Modelling massive sulphate aerosol pollution,following the large 1783 Laki basaltic eruption

Anne-Lise Chenet*, Frederic Fluteau, Vincent Courtillot

Institut de Physique du Globe and University Denis-Diderot Paris 7, 4 place Jussieu, 75252 Paris Cedex 05, France

Received 4 February 2005; accepted 12 April 2005

Available online 11 July 2005

Editor: K. Farley

Abstract

The climatic impact of volcanic eruptions depends mainly on the amount of sulphur-rich gases released into the stratosphere.

These are rapidly converted to sulphate aerosols, which result in cooling of the lower troposphere and warming of the lowerstratosphere. These gases are retained in the stratosphere and then concentrated by atmospheric processes, forming acid fog nearthe Earth’s surface, with potentially dramatic environmental consequences on plants, animals and humans. We have modelled

such emissions in the case of the unusually large 1783–1784 basaltic fissure eruption of Laki (Iceland), using an AtmosphericGeneral Circulation Model. Results show good agreement with historical observations, such as typical time taken by individualpulses to reach continental Europe or geographical extent of the deadly haze that covered much of the northern hemisphere. Themodel could be adjusted to predict the climatic consequences of very large eruptions, up to the scale of massive flood basalts,

and test the proposal that these are the main agent in most mass extinctions of life on Earth.D 2005 Elsevier B.V. All rights reserved.

Keywords: Laki; sulphate aerosols; environmental impact; climate modelling

1. Introduction

The year 1783 was marked by numerous geologi-cal and geophysical events: a high degree of seismicactivity in Europe, notably in Calabria, accompanyingan eruption of Etna (Italy) on 17 February 1783 and ofStromboli, a few days afterwards. These were fol-lowed by other eruptions: Asama volcano in Japan

from May to August, Vesuvius in August and mostimportantly Laki (Iceland) from June 1783 to Febru-ary 1784. Additionally, in late February 1783, a newisland appeared then rapidly disappeared on the south-west coast of Iceland after a submarine eruption. Lakiwas the second largest basaltic lava flow event in thelast 2000 yr, following the 934 Eldgja eruption [1,2].In a thorough reassessment of all aspects of the erup-tion, Thordarson and Self [2] noted that it producedwidespread sulphuric acid aerosol haze. They recon-structed the eruptive sequence and quantified sulphate

0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2005.04.046

* Corresponding author.

E-mail address: alchenet@ipgp.jussieu.fr (A.-L. Chenet).

Earth and Planetary Science Letters 236 (2005) 721–731

www.elsevier.com/locate/epsl

aerosol emissions, concluding in favour of a stronglink between volcanism and climate. Grattan and co-authors [3–9] vindicate the environmental impact ofvolcanoes. Early observations and accounts comefrom the personal journals of de Montredon [10],Franklin [11] and White [12], who pointed out (asearly as a few months after the onset of eruption for deMontredon and almost as fast for Franklin) the con-nexion between unusual climate and volcanic activityin Iceland. Descriptions of fog impact are also foundin van Swinden [13] and Brugmans (1787), in [7].These reports and hundreds of other accounts identi-fied for instance by Thordarson and Grattan in theirpapers (see references), together with Kington’s studyof weather in the 1780s over Europe [14], allow areconstruction of daily meteorological conditions be-fore, during and after the Laki eruption. The purposeof the present study is to attempt to reproduce in aquantitative way the distribution and propagation ofsulphate aerosols at the time of the Laki, and to see towhat extent the model can account for observationsreported during this bAnnus MirabilisQ.

2. The scenario of Laki injections into theatmosphere

Based on eyewitness accounts, a thorough searchof literature and chemical analyses, Thordarson andSelf [2] have reconstructed the time sequence of lavaeruption and gas emissions. Laki volcano belongs tothe Grimsvotn volcanic system in the southeasternpart of Iceland. Its 1783 eruption began on June8 and ended on February 7, 1784. During 8 months,14.7F1 km3 of lava were produced. The successivephases took place along a 25 km-long fissure, but alsoinvolved explosive events. Ten eruptions occurred inthe first 5 months (Fig. 1), each involving at first ashort (~1 day) explosive phase, during which tephraand gases (SO2, HCl, HF, CO2 and H2O) were emit-ted, followed by a longer phase of lava fountainingand flow emission. Sulphur and carbon dioxide are themain components of gases but only sulphur emissioncould significantly alter the natural atmospheric bal-ance. Given a composition of 75 wt.% of H2SO4 and25 wt.% of H2O for the aerosols in aqueous phase and

1783 1010 10 1020 20 20 20June July August Sept. Oct.

lowhight

II III IV V VI VII VIII IXI

? ?

1784178310 10 10 10 1020 20 20 20

Nov. Dec. Jan. Feb.

X

?

0.0E+00

1.0E+10

2.0E+10

3.0E+10

4.0E+10

5.0E+10

6.0E+10

7.0E+10

8 Jun. 31 Aug.25 Jun. 29 Jul. 24 Sep. 25 Oct.9 Jul.

Mas

s of

sul

phat

e ae

roso

l (kg

)

Fig. 1. (a) Eruptive time sequence of the 1783–1784 Laki eruption in Iceland (after Thordarson et al. [1]). Plumes represent explosive episodes

and dark shading the relative output of basaltic lava. (b) Reconstruction of sulphate aerosol output during the 10 explosive episodes [1], linked

to opening of 10 successive vents: note the quasi-uniform decrease from June 1783 to February 1784. These values were derived from

petrological analysis and assigned an uncertainty of 50% [38].

A.-L. Chenet et al. / Earth and Planetary Science Letters 236 (2005) 721–731722

assuming an ejected water mass which doubles theamount of SO2 released, the water vapour massdegassed by Laki activity was enough to convert allthe sulphur into sulphuric acid without drying up thetroposphere and lower stratosphere [2]. Followingthis, Thordarson and Self [2] concluded that completeconversion into sulphuric aerosols occurred in lessthan a month as the plume was dispersed.

The first three Laki phases occurred in the first 10days, accompanied by explosive phases in which 40%of the total sulphate mass was released. The next threeevents, approximately 2 weeks one after the other,injected almost as much sulphate aerosol as theirpredecessors (Fig. 1). After only 6 weeks, 60% ofthe total sulphate aerosols had been released into thelower stratosphere. Four other eruptive events oc-curred in September and October. Explosive activityended in late October. Close to 200 Tg of sulphateaerosols (i.e. about twice the amount of SO2) had beeninjected into the atmosphere in the first 5 months ofthe eruption. The amount of sulphate released by lavadegassing and due to thermal convection of air abovehot lava flows did not represent more than 20% of thetotal amount [2,15,16] and emission decreased fromJune 1783 till February 1784. In comparison with the122 Tg of sulphate aerosols induced by 76 Tg of SO2

ejected by fossil fuel and industry in 2001 [17], theburden of Laki appears very significant. Moreover, wenote that the Laki was a single point source, whereasthe modern figure is from all sources globally. If oneaverages Laki’s emissions over 6 weeks while themodern anthropogenic emissions are spread over 52weeks, the comparison becomes even starker.

Theoretical models of emission, eyewitnessaccounts in Iceland and reports of aerosol cloudsover Western Europe, North-eastern North America,Central Asia and Siberia in the 3 weeks following theinception of eruption [2,17,18] have led Thordarsonand Self [15] and Woods [19] to conclude that thecolumn height during explosive events reached atleast 13 km in the early phases and up to 10 km duringthe first 3 months: massive injection occurred into thelower stratosphere, and as a consequence volcanicproducts could be distributed at the hemispheric (ifnot global) scale. In contrast, Highwood and colleagues[21,22] considered that the Laki eruption injected gasand ash only in the troposphere. They suggested thatthe climatic impact of the Laki eruption came from the

long-lived effusion that led to a steady state of tropo-spheric aerosols. Grainger and Highwood [22] did notattempt to recreate the atmospheric patterns of 1783,ignoring Kington’s [14] mapping.

The climatic consequences of an eruption are large-ly due to the release of fine-grained submicron dust(tephra) and aerosols by volcanic eruptions into theatmosphere, which alter the Earth’s radiation budget.Tephra reduce incoming solar radiation, but this effectis restricted to the vicinity of the volcano: due to theirdensity and size, they are rapidly deposited. Sulphateaerosols backscatter incoming solar radiation, induc-ing cooling of the terrestrial surface, and absorb solarand terrestrial infrared radiation, leading to warmingof air masses wherever they are located. Aerosols alsohave an indirect effect: they act as condensation nucleifor water vapour, contributing to changes in cloudmicrophysics, radiative properties and lifetime. Thelifetime of sulphate aerosols depends on injectionheight: only 3 weeks in the troposphere but up to 3yr in the stratosphere. Longer lifetime leads to largerclimatic impact. Also, persistent recharging of thesulphate reservoir by the ongoing eruptions into theatmosphere due to the nature of the eruption is a keyfactor. Constraints on injection height are thereforeessential to any modelling attempt.

3. Modelling sulphate aerosol distribution inducedby the eruption

In order to propose a bhindQ prediction of volcano-driven climate change, we have simulated the distri-bution of sulphate aerosols both in space and time, inan attempt to reproduce the propagation of the unusualvolcanic haze. We have forced an Atmospheric Gen-eral Circulation Model (AGCM), coupled with anatmospheric chemistry module, under the scenariorepresenting the rate of sulphate injection into theEarth’s atmosphere as reconstructed by [2]. TheLMDzT-INCA model (for more detailed descriptionsee http://www.lmd.jussieu.fr/~lmdz/homepage.html)simulates atmospheric circulation in 3D at the globalscale. It is based on the equations that describe thedynamics, physics and thermodynamics of the atmo-sphere and includes a description of the exchangesbetween its different components. The chemistry mod-ule simulates atmospheric transport of sulphate and

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tephra and their fallout both in space and time. Simula-tions were bnudgedQ by a relaxation method (http://www.ipsl.jussieu.fr/modelisation/chimie-atmosphere),which consists in forcing the climatic model withEuropean Centre for Medium Range Weather Fore-casting re-analyses of relative humidity, temperatureand winds for the years 2000 and 2001. This allows abetter simulation of atmospheric circulation, particu-larly in the lower stratosphere. The correspondingmeteorological conditions are clearly different fromthose that may have prevailed during the years 1783–1784. Nevertheless, we will see that despite this lim-itation, our modelled results broadly conform to theobservations made at the time. We ran our simulationsover a total duration of 2 yr. This seems sufficient,given the actual duration of the eruption, the lifetime ofsulphate aerosols in the atmosphere and the observedduration of climatic effects due to the Laki (exceptionalevents are recorded only in the years 1783–1785).

Computation outputs consist of tracer simulationsas a function of latitude, longitude, altitude and time,allowing a space (or time) representation as maps,sections or time curves for each volcanic aerosol,from time of injection to total removal. Because theLMDzT-INCA model was primarily developed inorder to simulate the evolution of anthropic emissionsand mineral dust, we had to adapt it. First, the modelrequires input of monthly average injection rates,which results in some smoothing of eruption peaks.For the same reason, the start date for the eruption inthe model runs had to be set 2 weeks ahead of theactual date. Day-by-day knowledge of aerosol inputdoes not seem essential, and actually using dailyinputs does not fundamentally improve the accuracyof model results (C. Textor, personal communication,2003). Another key is sensitivity to injection height.We have seen that explosive events could inject parti-cles up to 10–13 km, which corresponds to an altitudeabove the Icelandic tropopause (around 9–10 km). Atthat altitude, particle transport is due to stratospheric

winds (which are, for instance, responsible for largescale propagation of the aerosol cloud toward Siberia).

Historical data also mention a low altitude cloud,and ash deposits have been found across eastern Ice-land [15]. This corresponds to a lower emission height(estimated at 5 km) for particles produced during thequiet phases emplacing lava flows. Therefore, oursimulation assumes that sulphate aerosols were emit-ted directly as such, in order to model rapid gas-to-particle conversion. Twenty per cent of sulphate aero-sols are injected at an altitude of 5 km and 80% at 10km (which is likely an underestimate), with two sub-scenarios with tephra injected either at 5 or 10 kmheight, in order to test model sensitivity to explosiveviolence of the eruption.

Next, all particles are considered as passive tracers:at this stage we do not model their role in the Earth’sradiative budget; therefore, they cannot alter climatethrough feedback on atmospheric circulation. Second-ly, because they are assumed to be chemically inert,they cannot interact with atmospheric molecules. Thisis not realistic, since we know that sulphur in associ-ation with chlorine (potentially issued from volca-noes) and hydroxyl molecules alter the photolysisratio of ozone [23]; for instance a 15% decrease inozone production has been observed after the Pina-tubo eruption [24]. In our numerical experiments, wesupposed that the behaviour of sulphuric acid aerosolsin the atmosphere was the same as that of sulphates.Sulphate particle density is taken to be 1769 kg m!3

and median diameter of sulphate particles 0.5 Am on amass basis: this is similar to the wavelength of visiblelight resulting in efficient light scattering. Sulphateparticle size distribution about the mean is log normal.The size range is realistic, given particle size resultingfrom gas-to-particle conversion of low-volatility gas-eous species as sulphuric acid formed by atmosphericoxidation. Corresponding parameter values for tephraare 2650 kg m!3 for density and 2.5 Am for mediandiameter on mass basis. Finally, we took into account

Fig. 2. Modelled sulphate aerosol distribution at 3 main altitude levels (near surface, 5 and 10 km) at six representative dates (from end-June

1783 to mid-February 1784). Concentration (i.e. aerosol mass per air mass) is displayed according to a logarithmic colour scale shown at

bottom. Maps are computed using the LMDzT-INCA model (http://www.lmd.jussieu.fr/~lmdz/homepage.html), with a standard resolution of

2.508 in latitude by 3.758 in longitude and including 19 vertical levels, (unequally) distributed between the surface and 40 km altitude. Note the

widespread (hemispheric) distribution of aerosols due to atmospheric circulation and small size and density of particles. Also note the

concentration maxima at ground level extending from Iceland to the Mediterranean after the main pulses of volcanism (see text, and, for more

visual inspection, the animation provided at http://www.ipgp.jussieu.fr/~alchenet).

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the way by which aerosols are brought back to theground by dry and wet deposition.

The various simplifications and assumptions intro-duced in our model in its current stage of developmentand summarized above may imply a number of limita-tions which should be borne in mind when analysingthe results and planning future work. As already men-tioned earlier, our results show that these do not pre-vent the model from accounting for many observations.

4. Results: aerosol distribution on a global scale

The output of our simulations allows us to drawmaps of tephra and sulphate aerosol distribution on aglobal scale or in any particular region. Rather expect-edly, we find that space and time dispersion of aerosolsdepends directly on atmospheric circulation and windpatterns at different atmospheric levels. The Icelandicdepression is a dominant factor driving troposphericcirculation over Greenland and Europe. Aerosolsinjected in the heart of the low-pressure zone arerapidly scattered within this dynamic system. Themodelled atmospheric circulation is in agreementwith meteorological reports over Europe in 1783[3,6,14]. Sulphate aerosol concentrations peak prefer-entially during the first week after eruption aboveIceland, Greenland, Northern and Western Europeand high concentration of aerosols are found all theway to Siberia, whichever the observed atmosphericlevel (Fig. 2). A particularly telling animation coveringthe entire eruptive period is available as supplementarymaterial (http://www.ipgp.jussieu.fr/~alchenet). At theend of July, after the six major explosive volcanicphases, significant aerosol concentrations at theEarth’s surface extended all over the Northern hemi-sphere, with two strong maxima above Iceland andWestern Europe and significant concentrations overRussia and Northern Africa. From August on, peakconcentrations remained important in the northernhemisphere, but total amounts decreased due to lesserinputs during the later events. Only Iceland remained afocus of high concentrations. A general decrease fol-lowed in the next months both related to waning stagesof the eruption and to strong removal at high latitudesdue to intense meteorological variability during thefall. This cleanout principally led to fallout near Ice-land and Western Europe.

5. Comparison between model results and data

5.1. Environmental impact of the sulphate aerosolhaze

We next compare our model output with reportedhaze observations [2]. We find the same sense ofpropagation, from Iceland alternately towards Asiaand North America. A large quasi-circular area, includ-ing Polar Regions, and approximately centred on thenorthern tip of Scandinavia, was covered by the famousdry haze of de Montredon and Franklin [2,10,11]. Thishas been reported on our late-June map (Fig. 3a). It fitsreasonably well with the aerosol concentrations de-duced from our model, except above Saharan regions.This could be due to lack of reports in this low popu-lation density area. The southern extension of the hazeapproximately follows the yellow to green concentra-tion level (i.e. 5.10–9 kg of sulphate aerosol per kg ofair or 5 ppb), allowing us to propose the critical sulphurconcentration level at which visible haze forms. Oursimulated aerosol distribution differs significantlyfrom that modelled by Highwood and colleagues[21,22], in which aerosol haze is so rapidly cleanedout that it does not significantly reach Western Europe,reducing potential climatic impact. Tephra wereinjected during the 10 explosive phases and couldfollow a global scale distribution. However, their sizeand physical properties lead to only proximal distribu-tion (not shown in Fig. 2). This is in agreement withEuropean observations which mention the acid fog inabundance but never tephra falls: Thordarson and Self[2] conclude that less than 10% of the tephra depositoccurs at a distance greater than 10 km away from Laki.

The climatic impact of a volcanic eruption natural-ly depends on meteorological conditions during andafter eruption: aerosol distribution and particle falloutare driven by air circulation. De Montredon [10] hasreported accurately in real time the daily arrival ofacid vapours (and associated weather) at Montpellier(France). These dates are in excellent agreement withpeaks in sulphate aerosol emission independently es-timated from contemporary data collected in Iceland[2]. Comparison of our modelled meteorological con-ditions with those reported by de Montredon [10],with English records of temperature measurements[25–27] and http://badc.nerc.ac.uk], and Kington’scompilation [14] shows that the LMDzT-INCA

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Fig. 3. (a) Modelling of aerosol distribution obtained with the LMDzT-INCA model for end-June 1783. Concentration near the surface, i.e. in

the first layer of the model, at the base of the boundary layer (aerosol mass per air mass) is displayed. The thick solid line corresponds to the

southern boundary of observed haze extension in late June, as reconstructed by Thordarson and Self [2] (shown as (b)).

A.-L. Chenet et al. / Earth and Planetary Science Letters 236 (2005) 721–731 727

model does satisfactorily reproduce the meteorologyand validates the large-scale aerosol distribution.However, our simulations are not able to reproducevolcanic pulses in detail because of lack of timeresolution in mass aerosol input files, which requiremonthly averages. Second, even if global air masscirculation is well modelled, we cannot generate theactual 1783–1784 wind patterns due to an evident lackof data for the global circulation. Yet, we observe inour simulation successive pulses of high aerosol con-centration in the first tens of meters of the atmosphere,which reproduce successive acid fallout arrivals asreported by de Montredon [10] (Fig. 4). The charac-teristic time of haze propagation from Iceland toFrance is less than 5 days, which corresponds totime delays estimated from pulses in Iceland andfallout in Montpellier.

Consequences of sulphuric vapours on the envi-ronment have been studied by Grattan and colleagues[3–9]. They demonstrated that, between August and

October 1783 and between December 1783 and May1784, marked mortality peaks occurred both in Franceand England. The main cause of death could beatmospheric pollution leading to respiratory diseases.Winter deaths appear to be due to strong cold andpossibly famine, after livestock and crops had becomeseriously affected. A recent study by Parker et al. [27]has further confirmed two periods of mortality crisisin England (and especially in the eastern part of thecountry). Mortality in August–September 1783 was40% higher than the 50-yr mean and that in January–February 1784 was 23% higher. Total extra deaths areestimated at about 20,000. Witham and Oppenheimer[28] further find that the first mortality peak correlateswith a very high summer temperature (July), and thesecond with a very cold winter (January–April). How-ever, temperature anomalies can account for only partof the extra deaths, however significant (30–50%).Other possible causes include fever epidemics, airpollution due to Laki or food shortages, all of which

Fig. 4. Latitudinal propagation of aerosols near the Earth’s surface from May to October 1783. Concentration (sulphate aerosol mass per air

mass) is averaged over the longitude band from 24.48 W (Iceland) to 5.68 E (Western Europe) and shown as a function of latitude between 08and 908 N. The map represents the evolution of the aerosol cloud in the first meters of the atmosphere. Maxima are located in Iceland and

dispersal occurs with propagation of volcanic pulses both toward high and middle latitudes (the latter effect being dominant). Slopes of iso-

concentration contours are in general quite steep and allow an estimate of propagation and receding velocities as a function of latitude. Aerosols

reach the latitude of Montpellier (France) only a few days after emission, which is the characteristic dispersal time resulting from atmospheric

circulation.

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by the way could be causally linked to the eruption.Witham and Oppenheimer [28] recall that for the Lakiemissions to be a direct cause of mortality requirestransport of the plume over England, presence of theplume in the boundary layer and harmful concentra-tions of pollutants in the plume. They do note that in[20–22] surface boundary layer concentrations of SO2

are less than 20 ppb, much lower than the detectableodour range (and possibly harmful level) of about 1ppm. But a number of observations, including thevery first ones reported by de Montredon [10], doimply high sulfur levels. And our own calculationsdo show pulses that could have exceeded 0.1 ppmover broad areas of western Europe (Fig. 2), with evenlarger local excursions. Moreover, the impact of finesulphuric aerosols on mortality may have been aworsening factor. On the other hand, as stated byWitham and Oppenheimer [28], the January to Aprilmortality may have been more prominently linked tothe cold temperatures, even though people may havebeen left weakened by the events of the previoussummer and fall.

5.2. Stratospheric injection confirmed

The winter 1783–1784 was among the most severeon record in the past three centuries. Mean wintertemperatures in England dropped by some 2.6 8Cwith respect to their 1659–1850 average [25–27]and http://badc.nerc.ac.uk]. This can be accountedfor by massive sulphur injection by a volcanicplume reaching the stratosphere. This is also docu-mented by high-latitude tree-ring data [29,30]. A dropin annual surface temperature seen in tree-ring widthrecords for 1783 can be linked to the presence ofaerosols in the stratosphere and therefore extremeconditions preventing normal tree development. Fall-out of sulphate aerosol also induced an acidity peak inGreenland ice-cores; such acidity peaks are generallyindeed found to occur on very cold years [31,32]. Inthe years after the 1783 Laki eruption, acidity levelsremained elevated, suggesting that significant sul-phate deposits occurred until 1 yr after the eruptionand again implying that emission must have reachedthe stratosphere.

Paradoxically, sulphate aerosols in Western Europewere accompanied by a heat wave during the summerof 1783 as discussed by Thordarson and Self [2] and

Grattan and Sadler [33] (e.g. a mean temperature of18.8 8C in July 1783 vs. the 1659–1850 July averageof 15.9 8C in central England [25-27] and http://badc.nerc.ac.uk; this caused an early wine harvest inFrance (Emmanuel Le Roy Ladurie, personal commu-nication, 2004). We speculate that, besides a possibleinfluence of warm tropical air masses [2], sulphatehaze observed over Western Europe may haveabsorbed lower tropospheric terrestrial infrared radia-tions, leading to warming of the Earth’s surface. Aero-sols close to the Earth’s surface could be due to falloutfrom higher altitude or tropospheric wind transport.

6. Conclusions

The first results of our model of climatic impact ofthe Laki eruption appear to be promising. TheLMDzT-INCA model reproduces with good accuracythe observed aerosol distribution. The reality of hemi-spheric distribution and propagation of sulphate aero-sols is attested by reports of environmental effects onlivestock, crops and humans. Therefore, that coolingwas mostly due to stratospheric sulphate aerosolsappears to be vindicated in some detail by our simu-lation. It should soon allow us to better constrain thetemperature decrease due to the eruption. With thiskind of benchmark, the model can be adjusted, im-proved and used in predicting expected climatic andpollution-related consequences of eruptions at otherlocations. This is relevant for instance to air trafficsafety: the likely occurrence of another Laki-likeeruption in Iceland in the centuries to come wouldcompletely halt traffic over much of the northernAtlantic [2].

It may be interesting to try and compare the impactof the 1783 Laki with that of the recent 1991 Pinatuboeruption (although of course this was an explosive andnot effusive event), which injected 20 Tg of SO2 at analtitude of 20 km. Pinatubo aerosols led to warming ofthe stratosphere by almost 1.5 8C and Earth surfacecooling of !0.7 8C during the year 1991–1992. Interms of quantities injected into the stratosphere, theLaki amounted to twice the Pinatubo (~40 Tg SO2) ifonly its first 10 days are considered, and 5 times(~100 Tg SO2) for the whole eruptive episode. Thisis yet another indication that Laki must have led tosignificant warming in the stratosphere and cooling in

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the lower layers of the troposphere. These effects maynot be proportional to the amounts of injected sul-phate aerosols, due to non-linear effects, clean out, orlatitude of injection. Given the extra deaths in Eng-land [3,27] and now those that can be estimated fromthe recent work of Grattan et al. in France [9], thedeath count of Laki may have been well in excess of50,000, possibly making it the most fatal volcaniceruption in history (disregarding the particular caseof the tsunami generated by the Krakatoa eruption).

An exciting development is to try, with properadaptations of the same model, to predict the climaticconsequences of large flood basalt magmatism. Arecent review of ages of these large igneous provincesshows an excellent correspondence with the ages ofthe largest mass extinction and ocean anoxia events,which punctuate the evolution of Earth and life [34],making flood basalts a primary candidate as being thecause of these events. However, final proof of thistheory may require modelling of the climatic conse-quences of these eruptions. Yet, it is quite staggeringto think that these lava piles contain tens to hundredsof superimposed flows, some of which are over ahundred times larger than the Laki flow! Preliminarywork using our model to predict the climatic conse-quences of the 30 million year old Ethiopian traps isquite encouraging [35], with excellent agreement withobservations of Ethiopian trap derived tephra found inSouthern Atlantic and Indian oceanic cores [36]. Thekey problem in that case is defining with sufficienttemporal accuracy the forcing function, i.e., the timingof flow emplacement. Such work is in progress for thecase of the Deccan traps, which coincided in time withthe famous mass extinction at the Cretaceous–Tertiaryboundary.

This would not be only of academic interest. Ifindeed flows reaching or even exceeding 1000 km3 involume have erupted in less than a few decades [37],the rates and amounts of CO2 and SO2 injection at thetime may have been on the same order or even largerthan those generated by man in the last two centuries.Each flow may thus have led to significant climatechange and repeated flows to massive climate change.Geologists may therefore hold some key evidenceneeded by climate modellers: the detailed stratigraphyof flood basalts and analysis of accompanying geo-logical and biological events may hold clues that canhelp benchmark their numerical predictions.

Acknowledgements

We are grateful to Yves Balkanski and DidierHauglustaine for providing the LMDz-INCA model.We thank Christiane Textor for her help with theLMDz-INCA model and Emmanuel Le Roy Laduriefor helpful discussion. We thank John Grattan, SteveSelf and Mike Widdowson for friendly and usefulinteractions on this research program. Alan Robockprovided incisive reading of an earlier version of thepaper, which helped to clarify assumptions andadvances made in this study. We are grateful toSteve Self and an anonymous reviewer for very help-ful comments and suggestions. This work was carriedout using the French Atomic Energy Commission(CEA) computing facilities and was partly fundedby the ECLIPSE program of INSU-CNRS. IPGPcontribution no. 2072.

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