep{Haywood95} and the forcing at each point is then
scaled by (1 -- cloud fraction) from the IGCM
climatology. Table~\ref{tab:uncert} gives the ratio of radiative
forcing compared to the base case scenario, for various sensitivity
tests for July 1783, when aerosol loading peaked.



In the base case we have assumed that the sulphate aerosol at all
altitudes is dry. If instead we assume that it has passed through
regions where the relative humidity is 80\%, the direct forcing is
approximately 2.6 times that found in the base case due to growth and
increased scattering of the aerosol. 2-stream radiation codes such as
that in the IGCM are sometimes considered inadequate to use in aerosol
forcing calculations since they do not allow for a good representation
of the aerosol phase function. \cite{Myhre98} demonstrated that using
a 2 stream code could introduce a difference of as much as 20\%. The
intercomparison project of \cite{Boucher98} suggests that 4 stream
models agree reasonably well with higher order versions. The Laki
forcing was recalculated using a 4 stream version of the radiative
transfer model, but the averaged forcing is less than 1\% different
from the 2 stream calculation. Most of these differences occur at high
latitudes.

The shortwave radiation scheme used in the IGCM has only 2 wavelength
bands. This can introduce errors in the aerosol optical properties
which must be averaged over quite large
wavebands. Table~\ref{tab:uncert} shows that increasing to a 24 band
solar scheme, the forcing is reduced by around 40\%, suggesting that
the IGCM forcing may in fact be too large.

The forcing due to changes in ammonium sulphate has also been
calculated assuming the same tropospheric aerosol size distribution
and humidity. However, in the pre-industrial atmosphere there are very
few spare ammonium ions \citep{Stevenson03} and so the SO$_2$ from the
eruption mainly forms sulphuric acid rather than ammonium
sulphate. The spatially averaged forcings are a few hundredths of that
due to the changes in sulphuric acid. The spatial patterns of forcing
due to ammonium sulphate are slightly different, with local peaks in
forcing over South America and southern Africa (spare ammonium ions
from biomass burning). Only over South America does the ammonium
sulphate forcing reach 10\% of the sulphuric acid forcing, which is in
any case very small in this region.

The combination of these effect suggests that the true direct
radiative forcing due to sulphate aerosol from the Laki simulations
could be up to 1.6 times (60\% of the factor 2.6) that calculated in
this study (without considering the uncertainties inherent in the
production of the sulphate aerosol field themselves as discussed in
STE03).

\subsection{Indirect effects}

We have not addressed the indirect effects of the sulphate aerosol
produced on cloud microphysics. The so-called first indirect effect,
whereby the presence of aerosols increases the number of small
droplets thereby increasing the albedo of the cloud, would produce a
potentially large additional negative radiative forcing, with a
pattern somewhat different to the direct forcing. Representation of
the indirect effects within the IGCM would be premature given the
large uncertainties in the indirect effects of aerosols in general
\citep{IPCC01} and the relatively coarse horizontal resolution of the
simulations. However, a test calculation using the aerosol
distributions for July 1783 and empirical relationships between
sulphate mass, aerosol number and cloud droplet effective radii
\citep[][]{Boucher95,Slingo81} and following the methodology of
\cite{IPCC01}, suggests that an indirect effect on cloud could result
in an additional Northern Hemisphere mean forcing of between
$-8$\,Wm$^{-2}$ (``Lo'' scenario) and $-14$\,Wm$^{-2}$, (``Hi''
scenario) i.e. considerably more than the size of the direct effect
calculated here. The spatial pattern of the indirect effect was
different to that of the direct forcing, peaks occuring over the
Atlantic and Pacific Oceans, in common with estimates of the indirect
effect due to industrial sulphates \citep{IPCC01}. A further
complication when considering the likely impact of the indirect effect
is that in the ``Hi" scenarios, much of the aerosol is in the upper
troposphere and lower stratosphere. While \cite{Jensen92} and
\cite{Karcher02} do consider the indirect of volcanic aerosols on
climate, almost all work on the indirect effect to date has examined
the effect of aerosol on low altitude clouds. The interaction of
aerosol with higher altitude clouds, particularly mixed phase clouds,
and the role of aerosol as ice nuclei is extremely uncertain
\citep{IPCC01}. Indeed, \cite{Luo02} found no evidence of any cloud
changes in satellite observations following the Pinatubo eruption, and
\cite{Lohmann2003} found significant changes in cloud properties only
when very large amounts of sulphate aerosol were used. The indirect
effects of aerosols from the Laki eruption are potentially large, but
impossible to quantify with any confidence at this moment in time. 

\subsection{Other possible radiative forcing agents}

We have not yet addressed the possible effect on climate of changes in
ozone resulting from changes in the tropospheric chemistry after the
eruption. The changes observed in \cite{Stevenson03} suggest a likely
small positive forcing due to increased tropospheric ozone polewards
of 70$^\circ$\,N and a small negative forcing due to decreases in the
tropics.

Sulphur dioxide is itself a greenhouse gas. To test whether the vast
amounts produced in the region of the volcano could have produced a
significant positive radiative forcing, the Narrow Band Radiation
Model of \cite{Shine91} was used together with a global mean
atmospheric profile of other radiatively important gases. For an
increase of 1\,ppbv sulphur dioxide throughout the atmospheric column,
the radiative forcing was 0.02\,Wm$^{-2}$. For the peak zonal mean
sulphur dioxide concentrations of 20\,ppbv, this amounts to only
0.4\,Wm$^{-2}$, which is less than 10\% of the large negative
direct radiative forcing from the sulphate aerosols. There would also
have been substantial amounts of tephra and ash in the eruptions which
fall out of the atmosphere more rapidly than the smaller sulphate
aerosols. At present, this aerosol is not included, but may have
modulated the forcing values close to the volcano itself.

\subsection{Climate sensitivity}

The magnitude of the temperature response of a model to an imposed
radiative forcing will depend strongly on the model itself. Model
intercomparisons show that the climate sensitivity parameter, $\lambda$, defined
for an equilibrium global mean temperature change, $\Delta T$, using:
$\Delta{T}=\lambda \,\Delta{F}$
where $\Delta F$ is the global mean radiative forcing, varies between
GCMs from 0.5 to 1.2\,K\,${(\chem{Wm}^{-2})}^{-1}$ \citep{IPCC01}. The climate
sensitivity parameter for this version of the IGCM (calculated using a
doubling of CO$_2$) is 0.6 K\,${(\chem{Wm}^{-2})}^{-1}$, placing it at the low
end of the range of models. A model with a higher climate sensitivity
parameter would tend to predict a higher equilibrium surface
temperature response to a given forcing. However, with a short
duration forcing such as that due to the Laki aerosol, experiments
with an energy balance model \citep[e.g.][]{Shine02} suggest that the
peak global mean temperature change would be very similar for all
values of $\lambda$ within the \cite{IPCC01} range. The duration of
the global mean temperature perturbation however would be
approximately twice as long for $\lambda=1.2$\,K\,(Wm$^{-2})^{-1}$ than
for a model having $\lambda=0.6$\,K\,${(\chem{Wm}^{-2})}^{-1}$. Thus, in so far
as an energy balance model is a good indication of the behaviour of
GCMs, the duration of the temperature response of the IGCM to Laki is
likely to be a lower limit. A longer duration response might be
consistent with observations of cold winters during subsequent years,
although there is no obvious evidence of a prolonged period of cold
temperatures \citep[e.g.][]{Briffa94}.

%see Fig.~\ref{fig:?}

%\begin{figure}
%  %\centering{\includegraphics{?}} % enter graphics file name here
%  \caption{
%    \label{fig:?}
%    ?}
%\end{figure}

\conclusions\label{sec:end}

This study has presented the first climate model simulation of some
potential effects of an eruption like that of Laki in
1783--1784. Substantial amounts of sulphuric acid aerosols
produced a large negative radiative forcing over much of the Northern
Hemisphere during June 1783 to Feburary 1784. The Northern Hemisphere
mean forcing peaked at $-5.5$\,Wm$^{-2}$. Sensitivity tests show that
this could be an underestimate and that the real direct effect could
be 1.6 times larger, mainly due to the assumptions about relative
humidity effects on the aerosol scattering properties. Indirect
effects on cloud microphysics are potentially large but impossible to
quantify reliably.

The direct radiative effect of Laki aerosol affected the climate of
the Northern Hemisphere in the IGCM at least throughout 1783 and 1784
in the simulations using the aerosol distribution from the ``Hi"
altitude SO$_2$ emission scenario. Northern Hemisphere mean
temperature anomalies for 1783 ($-0.21$\,K) due to the direct effect
of aerosols calculated using the IGCM are in reasonable agreement with
the somewhat limited observations after the 1783-84 eruption and are
significant at the 95\% level. The temperature response is spatially
complex.

Any mechanisms or circulation changes by which the temperature
response occurs remain to be determined, ideally in a GCM with
higher horizontal resolution. No significant changes in cloud
cover or surface pressure patterns were diagnosed from the IGCM
simulations. Northern Hemisphere mean snow depth did increase during
the winter following the eruption. Ideally, an ensemble of runs of the
chemical transport model and the IGCM should be performed, or
observations of surface pressure from that time period assimilated in
some way to ensure the correct distribution of eruption material and
the resulting aerosol through the Northern Hemisphere.

It is interesting that, given the likelihood that our estimate of
negative radiative forcing is somewhat smaller than reality (as
discussed in Sect.~5), we obtain a Northern Hemisphere mean
temperature response that is within the range of the observational
estimates when considering the ``Hi" scenarios. If the observations are
to be believed, and the indirect effects of the aerosol on clouds were
to produce a large negative forcing, simulated temperature anomalies
could be considerably larger than the observations. In that case there
would need to be some mechanism decreasing the cold anomaly in the
Northern Hemisphere, perhaps a widespread and persistent anomalous
circulation pattern independent of the volcanic effects?
Alternatively, the aerosol resulting from the ``Lo" scenario, together
with large indirect effects could also produce a temperature anomaly
in agreement with observations.

Despite the uncertainties detailed above and in Sect.~5, this study
has shown that large and long duration eruptions such as Laki,
producing aerosol mainly in the troposphere, can have a significant
effect on climate on at least hemispheric and seasonal scales through
the formation of large-scale tropospheric aerosol clouds that cause a
large and widespread negative radiative forcing. Such eruptions are
not restricted to history, nor to Iceland. As such, this study
represents a starting point for future simulations of the atmospheric
and climatic impact of these eruptions. It also highlights the areas where
more knowledge is needed in order to allow us to confirm or deny the
suggestion of \cite{Franklin85} regarding the Laki eruption and its
impact on climate.

%enter conclusions here

\balance % USE THIS TO BALANCE THE LAST TWO COLUMNS IN TWOCOLUMN MODE

\begin{acknowledgements}
EJH was funded in part through a personal NERC postdoctoral fellowship
(GT5/98/3/AS). DSS was funded by a NERC/Environment Agency fellowship
(P4-F02). M. Joshi is thanked for developmental work on the
IGCM. K. Shine is thanked for comments on an early version of this
manuscript.
% enter acknowledgements here

\end{acknowledgements}

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%\expandafter\ifx\csname natexlab\endcsname\relax\def\natexlab#1{#1}\fi
%\expandafter\ifx\csname url\endcsname\relax
%  \def\url#1{{\tt #1}}\fi
%\expandafter\ifx\csname urlprefix\endcsname\relax\def\urlprefix{URL }\fi
\renewcommand{\em}{\rm}

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\clearpage
\begin{table}[t]
  \caption[]
{\label{tab:means}Northern hemisphere and annual mean surface temperature anomalies for Laki simulations compared to a ``clean" pre-industrial atmosphere}
\vskip4mm
\begin{center}
\begin{tabular}{cccc}
\hline\noalign{\vskip1mm}
Year & ``Lo" & ``Hi/short" & ``Hi/Long"           \\
\hline\noalign{\vskip1mm}
1783 & $-0.01$ & $-0.21$ & $-0.21$  \\
1784 & +0.01 & $-0.17$ & $-0.19$   \\
1785 & $-0.08$ & $-0.12$ & $-0.14$   \\
\noalign{\vskip1mm}\hline
\end{tabular}
\end{center}
\end{table}

\clearpage


\begin{table}[t]
  \caption[] {\label{tab:uncert}Ratio of July 1783 Northern Hemisphere
mean radiative forcing for sensitivity studies compared to that of the
``Hi" base case with dry sulphuric acid, 2 wavelength band, 2
stream calculation of the IGCM. Base case value is $-5.5$\,Wm$^{-2}$.}
\vskip4mm
\begin{center}
\begin{tabular}{cc}
\hline\noalign{\vskip1mm}
Case & Forcing ratio           \\
\hline\noalign{\vskip1mm}
Base & 1.0  \\
80\% relative humidity & 2.6   \\
4 stream & 1.01   \\
24 band & 0.62\\
Ammonium sulphate & 0.004  \\
\noalign{\vskip1mm}\hline
\end{tabular}
\end{center}
\end{table}






\clearpage

\begin{figure}
\center\includegraphics[width=13cm]{delf}
\caption{\label{fig:forcing1} Time evolution of global and hemispheric
    mean direct radiative forcing in Wm$^{-2}$ due to sulphate
    aerosols from simulations of the Laki aerosol cloud. Top panel:
    Hi/Long, middle panel: Hi/Short, bottom panel: Lo. (as opposed to
explosive)}
\end{figure}

\clearpage

\begin{figure}
\center\includegraphics[width=13cm]{radf_lkhi_swlw_3yr_corr}
\caption{\label{fig:forcing2} Time evolution of zonal mean direct
    radiative forcing in Wm$^{-2}$ due to sulphate aerosols from
    simulations of the Laki aerosol cloud. Top panel: Hi/Long, middle
    panel: Hi/Short, bottom panel: Lo.}
\end{figure}

\clearpage

\begin{figure}
\center\includegraphics[width=13cm]{radf_jul_lkhi}
\caption{\label{fig:forcing3}
    Radiative forcing distribution (in Wm$^{-2}$) in Northern Hemisphere in July 1783 for {\bf (a)} ``Hi" and {\bf (b)} ``Lo" simulations.}
\end{figure}

\clearpage

\begin{figure}
\centering{\includegraphics[width=9cm]{glm_diff_3yr_acp_K_corr}}
\centering{\includegraphics[width=9cm]{nhm_diff_3yr_acp_K_corr}}
\centering{\includegraphics[width=9cm]{shm_diff_3yr_acp_K_corr}}
\caption{\label{fig:mean_delt}
   Temporal evolution of ensemble mean surface temperature anomaly (in K) relative to simulations with
   pre-industrial atmosphere. Panel A: Global mean, Panel B: Northern Hemisphere mean, Panel C:
   Southern hemisphere mean. Solid line is ``Lo" simulation, dashed line is ``Hi/short" simulation,
    dot-dashed line is ``Hi/long" simulation. Symbols above the curves denote the months where the difference
    in temperature is statistically different at the 95\% level from that using the pre-industrial atmosphere.}
\end{figure}

\clearpage

\begin{figure}
\centering{\includegraphics[width=12cm]{year1_st_lkhil_nobar_final}}
\centering{\includegraphics[width=12cm]{year2_st_lkhil_nobar_final}}
\centering{\includegraphics[width=12cm]{colbar}}
\caption{\label{fig:delt_map} Seasonally averaged ensemble mean
   surface temperature anomaly relative to pre-industrial simulation
   for the Laki ``Hi/Long" simulation, June--July--August (JJA) 1783 {\bf (a)}
   to March--April--May (MAM) 1785 {\bf (h)}. White contours show regions
   where the ensemble mean temperature is statistically different at
   the 95\% level from that in the pre-industrial simulation.}
\end{figure}

\end{document}
