TENTATIVE GENERAL CONCLUSIONS

T h e climates of the different epochs here discussed are amenable to interpretation largely

in

terms of (u) intensity changes and

(6)

latitude shifts of the main limbs of the zonal circulation, accompanied

by

appro- priate changes of wave length and trough positions in the belt of westerlies. T h e latitude and longitude changes must have had direct consequences

in

modifying climates in every latitude zone and have perhaps been more conspicuous than the changes of circulation strength.

There is some evidence €or supposing that both the circulation in general and the radiation available were weakened

during

the Little Ice A g e A.D.

1430-1850 by

a few per cent

in

the Northern Hemisphere and perhaps

by

about

1-2

per cent over the world, most pro- bably due to volcanic dust. Correspondingly, it might be reasonable to suppose that the circulation in general, and (more doubtfully) the radiation available, were slightly above their twentieth-century strength

in

the early Middle Ages (especially 1000-1200), though it would be safer to assume that they were not far different from modern values. Possibly the chief difference between the modern situation and that of the early Middle Ages w a r m epoch and of the major post-glacial climatic optimum around

5000-3000

B.C. lies in the duration of these w a r m epochs and the sea temperatures consequently attained

in

the Atlantic and Arctic.

A s the energy of the circulation increased (at least between

1800

and recent decades) the Northern Hemis- phere circulation seems in general to have shifted pole- ward, perhaps mainly controlled

by

the displacement of the main thermal gradient accompanying the shrinking Arctic ice and winter snow cover o n land.

It is clear that the extended Arciic ice and cold water

of

the period around A.D.

1800

and earlier modified the Atlantic sector circulation, causing peculiarly great latitude shifts there and even local strengthening of the circulation and thermal gradients near the southern extremities of the extended cold troughs-in an epoch w h e n over wider regions the energy was reduced.

Correspondingly, in the early Middle Ages w a r m epoch, when the energy generally available m a y be supposed to have been greater

(if only

because of higher sea temperatures), the circulation seems to have been not always stronger than n o w (and possibly sometimes weaker)

in

northern Europe. T h e likeliest reason for this seems to be the remoteness at that time of the Arctic ice limit. Indeed, the patterns of that time m a y

have some bearing upon what would happen

in

various latitudcs

if

the Arctic ice were artifically disposed o€.

W h e n the general atmospheric circulation increases in energy the strongest (most disturbing) effects should be expected

in

those sectors where a broad quasi- permanent ice or cold-water surface has protruded farthest towards low latitudes. A t times very slrong circulations and very abnormal patterns might then be produced and become very variable from year LO

year

if

the protrusion of Arctic ice were to break

up

or shrink rapidly. T h e peculiar climatic course of the 1830s in the North Atlantic and neighbouring regions should probably be M e w e d

in

this lighL. T h e decade was one o€ extreme variations between persis tent blocking and intense zonal circulations, most strongly developed

in

unusually

high

latitiides, so that the low values o€ some of the intensity indices

in

that decade m a y be misleading -indices taken in what are at most other times the best positions being just then unrepresentative.

Wexler has pointed out that any long-period changes in the insolation available should produce a m u c h quicker response over the great land masses than

in

the oceans and regions of quasi-permanent ice. This seems to be supported

by

our study of the state of the general circulation around 1800, w h e n a trend towards increasing energy m a y (according to some indications) have been already under w a y for about a century, and w e observe a peculiarly great equatorward displacement of the circulation over the North Atlantic Ocean.

Peculiar instability of the climate

in

the Atlantic- European sector at various times between

1250

and 1.550, with harsh alternations of wet and dry, w a r m and cold years

in

various (especially eastern) parts of Europe in the 1300s and (especially) 14ûOs, m a y perhaps be regarded as a phenomenon of the trend towards colder climate and increasing areas of snow and ice surface crudely corresponding to the vicissitudes of the 1830s during the recovery trend.

Clearly general trends of circulation strength must be judged

by

measures of the circulation in m a n y different parts of the world and allowance m a d e for passing phases in which misleading efjfects arise in regions affected

by

persistent ice or cold water-especially the North Atlantic.

T o

judge the position of the most recent decades in relation to longer-term trends it m a y be necessary to consider whether the appearance of rhythmic changes of wave position, w a v e length and circulation intensity affecting Europe-rises to maxima around

1100

and

1900-30

and a minimum around 165Q-represents some long-period oscillation.

So

far the evidence seems againsi.

this, since the fluctuation was probably asymmetric in time-the decrease spread over

300-500

years, the recovery nearly complete

in 150-200

years-and the Southern Hemisphere was probably noL airected.

Nevertheless there is a good deal of evidence that the general circulation has for the time being €allen away from its maximum in tensity around 1900-30-cvidenty

146

Clintatic epochs w h i c h differed Sfon the modern normal

not due to volcanic dust-and the trend

of

the North Atlantic circulation towards lower latitudes

in

winter

(and probably other seasons) needs watching and Baur

(1956, 1958)

cannot be ignored.

explaining.

In

this connexion the case m a d e out for solar weather relationships

by

Willett

(1949, 1961)

and

R É S U M É

De

la nature de certaines époques climatiques qui se sont 6CaTtéeS de la c normale x moderne (1900-1939)

(H. H.

Lamb)

L’auteur fait tout d’abord l’inventaire de nos connais- sances actuelles sur la répartition des principales périodes chaudes et froides depuis la dernière glaciation, en s’attachant plus particulierement aux indices de déplacements vers les pôles ou vers l’équateur des tra- jectoires de dépressions et des zones sèches. Quelques indices semblent montrer que les différences principales entre la récente période de réchauffement de l’Arctique et les périodes chaudes antérieures les mieux établies ont

été

: a) la durée; et par conséquent,

b)

les tempé- ratures océaniques superficielles et la superficie des glaces arctiques atteintes.

Les suggestions de Wexler concernant les anomalies de circulation générale

qui

apparaîtraient durant une période de réduction notable

du

rayonnement incident sont

à

nouveau examinées en relation avec les types d e circulation que l’on sait maintenant avoir existé snr les régions européennes et atlantiques et les longitudes avoisinantes a u cours de la période de climat froid aux environs de

1800,

Ces types de circulation tendent

à

confirmer les vues de Wexler sur les effets probables des poussières volcaniques : u) par l’action directe de voiles de poussigre répétés

; B)

indirectement, par suite de l’extension des glaces polaires et de l’abaissement

de

la température des mers.

Cette période voisine de

1800

se situe vers la

fin

d’une époque

qui

a duré plusieum siècles dans l’hémisphere nord, el

il

semble actuellement F e l’anomalie la plus évidente a

été

un déplacement vers l’équateur par rap- port à la normale actuelle de la circulation générale atmosphérique sur l’océan Atlantique-Nord.

Un

dépla- cement correspondant

du Gulf

Stream et de la dérive atlantique nord s’est apparemment produit. L’intensité de la circulation générale a

été

un peu inférieure aux valeurs actuelles, mais a

été

proballement encore moindre avant

1800. I1

semble possible

d’en

déduire une première estimation de la réduction relative clc l’insolation pendant le << petit âge glaciaire n (environ

D’autres indications sur la nature

du

«petit âge glaciaire >> et sur la période chaude

du

début

du

m o y e n âge (maximum vers

1000-1200) qui

l’a précédé sont tirées de l’étude systématique des documents de l’histoire climatique européenne depuis

800.

Les effets de ces diverses périodes climatiques sur les latitudes plus basses sont examinées.

U n e attention particulière est accordée

à

une décennie o u plus d’extrême désorganisation de la structure cli- matique normale sur l’Europe et la Méditerranée aux environ des années

1830.

Pour finir, une rapide estimation de la situation actuelle est tentée, en relation avec les principales tendances climatiques.

1430-1850).

D I S C U S S I O N

H. FLOHN. E.

L a m b has put before us an admirable amount of evidence including some more or less indirect results of large-scale variations in the upper-air pattern. Recent investi- gations based on the wind observations at the 700-metre level in the Alps (Sonnblick since 1886, Zugspitze since 1901) have revealed that the position of the upper troughs and ridges over Europe has undergone substantial variations which apparently fit well into the results of

Mr.

Lamb.

H. H.

L.4iin.

Dr.

Flohn’s study of the resultant wind directions in different groups of years since 1880-1900 at the approxi-

mately 3,000-metre-high summits of Sonnblick and Zugspitze nicely confkms the actual occurrence of longikdinal displace- ments of the mean positions of upper ridges and troughs in the European sector.

My

reconstructions of the mean pressure fields and circulation patterns of the last 250 years suggests that the variations of longitudinal position of the upper ridges and troughs are reflected in mean surface pressure and prevailing weather. From a very recent investigation

I

believe this lurkage m a y also be applied to the Southern Hemisphere westerlies : changes in the trough positions in the southern westerlies indicated at their northernmost fringe

147

Changes of climate

/

Les chnngements de climat

in July point to a general increase of wave length (and perhaps therefore of prevailing strength) from at least 1850-60 up to some time early in the present century, followed by some decline. This appears to confirm our measurements of the strength of the southern westerlies over Chile and N e w Zealand and perhaps carries our knowledge of the trend back a decade or two further.

R. FAIRBRIDGE.

T w o points should be made about the post- glacial climatic optimum of 5000 to 3000 B.C.:

1. The scale of

Mr.

Lamb’s four climatic phases should perhaps be plotted on logarithmic paper, since the oldest is 2,000 years, and the younger ones only a few hundred years.

2. The sea level in the climatic optimum was “similar to today” according to

Mr.

Lamb.

In

comparison to the Ice Age (with m.s.1. of 100 metres), this is broadly true, but geological evidence shows that the climatic optimum sea level was about f3 metres. Since 1 mm. of sea level change equals 0.36

x

1012m.3 of glacial melt-water, a sea level 3 metres higher than today means roughly 1

x

lO15m.3 less glacial ice on the mountains of that time; this is consistent with the paleobotanical climatic data.

H. H. LAMB. I

agree that the different durations of the out- standing phases of climate which

I

described is important and accounts for some of the differences between the respective w a r m epochs. Nevertheless, whatever periodic oscillations m a y or m a y not have been going on-e.g., on time scales from half a century to a few centuries in length-the periods

I

have indicated were undoubtedly the extreme ones, specially stressed perhaps by some extraneous variable such as the occurrence or non-occurrence of volcanic dust.

K. W. BUTZER.

This very interesting paper by Mr. L a m b is an occasion to comment with particular reference to the climatic anomalia associated with the so-called post-glacial climatic optimum.

The

evidence cited by Mr. L a m b in favour of a northward shift of the subtropical high pressure cells is from European Russia. The pollen data at its sourse is, however, without absolute dating and chronologically rather difficult to assess.

It

is, in m y opinion, inconclusive at this point.

Another aspect of the same problem: evidence of intensity of contemporary moist conditions in lowland Egypt decreases from latitude 250 to 200. This suggests the dry axis of the Sahara was still close to latitude 20°

N.

Again the Majorca record (near 400

N.)

shows noticeably moist conditions con- temporary to the maximum post-glacial sea level. This does not seem to warrant the reconstruction

Mr.

L a m b suggests for this, as yet, unsolved problem.

H.

H. LAMB. I

was hoping that some of those who are more familiar than

I

with the evidcnce and its limitations from fields outside meteorology would add comments of this kind showing where the reconstruction is weaker or firmer than elsewhere. It seems to me, however, that in the post-glacial optimum in western Europe the evidence points to a really high latitude for the near high pressure belt-perhaps 500

N.

or so. It m a y be that this high pressure system was an intrusion from the Atlantic, which commonly ended in northerly winds over Russia or a little farther east. Majorca could easily have a moist régime with this pattern.

I

understand that farther south on the southern half of what is now the western Sahara (in Niger territory especially) there were at that time lakes inhabited by fish in large numbers as far north of the source of the moisture in the

Gulf

of Guinea as 160-200

N. In

the eastern Sahara and in the Near Eastern desert areas,

I

well believe the changes from today’s pattern were less marked.

Another probable aspect of the situation in the western longitudes was less intense development of the arid zone than nowadays, perhaps because of more frequent variations of latitude of the high pressure system,

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150

O N T H E C L I M A T I C VARIATION

bY

L. L Y S G A A R D

Meteorological Service, Copenhagen, Denmark

During the

period of observation

by

means of instru- ments all elements of

the

weather have been subjected to a variation especially

in

the polar and temperate zones, where, at a lot of stations,

the

trend of tempera- winters m a y have become more frequent or rarer.

TABLE

1. The minimum and maximum mean temperature at Copenhagen for January, February, Júly, August and the year during the 30-year periods from 1811 up to 1960

The

coldest January occurred during the period

1871- 1900

and the coldest February

in 1811-40,

whilst the mildest January occurred

during

the period

1901-30

and the mildest February in

1931-60

(see Table

1).

The

absolute lowest temperature since

1860

occurred

The

absolute lowest temperature since

1860

occurred

Dans le document CHANGEMENTS DE CLIMAT OF CLIMATE (Page 149-163)