Sea Level, Ice, and Climatic Change (Proceedings of the Canberra Symposium, December 1979). IAHS Publ. no. 131.

The importance of ice sheets in long term

changes of climate and sea level

W. F. BUDD Meteorology Department, University of Melbourne, Parkville, Australia 3052

ABSTRACT A review of the effects of ice sheets on climate as indicated by modelling studies, reveals that the presence of the ice sheets themselves was the major factor causing the lower climatic temperatures prevailing during the ice ages. A review of other modelling studies of the climatic effects of the changing radiation regime, resulting from the Earth's orbital variations, indicates that the changes are large enough, and of sufficient duration, to cause the initiation and termination of the ice ages when the added feedback of the ice sheets oh the climate is included. To understand the effect of the orbital radiation changes it is important to different­iate between annual, summer and June-July periods all as functions of latitude and whether over land or sea. In particular the June-July changes over land at high latitudes are very high whereas the global annual changes are negligible. These concepts and modelling results resolve a number of anomalies in previous palaeo climate studies. A review of palaeo evidence for changes of ice extent, sea level and climate provides considerable support for the results of model calculations of ice sheet and climate response to the orbital variations. These responses show three major periods of lower temper­atures and greater ice extent over the last 120 OOO years, separated by periods of milder climate with low land ice volumes. In reconciling this pattern with sea sediment isotope records it is important also to consider a strong isotope component from the Antarctic ice sheet.

1 INTRODUCTION

The advance and retreat of the large ice sheets over North America and Europe were associated with large changes of climate and sea level. The causes of these dramatic changes have been obscure and controversial. The.palaeo evidence for these past changes has shown some spatial and temporal similarities, but also considerable variability so that a clear picture is diffi­cult to obtain. Problems in accurate dating and interpreting of palaeo evidence suggest that all the data should be continually reviewed to search for consistent patterns.

An initial review of theories and evidence for the ice age changes encounters many problems and inconsistencies which at first sight appear difficult to resolve. A re-examination of the

441

442 W.F. Budd

theories and the palaeo evidence in the light of recent develop­ments suggests that many of the problems can be resolved by consideration of the consequences of the Earth's orbital radiation changes combined with an albedo feedback effect of the ice sheets themselves. It will be shown that the role of the ice sheets is to modulate the climatic effects of the orbital radiation changes by amplifying the consequences of cooler summers and reducing the impact of warmer summers. Through this process the Northern Hemisphere ice volume largely controls the sea level but it is the sea level changes which control the extent of the Antarctic ice sheet. In this way the ice sheets play the central role in a complex interaction of feedback between climate, sea level, and the ice cover.

2 REVIEW OF PROBLEMS

The theory that the Earth's orbital radiation changes are the principal causes of the ice ages, as proposed by Milankovitch (1941), has received considerable support in recent years, from palaeo evidence such as ocean sediment analyses (Hays et al., 1976a; Imbrie & Imbrie, 1979) and sea level changes (Chappell, 1974a). However a number of tests of the theory using climate models have not obtained temperature changes considered large enough to cause ice ages, e.g. Shaw & Donn (1968), Schneider & Thompson (1979), North & Coakley (1979).

Consideration of the differences between seasonal and annual mean changes of temperature and snow or ice cover for different regions are needed to explain these apparent anomalies. The importance of the seasonal variations in the snow and ice cover with the orbital radiation changes has been demonstrated by Suarez & Held (1976, 1979). These variations allow the ice sheets to build up and decay as indicated by Weertman (1976). The ice sheets may then cause further climatic change which can delay their decay as modelled by Pollard (1978).

Indications of the effect of the ice sheets on the climate are provided by the modelling results of Gates (1976a, b) and Manabe & Hahn (1977) for typical northern summer conditions during the period near maximum ice extent ̂ 18 000 years BP (before present) (CLIMAP Project Members, 1976; Gates & Imbrie, 1975). The fact that the radiation regime, about 18 000 years BP, was similar to that of the present, makes these latter studies particularly valuable as indicators of the effects of the surface albedos and sea surface temperatures on the climate, in the absence of radiation changes.

Numerical values of the temperature changes from the radiation variations and from the albedo feedback have been examined by Budd & Smith (1981) in terms of the effects on the growth and decay of the ice sheets. I.t was concluded that the temperature changes have to be close to those obtained by Suarez & Held (1976) for the effects of the radiation changes, and close to those obtained by Gates (1976a, b) or Manabe & Hahn (1977) for the ice sheet albedo feedback, in order for the North American ice sheet to advance and retreat according to the inferred historical

Ice sheets and long term changes of climate and sea level 443

record as summarized, for example, by Prest (1969 and 1970), or Bryson et al. (1969).

As a result of this work it now becomes possible to provide explanations of the following apparent anomalies:

(a) Firstly why have the energy balance climate models using the orbital radiation changes consistently failed to obtain temperature changes large enough for ice ages?

(b) If the variations in summer total radiation over the northern temperate latitudes are primarily responsible for ice ages, then why have the climatic temperatures of the past not been much higher than those of the present, since over the last lOO OOO years the summer radiation in northern temperate lati­tudes has been further above than below present radiation levels?

(c) If the summer radiation totals are the primary forcing function for the ice ages, then how can the southern temperate glaciation be in phase with those of the north when the summer radiation totals there are out of phase?

(d) If temperature changes are the cause of the ice growth, rather than precipitation for example, then why are the ocean temperature changes at maximum ice extent generally much less than the temperature decrease over land areas, especially near the 20° latitudes (CLIMAP Project Members, 1976)?

(e) The radiation changes show a large radiation decrease by 115 000 years BP. Although a substantial drop in sea level has been reported for about that time by Steinen et al. (1973), why do the sea sediment (180/160) records of Broeker & van Donk (1970), or Hays et al. (1976a), not show a comparable large change also?

(f) Similar anomalies exist between the Northern Hemisphere land ice record and the sea level records, in that, for many locations, similar ice extents have been noted about 20 OOO and 60 OOO years BP, and perhaps also around 100 OOO years BP, whereas the ocean isotope records show increasing peaks to a maximum about 20 OOO years BP (cf. Fig. 9(b)).

(g) Hays et al. (1976b) reported an apparent phase lead of several thousand years for the extent of the Antarctic sea ice compared to the ice extent in the Northern Hemisphere. How can this lead come about?

(h) If the ice sheet albedo feedback is so important for the growth and maintenance of the ice sheets, what is the correspond­ing cause of their decay?

Finally, if these anomalies can be resolved is it also possible to unite the wide variety of palaeo evidence into some form of compatible history within the errors of dating and the ranges of possible interpretation?

The following sections aim to develop the orbital radiation changes as the framework and forcing function to which the other changes are related on a clear time scale.

3 ORBITAL RADIATION CHANGES

An important feature of the radiation changes associated with the Earth's orbital variations is that although the total incoming

444 W.F. Budd

solar radiation, over the whole Earth, for a full year, is not changed significantly, appreciable changes do occur for different latitudes at different times of the year. The variation in annual total radiation for different latitudes from -160 OOO to +20 000 years from present are shown in Fig. 1(a) from the data of Vernekar (1972). These results can be compared with the similar information for summer radiation totals as given by Budd & Smith (1981, Fig. 7) and the annual summer and June deviation at lat. 60°N of Fig. 1(b). The importance of the individual monthly changes has been stressed by Berger (1979) with his calculation of "insolation signatures".

Total Annual Insolation Deviations lyday4

Time 103a \20J WO , 80 , 6 0 | 40 20 0

-160 -140 ~^120 S o -80 -60 -40 -20 0 20

Fig. 1 (a) Annual total radiation changes. The deviations (from the present values) of the annual total solar radiation received at the Earth, outside the atmosphere, for 20° latitude increments are shown as a function of time from 160 000 years BP to +20 000 years. The units are for average daily values in ly day-1 (1 W m~2 = 2.07 ly day-1 ) as derived by Vernekar (1972).

(b) The radiation changes from the present at 60°N are shown for (1) the annual average (2) the summer (half year) average and (3) the June (average). Units are in ly day~'. 1 and 2 are from Vernekar (1972) and 3 from Berger's (1975) relation, calculated by D. Jenssen. A scale for the percentage change from the present for each is also shown.

The following points are worth noting. The annual total changes are due to the obliquity variations and are the same in each hemisphere, but opposite between tropics and poles. In polar regions the annual totals and summer totals are similar and the changes at the two poles are in phase.

The amplitude of the annual changes is much larger at the poles (e.g. lat. 80° - 5 x lat. 20° change), but the area receiv­ing the higher radiation is smaller, such that over the globe the totals are unchanged.

During periods of higher obliquity there could be a tendency for the temperature difference between tropics and poles to be decreased. The change in radiation gradient is calcuable.

The changes in the longitude of the perihelion do not

Ice sheets and long term changes of climate and sea level 445

appreciably affect the annual totals. In temperate zones the effect of the perihelion changes on the summer radiation totals is comparable to those of the obliquity changes, but in this case the different hemispheres are affected oppositely. With a period of about 40 OOO years for obliquity and about 22 OOO years for the perihelion cycles the periods when the two are in phase and out of phase are very important for the region between 40° and 70° lat. In particular the low summer radiation periods of 115 000, 75 OOO and 25 OOO years BP contrast markedly with the high summer radiation periods of 125 000 and 10 OOO years BP. The shift in the time of occurrence of the perihelion causes the peak radiation anomaly to migrate through the year as shown by Kukla (1978), from the calculations of Berger (1975). Further developments of this effect are given by Berger (1979) .

The present obliquity is near the midpoint of the range, and the longitude of the perihelion is close to the southern summer solstice. It is notable that the south temperate zone has received generally less summer radiation in the past whereas the north temperate zone has received generally more compared to the present. The polar regions are about half way through a cycle. These points are important when assessing various past climatic indicators with regard to their response to different effects such as seasonal and annual changes. The difference between high radiation and high temperature effects also needs to be considered, because the land and sea respond differently to radiation changes with different time constants.

The oceans, for example, will tend to integrate over a longer time than the land and therefore may reflect annual as well as seasonal effects. On the other hand, the land surface may respond very strongly to the seasonal changes. Similarly the extent of the snow on the ground is highly dependent on the summer radiation and the region (latitude and elevation) reached by the 0°C surface temperature isotherm.

For high northern latitudes the ice ablation is highly concentrated into the peak summer period. Ahlmann (1948) shows typical examples of polar glaciers for which 90% of the total ablation occurs from May to September, with 70% occurring over the 3 months June to August, and 30% during July. The radiation plays a dual role in its effect on ablation rates. Firstly there is the direct change in absorbed radiative energy and secondly there is the effect of the induced change of temperature which then determines the period for which the ice is at melting point. It is this latter effect which is most important for the variation of total ablation with elevation and latitude, cf. Budd & Allison (1975).

Thus although the total summer radiation can be considered as the primary feature of the radiation changes relevant to ablation, it is important also to consider the changes for the individual summer months. Figure 2 shows the comparison of a high summer radiation period (10 OOO years BP) with a low summer radiation period (115 OOO years BP) compared to the present. Although the summer total varies only by ^ 4% above and ^ 4% below the present, the June values reach ^ 10% above and ^ 8% below at high northern latitudes. Thus temperatures over land in

446 W.F. Budd

-10- Latitude, °N Jun 20 30 4,0 50 6p 7,0 8p

Fig. 2 Radiation deviations at peak periods. The annual, summer, June, and July average radiation deviations for the peak high and low northern radiation periods of 115 000 and 10 000 years BP are shown as a function of latitude in percent change from present values.

summer could be expected to show large changes due to the orbital variations, compared to the temperature changes in the ocean.

For the Antarctic the net ablation zones are very small areas generally near the coast. Most of the ice loss is by iceberg calving into the sea. At 60°S, in contrast to the Northern Hemisphere, the summer radiation has been much lower over the past 120 000 years but not much higher. It can be expected that high ablation rates have not been a dominant controlling factor in the extent of the Antarctic ice sheet. Temperature fluctu­ations from the past radiation and climate changes can be expected to be greatly damped by the time they reach close to the basal layer of the ice sheet. Thus the mass balance and flow of the Antarctic ice sheet cannot respond as readily to the orbital radiation changes as the northern ice sheets can be expected to have done.

On the other hand, the extent of the Antarctic ice sheet is strongly dependent on the sea level and the depths of the continental shelves. This means that the Antarctic can respond in some way to the ice volume and sea level changes associated with the northern ice sheets. Such responses will have feedback with sea level, and phase delays due to long buildup times of the ice and the subsequent depression of the bedrock.

Thus although the orbital radiation changes may be a primary driving mechanism for the ice age changes, the complex inter­actions, feedbacks and phase delays need to be analysed to develop a clear picture of the changes.

4 RESULTS FROM MODELLING STUDIES

Consider firstly the results of the thermodynamic climate model of Suarez & Held (1976) in terms of the reaction to the orbital radiation changes and the consequences regarding ice sheet growth and decay. The extent of the minimum residual coverage of snow on land after the summer can be considered as the region where ice sheets can grow. The results of Suarez & Held show the minimum seasonal extents of snow on land resulting from their

Ice sheets and long term changes of climate and sea level 447

model as a function of time since 120 000 years BP. Although for elevations near sea level the extent is too far south for the present the deviations are relevant and show that the minimum extent reached about 4° lat. further south and about 22° lat. further north, as shown in Fig. 3. The model is very sensitive to the exact value of the solar constant and can to some extent be tuned to the present climate. Alternatively the extent of the present minimum snow extent can be interpreted in terms of elevation. For example the present snow limit of 63°N given by the model could be considered valid for land above some eleva­tion, say ̂ 1.5 km depending on the precipitation rate at that location, cf. Fig. 4 (from Budd & Smith's modelling results).

A migration of the permanent snow line 4° lat. further south would then bring those elevations above the 1.5 km level into net accumulation zones upon which the ice sheets could begin to grow. The persistance of such a climatic situation for a period of 5000 to 10 000 years (as may have occurred around 115 OOO, 75 000 and 25 OOO years BP), could be expected to be long enough for the ice sheets to grow - especially if the additional summer ice cover then gives even lower temperatures from the albedo feedback. This is illustrated in Fig. 3 from the combination of the results of Budd & Smith with those of Suarez & Held.

The key regions then are those high bedrock zones above ̂ < 1 km elevation near latitude 60°N, e.g. Baffin Island, Ungava, Torngat Mts, Canadian Rockies, Scandinavia, Ural Mts, and Greenland, cf. for example Fig. 4, and also the results of Ives et al. (1975), Williams (1975, 1979) and Andrews & Barry (1978).

Similarly the mountain regions reaching above 3000 m as far south as 40°N (such as the European Alps, the Caucasus and the Himalayas) could also be centres of expanded glaciation according to Suarez & Held's model results.

The major discrepancies between the results of the Suarez & Held model and the palaeoclimate record concerns the apparent phase shift in the timing of the peaks and troughs together with the overemphasis of the warm periods and underemphasis of the cooling. These discrepancies can be explained by the presence of the ice sheets.

The results obtained by Gates (1976a and b) and Manabe & Hahn (1977) from modelling the ice age climate with atmospheric general circulation models (GCMs) simulating the summer climate near 18 OOO years BP, with a similar radiation regime to the present, showed that large temperature changes resulted over the land and ice areas.

Although some of the cooling with respect to the present could be attributed to the lower sea surface temperature, the primary difference can be considered due to the higher albedos of the snow and ice cover.

Table 1 summarizes the results of these studies. The modelling studies of Weertman (1976) have shown that the

ice sheets can build up and decay within the times available from the periods of the orbital radiation changes. The magnitudes he uses however, in terms of the latitudinal shifts south of the equivalent summer radiation totals, seem to be too large compared to those obtained by Suarez & Held. The assumption that the snow

(1)

(2)

(3)

(4)

120

Fig. 3 Radiation, snow line, temperature and ice volume changes. These features are shown as a function of time from 120000 years BP to the présentas follows.

(1) The summer radiation deviation averages (in ly day^1 ) for 70° and 40°N from Vernekar (1972).

(2) The maximum latitude of the snow line on land as derived by Suarez & Held (1976).

(3) The climatic summer (mean 3 months) temperature deviations from the present at 70°N including the ice sheet albedo feedback from Budd & Smith (1981).

(4) The corresponding computed ice volume for the North American ice sheet "close f i t " simulation.

90 80 70 60 50 4U 3U LATITUDE, °N

Fig, 4 Elevation of bedrock, ice sheet surface and net ablation rate for the region of North America including Baffin Island and Labrador. The bedrock height and ice surface height are shown from the modelling of Budd & Smith (1981) for the growth of ice from 120000 years BP at 3000, 6000 and 9000 years after commencement. The full (thin) curves illustrate the existing ice sheets (Barnes and Penny Ice Caps). The thick curves represent smooth mean annual ablation rates as a function of elevation and latitude for the present conditions.

line moves north and south with the summer radiation totals neglects the fact that ablation rates depend on temperature as well as radiation when these two parameters change separately and that the temperatures depend on other factors besides the summer

Ice sheets and long term changes of climate and sea level 449

Table 1 Ice age temperature drop ( C) from the GCM atmosphere modelling of Gates (1976b) (G), and Manabe & Hahn (1977) (M & H)

Northern Hemisphere G M & H

Southern Hemisphere G M & H

Global G M & H

Ice sheet

15.3

6.7

(2.6)

I ce free land

6.6 6.5

(2.9)

5.0 5.5

(2.6)

6.1 6.2

(4.0)

Land total

9.2 (3.3)

5.2 (4.1)

7.7

Sea ice

3.0*

9.3*

Open sea

2.2 2.6

(2.6)

3.2 2.3

(2.3)

2.7 2.4

Sea total

3.8 (3.2)

4.7 (4.8)

4.4 (4.1)

Average all surfaces

5.3 5.9

(3.2)

4.5 4.9 (4.7)

4.9 5.4

(4.0)

*lce age sea ice area considered. Note: The parentheses ( ) give the results for Manabe & Hahn's experiment with only the ice age sea surface temperatures changed and without the changed ice areas.

radiation. These other factors include the longer term response of the oceans and the albedo feedback of the ice sheets. Never­theless the latitude shifts used by Weertman for the last 150 OOO years were not unlike the shift in minimum snow line obtained by Suarez & Held, (i.e. 6°S to 8°N compared with 4°S to 22°N). Both studies however neglect effects of the large variations which exist in the elevation of bedrock topography and the feedback of the ice sheet on the climate.

The simulation of the ice sheet feedback effect was success­fully incorporated into a combined ice sheet atmosphere model by Pollard (1978). Pollard's results appear to be very promising for such coupled models and showed that the ice sheet causes the phase lag in climate and ice extent which was missed by Suarez & Held. The temperature differences obtained by Pollard between the cold and warm period summers were ̂ 9°C for polar regions and ^ 6° in mid latitudes, measured over land and sea. These compare favourably with the palaeo evidence.

Both Weertman and Pollard found it difficult to grow the ice sheets large enough and still have them disappear readily. Further difficulties were encountered due to the high sensitivity of the results to unknown parameters of uncertain magnitude.

Some of these problems are removed by including specific bedrock topography for the ice sheets and by using a dynamic ice flow model. Both of these features were included in a three dimensional ice flow model by Barry et al. (1975), and Andrews & Mahaffy (1976). An important result of this modelling was that, for a depressed equilibrium line altitude, ice sheets begin to develop and spread from the high regions in Baffin Island and Labrador. The initiation of the glaciation from this work resembled quite closely the pattern of increased glaciation

450 W.F. Budd

derived by Williams (1975, 1979) for the same region as likely to result from a cooler climate or reduced radiation.

A similar three dimensional ice flow model was used by Budd & Smith (1981) on a much larger grid covering all of mainland North America to study the growth and decay of the ice as a result of the orbital radiation changes. By considering as unknown the radiation-temperature relationship and the ice sheet albedo-temperature feedback relationship it was possible to derive values of these parameters which gave rise to good fits to the most recent advance and retreat of the ice sheet.

The values of the radiation-temperature factor (giving a maximum of 5°C lowering) and the ice sheet albedo feedback factor (giving a maximum of 6.6°C lowering) can be compared with the results of Suarez & Held (1976) , Gates (1976b) and Manabe & Hahn (1977) as shown in Tables 1 and 2.

The correlation of Suarez & Held's maximum northerly snow line latitude with the summer radiation deviations for 40° and 70°N latitudes gives a slightly nonlinear response increasing from 0.3° lat./dy day"1) at -20 ly day-1 to 0.5° lat./(ly day-1) (1 W m 2 = 2.07 ly day - 1). For a rate of change of mean summer temperature with latitude of 0.7°C/1° lat. (cf. Budd & Smith, Fig. 8) and a temperature-elevation gradient of 6.5°C/km, Suarez & Held1s result imply a range of the radiation-elevation factor (r of Budd & Smith,virtually relating the change in snow line to the radiation change) of 32 m/dy day-1) to 75 m/(ly day - 1). Even the smaller value compares favourably with the minimum radiation-elevation factor, f = 30 m/(ly day - 1), found by Budd & Smith as necessary to give a close match to the past ice sheet growth and decay. The higher value of f for large positive radiation deviations would tend to make the ice sheets disappear more readily.

A comparison of the relative magnitudes of the effects of the ice sheet albedo feedback and the direct radiation-temperature relation as obtained from the different modelling results is given in Table 2. Although some aspects of the different studies are not strictly comparable, at least some quantitative guide to the magnitudes of the major features is obtained.

It can be seen from Table 2 that the value of maximum temper­ature lowering for the ice sheet albedo feedback obtained by Budd & Smith is comparable to that implied by the GCM modelling by Gates. Similarly the radiation-elevation-temperature relation obtained by Budd & Smith corresponds, in the region of the appropriate latitude (near 60°N), to the snow line-iatitude-temperature relation implied by the results of Suarez & Held for the modelling of the climatic reaction to the orbital radiation variations.

It is also notable that the magnitude of the ice sheet albedo feedback effect, near the ice cover maximum (around 18 000 years BP), is somewhat larger than the maximum summer radiation reduction effect (around 2 5 000 years BP), although at other times (e.g. 115 OOO or 10 OOO years BP) the magnitude of the radiation change effect could be comparable. For both the ice sheet albedo feedback and the direct radiation variations the average net radiation-temperature change relations are comparable

Ice sheets and long term changes of climate and sea level 451

Table 2 Comparison of radiation and temperature changes from different studies

1. Gates (1976) GCM modelling for ice age conditions 'v 18000 years BP in the Northern Hemisphere

Bare land Open ocean Ice sheets Sea ice and lakes

Area changes from present X106 km2 -13.1 -13.6 +27.4 -0.7 % of hemisphere 4.9 - 5 . 1 +10.3 -0.3

Hemispheric mean albedo change = + 0.077 Mean hemispheric net radiation change = —24.5 ly day-1 = —6% Mean surface temperture change — all surfaces = — 5.3 C

over bare land = — 6.6 C

2. Budd and Smith (1981)

Temperature drop for maximum ice sheet albedo feedback (18 000 years BP) = - 6.6°C

3. Radiation changes from present at 60° N latitude (from Vernekar 1972, and Berger 1975, computed by D. Jenssen) (1 W nrf2 = 2.07 ly day-1 )

Time (X103 years BP) Annual (%) Summer (%) June (%) (ly day-1 ) (ly day"1 ) (ly day"1 )

-115 - 25 - 10

4. Suarez& Held (1976)

Advance of maximum snow line latitude at 25 000 years BP = 4°lat. Equivalent temperature change for 0.7°C/1° lat. = 2.8°C

5. Budd & Smith (1981)

Temperature decrease at 'v 25 000 years BP from a radiation-elevation-temperature factor of 30 m/(ly day-1 ) (for summer radiation) and a temperature-elevation gradient of 6.5°C km - 1

•4.5 3.5 3.5

-0.92 -0.72 +0.72

-32 - 1 3 +32

-4.1 -1.7 +4.1

-94 -27 +67

-9,7 -2.8 +6.9

Latitude <°N)

70 60 50

Radiation

Annual (ly day-1 )

-10.5 - 3.5 - 1

change

(%)

-2 .6 -0.72 -0.17

Summer (ly day"1 )

- 2 2 - 1 3 - 8

(%)

-3 .0 -1.7 -0.96

June (lyday"1)

- 4 7 - 2 7 - 1 8

(%)

-4.7 -2 .8 -1 .8

Temperature change (°C)

-4 .5 -2 .5 -1 .6

(̂ 1°C/1% radiation change). Similarly the high radiation period of 10 OOO years BP can be

compared with the results of the GCM study reported for similar conditions by Mason (1979), who obtained surface temperature increases above the present of 6° to 7°C in the northern

452 W.F. Budd

latitudes. Figure 2 shows that the June-July radiation levels are 6-9% greater in high northern latitudes during that period, again supporting the 1°C/1% radiation change.

These separate independent modelling studies thus indicate a common thread showing how the radiation changes together with the ice sheet albedo feedback, can account for the changes in the climate and the ice sheets as inferred from the palaeo evidence.

It is important to note that certain features needed to be included in the ice sheet model of Budd & Smith in order to obtain a reasonable match to the record of ice advance and retreat. These features included an appropriate bedrock distri­bution, an elevation-desert effect (similar to that used by Sugden (1977)) and a time delayed isostatic bedrock response. These results provide a useful guide to the phase shifts between radiation, climate, ice volume, sea level and bedrock depression.

In order to assess the role of the Antarctic two other model­ling studies are relevant.

Firstly the two dimensional flowline modelling of Budd & Mclnnes (1979) for East Antarctica showed that a much longer buildup time for Antarctica was required than for the North American ice sheet (viz.^ 30 000 compared to ^ 10 OOO years). It was also found that for a given accumulation distribution the extent of the ice and thickness at the edge were governed largely by the depth of water over the continental shelf. Thus the area and volume of the Antarctic ice sheet are dependent on the sea level and bedrock depression with a long buildup time but with the possibility of a rapid retreat time.

Secondly the study of the effect of the Antarctic sea ice extent on the Southern Hemisphere climate by Simmonds (1981) indicates that for fixed sea surface temperatures the effect of the present sea ice cover in September compared to no sea ice only lowers the hemispheric mean temperature by 1.2°C, even though the temperature difference over the sea ice zone between the open water and ice covered area was 13°C.

A similar study is provided by the additional GCM run performed by Manabe & Hahn with ice age sea surface temperatures, but present ice extent. Again for the Southern Hemisphere the effect of the sea surface temperature seems to be strong compared to the effect of the additional ice extent, cf. Table 1.

It is worth noting that both the ocean and sea ice extent react to winter and annual radiation regimes as well as the summer totals.

Suarez & Held (1976, 1979) showed that for their model the orbital variations only affected the Southern Hemisphere temper­ate or sea ice extent slightly but that this slight effect is in phase with the Northern Hemisphere changes. The strong effect of albedo feedback obtained by Suarez & Held (1979) suggests that the northern ice cover is also the important factor for the Southern Hemisphere temperatures. Thus although Manabe & Hahn (1977) only obtained a small additional cooling for the Southern Hemisphere from combining the ice age albedos with the ice age sea surface temperatures, the important factor seems to be that the Northern Hemisphere ice cover also influences the sea surface temperatures.

Ice sheets and long term changes of climate and sea level 453

From these various modelling results a consistent picture emerges as follows. The orbital obliquity variations seem to be the most important primary cause of the ice ages through their reduction of summer temperatures in the polar regions by ^ 3°C and by extending the summer snow line in elevated regions south­wards by 4°-5° lat. The longitude of the perihelion with high eccentricities can add to or subtract from the obliquity effects such that for cooler northern summers the reduced radiation could lower the summer temperatures by a further 1°-2°C and displace the snow line a further 2° lat. south for sufficiently high elevations.

The growth of the ice sheets adds substantially to the summer cooling with an albedo feedback effect as large as the initial cooling. The long times required for the ice sheet to grow and decay cause a lag in climate and ice extent from the summer radiation peaks.

The Southern Hemisphere, with its large sea surface area in mid latitudes, responds to annual total radiation changes and to the lower temperature caused by the Northern Hemisphere ice cover, with only a slight feedback from its changing sea ice extent.

This picture of ice and climate changes can now be used to clarify some of the apparent anomalies described in Section 2.

5 RESOLUTION OF ANOMALIES

The problems raised in Section 2 are now considered in the same order.

(a) The question of whether the orbital radiation changes are

sufficiently large to cause the onset and departure of the ice

ages can be answered as follows. The annual or hemispheric

changes are not large enough but the summer changes, particularly

June and July, in the north polar regions, can be large enough,

for sufficiently elevated land areas. Again the ocean response

is much less and must be considered separately. The important

feature is the shifting limit of the seasonal snow line which can

allow the land ice extent to spread, and which then gives rise to

albedo feedback effects which lower the temperature further.

(b) Although the summer radiation totals have been much

further above than below the present totals for the north temper­

ate regions, the reason that the temperatures have not been much

higher is that the high radiation periods occurred when ice

sheets were present. The high radiation levels were needed to

cause the ice sheets to disappear.

(c) The cold glacial periods were synchronous in the temper­

ate zones of each hemisphere, in spite of different summer

radiation regimes, because there is a dominant effect due to the

large northern land masses with their ice cover, and because the

polar sea surface temperatures respond to annual changes which

are in phase in the two hemispheres.

(d) For the same reason the ocean temperature changes were

much smaller than the land temperature changes. In fact the

18 OOO years BP temperature differences from the present

454 W.F. Budd

conditions given by CLIMAP Project Members (1976) reflects very well the concepts of cooler regions poleward of about 40° lat., and with greater differences in the north polar region.

(e) The ocean sediment 6 80 record and the sea level changes can be reconciled if the Antarctic ice contribution is adequately taken into account. The estimate made by Shackleton & Kennett (1975) of the isotope ratios for the mean additional Antarctic ice added during its expansion beyond the present is for a 6 80 of -50°/oo. This value compares well with a modern compilation of the present Antarctic surface isotope distribution by Morgan (unpublished) when the additional ice is added to the existing ice sheet. For the North American ice sheet the mean isotope depletion can be expected to be much less. Although the present day isotopic composition of the precipitation over the North American ice sheet region from observations reported by Dansgaard (1964) and Hage et al. (1975) may be only depleted by & 80 ~ -10°/oo, as argued by Emiliani (1971), the depletion can be expected to increase with ice sheet elevation and distance inland (Lorius & Merlivat, 1977; Siegenthaler & Oeschger, 1980). As the ice sheet began to buildup this depletion may not have changed greatly from that of the present precipitation until the thickness reached more than about 1000 m. In fact there may have been an initial decrease as the ice spread to lower elevations and latitudes. In Terre Adelie for example there is little isotopic change in precipitation up to 1 km elevation, cf. Lorius & Merlivat (1977).

As the North American ice sheet grew the 80 depletion in the precipitation over the high central regions could be expected to be reduced to below ô 80 ~ 30°/oo, but for only a small propor­tion of the ice. From the modelling results of Budd & Smith (1981) the ice sheet did not remain long enough to develop the "steady state" isotope distribution as approached by the Antarctic ice sheet. The average depletion then for the northern ice sheets should be considered as a function of volume or of sea level. Thus the sea sediment 6 0 record needs also to be considered as a nonlinear function of ice volume with a larger weighting for the larger volumes and for the Antarctic component. The following table gives an indication of the isotope volume relation:

Ice volume (106 km3) 5 10 20 30 Average 6180 depletion (°/oo) 12 10 15 17 Sea level contribution (m) 12 25 50 75

The Antarctic ice volume contribution to the sediment ô 80 record therefore needs to be weighted by about a factor of 3 with respect to the average northern ice contribution. Hence the sea level and sea sediment isotope record become more compatible if the Antarctic ice sheet continued building up from 114 000 to 20 000 years BP. This type of result has also been reproduced in a simple model by Imbrie & Imbrie (1980) using a long time constant component. The reason for the long time constant for the Antarctic ice sheet follows from its lower accumulation rate, its response to sea level lowering and its more dramatic reaction to sea level rising, only after the bedrock has been depressed.

Ice sheets and long term changes of climate and sea level 455

The relatively rapid change in the ice core 6 0 isotope profiles from the coastal Antarctic sites in Terre Adelie (Raynaud et al., 1979) and on Law Dome (Budd & Morgan, 1977) suggest an ice sheet response to sea level rise rather than a response to climatic change.

(f) The sixth question concerned the relation between the land ice record, sea level and the sea sediment 6 80. Again the successive maximum ice extents around 110 OOO, 60 OOO and 20 OOO years BP do not suggest greatly increasing ice extent as might be interpreted from the sediment sea level records. The above explanation involving a growing Antarctic ice sheet is consistent with similar ice extent at those times in the Northern Hemisphere.

(g) The apparent lead of the Antarctic sea ice reported by Hays et al. (1976b) relates to the dependence of the Antarctic sea ice extent on the annual radiation budget and on the extent of the Northern Hemisphere ice. This means that variation of the Antarctic sea ice can be expected to follow the climate curve derived by Budd & Smith (1981). This curve lags the radiation but leads the ice extent and thus also the sea level and the ocean sediment isotope record, cf. Figs 3, 5 and 9.

70 60 50 40 30 20 10 0 Fig. 5 Climate and sea level changes — empirical evidence and model comparison. The climate (a) and sea level (b) curves from Klein (1971) for the Siberia-Alaska region are shown together with climate (c) and sea level (d) results derived from the modelling of the North American ice sheet changes by Budd & Smith (1981). Although the empirical dating is not sound beyond 50 000 years BP, similarities exist in the lag of sea level following climatic changes. The line at —46 m represents the required sea lowering to form the Bering Strait land bridge.

Finally, if albedo feedback is so important for the growth ice ice sheets, what makes them disappear? Both Weertman and Pollard (1978) found that if their model ice sheets

became sufficiently large then they would not disappear readily. There are two major contributing factors involved. First, the summer radiation levels reached much further above the present

(h) of the (1976)

456 W.F. Budd

values than below them and from Suarez & Held1s results this causes a nonlinear increased retreat of the snow line. Second, a delayed isostatic bedrock depression was found by Budd & Smith to be a further major cause of an effective relative rise of the snow line with respect to the ice sheet. The reversal of the ice sheet albedo feedback and the rise in sea level, while the summer radiation levels are still high, add to the rapid disappearance of the ice during the final stages.

6 RE-EXAMINATION OF PALAEO EVIDENCE

A thorough examination of palaeo evidence in the various fields of sea level, ice extent and climate, whereby the original data are reinterpreted and the accuracy of the dating assessed, is a task for specialists. Here only a summary appraisal is attempted by making use of other previous summaries and by selecting a few examples of additional sources to try to tie the three fields of study together in the light of the foregoing arguments. The evidence is reviewed in the order of the land ice, the sea level and then the climate.

6.1 Land ice record Porter (1971) has attempted to relate the ice extent of different regions of North America including the Laurentide ice sheet near the Great Lakes, Northeast St Elias Mts in Yukon Territory, Puget-Fraser lowland in Washington and British Columbia, the north-central Brooks Range in Alaska, the Rocky Mts in Colorado and the Sierra Nevada Mts in California. Although minor differences in pattern occur and considerable uncertainty remains for dating beyond 40 OOO years- BP there is a considerable similarity in the major features.

The common features show a maximum ice extent around 20 000 years BP, or later, retreating by 10 000 to 8000 years BP. In most cases a greatly reduced extent occurred in the period 30 OOO-40 000 years BP, and an earlier advanced state was indicated prior to 50 OOO years BP. These features show close similarity with the ice extent obtained from the model by Budd & Smith (1981).

The pattern of retreat of the North American ice sheet given by Bryson et al. (1969) and Prest (1969) shows the termination of the last glaciation in great detail with a similar pattern to those of the above regional studies. A corresponding pattern of the retreat of the last European ice sheet is given by Zonneveld (1973). Ten Brink & Weidick (1974) report a similar chronology for the retreat of the Greenland ice sheet.

Kind (1969) and Klein (1971) show similar changes for the Siberian ice extent in comparison with that of North America and Europe. Longer term changes in the Beringa-Alaska region are given by Hopkins (1973) and von Huene et al. (1976).

A summary of the changes over different regions is presented by Dreimanis & Karrow (1972). Again the similarity of the patterns is quite striking with prominent peaks of ice extent around 20 OOO and 60 000 years BP.

Ice sheets and long term changes of climate and sea level 457

The pattern for the next earlier advance is not so well documented because it is well beyond the carbon dating range and very few other dates are. available. Nevertheless a pattern of three or more glaciations of which the third one was of similar, if not larger magnitude is a feature of several regions. The following references serve for illustration: White River valley in Alaska, Denton (1974); the northeast coast of Baffin Island, Miller et al. (1977); Europe and North America, Morner (1977), Andrews & Barry (1978); Mauna Kea in Hawaii, Porter et al. (1977); Tasmania, Colhoun (1976); Antarctica, Denton et al. (1971). At this stage it is still not clear how the ice extent matches with the ocean sediment isotope data back beyond 50 OOO years BP. Nevertheless the results of the ice sheet modelling from the radiation variations, and also the sea level changes, suggest that a maximum extent around 110 000 years BP was most likely. Figure 6 presents a summary compilation of a selection of these studies. Karlstrom (1966) has put forward a worldwide glacial classification system with three major glacial and interglacial periods since 130 000 years BP which provides a chronology in close agreement with the general picture of ice extent presented here.

6.2 Sea level, ice volume, and sea sediment isotopes Since reviews of past sea level data and the problems of 180/160 interpretations are given by Chappell (1981) and Newman et al. (1981) the aim here is just to attempt to tie together the sea level record with the ice volume and sea sediment records. Measurements of isotope ratios in sea sediment cores have pro­vided a valuable indicator of changes over the period of the ice ages. Although there are many problems such as variability bet­ween cores, errors of dating, and difficulties of interpretation, some patterns of general similarity are emerging. A discussion of the problem of interpretation is given by Duplessy (1978). Figure 9(b) shows a selection of sediment records given by Duplessy for the last 120 OOO years.

Emiliani (1958, 1966) first interpreted his 1 80/ 1 60 records as indicators of ocean surface temperature change and also suggested a close correspondence with the Milankovitch type radiation variations. Fairbridge (1960) indicated a further close similar­ity between the isotope records and his estimates of past sea level changes.

Rosholt et al. (1961) established a preliminary time scale for the sediment record from Z 3 1Pa/ 2 3 0Th dating. This dating has been disputed by Broecker & Ku (1969) and Broecker & van Donk (1970) who concluded that the original time scale should be stretched about 25% to place the most prominent previous minimum "ice extent" at about 120 OOO years BP rather than about 95 OOO years BP. These latter authors also claim close correspondence between the sediment record and the sea level dates from Barbados, (Broecker et al., 1968; Mesollela et al., 1969). Considerable controversy has arisen concerning these time scales and for the date of the "last interglacial", i.e. the last previous occasion with ice volumes and temperature comparable to those prevailing after the disappearance of the ice sheets, cf. e.g. Suggate

458 W.F. Budd

Alaska ,N ^

N. America # '->

N/ i f}

-,/i,/ Europe

N.E.I \

.' 1 (ft Jknl%--J/

' ; Hawaii

' '

s~^

•' \ Tasmania ^ - x

/

A v_/\

\^

r*\ \ i \ J l

\M\ A

\ ^

LJf\ r\

\A r\

\J \ 5,° , , . , 9

Age 10Ja 8 P

Fig. 6 Land ice changes. The variation in ice extent as a function of time over different parts of the globe are estimated from various studies as in: Alaska (Denton, 1974), North America and Europe (Môrner, 1977), Hawaii (Porter era/., 1977) and Tasmania (Colhoun, 1976 and unpublished). The dashed lines indicate indefinite dating control or ice edge position. Approximate distance scales for ice extent are given for Alaska, North America, and Europe. For Hawaii and Tasmania the scales show changes of equilibrium line elevation (ELA) and temperature respectively.

Fig. 7 Computed sea level changes. A total sea level curve (c) has been derived from ice volumes computed for (a) Antarctica (from the Byrd isotope curve with the Clark & Lingle, 1979, ice volumes), (b) the Northern Hemisphere ice sheets where 3 is the total, 2 is the North American contribution, and 1 is the remaining contribution.

Ice sheets and long term changes of climate and sea level 459

(1974). Emiliani's date for the interglacial was about 95 000 years BP while Broecker & Ku give ~ 125 000 years BP.

The Emiliani pattern with the sea level as portrayed by Fairbridge does look attractive when compared with the ice volume curves of Budd & Smith (1981) and the radiation changes.

Since Fairbridge's (I960) analysis further data have been obtained on dates of sea level stands such as on Barbados (Broecker et al., 1968; Steinen et al., 1973), Micronesia (Curray et al., 1970), New Guinea (Bloom et al., 197'4; Veeh & Chappell, 1970; Chappell, 1974b), and East Timor (Chappell & Veeh, 1978a). Despite difficult interpretation due to the inaccuracies in the dating and the estimates of the magnitude of the sea level changes with the tectonic movement removed, the results obtained by Steinen et al. (1973) for a lowering of ~ 70 m between 120 000 and 105 000 years match the calculated ice volume curve quite closely.

The high sea level at about 82 000 years BP corresponds closely to the computed minimum North American ice sheet volume. The same holds for the high ~ 40 000 years BP and the low levels around the periods 115 OOO, 70 000, and 20 OOO years BP. On the other hand, the high levels at 115 OOO, 60 000, or 30 OOO years BP are not obtained. The computed North American volume does show a still-stand about 95 000 years BP which may be within the range of the difference between the astronomic and radioactive dating time scales. With regard to magnitudes of the sea level changes, the difference from the 120 000 years BP level of the computed North American ice sheet volume (corrected for bed depression) gives about 30 m. If other ice masses are taken into account this change in level could be much larger. Such larger differences could be explained from the New Guinea data if a uniform uplift rate were used between the present and 120 OOO years BP.

The sea level data need to be interpreted in terms of changes in the different ice masses which can be expected to react quite differently, in particular the north polar ice sheets, the north temperate glacier areas (of the European Alps, Caucasus, and Himalayan complex), and the Antarctic ice sheet.

The Antarctic ice sheet can be expected to grow as sea level lowers, but not retreat dramatically with sea level rises until the bedrock is depressed. This gives an Antarctic ice volume change as shown in Fig. 8, where the Byrd ice core isotope data from Epstein et al. (1970), referred to a time scale and the difference from steady state flow deduced by Budd & Young (1981) has been used as a guide for ice volume change. The temperate glacier regions could give rise to fluctuations of ~ 3 m in sea level in phase with the temperate radiation curves as shown in Fig. 3.

The combined effect of these then gives a close agreement to the sea level variations as summarized by Bloom et al. (1974). Figure 8 summarizes past sea level estimates from recent studies which can be compared with the curves computed from the modelled ice sheet volume changes, Fig. 7.

For the sea sediment isotope records the above different ice mass volumes need to be weighted differently, and for the North

460 W.F. Budd

Fig. 8 Empirical sea level changes. Estimates of past sea levels are shown derived from various sources as follows: (a) Veeh & Chappell (1970), (b) Steinen eta/. (1973) (c) Bloom et al. (1974), (d) Shi & Wang (1981), (e) 1, Emery et al. (1971), 2, Geyh et al. (1979) averaged, 3, Chappell & Veeh (1978b), 4, combined dated points derived from Chappell (1974b), Steinen et al. (1973), Bloom et al. (1974).

Antarctic ^ ^ ^

^ ^

Yr^\/~\ V \y \ Northern Hemisphere

Total \ _ / \

Age 103 a B P \

/ Q2-

J 0.4-

/ 02-

/ 0.4-

0.6-

/ 02-

1 04-

f 0.6-

/ 0.8-

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120 100 80 60 40 20 0

Fig. 9 (a) Derived ocean isotope curves. Using the ice volume changes of Fig. 7 and separate isotope depletion factors for Antarctica (—50%o) an~' th-1 ^ r t h e r n Hemisphere (—17 /00) a total isotope curve is derived.

(b) Observed ocean sediment isotope profiles. Oxygen isotope changes (5180%o) in ocean sediments for different locations are shown as given by Duplessy (1978) as follows: 1, Atlantic Ocean (Caribbean); 2, Pacific Ocean; 3, Indian Ocean; 4, Mediterranean.

American and Eurasian ice sheets the increased depletion with age (or volume) needs also to be considered. Such a combination is shown in Fig 9 (a).

The low ice volume and relatively high sea level around 40 OOO years BP with values of the sea level lowering between O and 40 m are supported by many studies including Curray (1965) , Milliman & Emery (1968), Broecker et al. (1968), Bloom et al. (1974),

Ice sheets and long term changes of climate and sea level 461

Chappell (1974b), Shi & Wang (1981). Although Thorn (1973) shows there are problems in interpreting these data, Chappell (1981) indicates that the rapid uplift regions such as New Guinea where reliable dating is available, provide a sound basis for the relatively high sea level i.e. ̂ -40 m below present, compared to a maximum lowering to ̂ -150 m at ̂ 16 OOO years BP.

The problem of the discrepancies between the sea level data and the ocean sediment isotope data is diminished when the effect of the Antarctic changes is considered with appropriate weighting. Effects of sediment mixing as obtained by Peng et al. (1977), giving rise to apparent smoothing of the isotope records by 'v- 10 000 years, need also to be considered.

The increase in magnitude of the three peaks of the isotope and sea level records from 110 OOO to 70 OOO and 20 000 years BP compared to the similar sized peaks of the northern ice extent, which appear to be of more similar magnitudes, seems to be attributable to the growth of the Antarctic ice sheet, cf. Hughes (1973a, 1973b and 1975) and Clark & Lingle (1979). The common estimates of maximum sea level lowering range predominantly between 160 m (Veeh & Veevers, 1970) and 90 m (Shepard & Curray, 1967), of which the Antarctic could provide 'v 25 m. This growth of the Antarctic ice sheet could have been from 120 OOO to 20 000 years BP. The first part to 80 000 years BP may have included the reforming of the West Antarctic ice sheet with consequently less effect on the sea level or isotope records. Estimates of the ice volumes of the different ice masses for the most recent glacial maximum are given by Flint (1971), Hughes et al., 1977, and Grossvald (1979). A sea level lowering of at least 130 m seems plausible from these estimates.

6.3 Analysis of climatic indicators There are large numbers of sources of past climatic indicators but problems exist in all cases for first dating the record and secondly interpreting the climatic indicator. Some form of present day calibration and certain assumptions on how the situation may have changed in the past are also necessary. In some cases gross changes of interpretation of indicators have occurred. One example of such a change is the study of ocean sediment 80/ 0 ratios. These were first considered as changes of ocean temperatures, and then of volume of the ice sheets and now as a complex indicator of the integral of different changes in different ice sheets. Similarly the ice core isotope ratios originally suggested as climatic temperature indicators are now considered to have strong components of ice surface elevation change (partly due to ice flow and part due to ice thickness change) and also to reflect shifts in precipitation patterns. In spite of such difficulties many of the climatic indicators are showing similarities which reinforce the interpretations and the history obtained from other indicators such as the ice extent and sea level.

Only a few examples are discussed here in order to show how diverse indicators from different parts of the world have many features in common. For the details of the interpretation the original articles need to be referred to, as well as the

462 W.F, Budd

specialist articles on the methodology of interpretation. For example in the case of land indicators, Bowler et al. (1976) provide a guide, while for ocean material Imbrie & Kipp (1971) and CLIMAP Project Members (1976) outline the techniques used for the CLIMAP reconstructions.

For the north polar region Klein (1971) summarizes a wide range of different indicators including Kind's (1969) Siberian data and European data from Coope & Sands (1966) for England, and Zagwijn & Paepe (1968) for the Netherlands.

Sancetta et al. (1973) present a combined study of past indicators from a deep sea core in comparison with land record data. Similarly for the Southern Hemisphere Hays et al. (1976b) provide indicators of past changes north and south of the Antarctic convergence.

For the land record in the Southern Hemisphere a tropical situation has been given by Bowler et al. (1976) for the New Guinea highlands, and a temperate zone has been given by Heusser (1974) for southern Chile. The use of isotope measurements on speleothems has been used by Wilson (1978) to derive past temperature changes in New Zealand.

For the south polar regions ice core isotope ratios have been used as indicators of past temperature changes for Byrd (Epstein et al., 1970), Vostok (Barkov et al., 1977), Terre Adelie (Raynaud et al., 1979), Law Dome (Budd & Morgan, 1977), Dome C (Lorius et al., 1979). Again it is important to separate climatic temperature change from the changes in the ice sheet elevation. For the elevation changes the gas volumes of the cores have been used as indicators (Raynaud & Lorius, 1977; Budd & Morgan, 1977; Raynaud & Lebel, 1979).

Other indicators which have been used include loess layers (Kukla, 1970)., sand dunes and lake levels (Bowler, 1978), lake levels (Eardley et al., 1973; Grosswald, 1979), and archaeology (Rouse, 1976).

Besides the extent of ice,other glacial indicators of climate include: altitude of snow lines, elevation shifts of vegetation, migration of permafrost zones, dust and chemicals in deep ice cores.

A summary of a selection of some of the various climatic indicator records is given in Fig. 10.

A pattern tends to be common to most of these indicators and reinforces the history from the land ice and sea level records. It comprises a generally cold period ^ 60 000 years BP followed by a warmer period % 40 OOO years BP before the final maximum cooling between 25 000 and 20 000 years BP. The slightly warmer period compared with the present ^ 8000 years BP is also generally depicted.

The record of temperature and sea level changes given by Klein (1971) for Siberia shows the sea level lag very similar to the computed results of Budd & Smith, cf. Fig. 5. This phase delay was used by Klein to explain the limited conditions for migrations from Asia to North America. The conditions for communication of people north and south of the ice sheet over Canada as discussed by Rouse (1976) also seem to fit the general picture described here. These climate, ice and sea level changes

Ice sheets and long term changes of climate and sea level 463

Fig. 10 Climatic indicators. Various indicators of past climate changes are illustrated from the following sources. England (Coope & Sands, 1966) China (Shi & Wang, 1981), North Atlantic (Sancetta et al., 1973), New Guinea (Bowler et al., 1976) South America (Heusser, 1974) New Zealand (Wilson, 1978), Antarctica (Budd & Young, 1981) from the Byrd isotope data of Epstein et al. (1970). Broken curves indicate indefinite dating control. The units are °C from present conditions.

are also of interest to the study of migration of peoples of other lands such as Europe and Australasia.

Since these changes probably represent the most dramatic environmental changes that have taken place over the Earth during man's history, it is of considerable interest that the nature of the changes and their causes are understood.

7 CONCLUSIONS

Strong evidence now exists that the Earth's orbital radiation changes provide the primary cause for the initiation and termin­ation of the ice ages. However, for these events the ice sheets themselves play the major role by modulating the climatic conditions to an even greater degree than the primary radiation causes.

The various modelling studies indicate many factors which must be taken into account to simulate the changes of climate, the ice sheets and sea level. For the radiation conditions these factors include: the annual, summer and June-July radiation regimes, the effects of land-sea contrasts, latitudinal differences and the response of the snow and ice cover.

For the ice sheets it is essential to consider the effects of topography, snow line elevations, albedo feedback, elevation desert effects and a delayed bedrock depression rate. The sea level changes induced by the Northern Hemisphere ice sheets are

464 W.F. Budd

the primary driving force for changes of the Antarctic ice sheet. The slow buildup time and delayed bedrock depression are also important for the Antarctic ice sheet response.

The Northern Hemisphere ice sheets and the Antarctic ice sheet need to be considered separately for the interpretation of ocean sediment isotope records.

Indications of past changes of climate, sea level and ice extent are tending to show some similarity for the phases of warm and cold or high and low ice extent over the past 120 000 years. A concensus of these changes agrees reasonably with the results derived from modelling the ice sheet reactions to the orbital radiation changes.

Nevertheless some important problems still remain. Although some consistency prevails for large ice extent around 20 000 and 50 000-70 000 years BP the minimum occurring around 40 000 years BP is not yet clear. Similarly the extent of the changes from 120 OOO to 80 OOO years BP needs clarifying. These problems together with the progress made to date suggest that it is now appropriate to combine the various climate and ice models in the manner attempted by Sergin (1979) but including the specific geography and ice modelling of Budd & Smith (1981).

The North American, Eurasian and Greenland ice sheets need to be studied first and then a sea level forcing function can be used to model the reaction of the Antarctic ice sheet. Finally, a fully coupled global ice, climate and sea level model could be instituted. In such a study it is now clear that the ice sheets play the central role in the long term changes of climate and sea level.

It is of special significance that the large ice sheets of Greenland and Antarctica also contain in their sedimentary layers of ice the most promising records for unravelling the history of the past changes of sea levels, the ice sheets and the climate.

ACKNOWLEDGEMENTS The author is indebted to a number of colleagues for contributions to this work. Dr D. Jenssen carried out the computations of the detailed radiation deviations. Mr I. N. Smith provided results of further ice sheet modelling studies, and Dr I. H. Simmonds provided further results of his atmosphere-sea ice-climate modelling studies.

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