Agricultural and Forest Meteorology 115 (2003) 127–137

A review of the risks of sudden global cooling andits effects on agriculture

Kjeld C. Engvild∗Plant Research Department, PRD-313, Risoe National Laboratory, DK-4000 Roskilde, Denmark

Received 11 February 2002; received in revised form 28 November 2002; accepted 14 December 2002

Abstract

Global warming has received much attention, but evidence from the past shows that sudden global cooling has occurred withsevere failures of agriculture. Extrapolating from dendrochronological evidence, one can predict the following: Approximatelyonce per century there will be a drop of about 0.5–1◦C in mean temperature worldwide. In some of these cases, perhaps onceevery 200 or 300 years this might endanger agricultural production globally. About once per millenium there will be periods of5–20 years where the temperature is seriously below normal. The last major one year temperature drop was 1816, the year with-out a summer, probably caused by the cooling effect of the eruption of the volcano Tambora, Indonesia. The last decade-longcooling event wasa.d. 536–545 where dust veil, cold, famine, and plague was recorded in Byzantium and China. Very largevolcanic eruptions or a comet/asteroid impact have been suggested as cause. Nuclear winter after large-scale nuclear war is awell-known scenario, but climate instabilities may also be caused by changes in the sun, Milankovitch cycles, changes in oceancurrents, volcanoes, asteroid impacts, dusting from comets passing close, methane released from its hydrate, and pollution. Therisks associated with sudden global cooling are rather smaller than the risks of global warming, but they are real. A dangeroussudden cooling event will happen sooner or later. Ability to change to cold-resistant crops rapidly in large parts of the worldmay be necessary to avoid major famines. With some important exceptions, fundamental research in abrupt climate changeis in place, but agricultural or economic research on volcanic/comet-dusting/nuclear winters and their mitigation is lacking.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Dust veil; Dry fog; Haze; Global change; Nuclear winter; Volcanic winter

1. Unstable climate

Normally people believe that climate is quite sta-ble. Many recognize that the recent global warming isprobably due to increases in greenhouse gases, suchas methane and carbon dioxide. Many people also rec-ognize that there has been a “little ice age” from about1350–1850 where Londoners skated on the Thames,the Dutch painted vivid winter scenery from theircanals and armies crossed the straits of the Baltic sea

∗ Tel.: +45-46-77-41-44; fax:+45-46-77-42-02.E-mail address: kjeld.engvild@risoe.dk (K.C. Engvild).

on foot. The recurrence of ice ages is taught in schools,and we learn that we are probably living in an inter-glacial age. Even so, very few people think that climateis very variable, although Christmases were whiter inchildhood and hurricanes seem to be stronger now.Also many climatologists have had this concept of astable climate as a fundamental assumption.

This attitude started to change after 1980, whenmore and more scientific results emerged on Green-land and Antarctic ice cores (Dansgaard et al., 1993;Petit et al., 1999; Adams et al., 1999; Taylor, 1999;Stocker, 2000; Lockwood, 2001), on ocean sediments(Bond et al., 1992, 2001), on tree rings series spanning

0168-1923/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0168-1923(02)00253-8

128 K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137

6000 years (Baillie and Munro, 1988; Baillie, 1994,1995, 1999; Briffa et al., 1998; Briffa, 2000; D’Arrigoet al., 2001), and from archaeology. The evidenceis that climate changes quite often, sometimes veryrapidly. Climatologists talk about Dansgaard-Oesch-ger cycles during the ice age (Dansgaard et al., 1993;Rahmstorf, 2002), with changes in mean tempera-tures of 5–10◦C within 20–30–40 years. Some histo-rians talk about rapid climate changes as cause of thelarge migrations and falls of great empires (Hsu, 1998;DeMenocal, 2001; Weiss and Bradley, 2001). The cli-mate changes may vary from the scale of 1 year, overdecades, millennia and 100 000 years (Adams et al.,1999; Taylor, 1999). In the following I shall discusssome of the evidence that climate changes might hap-pen suddenly, the possible causes, and what the con-sequences might be.

1.1. Years without summer

The year without a summer was recorded inboth North America and Europe in 1816 (Stommeland Stommel, 1979; Stothers, 1984). There werecrop-killing frosts in July in Massachusetts and foodriots in France. The maize harvest was particularlyaffected while the wheat yields suffered less. Evi-dence from dendrochronology, the science of datingby tree rings, has shown that the year without a sum-mer was not a unique event. Such years have notbeen uncommon sincea.d. 1400.Briffa et al. (1998)have pinpointed five other years (Table 1). The meantemperature drop was 0.4–0.8◦C. Other evidence also

Table 1The mean summer temperatures on the northern hemisphere after very large volcano eruptions, deduced from low tree ring density

Year Mean summertemperature fall (◦C)

Volcano eruption Volcanic explosivityindex

1453 0.5 Kuwae, SW Pacific, 1452 61601 0.8 Huaynaputina, Peru, 1600 6?1641 0.5 Parker, Philipines, 1641 61695 0.4 Unknown

1816 0.5 Tambora, Indonesia, 1815 71817 0.4

1912 0.4 Katmai, Alaska, 1912 61991 (0.5)a Pinatubo, Philipines 6

Data according toBriffa et al. (1998)and Briffa, 2000. The Volcano explosivity index goes from 0 (no change) to 7 (super colossal).a Global mean temperature fall (McCormick et al., 1995).

shows strong yearly variability within the last 600years (Mann et al., 1998).

1.2. Decade-long cold excursions

It has long been recognized that climate changedseveral times during historic times (Denton andKarlen, 1973; Bond et al., 1997; DeMenocal et al.,2000). The climate was quite warm during the BronzeAge in Western Europe, and it was harsher and colderduring the Iron Age. There was a cold spell called thelittle ice age, which lasted from about 1350 to 1850.At least seven cases are known where temperaturesdropped suddenly and stayed low for 3–16 years(Table 2). Again important evidence comes fromthe dendrochronologists with their world wide netof tree ring series, spanning 6000 years, from Irish,English and German oaks, Scandinavian pines, andAmerican bristle cone pines and foxtails (LaMarcheand Hirschboeck, 1984; Baillie and Munro, 1988;Baillie, 1994, 1995, 1999; Keys, 1999; Briffa, 2000;D’Arrigo et al., 2001). The temperature fell froma.d.536 to 545, which was described both in Chinese andByzantine chronicles as a period of weak sun, visibleonly for a few hours around noon, and casting noshadow (Baillie, 1995, 1999). During the period therewas crop failure, famine and pestilence (Baillie, 1999;Keys, 1999). Holocene variability has also been doc-umented in glacier and tree line fluctuations (Karlén,1998). The evidence for the 536–545a.d. cooling(Table 2) is solid with independent tree chronologies,and historical confirmation from several places. The

K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137 129

Table 2Decade-long episodes of very narrow tree rings and frost ring anomalies, spanning most of the northern hemisphere

Year Tree ring anomalies Volcano

536–545a.d. Oak, N. Ireland; Scots pine, Fennoscandia; bristle conepine, USA; fox tail, USA; fitzroya, Argentina, Chile

Comet/asteroid? Baillie; Krakatoa? (Keys, 1999)

44–41b.c. Bristle cone pine, USA; oak, England Unknown (Stothers, 1999)208–204b.c. Oak, Germany; bristle cone pine, USA Unknown1159–1141b.c. Oak, N. Ireland; Juniper, wide rings, wet conditions, Turkey Hekla 3?1634–1627b.c. Bristle cone pine, USA; oak, N. Ireland; oak, England Santorini?2354–2345b.c. Oak, N. Ireland, extensive flooding?About 3190b.c. Oak, N. IrelandAbout 4370b.c. Oak, N. Ireland

Data taken fromBaillie (1995, 1999). Also compareLaMarche and Hirschboeck (1984), Baillie and Munro (1988), Briffa (2000).

evidence for the other decade-long coolings inTable 2is strong, but the dating of the events is weak dueto fewer tree chronologies. Both tree chronologiesand history point to periods of climate deterioration(Stothers, 1999; D’Arrigo et al., 2001).

1.3. Ice age climate changes

The first strong indication of sudden, rapid climatechanges came from the famous oxygen isotope stud-ies of the ice cores from Greenland and Antarctica(Dansgaard et al., 1993; Petit et al., 1999). It is pos-sible to deduce the temperature during which the iceformed from the ratios between the stable oxygenisotopes as measured by mass spectrometry. The icecores have covered more than 100 000 years. Thesemeasurements have shown that the Greenland averagetemperatures have fluctuated between two levels withdifferences in mean yearly temperatures of 7◦C over

Fig. 1. The very large variations in Greenland temperature during the last ice age. Courtesy: Johnsen and Dansgaard (unpublished), compareDansgaard et al. (1993).

periods of about 3000 years. These very large shiftsin mean temperatures occurred very rapidly over a pe-riod from 50 to 200 years. Over long time-scales, tem-perature highs and temperature lows occurred roughlysimultaneously in Greenland and the Antarctica. Onshorter time-scales, cooling in the arctic often coin-cided with warming in the Antarctic and vice versa,so-called see-sawing (Blunier et al., 1998; Stocker,2000; Rahmstorf, 2002) (Fig. 1).

2. Possible causes

Many causes for the temperature variations havebeen proposed (Crowley, 2000; Zachos et al., 2001):(1) changes in the solar constant, that is changes inthe energy output of the sun; (2) Milankovitch cycles,changes in the earth’s orbit altering the irradiationclose to the poles; (3) changes in ocean currents

130 K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137

which changes heat transport from equator to thepoles; (4) volcanic eruptions creating dust veils; (5)comet/asteroid impacts creating dust veils; (6) dust-ing from comets passing close, creating dust veilsas stratospheric ice. In the future can be added twonew causes of human origin: (7) changes caused bypollution haze and greenhouse gases and (8) nuclearwinter caused by dust veil after large-scale nuclearwar.

Several of the mechanisms act via changes in ra-diative forcing, i.e. via changes in the amount of en-ergy that reaches the surface of the earth (Eddy, 1976;Hansen et al., 1997; Hansen, 2000). Change in thisamount of less than one percent will cause changes inclimate especially if the these changes happen closeto the poles.

These various mechanisms interact in subtle ways,linked by positive and negative feedbacks, so thereare large disagreements about the importance of each.Large feedback exists between for example dust veils,ocean currents, presence or absence of permanentsnow cover and ocean pack ice, cloud cover, withresultant changes in the albedo of the earth.

2.1. Changes in the solar output

Most people assume that the energy output of oursun is constant. But there is good evidence that this isonly an approximation.Perry and Hsu (2000)believethat all important climate changes can be explained bychanges in solar output; this is probably too simplified.However, the peak of the little ice age from 1350 to1850 coincided with the “Maunder” minimum (Eddy,1976), a reduction in the number of sun spots whichseem linked to reduced solar output. It is a fact thatthe production of14C varies so much that age deter-mination by the counting of tree rings must be used tocalibrate14C age determination (Stuiver and Becker,1993). 14C is produced in the stratosphere by cosmicrays which are modulated by solar activity such thatlow solar activity causes high14C production (Beeret al., 2000). So there have clearly been changes insolar activity in the past.Bond et al. (2001)andVanGeel et al. (1999), have shown strong correlation be-tween climate cycles and the content of14C and10Bein sediments. There is also some evidence that thesolar wind/cosmic radiation changes influence cloudcover (Svensmark and Friis-Christensen, 1997).

2.2. Milankovitch cycles

So why has there been ice ages lasting about100 000 years interspersed with interglacials of10–20 000 years? Milankovitch suggested that thiscan be explained by an astronomic model (Muller andMacDonald, 1997; Alley and Clark, 1999; Rial, 1999;Zachos et al., 2001). The amount of sunshine receivedat the poles varies according to the tilt of the earth, theearth’s distance to the sun and whether northern sum-mer or southern summer occurs when earth is far fromthe sun. The three Milankovitch cycles have periodsof 100 000, 40 000 and 23 000 years. Modern modelstend to support the Milankovitch model (Alley andClark, 1999; Zachos et al., 2001), but a major difficultyseems to be that the 100 000 years ice age cycle coin-cides with the Milankovitch cycle which is weakest.

2.3. Ocean currents

Climate and ocean currents are intricately linked,as proven by el Niño and la Niña phenomena. Thepresent interglacial climate is probably maintainedby the “Thermohaline Conveyor” (Rahmstorf, 1995,2002; Broecker, 1995, 1997; Stocker, 2000). The con-veyor consists of the Gulf Stream plus the formationof very dense cold high-salt water close to Icelandand Greenland by evaporation and pack ice freezing.The dense water sinks and forms deep-water currentsflowing south to the Antarctic and then north into theIndian and pacific oceans. This total system of oceancurrents seems to be quite sensitive to small distur-bances, especially fresh water influx, causing someclimatologists to talk of “chaotic climate” (Rahmstorf,1995; Broecker, 1995, 1997).

2.4. Volcanoes, dust veils and SO2

Large volcano eruptions can have effects on cli-mate through the formation of dust veils and sulfuricacid clouds in the stratosphere, formed from SO2 andwater vapor (Lamb, 1970). Dust particles and the sul-furic acid droplets both act on climate by reflecting orabsorbing light; they increase the earth’s albedo, andthe overall effect is measurable cooling (Andreae andCrutzen, 1997; Bertrand et al., 1999; Stothers, 1999;Robock, 2000; Zielinski, 2000; Ramanathan et al.,2001). Outbreaks in the tropics have larger climate

K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137 131

effects than outbreaks close to the poles, because thehaze will quickly spread north and south and coverboth hemispheres. The most recent example is the eru-ption of Pinatubo, which caused a cooling of about0.5◦C in 1991 (McCormick et al., 1995). The Tamboraeruption in Indonesia in 1815 was about 100 times big-ger (Stothers, 1984). It was probably the cause of theyear without a summer in 1816 (Stommel and Stom-mel, 1979) and the very bad years after that. The Tobaerution about 75 000 years ago was 100 times largerthan Tambora (Rampino and Self, 1992). It probablycaused a decade-long deep plunge in temperature.

It has been proposed that the eruption and de-struction of Santorini caused a major climate changealong with the elimination of the Minoan civilization(LaMarche and Hirschboeck, 1984; Rampino et al.,1988; Baillie and Munro, 1988; Baillie, 1999). Tracesof large eruptions are numerous in the Greenland icecores as sulfuric acid acidity (Hammer et al., 1980). Ifseveral very large eruptions occur within a short time-span, there could be decade-long cooling of several de-grees centigrade, with complete destruction of crops.

The decade-long cooling 536–543 is very well doc-umented in dendrochronological records (LaMarcheand Hirschboeck, 1984; Briffa, 2000; D’Arrigo et al.,2001) and the presence of haze/dust veil is evident inthe historical records (Baillie, 1999; Keys, 1999). Ithas, however, been difficult to find a correspondinglarge acid layer in the arctic or Antarctic ice pointingto large-scale volcanism (Baillie, 1995, 1999).

2.5. Comets and asteroids

That the Dinosaurs died out because of an asteroidimpact 65 million years ago has caught the imagina-tion of the public (Alvarez et al., 1980; Cockell andStokes, 1999). Each year the earth is hit by 5-m me-teoroids delivering an explosion 50 km above groundlevel equivalent to one kiloton TNT, comparable toa small nuclear weapon. The small country of Es-tonia has three crater fields, Kaalijärv, Ilumetsa andTsoorikmäe from three separate impacts during thelast 10 000 years (Raukas, 2000). Most asteroid/cometimpacts never hit the ground. Instead there are “highair-burst impacts” in the military language. Last timethere was a very large asteroid impact was in 1908,where a 30–50 m large bolide exploded in about 10 kmaltitude over Tunguska in Siberia with an energy of

about 10 megatons of TNT (Chyba et al., 1993) andflattened the forest over 2000 km2.

The impacts of comet Shoemaker–Levy 9 frag-ments on the gas planet Jupiter in 1994 (Bosloughand Crawford, 1997; Crawford, 1997; Toon et al.,1997; Levy, 1998) have shown that comet air-burstimpacts are quite complex. For example, an out-gassed comet, a “flying rubble pile” 0.3-km diameter,mass about 14 million tons, at 15 km/s would hit theearth about once per 3700 years (Solar System Col-lisions, http://janus.astro.umd.edu/astro/impact.html,Shoemaker, 1983; Chapman and Morrison, 1994;Kring et al., 1996). Such a rubble pile would ex-plode in a few kilometers altitude with an energy of300 Megatons TNT and produce a fireball more than1000 km high. The fine dust and vapor condensateswould cover the entire globe above the stratosphereat heights of 50–200 km within days. This dust veilwould slowly drift to the ground over the next coupleof years. The dust in the stratosphere would weigh10–12 million tons. Supplemented with atmosphericwater vapor and newly synthesized nitrogen oxidesthis would give 30–50 mg of smoke for every squaremeter earth surface, which could reduce insulationespecially near the poles. The seriousness of such animpact would to a very large degree depend on thenumber of fires ignited. An impact over the pacificwould not be so dangerous, but if many thousandsof square kilometers were ignited simultaneously, theearth might be covered in smoke and soot comparableto the nuclear winter scenarios.

2.6. Comet dusting

In addition to the standard risks of comet/asteroidimpacts (Near Earth Object Program,http://neo.jpl.nasa.gov; Kring et al., 1996; Toon et al., 1997) theearth might be hit by large numbers of meteoritesfrom a comet passing very close—within the orbitof the moon—but not hitting the earth (Hoyle andWickramasinghe, 1978; Clube and Napier, 1990;Clube et al., 1996). The resulting dust veil from, e.g.millions of tons of meteorites would cause majorcooling. The probability of a comet dusting is muchlarger than the probability of an impact. In the firstcase the comet should pass within a circle of 100 000–200 000 km radius from the earth, in the second casethe comet/asteroid should hit within a circle of about

132 K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137

7–8000 km radius. There is some evidence that theaccretion rate of interplanetary dust varies (Farleyet al., 1998; Kortenkamp and Dermott, 1998).

2.7. Other climate factors

The location and the altitude of continents areimportant factors in the overall climate of the earthon a scale of millions of years (Crowley and Burke,1998; Ruddiman and Kutzbach, 1991). Like theMilankovitch cycles they are only in indirect waysimportant in sudden global cooling. The same is truefor greenhouse gases, carbon dioxide and methane,which are decreased about one third during ice ages(Petit et al., 1999). There are great uncertainties of thereasons for that. There are large reserves of methanehydrate on the ocean bottoms (Kvenvolden, 1999;Suess et al., 1999). Some people believe that a catas-trophic release of methane from this methane hydratecould lead to a runaway greenhouse effect (Suesset al., 1999; Kennett et al., 2000).

2.8. Nuclear winter

Now, the technology of mankind has reached such alevel that we are ourselves able to cause sudden globalcooling. This was first suggested byCrutzen and Birks(1982), and followed up by Carl Sagan and his students(Turco et al., 1983, 1990) and many others. The initialvery pessimistic forecasts of nuclear winter after aserious nuclear war have been modified, so that thepredictions now are nuclear falls in all but the most se-rious of cases. However, nuclear fall during the grow-ing season would still cause huge crop losses becauseof frost damage. The year without a summer 1816had a global scale mean summer temperature drop of<1◦C (Stothers, 1984; Mann et al., 1998, Table 1).

2.9. Pollution

Much work is done on the influence of the pollu-tion on the climate in the next century. The warmingof the last decade is almost certainly due to green-house gases (Mann et al., 1998; Free and Robock,1999; Crowley, 2000). However, many of the fore-casts based on models of the effects of the increaseof greenhouse gases in the atmosphere yield muchlarger temperature increases than actually measured

(Ramanathan et al., 2001). This is probably caused bythe cooling effect of aerosols (Rampino et al., 1988;Capaldo et al., 1999; Robock, 2000; Stanhill andCohen, 2001; Ramanathan et al., 2001). Smoke fromburning biomass can influence climate on a regionalscale (Hobbs et al., 1997; Shine and Forster, 1999).

3. Risk assessment

There is at least some experimental or modelingevidence for all of the mechanisms referred to above.It is very unlikely that any one mechanism can ex-plain all sudden climate changes. It is likely thateffects of any one major cause are strongly modifiedby the combined influence of many other factors. Theempirical climate data that we have from the past7000 years can be used for a rough estimate of therisk of sudden global cooling. During the last 600years there have been seven very cold years akin tothe year without a summer 1816 (Table 1). Duringthe last 7000 years there have been about 8 very coldperiods lasting 3–15 years like the 536–545 coolingevent (Table 2). This extrapolates roughly to:

• One very cold year per century, perhaps with dis-ruptions in agriculture.

• One very cold decade per millennium with break-down of agriculture.

Most reconstructions of past temperatures involveextensive statistic smoothing which is as it should befor most purposes. However, when one wants to assessthe crop failure risks associated with lowered meansummer temperatures, it is adverse weather extremesduring the growing season which are important. Therisk of a serious 1 year temperature drop is perhapsrather smaller than once per century today becauseof the recent global warming trend. The eruption ofPinatubo in 1991 resulted in a mean temperature dropof about 0.5◦C (McCormick et al., 1995), but therewere no reports of disturbances in world agriculture.

3.1. Changes in rainfall

Another possible consequence of dust/smoke veil ischanges in precipitation. Aerosols act as condensationnuclei in cloud formation; too much aerosol leads tothe formation of many, but very small droplets whichcoalesce poorly to raindrops (reference inRamanathan

K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137 133

et al., 2001; Kaufman et al., 2002). It is quite difficultto sort out the effects of the various types of aerosols. Itis known that troposphere carbon and organic aerosols(smoke from burning) inhibits precipitation strongly(Kaufman et al., 2002; Rosenfeld et al., 2002). Organicaerosols can be cleansed from the atmosphere at seaby salt spray induced rain (Rosenfeld et al., 2002).

3.2. Risk factors

Some of the risk factors are fairly easy to estimateand model, other factors are almost impossible topredict. There are also so many feedback mechanismsthat the end results from stipulated causes becomedifficult to estimate. The most common cause forcooling events is probably volcanic eruptions (Lamb,1970; Hammer et al., 1980; Rampino et al., 1988;Zielinski, 2000). These are notoriously difficult topredict, although large volcanic eruptions will hap-pen eventually, for example in Iceland or the Aleuts.Another parameter for global cooling, ocean currents,seem actually to be chaotic in nature (Rahmstorf,1995, 2002; Broecker, 1997). This means that verysmall differences in initial conditions can end up inalmost opposite effects. The effect of other factors,clouds, albedo, dust veils, ice cover e t c on theglobalconveyor requires large-scale modeling (Crowley,2000; Zachos et al., 2001). With the present knowl-edge it is not possible to give an assessment of therisk of a new ice age starting.

Paradoxically, the most farfetched risk, thatof comet/asteroid impact is quite easy to predict(Shoemaker, 1983; Chapman and Morrison, 1994).The long-term effects of all types of impacts havenot yet been calculated (Levy, 1998; Boslough andCrawford, 1997). Most efforts have been concentratedon very large “dinosaur” or very small “Tunguska”impacts. At present, about 2300 near earth objects areknown (Fig. 2); 650 of these are larger than 1 km indiameter. More than one new one is discovered everyday; 500 potentially hazardous objects are known;they are larger than 150 m and may approach the earthcloser than 20 times the distance to the moon (NearEarth Object Program,http://neo.jpl.nasa.gov, Stuart,2001). Asteroid 1950 DA has a 0.3% chance of col-liding with the earth in 2880 (Giorgini et al., 2002).Within the decade we expect to know the orbits of90% of the kilometer size near earth objects, provided

Fig. 2. The asteroids in the vicinity of the earth on 15March 2001. Three months later an entirely different aster-oid population was close to the earth. From Scott Manley.http://szyzyg.arm.ac.uk/∼spm/neomap.html.

the projected doubling of the modest resources is re-alized. This means that we can predict asteroid/cometimpacts centuries in advance and may be able to de-flect dangerous objects. Only long period comets willremain unforeseen until a few months before impact.The >300 short-period comets and centaurs (asteroidsin orbits between Jupiter and Neptune) may change toearth crossing orbits after close passages to Jupiter orSaturn. Significant orbit changes occurred more than50 times during the last century (Kronk, 2001). Themost famous examples are the comets Hale–Boppand Shoemaker–Levy 9 (Levy, 1998). Both long- andshort-period comets can dust the earth when passingclose, but the risks of short-period comets are larger,because they return frequently and most of them movein the same plane as the planets.

The risk of nuclear winter (Crutzen and Birks,1982; Turco et al., 1990; Robock, 1996) has decreasedvery much since the ending of the cold war. Butthat situation may change very quickly, especially ifeconomic conditions deteriorate in any of the largercountries with nuclear capability. Under such circum-stances there are great risks that leaders with the evilpolitical genius of a Hitler or Stalin may emerge andbring nuclear winter back to reality.

134 K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137

4. Possible mitigation

In general, it will be much easier to deal with theconsequences of cooling after dust veils from volca-noes or comet/asteroid impacts than after nuclear win-ter, because the general infrastructure and energy sup-ply of society will remain intact.

The major risks (Smith, 2000) of global coolings arethe famines connected with crop failures. The worldcereal stock levels are about 30% of the yearly produc-tion (FAO, 2001). Difficulties in developing countrieswill begin already a few months after the first crop fail-ures. In case of a cooling event lasting more than a yearfamines will also hit the developed countries (Harwelland Cropper, 1985); in that case crop losses in thenorthern hemisphere cannot be compensated withimports from the southern hemisphere or vice versa.

A general awareness that there may be a problemis an important beginning. For example, necessaryseed supplies of appropriate frost/cold-resistant cropsshould be available. Sufficient knowledge among agri-cultural extension agents is necessary, and the nec-essary agricultural and economic research should bein place. The warning times for the formation of vol-canic dust veils are of the order of months. Asteroidor comet dusting may happen instantaneously, if theparent body has not been seen. With the current auto-mated surveys (Stuart, 2001) more and more threaten-ing objects are found and warning times may becomecenturies. Warning time for nuclear war can be veryshort, probably only weeks; nuclear war probabilitieswax and wane with the current political climate.

The developed countries with large meat produc-tion can to some degree extend their food resources byslaughtering most of their animals early and changeto primarily eating plant products. This option is notavailable in most of the developing countries, espe-cially not where two or three crops are grown eachyear.

The lessons of history have been that it is weatherextremes, rather than the means that result in cropfailure; frost in July had serious consequences inUSA in the year without summer 1816 (Stommel andStommel, 1979). In other areas of the world droughtor flooding might follow sudden global cooling.

There is a need for stress tolerant crops, and the or-dinary definitions may have to be extended. It is notenough that a wheat cultivar is cold tolerant during

winter, it must also be cold tolerant during growth, an-thesis and grain filling. Finding such cultivars is diffi-cult, perhaps impossible as frost resistance depends onproper hardening. Probably the strongest detrimentaleffects on crops will be where crops are grown undermarginal conditions, either on marginal land or undermarginal temperatures.

The change to frost/cold-resistant crops might befairly easy in some countries, such as growing hardypotatoes or beets instead of cereal grain, or growingwheat instead of maize. In some developing countrieshardy alternative crops have been known for centuries,such as grass pea (Lathyrus sativus) in Bangladesh,India, and China, and Cassava tubers (Manihot escu-lenta) in Africa and South America. Most research onthe effect of global cooling on agriculture was donein relation to “nuclear winter” (Ehrlich et al., 1983;Harwell and Cropper, 1985; Myers, 1989) and verylittle has been done since then (Robock, 1996). Thelarge-scale research in the agricultural consequencesof global warming (Rötter and vandeGeijn, 1999) haslittle relevance in the global cooling case. There isa need to for agricultural and economic research ad-dressing the issues of sudden global cooling and thefollowing famines.

Small cooling events are much more frequent thancatastrophic events. From observation of the smallerevents, we might learn what the large catastropheswould be like. It is necessary that all possible causesare investigated. Much of the relevant research effortis already in place in climatology, space sciences,volcanoes, and geology; but redirection is needed inseveral areas. For example Hoyle’s comet dustinghypothesis (Hoyle and Wickramasinghe, 1978; Clubeand Napier, 1990; Clube et al., 1996) needs seriousinvestigation. Also the dendrochronological evidencefor several decade-long coolings during the last mille-niums needs confirmation and extension (Baillie andMunro, 1988; Baillie, 1994, 1995, 1999).

References

Adams, J., Maslin, M., Thomas, E., 1999. Sudden climatetransitions during the quaternary. Prog. Phys. Geogr. 23, 1–36.

Alley, R.B., Clark, P.U., 1999. The deglaciation of the northernhemisphere: a global perspective. Annu. Rev. Sci. Earth Planet.Sci. 27, 149–182.

K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137 135

Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980.Extraterrestrial causes of the K/T extinction. Science 208, 1095–1108.

Andreae, M.O., Crutzen, P.J., 1997. Atmospheric aerosols:biogeochemical sources and role in atmospheric chemistry.Science 276, 1052–1058.

Baillie, M.G.L., 1994. Dendrochronology raises questions aboutthe nature of thea.d. 536 dust-veil event. Holocene 4, 212–217.

Baillie, M.G.L., 1995. A Slice Through Time. Batsford, London,UK.

Baillie, M., 1999. From Exodus To Arthur. Batsford, London, UK.Baillie, M.G.L., Munro, M.A.R., 1988. Irish tree rings, Santorini

and volcanic dust veils. Nature 332, 344–346.Beer, J., Mende, W., Stellmacher, R., 2000. The role of the sun

in climate forcing. Q. Sci. Rev. 19, 403–415.Bertrand, C., VanYpersele, J.P., Berger, A., 1999. Volcanic and

solar impacts on climate since 1700. Climate Dyn. 15, 355–367.Blunier, T., Chappellaz, J., Schwander, J., Dallenbach, A., Stauffer,

B., Stocker, T.F., Raynaud, D., Jouzel, J., Clausen, H.B.,Hammer, C.U., Johnsen, S.J., 1998. Asynchrony of Antarcticand Greenland climate change during the last glacial period.Nature 394, 739–743.

Bond, G., Heinrich, H., Broecker, W., Labeyrie, L., McManus,J., Andrews, J., Huon, S., Jantschik, R., Clasen, S., Simet, C.,Tedesco, K., Klas, M., Bonani, G., Ivy, S., 1992. Evidence formassive discharges of icebergs into the North Atlantic oceanduring the last glacial period. Nature 360, 245–249.

Bond, G., Showers, W., Cheseby, M., Lotti, R., Almasi, P.,DeMenocal, P., Priore, P., Cullen, H., Hajdas, I., Bonani, G.,1997. A pervasive millennial-scale cycle in North Atlanticholocene and glacial climates. Science 278, 1257–1266.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N.,Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I., Bonani,G., 2001. Persistent solar influence on North Atlantic climateduring the holocene. Science 294, 2130–2136.

Boslough, M.B.E., Crawford, D.A., 1997. Shoemaker–Levy 9 andplume-forming collisions on earth. Ann. N. Y. Acad. Sci. 822,236–282.

Briffa, K.R., 2000. Annual climate variability in the holocene:interpreting the message of ancient trees. Q. Sci. Rev. 19, 87–105.

Briffa, K.R., Jones, P.D., Schweingruber, F.H., Osborn, T.J., 1998.Influence of volcanic eruptions on the northern hemispheresummer temperature over the past 600 years. Nature 393, 450–455.

Broecker, W.S., 1995. Chaotic climate. Sci. Am. 273 (November),44–50.

Broecker, W.S., 1997. Thermohaline circulation, the Achilles heelof our climate system: will man-made CO2 upset the currentbalance? Science 278, 1582–1588.

Capaldo, K., Corbett, J.J., Kasibhatla, P., Fischbeck, P., Pandis,S.N., 1999. Effects of ship emissions on sulphur cycling andradiative climate forcing over the ocean. Nature 400, 743–746.

Chapman, C., Morrison, D., 1994. Impacts on the earth by asteroidsand comets: assessing the hazard. Nature 367, 33–40.

Chyba, C.F., Thomas, P.J., Zahnle, K.J., 1993. The 1908 Tunguskaexplosion: atmospheric disruption of a stony asteroid. Nature361, 40–44.

Clube, V., Napier, B., 1990. The Cosmic Winter. Blackwell,Oxford.

Clube, S.V.M., Hoyle, F., Napier, W.M., Wickramasinghe, N.C.,1996. Giant comets, evolution and civilization. Astrophys.Space Sci. 245, 43–60.

Cockell, C.S., Stokes, M.D., 1999. Polar winter: a biologicalmodel for impact events and related dark/cold climate changes.Climatic Change 41, 151–173.

Crawford, D.A., 1997. Comet Shoemaker–Levy 9 fragment sizeestimates: how big was the parent body? Ann. N. Y. Acad. Sci.822, 155–173.

Crowley, T.J., 2000. Causes of climate change over the past 1000years. Science 289, 270–277.

Crowley, T.J., Burke, K.C. (Eds.), 1998. Tectonic BoundaryConditions for Climate Reconstructions. Oxford UniversityPress, Oxford.

Crutzen, P.J., Birks, J.W., 1982. The atmosphere after a nuclearwar: twilight at noon. Ambio 11, 114–125.

D’Arrigo, R., Frank, D., Jacoby, G., Pederson, N., 2001. Spatialresponse to major volcanic events in or abouta.d. 536, 934 and1258: frost rings and other dendrochronological evidence fromMongolia and northern Siberia—comment on R.B. Stothers,volcanic dry fogs, climate cooling and plague pandemics inEurope and the middle east (Climatic Change 42, 1999).Climatic Change 49, 239–246.

Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D.,Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen,J.P., Sveinbjörnsdottir, A.E., Jouzel, J., Bond, G., 1993.Evidence for the general instability of past climate from a250-kyr ice-core record. Nature 364, 218–220.

DeMenocal, P.B., 2001. Cultural responses to climate changeduring the late holocene. Science 292, 667–673.

DeMenocal, P., Ortiz, J., Guilderson, T., Sarntheim, M., 2000.Coherent high- and low-latitude climate variability during theholocene warm period. Science 288, 2198–2202.

Denton, G.H., Karlen, W., 1973. Holocene climatic variations—their pattern and possible cause. Q. Res. 3, 155–205.

Eddy, J.A., 1976. The Maunder minimum. The reign of Louis XIVappears to have been a time of real anomaly in the behaviorof the sun. Science 192, 1189–1202.

Ehrlich, P.R., Harte, J., Harwell, M.A., Raven, P.H., Sagan, C.,Woodwell, G.M., Berry, J., Ayensu, E.S., Ehrlich, A.H., Eisner,T., Gould, S.J., Grover, H.D., Herrera, R., May, R.M., Mayr,E., McKay, C.P., Mooney, H.A., Myers, N., Pimentel, D., Teal,J.M., 1983. Long-term biological consequences of nuclear war.Science 222, 1293–1300.

FAO, 2001. The State of Food and Agriculture (SOFA) 2001.http://www.fao.org/es/ESA/sofa.htm.

Farley, K.A., Montanari, A., Shoemaker, E.M., Shoemaker, C.S.,1998. Geochemical evidence for a comet shower in the lateeocene. Science 280, 1250–1253.

Free, M., Robock, A., 1999. Global warming in the context ofthe little ice age. J. Geophys. Res. Atmos. 104 (D16), 19057–19070.

Giorgini, J.D., Ostro, S.J., Benner, L.A.M., Chodas, P.W., Chesley,S.R., Hudson, R.S., Nolan, M.C., Klemola, A.R., Standish,E.M., Jurgens, R.F., Rose, R., Chamberlin, A.B., Yeomans,

136 K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137

D.K., Margot, J.-L., 2002. Asteroid 1950 DA’s encounter withearth 2880: physical limits of collision probability prediction.Science 296, 132–136.

Hammer, C.U., Clausen, H.B., Dansgaard, W., 1980. Greenlandice sheet evidence of post-glacial volcanism and its climaticimpact. Nature 288, 230–235.

Hansen, J.E., 2000. The sun’s role in long-term climate change.Space Sci. Rev. 94, 349–356.

Hansen, J., Sato, M., Ruedy, R., 1997. Radiative forcing andclimate response. J. Geophys. Res. Atmos. 102 (D6), 6831–6864.

Harwell, M.A., Cropper, W.P., 1985. Potential effects of nuclearwar on agricultural productivity. In: Harwell, M.A., HutchinsonT.C. (Eds.), Environmental Consequences of Nuclear War.Ecological and Agricultural Effects, vol. 2. Wiley, New York,pp. 271–355.

Hobbs, P.V., Reid, J.S., Kotchenruther, R.A., Ferek, R.J., Weiss, R.,1997. Direct radiative forcing by smoke from biomass burning.Science 275, 1776–1778.

Hoyle, F., Wickramasinghe, C., 1978. Comets, ice ages, andecological catastrophes. Astrophys. Space Sci. 53, 523–526.

Hsu, K.J., 1998. Sun, climate, hunger, and mass migration. Sci.China, Ser. D: Earth Sci. 41, 449–472.

Kaufman, Y.J., Tanré, D., Boucher, O., 2002. A satellite view ofaerosols in the climate system. Nature 419, 215–223.

Karlén, W., 1998. Climate variations and the enhanced greenhouseeffect. Ambio 27, 270–274.

Kennett, J.P., Cannariato, K.G., Hendy, I.L., Behl, R.J., 2000.Carbon isotopic evidence for methane hydrate instability duringquaternary interstadials. Science 288, 128–133.

Keys, D., 1999. Catastrophe: An Investigation into the Origins ofthe Modern World. Century, London.

Kortenkamp, S.J., Dermott, S.F., 1998. A 100,000 years periodicityin the accretion rate of interplanetary dust. Science 280, 874–876.

Kring, D.A., Melosh, H.J., Hunten, D.M., 1996. Impact-inducedperturbations of atmospheric sulfur. Earth Planet. Sci. Lett. 140,201–212.

Kronk, G.W., 2001. Cometography. Cambridge. Also web:http://cometography.com.

Kvenvolden, K.A., 1999. Potential effects of gas hydrate on humanwelfare. Proc. Natl. Acad. Sci. U.S.A. 96, 3420–3426.

LaMarche, V.M., Hirschboeck, K.K., 1984. Frost rings in trees asrecords of major volcanic eruptions. Nature 307, 121–126.

Lamb, H.H., 1970. Volcanic dust in the atmosphere with achronology and assessment of its meteorological significance.Phil. Trans. R. Soc. Lond. Ser. A 266, 425–533.

Levy, D.H., 1998. The collision of comet Shoemaker–Levy 9 withJupiter. Space Sci. Rev. 85, 523–545.

Lockwood, J.G., 2001. Abrupt and sudden climatic transistionsand fluctuations: a review. Int. J. Climatol. 21, 1153–1179.

Mann, M.E., Bradley, R.S., Hughes, M.K., 1998. Global-scaletemperature patterns and climate forcing over the past sixcenturies. Nature 392, 779–787.

McCormick, M.P., Thomason, L.W., Trepte, C.R., 1995.Atmospheric effects of the Mt-Pinatubo eruption. Nature 373,399–404.

Muller, R.A., MacDonald, D.J., 1997. Glacial cycles andastronomical forcing. Science 277, 215–218.

Myers, N., 1989. Nuclear winter: potential biosphere impacts inBritain. Ambio 18, 449–453.

Perry, C.A., Hsu, K.J., 2000. Geophysical, archaeological, andhistorical evidence support a solar output model for climatechange. Proc. Natl. Acad. Sci. U.S.A. 97, 12433–12438.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.M.,Basile, I., Bender, M., Chapellaz, J., Davis, M., Delaygue, G.,Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y.,Lorius, C., Pepin, L., Ritz, C., Saltzman, E., Stievenard, M.,1999. Climate and atmospheric history of the past 420,000 yearsfrom the Vostok ice core, Antarctica. Nature 399, 429–436.

Rahmstorf, S., 1995. Bifurcations of the Atlantic thermohalinecirculation in response to changes in the hydrological cycle.Nature 378, 145–149.

Rahmstorf, S., 2002. Ocean circulation and climate during the past120,000 years. Nature 419, 207–214.

Ramanathan, V., Crutzen, P.J., Kiehl, J.T., Rosenfeld, D., 2001.Aerosols, climate and the hydrological cycle. Science 294,2119–2124.

Rampino, M.R., Self, S., Stothers, R.B., 1988. Volcanic winters.Annu. Rev. Earth Planet. Sci. 16, 73–99.

Rampino, M.R., Self, S., 1992. Volcanic winter and acceleratedglaciation following the Toba super eruption. Nature 359, 50–52.

Raukas, A., 2000. Investigation of impact spherules—a newpromising method for the correlation of quaternary deposits.Q. Int. 68–71, 241–252.

Rial, J.A., 1999. Pacemaking the ice ages by frequency modulationof the earth’s orbital eccentricity. Science 285, 564–568.

Robock, A., 1996. Nuclear Winter. In: Schneider, S.H. (Ed.),Encyclopedia of Weather and Climate, vol. 2. Oxford UniversityPress, New York, pp. 534–536.

Robock, A., 2000. Volcanic eruptions and climate. Rev. Geophys.38, 191–219.

Rosenfeld, D., Lahav, R., Khain, A., Pinsky, M., 2002. The roleof sea spray in cleansing air pollution over ocean via cloudprocesses. Science 297, 1667–1670.

Rötter, R., vandeGeijn, S.C., 1999. Climate change effects on plantgrowth, crop yield and livestock. Climatic Change 43, 651–681.

Ruddiman, W.F., Kutzbach, J.E., 1991. Plateau uplift and climaticchange. Sci. Am. 264 (March), 42–50.

Shine, K.P., Forster, P.M.d.F., 1999. The effect of human activityon radiative forcing of climate change: a review of recentdevelopments. Global Planet. Change 20, 205–225.

Shoemaker, E.M., 1983. Asteroid and comet bombardment of theearth. Ann. Rev. Earth Planet. Sci. 11, 461–494.

Smith, J.V., 2000. Natural hazards: geology, engineering,agriculture and sociopolitical/humanitarian considerations forthe twenty-first century. Int. Geol. Rev. 42, 617–656.

Stanhill, G., Cohen, S., 2001. Global dimming: a review of theevidence for a widespread and significant reduction in globalradiation with discussion of its probable causes and possibleagricultural consequences. Agric. For. Meteorol. 107, 255–278.

K.C. Engvild / Agricultural and Forest Meteorology 115 (2003) 127–137 137

Stocker, T.F., 2000. Past and future reorganizations in the climatesystem. Q. Sci. Rev. 19, 301–319.

Stommel, H., Stommel, E., 1979. The year without a summer. Sci.Am. 240 (June), 134–140.

Stothers, R.B., 1984. The great Tambora eruption in 1815 and itsaftermath. Science 224, 1191–1198.

Stothers, R.B., 1999. Volcanic dry fogs, climate cooling, and plaguepandemics in Europe and the middle east. Climatic Change 42,713–723.

Stuart, J.S., 2001. A near-earth asteroid population estimate fromthe LINEAR survey. Science 294, 1691–1693.

Stuiver, M., Becker, B., 1993. High precision decadal calibrationof the radiocarbon time scale,a.d. 1950–6000b.c. Radiocarbon35, 35–65.

Suess, E., Bohrmann, G., Greinert, J., Lausch, E., 1999. Flammableice. Sci. Am. 281 (November), 52–59.

Svensmark, H., Friis-Christensen, E., 1997. Variation of cosmic rayflux and global cloud coverage—a missing link in solar–climaterelationships. J. Atmos. Solar-Terr. Phys. 59, 1225–1232.

Taylor, K., 1999. Rapid climate change. Am. Sci. 87 (4), 320–327.

Toon, O.B., Zahnle, K., Morrison, D., Turco, R.P., Covey, C.,1997. Environmental perturbations caused by the impacts ofasteroids and comets. Rev. Geophys. 35, 41–78.

Turco, R.P., Toon, O.B., Ackerman, T.P., Pollack, J.B., Sagan, C.,1983. Nuclear winter: global consequences of multiple nuclearexplosions. Science 222, 1283–1292.

Turco, R.P., Toon, O.B., Ackerman, T.P., Pollack, J.B., Sagan,C., 1990. Climate and smoke: an appraisal of nuclear winter.Science 247, 166–176.

Van Geel, B., Raspopov, O.M., Renssen, H., vanderPlicht, J.,Dergachev, V.A., Meijer, H.A.J., 1999. The role of solar forcingupon climate change. Q. Sci. Rev. 18, 331–338.

Weiss, H., Bradley, R.S., 2001. What drives societal collapse.Science 291, 609–610.

Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001.Trends, rhythms, and aberrations in global climate 65 Ma topresent. Science 292, 686–693.

Zielinski, G.A., 2000. Use of paleo-records in determiningvariability within the volcanism–climate system. Q. Sci. Rev.19, 417–438.